For calculating internal cabinet volumes, how do you factor in cabinet fillers? For solid rubber matting or something dense like XPS foam, it seems reasonable to just subtract the volume used from the total volume. But what about polyfill or denim matting? Maybe for each cubic foot of polyfil, subtract something like 0.5 cf? Then a little more for denim, like -0.6 or -0.7? Some of the air in the polyfill has to contribute to the overall volume, right? Thanks for the help in advance! -Tony
This is a much debated subject, with his supporters and his detractors... 😉
According to MM. John d'Appolito, Vance Dickason and Martin Colloms in their respective books, the use of a damping/filler material can increase the acoustic volume of the enclosure of a theorical factor of 41%, but in practice at least 15% (M. Colloms), and usually 20-25% are easily obtained (J. d'Appolito, V. Dickason).
I personally use Cotton Wool for filling, and I use it generously, no matter on Bass-Reflex or Sealed enclosures. So I am rather in the supporter's camp ! 😀
Consequently, I substract nothing to the internal volume to take in count the addition of filling material inside...
But it's me, OK ?🙂😉
T
According to MM. John d'Appolito, Vance Dickason and Martin Colloms in their respective books, the use of a damping/filler material can increase the acoustic volume of the enclosure of a theorical factor of 41%, but in practice at least 15% (M. Colloms), and usually 20-25% are easily obtained (J. d'Appolito, V. Dickason).
I personally use Cotton Wool for filling, and I use it generously, no matter on Bass-Reflex or Sealed enclosures. So I am rather in the supporter's camp ! 😀
Consequently, I substract nothing to the internal volume to take in count the addition of filling material inside...
But it's me, OK ?🙂😉
T
@tubelectron & @GM
Thanks for the replies! So soft, damping materials like cotton wool seem to effectively increase the internal volume (to varying degrees.)
Ok, any ideas how a hard foam like XPS might affect the apparent volume? If it's "mostly" reflective, I would assume it would subtract rather than add. What about materials like bitumen pads or mass loaded vinyl? Then based on the fact that they dampen and can convert acoustic energy to heat they would also have a volume positive effect?
Thanks!!
Thanks for the replies! So soft, damping materials like cotton wool seem to effectively increase the internal volume (to varying degrees.)
Ok, any ideas how a hard foam like XPS might affect the apparent volume? If it's "mostly" reflective, I would assume it would subtract rather than add. What about materials like bitumen pads or mass loaded vinyl? Then based on the fact that they dampen and can convert acoustic energy to heat they would also have a volume positive effect?
Thanks!!
I think some of the posts are misunderstanding your question and are discussing the determination of an appropriate cabinet volume and/or what adding fill seems to do. BUT I think you are asking: Once you have net volume of the bare cabinet you want to determine the volume displaced by the bracing and the diver to determine the actual net volume of air affecting the woofer. BUT...should you also attempt to determine the air displaced by the damping material.
I'll be interested to see what people tend to do...but I THINK the practical answer is...for closed cell material (like rubber and closed cell foams) you should attempt to determine the volume and deduct that...none of that volume is air. For denim insulation, polyfill, open cell foam, they are actually almost all air, so I have never seen anyone reduce the volume further for those. However, I have considered the same thing as you...for example denim damping material and especially thick wool felt are quite dense and surely do displace some air, but I have never seen anyone mention that they try to account for that.
If you have a total of 104 liter inside of a box, and then counting off driver and port maby you end up with 99 liter netto.
Then all isolationmaterial/filling is mostly "air" so not so important to calculate regards to liter "used". (if you dont trick a sealed woofer to "se" other)
Filling It´s about resistance and converting energi into heat, and don´t let resonantes get out of the box.
But it only regards to "higher" resonanses....you don´t stop 20-200 hz.
@A4eaudio
Yes, I think you have the idea of my question. I want to simulate/calculate the frequency response of a bass reflex enclosure. I have the internal volume of the bare cabinet and the parameters of the woofers I am interested in. One of the critical parameters is the volume of the cabinet. Should I subtract the air displaced by any damping material used inside the box from the measured internal volume when calculating the frequency response of the cabinet? Also, how does the choice of internal materials affect the apparent volume?
It appears that damping/filler material that convert acoustical energy to heat (polyfill, cotton wool) increases the apparent volume of the cabinet.
