So I'm confused about how people are calculating the amount of material they put in a cabinet when doing a build. I've seen recommendations of Whispermat, Sonic Barrier, and Black Hole, but not only do these damp the cabinet and increase rigidity, but also absorb sound. Therefore, you are effectively reducing the amount of air to the port in a ported design.
This said, is there a calculation that people are doing, or is everyone just throwing material at the designs and hoping for the best? I was under the impression that it would be best to "right-size" the cabinet and only use damping mats.
Tks,
Paul
This said, is there a calculation that people are doing, or is everyone just throwing material at the designs and hoping for the best? I was under the impression that it would be best to "right-size" the cabinet and only use damping mats.
Tks,
Paul
I could (probably am) well be wrong here but I would have thought that sound absorbtion and port air velocity were unrelated. Basically the absorbtion is helping to stop sound reflections which is desirable. What is coming out of/going into your port should be air that is being displaced by the cone movements.
Sooooo apart from the fact that the absorbtion layer will tend to increase the volume (not intuitive I know) of the cabinet, it shouldn't have any negative effects (provided your port is a good distance away from any damping material, to allow adequate air flow.
Tony.
Sooooo apart from the fact that the absorbtion layer will tend to increase the volume (not intuitive I know) of the cabinet, it shouldn't have any negative effects (provided your port is a good distance away from any damping material, to allow adequate air flow.
Tony.
Thanks Wintermute. I noticed the values of absorption rates on the Whispermat and they are frequency dependant just like any other. Therefore, you're getting a great reduction of SPL in the higher frequency range, but you can still see a reduction in the low frequency range (same as room acoustic absorption panels).
The pupose is to change sound into heat usually (you have to do something with the energy), and I would thus believe that the velocity and frequency that exits the port are both impacted.
In conclusion, aren't we tuning the cabinets using this stuff, and if so, very similar to a port size determination, how do we determine where and how much we want to change?
Rgds,
Paul
The pupose is to change sound into heat usually (you have to do something with the energy), and I would thus believe that the velocity and frequency that exits the port are both impacted.
In conclusion, aren't we tuning the cabinets using this stuff, and if so, very similar to a port size determination, how do we determine where and how much we want to change?
Rgds,
Paul
Tuning of the cabinet in a ported speaker is based upon both the volume of the cabinet as well as the port dimensions. To the effect that stuffing changes the apparent volume of the box, this may impact the tuning. The speed of the air exiting the port is estimated in Mach by: 13.7*SQRT(Reference Efficiency of the bass driver*power rating in watts of the driver)/(tuning frequency of the enclosure*radius of the port^2). Using non porous damping materials on the walls will not increase and may decrease the effective size of the enclosure while using porous damping materials may slightly increase the effective box size but only at higher frequencies (roughly where the wavelength is at least 25% of the thickness of the damping material) so this will have little impact on the frequencies coming from the port.
HTH,
Jay
HTH,
Jay
The formula 0.62........1........1.62 used for typical box loudpeakers isn't bad as it suppresses the 5th harmonic. Generally with well constructed cabinets (min 30mm) wall thickness I use surplus egg crate fiber boxes (which is excellent for HF absorption) and 1" glass fiber glued around whole box except front. See enclosed photo in early stages of 180 liter cab for 2x10" units. Due to oblique construction, (recommended for large cabinets) bracing can be minimised but essential for front/rear.
Oblique construction cabinet permits simple 6dB/oct crossover without upper bass colouration.
Don't get confused..... inadequate bracing can lead to a drummy panel resonance/colour sound and temptation is to add more damping material.....this can swallow critical lower midrange frequencies.
richj
Oblique construction cabinet permits simple 6dB/oct crossover without upper bass colouration.
Don't get confused..... inadequate bracing can lead to a drummy panel resonance/colour sound and temptation is to add more damping material.....this can swallow critical lower midrange frequencies.
richj
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Very interesting concept on the egg crate routine. I guess using that to diffuse the sound prevents the signal from returning to the rear of the cone with any velocity that would interrupt the signal exiting the front of the cone. I thought about diffusion, but wasn't sure how effective it would be in such close quarters.
Now I'm thinking that egg crate open cell foam would be a good idea for the rear of the cabinet on top of a damping sheet (vinyl, asphalt, whatever you like), sort of a diffuser/absorber due to the shape.
Now I'm thinking that egg crate open cell foam would be a good idea for the rear of the cabinet on top of a damping sheet (vinyl, asphalt, whatever you like), sort of a diffuser/absorber due to the shape.
We really need to stop treating damping/absorption material as the same thing, since particularly in cases like this, they have 2 very different effects. If we are talking cabinet resonance damping material, IE a rubbery or otherwise lossy material attached to cabinet walls, it will reduce box volume. If we are talking about absorption material, IE fiberglass, open cell foam lining the walls, polyfill, etc, it will increase apparent box volume. In simplest terms - if the material in question is air-permeable, it will increase volume (if anything). If it is airproof (ie any sort of putty, rubber, closed cell foam), it will reduce box volume by an amount equal to its own volume, which should be simple to calculate.
