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Title: Preferred Subwoofer Damping Material?
Post by: Tim Gurske on April 25, 2017, 06:12:55 pm
It seems like there are many options. Is there a preferred material that "the pros" use or does everybody have a different preference? As always I am looking for cheap and best, although I realize that those might be mutually exclusive in most situations.

And, before the "it depends" answer :-), I will give some details about the application: I am building a basic ported cabinet for a single 18" subwoofer that will be used for playing general music (top 40 mostly) sometimes indoors and sometime outdoors.

Thank you in advance!
Title: Re: Preferred Subwoofer Dampening Material?
Post by: Lee Buckalew on April 25, 2017, 06:16:55 pm
It seems like there are many options. Is there a preferred material that "the pros" use or does everybody have a different preference? As always I am looking for cheap and best, although I realize that those might be mutually exclusive in most situations.

And, before the "it depends" answer :-), I will give some details about the application: I am building a basic ported cabinet for a single 18" subwoofer that will be used for playing general music (top 40 mostly) sometimes indoors and sometime outdoors.

Thank you in advance!

I like water for dampening. 
Now for damping...  ;D

I will let some of the speaker builders chime in with their preferences and why and in what context they like/don't like the various options.

Lee
Title: Re: Preferred Subwoofer Dampening Material?
Post by: Tim Gurske on April 25, 2017, 08:49:45 pm
I like water for dampening. 
Now for damping...  ;D

I will let some of the speaker builders chime in with their preferences and why and in what context they like/don't like the various options.

Lee

Ha! I fixed it.
Title: Re: Preferred Subwoofer Damping Material?
Post by: Michael Thompson on April 25, 2017, 09:55:59 pm
It seems like there are many options. Is there a preferred material that "the pros" use or does everybody have a different preference? As always I am looking for cheap and best, although I realize that those might be mutually exclusive in most situations.

And, before the "it depends" answer :-), I will give some details about the application: I am building a basic ported cabinet for a single 18" subwoofer that will be used for playing general music (top 40 mostly) sometimes indoors and sometime outdoors.

Thank you in advance!
It depends!  LOL  Seriously though...Manufactures use all different things.  The first question is does your design need any at all?  Many don't.  Personally if I need it, I like polyester because it doesn't rot from ozone exposure like charcoal foam, but it does hold more moisture than the foam.  Fiberglass works, but it can be a mess and heaven forbid your design has open ports and a mouse gets in there.  They love that stuff.

some examples:
JBL SRX712m (polyester)
Various D&B cabinets (Foam)
JBL SRX728 (None)
Title: Re: Preferred Subwoofer Damping Material?
Post by: Tim Weaver on April 26, 2017, 04:06:08 am
For subs I wouldn't use any. Fiber material won't do anything at those frequencies. You're better off building a solid, dead, rock-hard cabinet.

So I guess the best damping material for subs is more wood for bracing!
Title: Re: Preferred Subwoofer Damping Material?
Post by: Ivan Beaver on April 26, 2017, 06:03:05 am
The first question is does your design need any at all?  Many don't. 
Think of it like this.

Low large are the waves being produced in the cabinet?  Think 10-30' long.

Is 2" of "stuff" going to do anything?

Stuffing can help on higher freq freq that are shorter, but will not do anything at the longer lower freq
Title: Re: Preferred Subwoofer Damping Material?
Post by: Stephen Kirby on April 26, 2017, 07:25:10 pm
In the box modeling software I have, adding "stuffing" material has the effect of a larger air volume.  My understanding is that the resistance to air flow kind of acts like a capacitor lowering the resonant frequency of the volume.
Title: Re: Preferred Subwoofer Damping Material?
Post by: Tim Weaver on April 26, 2017, 07:59:55 pm
In the box modeling software I have, adding "stuffing" material has the effect of a larger air volume.  My understanding is that the resistance to air flow kind of acts like a capacitor lowering the resonant frequency of the volume.

Yes, but it's not enough to worry about when dealing with subs.

Imagine this. You have a big sub playing 10 feet in front of you. Now hold up a blanket of whatever material you want. Did the sub get any quieter? Did it change in tone?

