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Car Forum / Driving, Maintenance, Tuning / Car Audio / June 2006Acoustic wadding.
I'm a novice as far as audio is concerned, so bear with me. Have a 12" bass speaker fitted in a homemade infinite baffle cabinet. Speaker manufacturer recommends cab is filled with acoustic wadding. Got some wadding. Now I need to know how it should be fitted. I notice the 3/4" thick stuff I bought is very open and springy. Just stuffing it in would obviously interfere with the speaker cone, so should it be cut so it doesn't touch the cone at all, or what? If someone can point me at a website, or offer any advice. As an aside what is the effect of having acoustic wadding in a speaker cabinet? As opposed to leaving it empty. TIA. Mike. What you are calling "acoustic wadding" I think is the same as adding padding to the walls of a subwoofer enclosure. I have never heard the term "wadding" before, hmmm. Another common technique is to fill the sub enclosure with polyfil (the same stuff as in pillows). Here's the purpose, and I'm going to start at the basics: Any cone type speaker (including subwoofers) produces soundwaves both forward and behind. The sound waves produced behind are 180 degrees out-of-phase with the sound waves produced in front. This makes sense if you think about it, as the cone moves forward in the front of the sub, it is moving backwards behind the sub. This produces a sound wave exactly out of phase. If these two sound waves were allowed to interact, they would cancel each other out. This is THE MAIN REASON why enclosures are necessary when building subwoofer systems, to keep the rear sound waves from canceling out the waves produced by the front. So inside the subwoofer enclosure, obviously much sound is being created. The purpose of acoustic wadding or polyfil is to absorb this sound energy. Why is this important? Because if you don't you can have very strange acoustic phenomena going on in your enclosure, the main concern being what are called "standing waves". This can create strong reinforcement (boosts) at certain frequencies and effect the motion (and sound) of the cone. Also, you can have cancellation effects going on inside the enclosure which will again affect the motion of the cone. Remember, ideally, the subwoofer enclosure should JUST be a volume of air to act as a kind of a spring for the subwoofer (acoustic suspension). Ported enclosures are a little different as they utilize a tuned port to emphasize certain frequency bands (bass reflex) and this port has the effect of realigning the sound coming out of the port so they are in-phase with what is being produced by the front of the sub. Ported enclosures can produce more SOL (they are a little louder) because they are able to utilize some of the sound energy that is always lost in a sealed enclosure. But THE MAIN POINT of any kind of polyfil or "acoustic wadding" or whatever you want to call it is to try and produce (as close as possible) an anechoic (no echoes) chamber as echoes CAUSE PROBLEMS (as I already mentioned). I'm not an engineer so I've probably flubbed a few things, but basically I think what I have said is correct. I hope this helped. MOSFET > I'm a novice as far as audio is concerned, so bear with me. > Have a 12" bass speaker fitted in a homemade infinite baffle cabinet. [quoted text clipped - 9 lines] > TIA. > Mike. > What you are calling "acoustic wadding" I think is the same as adding > padding to the walls of a subwoofer enclosure. I have never heard the [quoted text clipped - 52 lines] >> TIA. >> Mike. As a side note, if the speaker is in a box, it isn't infinite baffle. Rob >>> I'm a novice as far as audio is concerned, so bear with me. >>> Have a 12" bass speaker fitted in a homemade infinite baffle cabinet. [quoted text clipped - 11 lines] > > As a side note, if the speaker is in a box, it isn't infinite baffle. I thaught that was the common name for a sealed speaker enclosure. Although I understand that technically it is incorrect. Mike. > What you are calling "acoustic wadding" I think is the same as adding > padding to the walls of a subwoofer enclosure. I have never heard the > term "wadding" before, hmmm. Another common technique is to fill the sub > enclosure with polyfil (the same stuff as in pillows) Speaker acoustic wadding was how it was described in the shop. > Here's the purpose, and I'm going to start at the basics: > [quoted text clipped - 15 lines] > you can have cancellation effects going on inside the enclosure which will > again affect the motion of the cone. So far I'm with you. It all makes sense. > Remember, ideally, the subwoofer enclosure should JUST be a volume of air > to act as a kind of a spring for the subwoofer (acoustic suspension). [quoted text clipped - 5 lines] > utilize some of the sound energy that is always lost in a sealed > enclosure. A little more technical, but I think I'm still with you. :-) > But THE MAIN POINT of any kind of polyfil or "acoustic wadding" or > whatever you want to call it is to try and produce (as close as possible) > an anechoic (no echoes) chamber as echoes CAUSE PROBLEMS (as I already > mentioned). I'm not an engineer so I've probably flubbed a few things, > but basically I think what I have said is correct. I hope this helped. Certainly did. So If I've understood correctly. Ideally the volume behind the cone should be filled as completely as possible without actually interfering with the movement of the cone. Thank you. I think I understand speakers a little better now. Possibly it explains why, with the speaker in an empty cabinet, certain frequencies seem distorted. Sort of boomy, without any clarity, for want of a better discription. Thanks again. Mike. >What you are calling "acoustic wadding" I think is the same as adding >padding to the walls of a subwoofer enclosure. I have never heard the term [quoted text clipped - 20 lines] >going on inside the enclosure which will again affect the motion of the >cone. Unless the woofer is reproducing up into the midbass region, stuffing is not required. The bass frequencies do not need to be damped. >Remember, ideally, the subwoofer enclosure should JUST be a volume of air to >act as a kind of a spring for the subwoofer (acoustic suspension). Ported [quoted text clipped - 4 lines] >more SOL (they are a little