But does the air inside a close-celled foam interact with the volume of air the same way? I would assume it does not and basically is similar to the walls of the cabinet, so as you mentioned, the volume of the close-celled foam would be subtracted from the total internal volume. I assume denim has a similar density and air flow as cotton wool, so maybe it too has a similar additive effect?
Yes, I think you have the idea of my question. I want to simulate/calculate the frequency response of a bass reflex enclosure. I have the internal volume of the bare cabinet and the parameters of the woofers I am interested in. One of the critical parameters is the volume of the cabinet. Should I subtract the air displaced by any damping material used inside the box from the measured internal volume when calculating the frequency response of the cabinet? Also, how does the choice of internal materials affect the apparent volume?
It appears that damping/filler material that convert acoustical energy to heat (polyfill, cotton wool) increases the apparent volume of the cabinet.
But does the air inside a close-celled foam interact with the volume of air the same way? I would assume it does not and basically is similar to the walls of the cabinet, so as you mentioned, the volume of the close-celled foam would be subtracted from the total internal volume. I assume denim has a similar density and air flow as cotton wool, so maybe it too has a similar additive effect?
Yes. It's about density or "air contenance or transparence" of the filling material. So I would count bitumen foils or pads as taking volume, like bracing.
I'd empirically say : if the damping material is "respirable", or if a cigarette smoke passes/flows through it - I mean it is non-tight to air circulation - I would not substract anything about volume, and consider this material as suitable for sound waves damping / not vibration waves damping (where bitumen and bracing are efficient, but have to be deducted from internal volume then).
Here's what I did in my HERESY I for a 100% filling with Cotton Wool - some will say Sacrilège, I know, but the tone improvement in the midrange / bass-midrange is obvious...
But it's me, OK ? 🙂 😉
T
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Actually the level of acoustic internal material and packing style you use reminds me of how my Focal Electra speakers are assembled. I think Focal may subscribe to a similar viewpoint as you. Thanks again for the very nicely detailed replies.
Well, @oranda : not everybody agrees with such a damping / filling strategy...
Some says that it impairs the efficiency of the loudspeaker, and this his is possibly true to a certain extent. Conversely, I'd say that it impairs the frequency response by the internal resonances, giving noticeable peaks and dips, not to say mounts and valleys sometimes - at least in conventional shapes of enclosures.
In his book (The Loudspeaker CookBook), Vance Dickason has made a great number of tests about damping materials, mixes and quantities, and came to interesting conclusions about the benefits of damping. Martin Colloms (High Performance Loudspeakers) and John d'Appolito (Testing Loudspeakers) also draw similar conclusions. That said, Mr. Paul Wilbur Klipsch - among others - was not an advocate for filling / stuffing enclosures...
Avoiding the use of filling material will require a careful research about the shape, proportions and panel materials of the enclosure itself. Going instead for generous internal filling / damping could be then considered as an easy solution to get around the problem... So there's possibly a compromise to find ! 😉
T
Some says that it impairs the efficiency of the loudspeaker, and this his is possibly true to a certain extent. Conversely, I'd say that it impairs the frequency response by the internal resonances, giving noticeable peaks and dips, not to say mounts and valleys sometimes - at least in conventional shapes of enclosures.
In his book (The Loudspeaker CookBook), Vance Dickason has made a great number of tests about damping materials, mixes and quantities, and came to interesting conclusions about the benefits of damping. Martin Colloms (High Performance Loudspeakers) and John d'Appolito (Testing Loudspeakers) also draw similar conclusions. That said, Mr. Paul Wilbur Klipsch - among others - was not an advocate for filling / stuffing enclosures...
Avoiding the use of filling material will require a careful research about the shape, proportions and panel materials of the enclosure itself. Going instead for generous internal filling / damping could be then considered as an easy solution to get around the problem... So there's possibly a compromise to find ! 😉
T
I wouldn't bother.
Don't worry about the "apparent volume of the cabinet". The box software should be able to take care of everything pretty closely. Terms like "apparent volume" are just intuitive ways to think about it, especially if you are just following a rule-of-thumb and not simulating it. The correct way to account for the filling is to include it in your software. In WinISD, there is an "advanced" menu in the BOX section. Qa is for absorption and the default value is 100. Change it to 50 for a lined, ported box and 10 for a lightly stuffed sealed box. You will be able to see that damping will change the "Q", i.e., the roll-off, of the woofer in a similar way as if you changed the volume, thus terms like "apparent volume". But if you are looking at other things like the port tuning, the length of the port, port air velocity, you will want to have the "actual" volume not an "apparent" volume.