BTW, nice idea with the eggcrate material, I have used something similar in a recent project, I found some 5' x 1" strips of 'wavy' wood, which undulates from .25" to 1" in height. I think they are intended to be put on the top of decorative fences and the like. I cut these into smaller strips, then glue them to the back wall and one side of the cabinet, staggered so that the waves are not all lined up. This gives a fairly randomized surface to the walls in question, though since the maximum wave height is only about 1" it will probably not do much at the lower frequencies. I will try to post a picture of this soon. In my next box I am going to greatly oversize it, and put some angled panels and the like inside to make the inner geometry very different than the outer geometry.
BTW, nice idea with the eggcrate material, I have used something similar in a recent project, I found some 5' x 1" strips of 'wavy' wood, which undulates from .25" to 1" in height. I think they are intended to be put on the top of decorative fences and the like. I cut these into smaller strips, then glue them to the back wall and one side of the cabinet, staggered so that the waves are not all lined up. This gives a fairly randomized surface to the walls in question, though since the maximum wave height is only about 1" it will probably not do much at the lower frequencies. I will try to post a picture of this soon. In my next box I am going to greatly oversize it, and put some angled panels and the like inside to make the inner geometry very different than the outer geometry.
in the case of black hole and wispermat you can't separate the two (into damping or absorbtion) because they are both 😉 they have a mass loaded vinyl layer for damping and an open cell foam layer for absorbtion. I haven't used either (but have read about both). I have just obtained a sample of something similar (soundsorber barrier) which I am going to test.
I have tried egg crate foam and found that whilst it had some effect (in reducing rear wall reflections) it was only marginally better than without. I also tried some packing foam which is completely flat but is less dense and it was much more effective at reducing the reflections than the egg crate foam.
I'm still not convinced that the foam will reduce your port air speed 😉 I'd need to do more research but my gut feeling is no.
Tony.
I have tried egg crate foam and found that whilst it had some effect (in reducing rear wall reflections) it was only marginally better than without. I also tried some packing foam which is completely flat but is less dense and it was much more effective at reducing the reflections than the egg crate foam.
I'm still not convinced that the foam will reduce your port air speed 😉 I'd need to do more research but my gut feeling is no.
Tony.
Thanks guys, this is good information. I think I'm going to try the packing foam behind the woofer. I have some 3 inch foam that should work well. I'll do a damping mat on the rear wall, then the square of foam to diffuse the rear wave. The rest of the walls will get whispermat or something similar as a damping/absorption layer. So, damping diffusion on the rear wall, and damping absorption on the remaining walls. Some needs to do some empirical testing on this. I guess I'll looking at incremental improvements at best, but anything is better than nothing.
Paul
Paul
Now I need to figure out where I can fit the crossover with all of this material inside the box! 
😉
😉
hehehe you could go for an external xover like I am 😉 one thing I didn't say before two layers of the packing foam I used was better than one layer... mine was 1" thick...... If you have a mic and can do FR measurements you can very easily see what effect the absorbtion is having.... I was amazed at how big the iregularities were with no material on the back of my box....
attached is an example of a nearfield measurement with and without the packing foam at the back of the box. the yellow (appologies if it is green, I'm colour blind) trace is with the foam, as you can see it is much smoother.
Tony.
attached is an example of a nearfield measurement with and without the packing foam at the back of the box. the yellow (appologies if it is green, I'm colour blind) trace is with the foam, as you can see it is much smoother.
Tony.
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hmmmm I just went back to look at my results for eggcrate foam compared to the packing foam, and they were almost identical.... though I clearly remember them being quite far apart. I think I may have to do some more tests 🙂 Have to test the new stuff anyway, but probably won't get a chance till next week.
Tony.
Tony.
OK dug up some pics of the reflective interior walls I mentioned in my earlier post. I will be experimenting more with this approach in my future projects as this test run worked very well. Not that the driver mounted in this small airspace was a TB871 3" fullrange, playing from ~ 500hz - 20khz.
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wintermute said:
Now that's just odd. Going from reflective diffusion to absorptive diffusion with no change. I think you misnamed the files when you saved the results. I bet you forgot to save the changed results. Eek.
morbo said:
Now that is interesting. So you decreased volume and diffused with your approach, because I don't think floor padding is absorptive, only damping correct?
xyrium said:
Yes I think that I may have actually done a new test but used the old dataset, saving the new test over the old... both were much better than the no foam test the problem was that the two foams looked identical, which is not my recollection at all..... I think from memory there was at least 3db more attenuation in the better foam. Will test again. The differences between the two (types of foam) datasets was so small that you would put it down to differences in measurement not differences in the materials.
Tony.