louder) because they are able to utilize some of >the sound energy that is always lost in a sealed enclosure. A port is not in phase with the drivers output. Below the frequency where they share output, most all of the lower bass comes out of the port only. >But THE MAIN POINT of any kind of polyfil or "acoustic wadding" or whatever >you want to call it is to try and produce (as close as possible) an anechoic >(no echoes) chamber as echoes CAUSE PROBLEMS (as I already mentioned). I'm >not an engineer so I've probably flubbed a few things, but basically I think >what I have said is correct. I hope this helped. Thats true, and stuffing can also damp flimsy enclosurers. There is another effect of course, a very important one. In that stuffed enclosure the manufacture may have specified a volume, and the stuffing acts to increase that volume.If you don't stuff that enclosed box, it may be too small. Stuffing is all very different and has different effects at different frequencies. I make use of cotton, wool, fiberglass, and Dacron, and very important, FOAM. greg >MOSFET > [quoted text clipped - 11 lines] >> TIA. >> Mike. > There is another effect of course, a very important one. In that stuffed > enclosure [quoted text clipped - 7 lines] > > greg OK, this might be a gap in my knowledge, but I have ALWAYS been confused by the assertion that in a box "stuffing acts to increase that volume" to use your words. To me, that defies the laws of physics and I flat out refuse to use that term. I have ALWAYS believed that this was just a shorthand way of describing the effects stuffing can have (reduced acoustical interchange that causes problems like standing waves) which I DID address in my explanation. By reducing sound waves bouncing around in a box, THAT IS like having a larger box. So in a sense, I did cover that in my explanation. Is there something else I'm missing? MOSFET Adding loosely packed materials like fiberglass or long fiber wool to the interior of a box changes the the effective air mass which affects box resonance. It also absorbs somewhat the higher frequency standing waves (although in car sub boxes that is rarely an issue) but that is not the reason for it making the box act acoustically larger, its the change to resonant mass. That is why materials like the wool or fiberglass work better than polyfill which has lower mass per given volume. To the prevous poster, high-quality plywood can make an excellent enclosure. Generations of DIY home speaker builders have known this, and in fact a lot of older hifi enclosures are made with plywood. 13-ply Baltic birch is a lot tougher lb. for lb. than high-density or medium-density fiberboard. It's mostly the kids raised on car audio in the 90's and subsequently that think MDF is the god's choice of box material. It takes more experience and skill to craft a box well with plywood but it will also probably outlast the MDF box by many years if you take the time to do it right. JD >>There is another effect of course, a very important one. In that stuffed >>enclosure [quoted text clipped - 20 lines] > > MOSFET > Adding loosely packed materials like fiberglass or long fiber wool to the > interior of a box changes the the effective air mass which affects box > resonance. Box resonance? You DON'T WANT the box to resonate! It also absorbs somewhat the higher frequency standing > waves I DID address this already. (although in car sub boxes that is rarely an issue) but that is > not the reason for it making the box act acoustically larger, its the > change to resonant mass. That is why materials like the wool or fiberglass > work better than polyfill which has lower mass per given volume. Hmmmm. I really don't think changing the resonant mass has a lick to do with ANYTHING. The ideal speaker enclosure has ABSOLUTELY NO RESONANCE to it. Have you ever rapped your knuckles against a REALLY EXPENSIVE set of high-end speakers? It's like knocking on concrete. I mean, THINK ABOUT IT, if your enclosure is resonating in any way, this represents energy that is NOT being turned into sound. This is why we use sound dampening material in our cars, to reduce resonance as this will increase SPL's. Have you ever been to an SPL competition? You will see these guys leaning against their cars to try and reduce the body panels from flexing, body panels flexing wastes energy that SHOULD be sound waves, by leaning against their cars they can stop the panels from resonating and gain a few more decibels. The same goes for speakers. I think your explanation has some holes. MOSFET Not the box, dude, the air mass. When you lower the resonant point of the air mass by adding stuffing to it, you lower the Fb or tuning point of the box at the same time. Has the same effect as increasing the box size assuming the box starts out sized below nominal flat response, which is typically the case in car enclosures. If it's already sized above that, increasing the box volume will have the opposite effect, and make it sound more anemic at low frequencies. You should forget all this crap about the box panels resonating, as you have to build a pretty flimsy enclosure for a subwoofer before panel resonance comes into play. It's an issue for full-range enclosures where the panel resonance causes coloration at higher frequencies but that's not going to happen in the sub-100 Hz range where car sub boxes are being tuned. Pretty nice rant though, given how wrong you are. I recommend picking up a copy of Vance Dickason's Loudspeaker Design Cookbook & giving it a read. I was quoting from another good read on speakers and enclosures, Collums' High Performance Loudspeaker, but the LDC has a lot more car audio-specific content. http://www.amazon.com/gp/product/1882580338/002-6336208-9252046?v=glance&n=283155 JD >>Adding loosely packed materials like fiberglass or long fiber wool to the >>interior of a box changes the the effective air mass which affects box [quoted text clipped - 28 lines] > > MOSFET >>There is another effect of course, a very important one. In that stuffed >>enclosure [quoted text clipped - 18 lines] > > Is there something else I'm missing? Nope, you pretty much nailed it. The stuffing DOES have the effect of slowing down the sound waves as well, which makes the box "appear" larger to the driver (internal waves take longer to reflect back). "coughbullshitcough" Guys, all