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Author Topic: Preferred Subwoofer Damping Material?  (Read 2399 times)

Jeff Bankston

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Re: Preferred Subwoofer Damping Material?
« Reply #20 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.
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Chris Grimshaw

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Re: Preferred Subwoofer Damping Material?
« Reply #21 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

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Len Zenith Jr

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Re: Preferred Subwoofer Damping Material?
« Reply #22 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.
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Len Zenith Jr

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Re: Preferred Subwoofer Damping Material?
« Reply #23 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.
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Chris Grimshaw

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Re: Preferred Subwoofer Damping Material?
« Reply #24 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
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