No. Because the wavelengths are huge. You would need about 2.5 FEET of damping material to even begin to have an effect at 100 hertz. At 50 hertz, you'd need 5 feet of the stuff. Are you going to add 5 feet of stuffing to every wall of your cabinet? Probably not.
Title: Re: Preferred Subwoofer Damping Material?
Post by: Stephen Kirby on April 26, 2017, 08:15:26 pm
I don't think it's about wavelengths.  It's about the effective volume of the box and the tuning.  Air is a spring and the larger the volume of air the lower the natural frequency of that spring.  If stuffing actually does behave as a way of faking additional volume then that changes the tuning of the box.  Having done bunches of models of boxes, it does show up in the software as extended LF.
Title: Re: Preferred Subwoofer Damping Material?
Post by: Tim Weaver on April 26, 2017, 08:31:25 pm
I don't think it's about wavelengths.  It's about the effective volume of the box and the tuning.  Air is a spring and the larger the volume of air the lower the natural frequency of that spring.  If stuffing actually does behave as a way of faking additional volume then that changes the tuning of the box.  Having done bunches of models of boxes, it does show up in the software as extended LF.


How many boxes have you built and measured with and without stuffing?

Stuffing in subs used to be a thing. Now it's not so much. The drivers are so much stronger that the box has much less effect on low frequency output nowadays. Sure, it still needs to be a tuned system, but the motor driving that system is a big-block chevy compared to the old days. IOW, the woofer's now have all the low end you need. The box doesn't need to extend things any lower. In fact, often times you can extract a good, flat low corner freq by tuning the box extra low. Since most modern woofers will easily hit under 40hz. You don't need to tune the box (and have a group delay hump) at 40ish hertz. You can tune it lower, push the GD lower so that the port tuning is below the HP filter of the cabinet. You'll have better phase response down low like this.

Older drivers needed all the help they could get to hit under 40. So we lived with group delay and itchy fiberglass.
Title: Re: Preferred Subwoofer Damping Material?
Post by: Tim Gurske on April 27, 2017, 12:52:09 am
The specifications are from Eminence and say "fill = minimal." Their phone support said that they snap their fingers in the box and when it stops ringing/echoing then you have enough fill.

So, it sounds like they are using fill like you would in a studio, rather than as a way to slow down the air and emulate a bigger box. I'm not sure what that accomplishes in a sub but maybe it cuts out noice or something.

I think I will try it without foam and then throw some foam in it to see if I can tell the difference.

Also, I am building it with 1" think baltic birch and biscuits so hopefully it'll be a "solid, dead, rock-hard cabinet" from the construction alone.

Thank you for the help!
Title: Re: Preferred Subwoofer Damping Material?
Post by: Chris Grimshaw on April 27, 2017, 03:31:46 am
The information about filling/lining cabinets is freely available online. Rod Elliott's website is a good source of info: http://sound.whsites.net/articles/boxstuff.htm

For subs, any stuffing or lining is going to absorb energy. With sealed boxes, the driver might feel like it's in a slightly (of the order of 5-10%) bigger cabinet, while ported boxes will find a loss in output around the port tuning frequency, since the port is activated by pressure inside the cabinet.

Ports often have secondary resonances somewhere in the kHz range, and filling or lining will help attenuate those at the expense of a little low-frequency output. As a result, you'll often find some lining in "full-range" speakers where the cone is producing a wide bandwidth, but for low-frequency-only situations, I'd leave it out.
If you really want to add some, go for lining, not stuffing, since the latter will really kill port output.

With any of these, make sure that whatever you use will not disintegrate or expel dust. Modern sub drivers pump a lot of air through the motor at high power levels - getting the vents clogged up will take it's toll on the driver's thermal power handling and you might find they burn out unexpectedly. A layer or two of cloth over the lining will keep everything in place.

Chris
Title: Re: Preferred Subwoofer Damping Material?
Post by: lindsay Dean on April 27, 2017, 03:27:32 pm
     Building a home brew sub starts with proven designs,
quality materials
and construction.
   Any  movement in the box is wasting output energy.
  brace,brace, brace.
Title: Re: Preferred Subwoofer Damping Material?
Post by: Mark Wilkinson on April 27, 2017, 06:02:43 pm
     Building a home brew sub starts with proven designs,
quality materials
and construction.
   Any  movement in the box is wasting output energy.
  brace,brace, brace.

Amen to all points. 