this hokum-smokum is making me go TWFER on you... I had to bust out the textbooks. Here's some facts: Quote: "Chipboard is the densest of all the wood materials and as it comes in reasonably priced, uniform sheets which resist warping [sic] it is not surprising that the great majority of speaker cabinets today are constructed from this material." Please note emphasis on cost. Quote: "high quality birch plywood is also frequently encountered as an alternative to chipboard, although both its 'Q' and more particularly its cost are greater." Again, emphasis on cost. The density of chipboard (MDF is one form of chipboard, and stands for Medium Density Fiberboard, not High Density)) is given as 0.81 x 10^3, where plywood is slightly lower at 0.67 x 10^3. Solid oak is between the two, solid pine well below plywood. Aluminum is rated at 2.6 x 10^3 and steel at 7.7 x 10^3 yet you do not run into many enclosures made from those, at least not in car audio. Aluminum is used a lot in small bookshelf speakers where it provides denser enclosures in small cabinets than you can build in wood. Concrete, brick, and sand all have better densities than chipboard but again not used a lot in car audio, although I did have the privilege once of hearing a Caddy hearse with an enclosure stuffed full of 15's that was built from two layers of 1" MDF with sand poured between them. That was one dead enclosure as far as resonance. Quote: "Speaker enclosures [sic] may employ self supporting volume fillers such as wool-felt, resin-bonded fiberglass, or long-haired wool, these also providing a degree of damping at the fundamental resonance." This is due to the increased resonant mass which includes box air mass plus stuffing mass, not because the stuffing "reduces sound waves bouncing around in a box". Standing waves are essentially a non-issue in subwoofer enclosures for cars anyway; they tend to be at higher frequencies than you are sending the box in the first place, assuming a reasonable LP filter in the systme ahead of the box. Using damping pads or other means to reduce coloration caused by box panel vibration is really only meaningful in full-range systems. Stuffing the box just lets you push the envelope by a little on box size, just as using passive radiators lets you tune small boxes to lower frequencies than a comparable vented box could achieve without extremely long ports or excessive windage (port noise). JD thus endeth the lecture >>> There is another effect of course, a very important one. In that >>> stuffed enclosure [quoted text clipped - 23 lines] > slowing down the sound waves as well, which makes the box "appear" > larger to the driver (internal waves take longer to reflect back). WHAT THE HELL DOES RESONANCE HAVE TO DO WITH THE INSIDE OF A SUBWOOFER? Sure, the subwoofer itself has a natural resonant frequency that one needs to know when calculating T/S parameters. But resonance is a BAD THING when it comes to enclosures. IT IS WASTED ENERGY!!!! THINK ABOUT IT! MOSFET > "coughbullshitcough" > [quoted text clipped - 77 lines] >> slowing down the sound waves as well, which makes the box "appear" larger >> to the driver (internal waves take longer to reflect back). > WHAT THE HELL DOES RESONANCE HAVE TO DO WITH THE INSIDE OF A SUBWOOFER? > [quoted text clipped - 3 lines] > > MOSFET Drivers resonate, boxed or not. Panels of an enclosure resonating are another matter.  Signature-Cyrus *coughcasaucedoprodigynetcough* > Drivers resonate, boxed or not. > > Panels of an enclosure resonating are another matter. Exactly right, well said. MOSFET Resonance is wasted energy? Jeez, I better dump my JBL L150A's with passive radiators & L112's with vents, cause they rely on WASTED ENERGY to extend the usable LF response a good half-octave and that's apparently a BAD THING. Again, just forget everything you ever heard about box resonance that somehow manifested itself into a concern about the enclosure panels vibrating, at least as regards car audio subwoofer applications. It's a non-issue at these frequency ranges. You need to crack a book and learn about enclosure physics. What did you think that abbreviation Fb means? It's the box resonant frequency, and the mass of air in the box (or in a port for that matter) is what determines the frequency. Adding stuffing changes that mass, lowering Fb. I don't understand how any of this is confusing... as RG points out, all of these issues are well understood in the speaker community for years now. Any decent text on enclosure design would tell you the same thing I am. JD > WHAT THE HELL DOES RESONANCE HAVE TO DO WITH THE INSIDE OF A SUBWOOFER? > [quoted text clipped - 85 lines] >>>slowing down the sound waves as well, which makes the box "appear" larger >>>to the driver (internal waves take longer to reflect back). OK, OK. So you're saying adding stuffing reduces the resonance of the volume of air inside the box (not the box itself). Is that right? And this simulates a larger box? Is this correct? MOSFET > Resonance is wasted energy? > [quoted text clipped - 109 lines] >>>>slowing down the sound waves as well, which makes the box "appear" >>>>larger to the driver (internal waves take longer to reflect back). It lowers the resonant frequency of the air mass, similar to putting the sub in a larger box. I don't recall whether the amplitude of the resonance is altered in the process but the frequency is. It's not a matter of absorption as I think you may have been thinking, it's simply changing the behavior of the air mass. JD > OK, OK. > [quoted text clipped - 117 lines] >>>>>slowing down the sound waves as well, which makes the box "appear" >>>>>larger to the driver (internal waves take longer to reflect back). > Resonance is wasted energy? > > Jeez, I better dump my JBL L150A's with passive radiators & L112's with > vents, cause they rely on WASTED ENERGY to extend the usable LF response a > good half-octave and that's apparently a BAD THING. Nice! You don't see the L150A's too often! I too, Own a set of L112's :) Chad Nice pair of L150's going on ebay for $721 with a few hours left... you gotta love the way these things have held their value, not to mention how damn good they still sound. I'm up to 3 pair of the 150A and 1 pr. of L112. One of the pair of 150A's got aborted by someone with eyebolts through the back (4 per cabinet) so I