Yes.
Wouldn't worry about lost/ displaced volume
with absorption material.
Which pressure/air easily passes through.
If material is more solid then subtract it.
I calculate Cabinet Qtc at .707 and .8
Depending on the volume range you get
for a specific driver. If volume covers a wide range
just between .7 and .8
Then dont need to worry much about " exact"
volumes. Since just Qtc between those variances
usually somewhat wide. And if your in that ballpark
it is friendly to the driver.
Port gets tuned few Hz below or above driver Fs.
That is basically it. You get what you get.
People over analyze cabinet volume wayyyy to much
.7 to .8 Qtc if the driver can do it.
Few Hz above / below Fs for tune +/_ 3 to 6 Hz
and that is it.
Speaker is what a speaker is.
Also in modeling there is advanced parameters
for absorption ( Qa )
or absorption losses basically.
Qa 100 being no absorption Qa 10 to 15 heavily stuffed.
People will argue what is considered " in the middle"
60 to 80 basically to lined and medium stuff.
Anyways point is absorption is absorption.
In models youll notice adding more absorption
usually. Has behavior no where close to making things " appear"
larger. But the myth still carries I guess?
Theile / Small found out many moons ago.
The basic formulas worked. But was a margin
between real world and calculated.
Still is, but they came up with a few " error correction"
values to weasel the formulas a little.
Hence advanced parameters Leakage /Absorption losses/ port losses
Ql , Qa , Qp
Even basic 20 year old models like Winisd has them to be adjusted.
with absorption material.
Which pressure/air easily passes through.
If material is more solid then subtract it.
I calculate Cabinet Qtc at .707 and .8
Depending on the volume range you get
for a specific driver. If volume covers a wide range
just between .7 and .8
Then dont need to worry much about " exact"
volumes. Since just Qtc between those variances
usually somewhat wide. And if your in that ballpark
it is friendly to the driver.
Port gets tuned few Hz below or above driver Fs.
That is basically it. You get what you get.
People over analyze cabinet volume wayyyy to much
.7 to .8 Qtc if the driver can do it.
Few Hz above / below Fs for tune +/_ 3 to 6 Hz
and that is it.
Speaker is what a speaker is.
Also in modeling there is advanced parameters
for absorption ( Qa )
or absorption losses basically.
Qa 100 being no absorption Qa 10 to 15 heavily stuffed.
People will argue what is considered " in the middle"
60 to 80 basically to lined and medium stuff.
Anyways point is absorption is absorption.
In models youll notice adding more absorption
usually. Has behavior no where close to making things " appear"
larger. But the myth still carries I guess?
Theile / Small found out many moons ago.
The basic formulas worked. But was a margin
between real world and calculated.
Still is, but they came up with a few " error correction"
values to weasel the formulas a little.
Hence advanced parameters Leakage /Absorption losses/ port losses
Ql , Qa , Qp
Even basic 20 year old models like Winisd has them to be adjusted.
I am trying to calculate a box with Vb=124 liter
Now there is one 6.5" driver and one 15" driver.
Calculating includes displacement of those drivers.
Using https://www.diyaudioandvideo.com/Calculator/SpeakerDriverDisplacement/
I get the displacement for those 2 drivers is ~4 liter together.
Now there is one 6.5" driver and one 15" driver.
Calculating includes displacement of those drivers.
Using https://www.diyaudioandvideo.com/Calculator/SpeakerDriverDisplacement/
I get the displacement for those 2 drivers is ~4 liter together.
I wouldn't regard closed cell acoustic foam as being solid like the walls of the cabinet.
Its structure absorbs energy when the cell content, air, is compressed and rarefied due to the sound pressure wave.
This leads me to consider that perhaps the closed cell foam volume should be regarded as an active component of the enclosure volume.
Yes, it's interesting to consider. The topic of closed cell foam has come up before and it's not the kind of thing you'd normally choose to use in a speaker, but it might potentially act between a solid and a pressure absorber if movement produces heat.