I just built some home brew single 18"s, designed using the great software floating around.
4 corner ports, well braced, and still vibrates like no tomorrow. 
Told my girlfriend to have a seat on one when I cranked up Crystallize by Lindsey Stirling.  She blushed deeply.... ;D

I don't know how much bracing it would take to completely tame today's drivers...other than I would want Tim's forklift handles...!
Title: Re: Preferred Subwoofer Damping Material?
Post by: Stephen Kirby on April 27, 2017, 09:32:45 pm
I built some similar sized cabinets some years ago.  I turned the handles the other way though.  It was easier to hold them against my hips while moving them around and much easier to do a two person move.

I did make the mistake of tuning them too low thinking I could get some extension out of them only to get into over excursion issues and having to raise the HP.

My current cabs have the clover four side cut outs that allow for grabbing from multiple directions.
Title: Re: Preferred Subwoofer Damping Material?
Post by: Mark Wilkinson on April 28, 2017, 12:11:46 pm
I built some similar sized cabinets some years ago.  I turned the handles the other way though.  It was easier to hold them against my hips while moving them around and much easier to do a two person move.

I did make the mistake of tuning them too low thinking I could get some extension out of them only to get into over excursion issues and having to raise the HP.

My current cabs have the clover four side cut outs that allow for grabbing from multiple directions.

Yeah, I can't say I really like my handle placement / orientation all that much.  I'm just not sure what would be substantially better, and their location didn't interfere with ports or efficient bracing.

The subs are kinda big for a single 18" at 23x23x30 because I wanted plenty of port area and length.  They are tuned to 31 Hz, initial measurements look great.


To the OP's question, I use acoustic foam for port or box resonances .https://b2b.parts-express.com/sonic-barrier-1-acoustic-sound-damping-foam-with-psa-18-x-24--260-525
HornResp is a fantastic modeling program, and shows both. http://www.diyaudio.com/forums/subwoofers/119854-hornresp.html
That said, I didn't use any foam in mine.....

As far as fill, like the guys said, the models show it effectively increases box volume.  But I really think it's much more for use with sealed subs, than ported. 
I also modeled and built some sealed boxes for use with the same drivers that are in my ported boxes (bms18n862).  Fill did make a difference, but not a lot.

Title: Re: Preferred Subwoofer Damping Material?
Post by: Jeff Robinson on April 28, 2017, 05:02:56 pm
It seems like there are many options. Is there a preferred material that "the pros" use or does everybody have a different preference? As always I am looking for cheap and best, although I realize that those might be mutually exclusive in most situations.

And, before the "it depends" answer :-), I will give some details about the application: I am building a basic ported cabinet for a single 18" subwoofer that will be used for playing general music (top 40 mostly) sometimes indoors and sometime outdoors.

Thank you in advance!

Here is a single word to research: Adiabatic (as it relates to compression/rarefaction of air), Fiberglas fill enables this characteristic (in sealed boxes).
I also suggest reading "Bullock on Boxes".

HTH,
Jeff Robinson
Title: Re: Preferred Subwoofer Damping Material?
Post by: Jeff Bankston on May 04, 2017, 05:10:02 pm
I have 15" and 18" woofers in ported boxes. I dont use fill. Use 1" no void batic birch with a few side to side and top to bottom braces for additional cabinet stiffness.
Title: Re: Preferred Subwoofer Damping Material?
Post by: Chris Grimshaw on May 04, 2017, 05:33:03 pm
Amen to all points. 

I just built some home brew single 18"s, designed using the great software floating around.
4 corner ports, well braced, and still vibrates like no tomorrow. 
Told my girlfriend to have a seat on one when I cranked up Crystallize by Lindsey Stirling.  She blushed deeply.... ;D

I don't know how much bracing it would take to completely tame today's drivers...other than I would want Tim's forklift handles...!

Depends which way it's vibrating. If the cabinet itself is moving due to the reaction force of the driver, then you simply need a heavier cabinet, or a better way of anchoring it to the ground. Bracing will stop the cabinet ballooning in response to the pressure, but it won't stop the whole cabinet moving the opposite way to the cone.

I had a Beyma 15P1200Nd in a fairly lightweight box for a while. Bridged a PV2600 into it for around 2KW, and found the cabinet (on wooden feet) was lifting itself up and rattling on the concrete floor. Fortunately, I had a sports bag with lots of SpeakOn to weigh it down. Wasn't pretty, but it was very early days for me so I probaly got away with it.