figured I'd hang them in a dead space above the doors in the garage, that should be sufficient to screw with the neighbors :-) May also try throwing together a 5.1 surround system upstairs just for the hell of it, think about three L150A's for L/C/R and a pair of L112's on orig. JBL stands for rear surround, with 300W bridged Sony TA-55ES amps on each of the front channels & a Sansui 717 to drive the rear surround. Only problem is what to use for a sub to keep up with all that - maybe the 18 some guy was selling in a 6 cu. ft. enclosure recently. Probably cost me a wife if I build this thing & fire it up, but might be worth it! JD >>Resonance is wasted energy? >> [quoted text clipped - 7 lines] > > Chad That's frankly amazing. $721 for a pair of 25 year old JBL speakers (let's face it, JBL is no Martin Logan) which appear to be banged up a bit. Holy cow! Those must be REALLY good speakers. Some of those older JBL speakers are really great. I own a pair of JBL P10 bookshelf speakers that I bought about 15 years ago and I have NEVER yet heard a speaker of it's size that I felt sounded better. I used to use them (with a subwoofer, of course) on 4' sand-filled stands in my "listening room" and the imaging was ABSOLUTELY STARTLING. To show them off I used to play the beginning of Pink Floyd's "Time". I remember for most of my friends, this was the first time they had ever heard two speakers "disappear" and TRULY create a near 360 degree soundfield. Those speakers are now relegated to surround duties in my home-theater, but they are still going strong AND they handle all the power my Denon AVR-3600 can dish out (on a full range setting!). I believe JBL continues to produce high-quality speakers, but I agree there was something great about those older ones (especially when considering what they compared to at the time). MOSFET > Nice pair of L150's going on ebay for $721 with a few hours left... you > gotta love the way these things have held their value, not to mention how [quoted text clipped - 28 lines] >> >> Chad > I believe JBL continues to produce high-quality speakers, but I agree > there was something great about those older ones (especially when > considering what they compared to at the time). > > MOSFET Their best stuff is the older stuff because that's when the consumer and professional division ACTUALLY got along. There WAS a lot of shared engineering. Not anymore :( Chad Closed at $990... and this is the L150, which I think is not quite the equal of the 150A due to difference in tweeter design plus with some cabinet damage. That's damn near 0% depreciation regardless... Newer JBL speakers lack any of the unique and special qualities that make that kind of resale value stand up, my opinion. Not that they suck, but they just lost the aura they had back then when the guys in engineering had the reins instead of the sales & marketing dept's. JD > That's frankly amazing. $721 for a pair of 25 year old JBL speakers (let's > face it, JBL is no Martin Logan) which appear to be banged up a bit. Holy [quoted text clipped - 51 lines] >>> >>>Chad > Nice pair of L150's going on ebay for $721 with a few hours left... you > gotta love the way these things have held their value, not to mention how [quoted text clipped - 16 lines] > > JD That will be a nice theater! Most of my power at the ranch comes from Crown, with spatterings from Carver (Clair Bros. Modified) and Phase Linear (FlameLinear). For A sub I use an Electro Voice EVX180B in a big TL enclosure and it will shake your bones and play flat VERY low. For aesthetics may I suggest 1-2 JBL 2245's in a big-ol enclosure? They are great drivers and will play very L-O-W! They were used mostly in theatres as effect drivers, you will need to find original baskets and have them reconed. Most of them have foam-rot. Recone Kits are available. The part is C8R2245. Orange county speaker can hook you up. I THINK the 2240 baskets are the same but I can't remember. 2240's will be easier to come across as they sold an a.s load of them. The 2245 is not a power handling monster and care must be taken in setting the HP filter for the box tuning. They have mass rings and when they unload they go a-bounding! Don't worry about the handling though. They are quite efficient and can handle the peaks with ease. I'm sure you know what you are doing :) Chad > Nope, you pretty much nailed it. The stuffing DOES have the effect of > slowing down the sound waves as well, which makes the box "appear" larger > to the driver (internal waves take longer to reflect back). Thanks Matt. That has always been my suspicion, but I appreciate you validating my theory. MOSFET It may be your suspicion, but it is still wrong and is far from being "validated". Mr. Durbin's explanation is correct. The loose stuffing inside a box does not make the box appear larger by reducing so-called standing waves. The concept of how it works is not new and has been studied by speaker builders for years. Do some research and stop trying to change the subject by arguing whether resonance is good or bad. That was not what Durbin was saying. - RG >> Nope, you pretty much nailed it. The stuffing DOES have the effect of >> slowing down the sound waves as well, which makes the box "appear" larger [quoted text clipped - 4 lines] > > MOSFET > It may be your suspicion, but it is still wrong and is far from being > "validated". Mr. Durbin's explanation is correct. The loose stuffing [quoted text clipped - 3 lines] > change the subject by arguing whether resonance is good or bad. That was > not what Durbin was saying. Yes, but if I keep the subject narrowly framed around box resonance then I'm right. You need to do some research regarding debating (the trick is to answer the question you WISH had been asked, politicians do it ALL THE TIME). I mean, who wants to be wrong? ;) MOSFET >Yes, but if I keep the subject narrowly framed around box resonance then >I'm right. MOSFET, you need to do to some research .... not me. Answering the question you wished had been asked is called a "straw man argument" in debating circles. Not a good tactic as you automatically lose :-) BTW, adding loose filling changes the operation of the box from adiabatic to isothermal. Nothing to do with standing waves as far as changing the effective cubic volume of the box. - RG >> It may be your suspicion, but it is still wrong and is far from being >> "validated". Mr. Durbin's explanation is correct. The loose stuffing [quoted text clipped - 10 lines] > > MOSFET > MOSFET, you need to do to some research .... not me. Answering the > question you wished had been asked is called a "straw man argument" in > debating circles. Not a good tactic as you automatically lose :-) I know, I was joking. Jeeez, lighten up. Smile. MOSFET LOL ... OK, I missed that. - RG >> MOSFET, you need to do to some research .... not me. Answering the >> question you wished had been asked is called a "straw man argument" in [quoted text clipped - 5 lines] > > MOSFET > MOSFET, you need to do to some research .... not me. Answering the question > you wished had been asked is called a "straw man argument" in debating > circles. Not a good tactic as you automatically lose :-) Yeah, but we're not stuffing the box with straw... <running away...