Damping in cabinets
This is my first post gentlemen. I have been reading and learning for about 2 years . This thread was the first time I thought I might have something constructive to say. Last year I built a pair of speakers using a GR research design. I used Black hole 5. I decided that the recommended amount was not enough so I bought and installed an extra sheet. I was reading about a new line of DIY speakers from a site at the audiocircle and the designer stated that he thought BH5 was a ripoff. He went on to state that it affected cabinet volume to the detriment of port tuning. I called GR's owner Danny Ritchie and he explained that the dense inner barrier layer damped panel resonance but reduced cabinet volume. The thick open cell top layer slows airflow and adds back an apparent volume very close to equal that lost by the first layer. The end result is a wash volume wise but nets a significant improvement in sound. You can accomplish a very similar thing on the cheap using a layer of stick-on tile against the cabinet walls with a layer of egg crate foam from a mattress cover over that.
This is my first post gentlemen. I have been reading and learning for about 2 years . This thread was the first time I thought I might have something constructive to say. Last year I built a pair of speakers using a GR research design. I used Black hole 5. I decided that the recommended amount was not enough so I bought and installed an extra sheet. I was reading about a new line of DIY speakers from a site at the audiocircle and the designer stated that he thought BH5 was a ripoff. He went on to state that it affected cabinet volume to the detriment of port tuning. I called GR's owner Danny Ritchie and he explained that the dense inner barrier layer damped panel resonance but reduced cabinet volume. The thick open cell top layer slows airflow and adds back an apparent volume very close to equal that lost by the first layer. The end result is a wash volume wise but nets a significant improvement in sound. You can accomplish a very similar thing on the cheap using a layer of stick-on tile against the cabinet walls with a layer of egg crate foam from a mattress cover over that.
Thanks LFM and welcome 🙂 The comments on BH5 are very usefull, nice to here what an expert has said on the matter 🙂 also a good low cost suggestion, though I'm suspecting it won't reach the performance of the BH5.....
I'm just doing some tests at the moment with eggcrate foam, packing foam and some sound deadening (probably closer to wispermat then bh5) material, I must say that the initial test with the sound deadening material is very impressive compared to the eggcrate foam (and that is with a foil sheild on it which I'm going to remove). There are so many different types of eggcrate foam that my findings may differ substantially from someone elses... I have found that two layers does make a difference compared to one with the foam I have, thickness seems to be more important than the surface shape.....
attached are my initial tests on bare, eggcrate and "soundsorber barrier" the soundsorber is the dark blue trace the bare cabinet is the light blue, and the eggcrate foam is the yellow trace. In this test the packing foam was slightly worse than the egg-crate foam, though it is a different cabinet to the one when I tested before.... as you can see the soundsorber is much more effective than the egg crate foam! probably makes about 3db difference on the ripples.
Tony.
edit: I have only put the material on the back wall... iregularities above 1Khz are probably either the driver or side/top bottom reflections.
I'm just doing some tests at the moment with eggcrate foam, packing foam and some sound deadening (probably closer to wispermat then bh5) material, I must say that the initial test with the sound deadening material is very impressive compared to the eggcrate foam (and that is with a foil sheild on it which I'm going to remove). There are so many different types of eggcrate foam that my findings may differ substantially from someone elses... I have found that two layers does make a difference compared to one with the foam I have, thickness seems to be more important than the surface shape.....
attached are my initial tests on bare, eggcrate and "soundsorber barrier" the soundsorber is the dark blue trace the bare cabinet is the light blue, and the eggcrate foam is the yellow trace. In this test the packing foam was slightly worse than the egg-crate foam, though it is a different cabinet to the one when I tested before.... as you can see the soundsorber is much more effective than the egg crate foam! probably makes about 3db difference on the ripples.
Tony.
edit: I have only put the material on the back wall... iregularities above 1Khz are probably either the driver or side/top bottom reflections.
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Hey Tony, I see the xover suckout at 2k, but what is the story with that dropoff at 5k? Is this a measurement of the woofer only, or limitation/configuration of the measurement method?
Rgds,
Paul
Rgds,
Paul
It is the woofer running full range.... the drop at just after 2K is something I've been chasing for a while but I think it is either my measurements or the speaker itself. Nearfield is only really good up to about 300Hz, after which it starts to get inaccurate (spl wise, which is why it appears spl is decreasing with increasing freq). But it is still good IMO for showing what is happening with the various damping/absorbing compounds.
The good thing is (so I have read), apparently large (but narrow) dips in freq response aren't particularly audible.
I did another test removing the foil layer off the soundsorber, and to my surprise it was worse!!! not a lot but definitely not as good... this surprised me a lot because I thought the foil layer would have been (at least partially) reflective.... I can only guess that because it is bonded to the open cell foam underneath that it absorbs rather than reflects, and has some property that makes it more absorbive than the foam alone.
Tony.
The good thing is (so I have read), apparently large (but narrow) dips in freq response aren't particularly audible.
I did another test removing the foil layer off the soundsorber, and to my surprise it was worse!!! not a lot but definitely not as good... this surprised me a lot because I thought the foil layer would have been (at least partially) reflective.... I can only guess that because it is bonded to the open cell foam underneath that it absorbs rather than reflects, and has some property that makes it more absorbive than the foam alone.
Tony.
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