Modern sub drivers have very strong (heavy) cones, and a lot of motor force, so the cabinet needs to be solid and heavy to avoid walking around. Alternatively, opposed drivers (not isobaric) will get you some force cancellation, but now you're moving double-18" cabs.

Chris
Title: Re: Preferred Subwoofer Damping Material?
Post by: Len Zenith Jr on May 04, 2017, 07:26:01 pm
Depends which way it's vibrating. If the cabinet itself is moving due to the reaction force of the driver, then you simply need a heavier cabinet, or a better way of anchoring it to the ground. Bracing will stop the cabinet ballooning in response to the pressure, but it won't stop the whole cabinet moving the opposite way to the cone.

I had a Beyma 15P1200Nd in a fairly lightweight box for a while. Bridged a PV2600 into it for around 2KW, and found the cabinet (on wooden feet) was lifting itself up and rattling on the concrete floor. Fortunately, I had a sports bag with lots of SpeakOn to weigh it down. Wasn't pretty, but it was very early days for me so I probaly got away with it.

Modern sub drivers have very strong (heavy) cones, and a lot of motor force, so the cabinet needs to be solid and heavy to avoid walking around. Alternatively, opposed drivers (not isobaric) will get you some force cancellation, but now you're moving double-18" cabs.

Chris

I just did some calculations the other day on this matter and I'm going to have to say that the inertial force of the cone was not at play. The cabinet walking was either the bottom feet vibrating due to panel flex or air pressure changes from the sound waves lifting the cabinet. Here are my calculations, feel free to correct me if I missed something:

Newtons 3rd law states that for every action there is an equal and opposite reaction;

force=mass x acceleration or f=ma

so using newtons 3rd law;

mass(cabinet) x acceleration (cabinet) = mass(cones) x acceleration (cones)

Assuming the mass of the cones of a double 18" cabinet is ~210 grams each so mass(cones) = 420 grams for the both of them or mass(cones) = 0.42 kg

and acceleration = 2 x distance / time squared

lets find acceleration of the cones at 60 hz:

time @ 60 hz = 1/60 = 0.0167 seconds
distance of cone travel (x-max) lets assume full bore is 18mm = 0.018 meters

acceleration (cones) = 2 x 0.018 m / (0.0167 s)squared
acceleration(cones) = 129 m/s2

assume the mass(cabinet) = 112 kg

back to newtons 3rd law:
mass(cabinet) x acceleration (cabinet) = mass(cones) x acceleration (cones)
112 kg x acceleration (cabinet) = 0.42 kg x 129 m/s2

gives us acceleration (cabinet) = 0.484 m/s2

now distance the cabinet moves d = 1/2at2
d= 1/2 x 0.484 m/s2 x 0.0167 s x 0.0167 s
d= 0.0000674 m
d = 0.067 mm

So there you have it, hanging in free space with a 60 hz tone playing at full balls to the wall 18mm x-max the cabinets are vibrating back and forth 0.067 mm or less than 3 thousands of an inch.

If you think about it, a driver sitting on the workbench playing at x-max doesn't even move the driver, nevermind the whole subwoofer cabinet.

Edit: Although if the cabinet had a natural resonance frequency within the drivers passband, the vibrations could add constructively like pushing someone on a swing.
Title: Re: Preferred Subwoofer Damping Material?
Post by: Jeff Bankston on May 05, 2017, 02:22:39 am
My 18" will wiggle around by themself. Strap a 2-12" mid cabinet and 2" horn on top and they stay in place. The heavier the stack the less they move around the floor. On a flatbed trailer you can screw a few wood screws against the sides , front and back into the wood trailer floor and it aint going anywhere.
Title: Re: Preferred Subwoofer Damping Material?
Post by: Chris Grimshaw on May 05, 2017, 05:48:48 am
I just did some calculations the other day on this matter and I'm going to have to say that the inertial force of the cone was not at play. The cabinet walking was either the bottom feet vibrating due to panel flex or air pressure changes from the sound waves lifting the cabinet. Here are my calculations, feel free to correct me if I missed something:


I ran a simulation in Hornresp which suggests you're an order of magnitude out on the acceleration.

You've made the following (inaccurate) assumption:

For a 60Hz sine wave, you've said that the cone accelerates linearly from rest to Xmax in 1/60th of a second. This is false in two ways.