> Now there's an idea... who knows, eh ? - RG >> MOSFET, you need to do to some research .... not me. Answering the >> question you wished had been asked is called a "straw man argument" in [quoted text clipped - 3 lines] > > <running away...> >>>There is another effect of course, a very important one. In that stuffed >>>enclosure [quoted text clipped - 22 lines] >slowing down the sound waves as well, which makes the box "appear" >larger to the driver (internal waves take longer to reflect back). Stuffing does slow sound waves. Reflecting back, really has nothing to do with anything. It just a pressure- contraction of the box, period. I breifly read some of the posts, and its seems very strange the reason most any speaker book might explain the most accepted version has not been addressed. When the air is pressurized, it heats up. When it contracts, it cools down. If you add fiber, the fiber absorbs these transistion somewhat. When the transistions of temperature are minimized the net effect is a reduction of pressure and contraction. Bigger box!! Polyfill has limited effect in this area. Fiberglass works much better as well as foam. I have got up to the theroetical increase in box size of 40% but much less is the norm. You can also use a heavier gas inside which will accomplish the same bigger box effect. greg >What you are calling "acoustic wadding" I think is the same as adding >padding to the walls of a subwoofer enclosure. I have never heard the term >Remember, ideally, the subwoofer enclosure should JUST be a volume of air to >act as a kind of a spring for the subwoofer (acoustic suspension). Ported [quoted text clipped - 4 lines] >more SOL (they are a little louder) because they are able to utilize some of >the sound energy that is always lost in a sealed enclosure. This can be confusing. A port will let the bass out, so to speak. The actuall efficiency is determined solely by the driver. There are two classes of drivers, those for sealed boxes, and those for ported, and of course some inbetween. The port increases the bandwidth but does not increase efficiency. However a bandpass box can increase the apparent efficiency by doing it over a narrow range of frequencies. In other words its response can look like a mountain top if desired. greg > What you are calling "acoustic wadding" I think is the same as adding > padding to the walls of a subwoofer enclosure. I have never heard the > term "wadding" before, hmmm. Another common technique is to fill the sub > enclosure with polyfil (the same stuff as in pillows). http://snipurl.com/s6zd Mike. Sure enough, there's the word "wadding", but I noticed the prices were all in British Pounds. I think the reason I have never heard of it is because it is a British term. MOSFET >> What you are calling "acoustic wadding" I think is the same as adding >> padding to the walls of a subwoofer enclosure. I have never heard the [quoted text clipped - 4 lines] > > Mike. > Sure enough, there's the word "wadding", but I noticed the prices were all > in British Pounds. I think the reason I have never heard of it is because > it is a British term. There you go. I've added something to your international audio terminology vocabulary. :-) I thaught you'd guess I was british from my address. Anyway, my thanks to you and all the others who replied with helpful information. Whatever their nationality. :-) Mike. > I'm a novice as far as audio is concerned, so bear with me. > Have a 12" bass speaker fitted in a homemade infinite baffle cabinet. [quoted text clipped - 7 lines] > cabinet? > As opposed to leaving it empty. Well first, you're confusing your terms... "infinite baffle" installations don't have a cabinet. Such an installation would be like a speaker mounted in the rear deck of a car. Door speakers are typically infinite-baffle configuration. Damping material in a speaker enclosure has the effect of slowing down the internal sound waves and makes the speaker "think" it's got a larger box behind it. It's useful in situations where the proper size box for a given speaker is impractical or won't fit, although it's not an ideal substitute for a properly sized box. The stuff you've got would probably work best just lining the inner walls of the box. >> I'm a novice as far as audio is concerned, so bear with me. >> Have a 12" bass speaker fitted in a homemade infinite baffle cabinet. [quoted text clipped - 12 lines] > mounted in the rear deck of a car. Door speakers are typically > infinite-baffle configuration. Looks like the terminology might have changed since I dabbled in building simple home speaker cabinets a good few years ago. There were basically only two types. Sealed and ported. where the sealed types were known as infinite baffle. > Damping material in a speaker enclosure has the effect of slowing down the > internal sound waves and makes the speaker "think" it's got a larger box > behind it. It's useful in situations where the proper size box for a > given speaker is impractical or won't fit, although it's not an ideal > substitute for a properly sized box. I made the box with the internal volume that the speaker manufacturer recommended for a sealed cabinet. Had some 20mm multiply wood knocking around so I used that. > The stuff you've got would probably work best just lining the inner walls > of the box. Thanks. that sounds easy enough. :-) Mike. > I made the box with the internal volume that the speaker manufacturer > recommended for a sealed cabinet. > Had some 20mm multiply wood knocking around so I used that. See, that's just asking for problems... plywood flexes too much and can cause all kinds of nasty issues for a sub box, including weak and/or boomy bass. The whole idea of enclosing the speaker in a box is so the sound waves from the back don't cancel out those from the front, and if the box is going to flex and produce sound waves itself, it sort of negates the whole purpose. Medium-density fiberboard (aka MDF or Medite) is the recommended material. Glue well... don't use nails, use pre-drilled, counter-sunk screws to put it together, but don't go too nuts with the screws: they're really only there to hold everything together until the glue sets up. A good wood glue should produce joints that are stronger than the wood itself. >> I made the box with the internal volume that the speaker manufacturer >> recommended for a sealed cabinet. [quoted text clipped - 3 lines] > cause all kinds of nasty issues for a sub box, including weak and/or boomy > bass. I know what you mean. I remembered that much from previous experience. It wasn't ordinary ply I used. It was good quality multi ply, made up of maybe a dozen layers, and over 3/4" thick. Very strong and very stiff. The whole idea of enclosing the speaker in a box is so the > sound waves from the back don't cancel out those from the front, and if > the box is going to flex and produce sound waves itself, it sort of > negates the whole purpose. The box is about 14" square. Tailored to fit my BM. The sound coming through the ski slot in the back seat. It's extremely strong and heavy.You could jump up and down on it, and I doubt it would flex at all.. > Medium-density fiberboard (aka MDF or Medite) is the recommended material. > Glue well... don't use nails, use pre-drilled, counter-sunk screws to put > it together, but don't go too nuts with the screws: they're really only > there to hold everything together until the glue sets up. A good wood > glue should produce joints that are stronger than the wood itself. You're right though. I should have used MDF. The box is far heavier than than it needs to be, but seeing as I already had the ply I used it. It is glued and screwed, with 1/2" quadrant glued to the internal corners. Then sealed with varnish. On reflection its way OTT. When I get round to it, I may replace it with a lighter box made from MDF. Thanks for the info though. All useful stuff. Mike. >> I made the box with the internal volume that the speaker manufacturer >> recommended for a sealed cabinet. [quoted text clipped - 6 lines] >the box is going to flex and produce sound waves itself, it sort of >negates the whole purpose. There is a factor in the formula for calculating boxes, specifically for loss. However in sealed boxes, losses can often improve things. Think if it as a very tiny port. >Medium-density fiberboard (aka MDF or Medite) is the recommended >material. Glue well... don't use nails, use pre-drilled, counter-sunk >screws to put it together, but don't go too nuts with the screws: >they're really only there to hold everything together until the glue >sets up. A good wood glue should produce joints that are stronger than >the wood itself. I have been building speakers for a long time, and never have yet used MDF. I'm not saying MDF is not good, its just that there are many considerations when selecting materials including availability. greg >>>I made the box with the internal volume that the speaker manufacturer >>>recommended for a sealed cabinet. [quoted text clipped - 10 lines] > loss. However in sealed boxes, losses can often improve > things. Think if it as a very tiny port. Except a port directs a single wavefront in a single direction... a flexing box radiates in all directions. >>Medium-density fiberboard (aka MDF or Medite) is the recommended >>material. Glue well... don't use nails, use pre-drilled, counter-sunk [quoted text clipped - 6 lines] > I'm not saying MDF is not good, its just that there are many considerations > when selecting materials including availability. There's been a lot of debate over MDF vs. other materials for speaker boxes in general; however, we're talking about sub boxes specifically here. Resonant acoustics of the box itself aren't as much a consideration as its raw rigidity. >> In article <QyKmg.76925$iF6.35576@pd7tw2no>, Matt Ion > <soundy@moltenimage.com> wrote: [quoted text clipped - 16 lines] >Except a port directs a single wavefront in a single direction... a >flexing box radiates in all directions. Flexing of the box is not usually wanted, especially if the panels resonate, which is a big problem. There was a article in Speaker Builder mag, about 20 years ago. A guy was going over a AR3 speaker which is a sealed box. After modifying the cone with various treatments, tests were run after installing the drivers. For a final tune, he started drilling small holes in the box to reduce the frequency of resonance. Some of the treatments of the cone, were, installing foam inside the dust cover. Varnishing the inner area of the cone. Placing foam strips radially around the cone. These basic steps I have done many times. >>>Medium-density fiberboard (aka MDF or Medite) is the recommended >>>material. Glue well... don't use nails, use pre-drilled, counter-sunk [quoted text clipped - 11 lines] >here. Resonant acoustics of the box itself aren't as much a >consideration as its raw rigidity. Rigidity is basically wanted for all enclosures. Where MDF shines is in the midrange where panel resonances color the sound. Drivers exciting the 300-500 Hz range have the most influence on vibrating the cabinet. Wow. I just got done reading this entire thread. I can see how it would be VERY confusing for a newbie! My general understanding over the years regarding this topic has been this: If you properly construct a box for your subwoofer, there should be absolutely zero need for stuffing it. If you've made a box with an internal volume of .75 cu ft, as recommended by the manufacturer of the subwoofer you're using (let's assume that the basket volume is a non-issue here), then adding polyfill stuffing would essentially increase the intermal volume of the box - lowering the desired tuned frequency. How much this lowers the frequency, if I remember correctly, is very small....even if one were to stuff the box almost entirely with loosely-packed polyfill. I wish I could remember where, but I read about an experiment where they measured output for boxes with and without stuffing and the audible differences were barely negligible. Similar results were found to be true over and over with several different boxes, subs, etc. Anyone remember this? Anyway, bottom line is this: No one really "needs" to stuff a box. If you think it sounds better with it, then by all means go for it! I myself have stuffed my boxes all my life until recently, and the last several I've made have been completely empty inside and they sound the same, if not better, than the previous ones. I say put more emphasis on box construction and less on whether to stuff or not to stuff. Tony Signature2001 Nissan Maxima SE Anniversary Edition Clarion DRZ9255 Head Unit, Phoenix Gold ZX475ti, ZX450 and Xenon X1200.1 Amplifiers, Dynaudio System 360 Tri-Amped In Front and Focal 130HCs For Rear Fill, Image Dynamics IDMAX10 D4 v.3 Sub 2001 Chevy S10 ZR2 Pioneer DEH-P9600MP Head Unit, Phoenix Gold Ti500.4 Amp, Focal 165HC Speakers & Image Dynamics ID8 D4 v.3 Sub 2006 Mustang GT Coupe Alpine IVA-D310 DVD Head Unit, Alpine MRA-550 Digital 5.1 Amp, Boston Acoustics Z-Series Speakers, Alpine SBS-05DC Center Channel Speaker, Amplified MTX Thunderform Sub > Wow. I just got done reading this entire thread. I can see how it would be > VERY confusing for a newbie! [quoted text clipped - 19 lines] > than the previous ones. I say put more emphasis on box construction and > less on whether to stuff or not to stuff. The fact is, you can do all the math, but there's still an element of voodoo magic to it all. As much care as you put into the design and construction of the box, the interior of the car is still a major part of the "SYSTEM" and will have its own effect on the overall bass production. To some degree, even the positioning and aiming of the sub and box affects the final outcome. Measuring and quantifying the acoustics of the SPECIFIC car isn't impossible, but it's damn close to it, and is certainly far beyond impractical in most cases. You may build a box exactly to manufacturer specs and stil find it's not quite right... adding some stuffing may change the resonant frequency just enough to make it match the car's acoustics better. Then again, simply turning the whole unit around may make the difference. In the end a certain degree of hit-and-miss experimentation may be needed to find the "sweet spot" of the whole setup. I know in my '87 Accord hatchback, my little 10" JL sub in a 1.2cu.ft. box doesn't really rattle much on its own - place it right behind the driver's seat and you barely hear it, let it alone feel it. Stick it in the back of the hatch, though, with the seats up and the cover panels in place, and the shape of the car nicely boosts and amplifies the mid-sub range (80-100Hz) and shakes things quite handily inside the car, yet without setting off seismographs outside the car. If you've made a box with an internal > volume of .75 cu ft, as recommended by the manufacturer of the subwoofer > you're using (let's assume that the basket volume is a non-issue here), > then adding polyfill stuffing would essentially increase the intermal > volume of the box - lowering the desired tuned frequency. Yes, Tony, but the tuned frequency of what? The volume of air inside the enclosure? In a sealed enclosure, what effect does the tuning frequency of the volume of air inside the enclosure make any difference to anything? That has been my entire point this whole thread. My understanding has always been that in a sealed enclosure, the volume of air inside a box acts as a spring to the woofer. That's it. I would think that in a well constructed enclosure the volume of air inside a box would have the same air pressure as the outside air. How would the resonant frequency of this volume of air have any bearing on anything! I'm sorry, I just don't understand. I'm so confused! Nick Resonant frequency of the box changes the output of the system at that frequency, also affects the driver's impedance as it appears to the amp driving the system, etc. If you sweep the driver impedance curve in free air & then again in the box and compare the two, you will see a large change. Think about it this way: in open air the driver compliance is a combination of factors that includes the relative stiffness of the spider and the surround. When you put it in a box, the effect of the air mass (air spring if you will) changes that compliance and your system tuning changes with it. This happens in a vented box too but is more complicated because the air mass behaves differently at port resonance than it does above it. JD > If you've made a box with an internal > [quoted text clipped - 15 lines] > > Nick > Resonant frequency of the box changes the output of the system at that > frequency, also affects the driver's impedance as it appears to the amp [quoted text clipped - 8 lines] > This happens in a vented box too but is more complicated because the air > mass behaves differently at port resonance than it does above it. And I thaught I was asking a simple question. :-) I've been following the debate with interest. I think I can now waffle with the best of them, even though I wouldn't know what I was talking about. :-) Mike. > Resonant frequency of the box changes the output of the system at that > frequency, also affects the driver's impedance as it appears to the amp [quoted text clipped - 5 lines] > (air spring if you will) changes that compliance and your system tuning > changes with it. Yes, I know that, but common sense would say that putting ANYTHING in the enclsoure would mean LESS air therefore a tighter "spring", like a SMALLER BOX, NOT A LARGER BOX. I guess this is why this saying "putting fill in a box will make it seem like a larger box" just seemed to go against common sense to me. But I do understand the first part of your explanation. By adding fill you change the resonant frequency of the volume of air in the enclosure. This will have an effect (small I would imagine) on the frequency response of the system. This makes sense. Thank you for that explanation. MOSFET > This happens in a vented box too but is more complicated because the air > mass behaves differently at port resonance than it does above it. [quoted