First up, the acceleration will vary a lot according to where in the cycle you are. Acceleration is inversely proportional to displacement.
Secondly, the cone needs to move from rest to Xmax to -(Xmax) to rest to complete one cycle, so you've only got 1/240th of a second to go from rest to Xmax.

It would appear that the discrepancy largely cancels out, this is just in the interest of doing things right.


Here's a nice trick.

First, I'll define some variables, where a "1" following a letter indicates that's the property of the cone, and "2" indicates that of the cabinet.
a=acceleration
d=peak amplitude
F=force
m=moving mass
f=frequency

Also, a=d*(2pi*f)^2 from https://en.wikipedia.org/wiki/Simple_harmonic_motion, looking at the maximum acceleration (we're not interested in any other case, since maximum acceleration produces maximum force).

Since, for both the cabinet and the cone, f is equal, we can write:

a1/d1=a2/d2

Then, from Newton's 3rd, F1=F2, m1*a1=m2*a2, so a1=m2*a2/m1

With some substitution and re-arranging, we can derive:
m1*d1=m2*d2

ie, the mass of the cone multipled by its peak excursion is equal to the mass of the cabinet multiplied by its peak excursion.

So, d2=m1*d1/m2.

In your case, d2=0.0675mm. You can go the long way around and work it out as you did. I got the same result both ways.


For the case of my 15" driver (mass of cone and voicecoil=160g*) in a 20kg box and 10mm one-way travel...
d2=0.08mm.
* should have used Mms on the T/S sheet, but I only had Mmd to hand, which doesn't include the air load of the cone.
Hardly seems enough to make the cabinet move much. I did hear it jumping around, though, and I do know that putting something heavy on top stopped it. Perhaps there was another mechanism at work. For instance, the distance of the driver from the floor could've produced a turning moment on the cabinet.

Out of interest, there's been at least one study about feeling vibrations: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC151682/
Looks like we can detect vibrations with an amplitude of the order of 10 microns, so something approaching 1/10th of a mm would be quite obviously vibrating when touched, even if we're unlikely to see the cabinet moving.

Chris

Title: Re: Preferred Subwoofer Damping Material?
Post by: Len Zenith Jr on May 05, 2017, 10:15:19 am


You've made the following (inaccurate) assumption:

For a 60Hz sine wave, you've said that the cone accelerates linearly from rest to Xmax in 1/60th of a second. This is false in two ways.

First up, the acceleration will vary a lot according to where in the cycle you are. Acceleration is inversely proportional to displacement.
Secondly, the cone needs to move from rest to Xmax to -(Xmax) to rest to complete one cycle, so you've only got 1/240th of a second to go from rest to Xmax.

It would appear that the discrepancy largely cancels out, this is just in the interest of doing things right.

I'm sure I made a lot of (inaccurate) assumptions, especially since the force of the motor probably isn't linear as the voltage changes which probably means a lot of calculus would be required to do an accurate calculation. I just copied and pasted my napkin calculations that I had from the other day. It started when a sound operator said that ground stacked arrays sound better than flown arrays because in a flown array the inertial force of the drivers moved the array back and forth causing doppler distortions. I rolled my eyes and said impossible and then I had to prove my rational to him. Once the answer was so small I stopped thinking as I proved my point.

My statement in my edit could very well be at play. A small force at a resonant frequency can add up to shake the whole system. Machines or even HVAC systems can shake the whole house from small vibrations at the right frequency. Also just from the sound, If your sub can vibrate the drywall and ceiling, those same sound waves can vibrate your box.
Title: Re: Preferred Subwoofer Damping Material?
Post by: Len Zenith Jr on May 05, 2017, 10:25:57 am

* should have used Mms on the T/S sheet, but I only had Mmd to hand, which doesn't include the air load of the cone.

If you really want to get that crazy then you would also have to figure out the inertial forces of the mass of the air in the ports which is opposite of the cones so they would cancel instead of add.
Title: Re: Preferred Subwoofer Damping Material?
Post by: Chris Grimshaw on May 06, 2017, 05:10:58 am
If you really want to get that crazy then you would also have to figure out the inertial forces of the mass of the air in the ports which is opposite of the cones so they would cancel instead of add.

Only when the ports are active.

Besides, at port tuning cone excursion is very minimal.

Chris