text clipped - 20 lines] >> >> Nick Actually, the effect is not "small" at all. In theory the effect is like having a 40% larger box. In practice, however it comes out to closer to 20%. - RG >> Resonant frequency of the box changes the output of the system at that >> frequency, also affects the driver's impedance as it appears to the amp [quoted text clipped - 45 lines] >>> >>> Nick > Actually, the effect is not "small" at all. In theory the effect is like > having a 40% larger box. In practice, however it comes out to closer to > 20%. > > - RG By small, I meant small change in the overall performance of the system (frequency response curve for instance). But I see your point. MOSFET Nick, Check out http://www.bcae1.com under chapter 115, Speaker Respose Curves. There's graph (and explanation) about 3/4 of the way through the chapter that shows the response curve for an unstuffed box vs. a stuffed one. Just FYI. Tony Thanks Tony. Aren't you a fountain of information!!! The two curves (box with Polyfill and box without) are nearly identical so, again, this seems to prove what I believe you said before (and what I said), Polyfill makes very little difference. I think we can now put this one to bed thanks to you. Nick > Nick, > [quoted text clipped - 4 lines] > > Tony "Standing waves are not usually a big problem in a subwoofer because it is usually crossed over. In other situations, like home speakers, poly-fill can be used to damp standing waves. In home speakers, the woofer is often used to reproduce a wider frequency range than it would be in a subwoofer. This creates the opportunity for standing waves to create problems. The wavelength is too long in the lowest frequencies to cause a problem but frequencies above 300 or 400hz will start to cause problems." I reckon so .... There goes YOUR theory and validation on what polyfill does or doesn't do in a subwoofer box .... right from the fountain of information itself :-) - RG > Thanks Tony. Aren't you a fountain of information!!! > [quoted text clipped - 14 lines] >> >> Tony That's funny because there sure are a WHOLE LOTTA FOLKS out there who seem VERY concerned about standing waves in their sub enclosure. For instance: http://www.the12volt.com/caraudio/boxes2.asp I guess they are all wrong according to you as standing waves are not a problem below 300-400Hz. So, to be honest, you HAVE NOT convinced me RG. MOSFET > "Standing waves are not usually a big problem in a subwoofer because it is > usually crossed over. In other situations, like home speakers, poly-fill [quoted text clipped - 29 lines] >>> >>> Tony Check again .... those were actually Perry Babin's words (direct quote). - RG > That's funny because there sure are a WHOLE LOTTA FOLKS out there who seem > VERY concerned about standing waves in their sub enclosure. For instance: [quoted text clipped - 42 lines] >>>> >>>> Tony BTW, the "spring" effects of air are not constant. Refer to my previous post on adiabatic versus isothermal. This changes the "characteristic" of the air considerably. Just remember, the earth is not flat .... - RG >> Resonant frequency of the box changes the output of the system at that >> frequency, also affects the driver's impedance as it appears to the amp [quoted text clipped - 45 lines] >>> >>> Nick Correct, it is not a huge effect but measurable & audible if you use stuffing that has enough mass. People tend to use polyfill because it's readily available but wool or fiberglass will give you better results. If you put too much in you actually interfere with the resonance of the air & stuffing mass & it works against you instead of helping squeeze a little more LF extension from the system. JD >>Resonant frequency of the box changes the output of the system at that >>frequency, also affects the driver's impedance as it appears to the amp [quoted text clipped - 45 lines] >>> >>>Nick > If you put too much in you actually interfere with the resonance of the > air & stuffing mass & it works against you instead of helping squeeze a > little more LF extension from the system. > > JD I concur. There have been instances where I have had to play with certain amounts of stuffing in an enclosure to get my desired response, whether in my head or not. This seems moreso with midbass/mid drivers. Signature-Cyrus *coughcasaucedoprodigynetcough* > I concur. There have been instances where I have had to play with > certain amounts of stuffing in an enclosure to get my desired response, > whether in my head or not. This seems moreso with midbass/mid drivers. This may have more to do with standing wave cancellation than anything else as RG has pointed out (in a snippy way) that standing waves become more of a problem in the midrange-midbass region (300-400 Hz). Just a thought. MOSFET > > I concur. There have been instances where I have had to play with > > certain amounts of stuffing in an enclosure to get my desired response, [quoted text clipped - 7 lines] > > MOSFET I try to keep my ideas/help vague and open for a reason. Some are very serious about their audio addictions. Wave lengths are shorter at higher frequencies. There's more of a chance of standing waves happening with midbass/mid drivers, as has I'm sure been beaten to death by now. Then again there are too many factors to even know whether or not its happening while communicating with someone via usenet. In any case, its just audio. I say enjoy the gear you have, your hearing and knowledge. Nobody really cares about this sorta thing except us, as evidenced by the last wedding reception I was at and its 'dj'. Signature-Cyrus *coughcasaucedoprodigynetcough* >> > I concur. There have been instances where I have had to play with >> > certain amounts of stuffing in an enclosure to get my desired response, [quoted text clipped - 20 lines] >and knowledge. Nobody really cares about this sorta thing except us, as >evidenced by the last wedding reception I was at and its 'dj'. Most of the time when things sound bad, people don't say mnuch. When it sounds good, they say more. At least thats my experiance. Midbass and midranges. Midranges have the most effect. Its like trying to have a speaker in the door with no sound treatment in back of it. Its the pits. OEM speakers usually don't have anything, but us hifi persons know better. greg |
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