Acoustics Intuition of Subwoofer Mounting

  • #1
coby_rumblewood
7
1
TL;DR Summary
I have a high-power sealed subwoofer box with flanges resting on 30 Duro Isolating Pads. I want to secure it using #4 bolts, which pass through the flanges and have their own isolating pads beneath their heads. Will the subwoofer vibrations be transmitted to the base through the bolts, even with the isolating pads in place?
I have an acoustics question regarding mounting of a subwoofer enclosure, here is wireframe view of situation:

1695143363598.png

I have a sealed subwoofer box lets assume it is running high power 250 W RMS. It has flanges that come out that have 30 Duro Isolating Pads between the flanges and the base board below. Instead of letting it just rest there I want to securely mount it using some bolts, so I passed a #4 bolt through the top with a washer on smaller 30 Duro Isolating Pads, the flanges and plates have 1/8" holes which is bit larger than #4 bolt. Then a locknut on bottom. When the subwoofer shakes the box hard, will the vibrations pass through the bolt into the base structure? I believe because I put the bolt head on its own smaller pad it will not, but wanted to get other opinions.
 
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  • #2
You have to ask yourself, when the enclosure moves, say 1mm, how much force is imparted to the floor in each case? The smaller pads will probably have more stiffness than the large ones so will transmit more force to the floor.
 
  • #3
tech99 said:
You have to ask yourself, when the enclosure moves, say 1mm, how much force is imparted to the floor in each case? The smaller pads will probably have more stiffness than the large ones so will transmit more force to the floor.
The floor is 0.75" thick plywood and on the ledge of 1.75" frame pieces. It should really be solid to take that force. Maybe it is obvious, but could you explain why this will transmit more force and what type of force? I figured they are the same stiffness because of same 30 duro material. Are you saying because it is smaller will not isolate as much vibration and more will get through, so more "dynamic" force? Or because it is smaller will have larger static forces? I am not worried about static forces really, I am assuming everything will hold statically. But am trying to eliminate vibrations going through my structure.
 
  • #4
Welcome to PF. :smile:

coby_rumblewood said:
I want to securely mount it using some bolts, so I passed a #4 bolt through the top
Can you say why you chose #4 bolts? That seems pretty thin from my perspective...
 
  • #5
berkeman said:
Welcome to PF. :smile:Can you say why you chose #4 bolts? That seems pretty thin from my perspective...
Thank you!

Didn't want to go with 1/2" and the isolating rubber washers are 1/2" and 1/8" I think. #4 fits 1/8" well. Haven't thought about if strong enough yet
 
  • #6
In my experience, #4 bolts are only used for light fastening. For a high vibration application, I'd be inclined to go for the 1/2" option if possible.
 
  • #7
berkeman said:
In my experience, #4 bolts are only used for light fastening. For a high vibration application, I'd be inclined to go for the 1/2" option if possible.
Makes sense, I'll switch it over. Thank you
 
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  • #8
If you want to isolate a vibrating object (the subwoofer) from the base, you need to mount it using soft vibration mounts. The mounts need to be soft enough that the natural frequency of the subwoofer is lower than the lowest frequency at which the subwoofer will be driven.

For example, assume that you want to drive the subwoofer at frequencies down to 20 Hz. Then the subwoofer mounting should have a natural frequency lower than 10 Hz, and preferably lower than 5 Hz. This will probably require mounting the subwoofer on some soft springs.

Good search terms to learn more are vibration isolators and lord vibration isolators. Lord Corporation specializes in vibration control products. This link is a good source for vibration control theory: https://lordfulfillment.com/pdf/44/PC6116_AerospaceandDefenseIsolator.pdf. Pay attention to the section titled Modifications to Theory Based on the Real World. A good source for isolation mounts is McMaster-Carr Supply Company: https://www.mcmaster.com/products/isolation-mounts/.
 
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  • #9
jrmichler said:
If you want to isolate a vibrating object (the subwoofer) from the base, you need to mount it using soft vibration mounts. The mounts need to be soft enough that the natural frequency of the subwoofer is lower than the lowest frequency at which the subwoofer will be driven.

For example, assume that you want to drive the subwoofer at frequencies down to 20 Hz. Then the subwoofer mounting should have a natural frequency lower than 10 Hz, and preferably lower than 5 Hz. This will probably require mounting the subwoofer on some soft springs.

Good search terms to learn more are vibration isolators and lord vibration isolators. Lord Corporation specializes in vibration control products. This link is a good source for vibration control theory: https://lordfulfillment.com/pdf/44/PC6116_AerospaceandDefenseIsolator.pdf. Pay attention to the section titled Modifications to Theory Based on the Real World. A good source for isolation mounts is McMaster-Carr Supply Company: https://www.mcmaster.com/products/isolation-mounts/.
This is good material, thank you for the reference. I think General Purpose Vibration-Damping Sandwich Mounts with Studs is exactly what I am looking for. Can just put lock nuts on top and bottom of plates. Guessing you want the natural frequency to be lower than 20 so the subwoofer doesn't hit the natural frequency of the rubber? What exactly would happen if it does?
 
  • #11
coby_rumblewood said:
Guessing you want the natural frequency to be lower than 20 so the subwoofer doesn't hit the natural frequency of the rubber? What exactly would happen if it does?
You have likely experienced resonance if you have ever been on a swing in the playground.

If someone pushes you at the end of the backward swing they are "in phase" or "resonate" with your motion; that is they are adding forward energy just as you start swinging forward.

A problem can arise if the speaker cabinet with its mounting resonates at 20Hz and you are playing some heavy bass also at 20Hz.

Just like the swing, the heavy bass will add a little bit of energy to the shaking speaker at the same point in every cycle. Eventually the speaker cabinet will have enough energy to visibly shake, if the mounting doesn't break first!

That's why @jrmichler recommended that the natural (or resonate) frequency of the speaker with its mounting be much lower than the lowest frequency sound you expect.

Hope this helps!

Cheers,
Tom
 
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  • #12
Tom.G said:
You have likely experienced resonance if you have ever been on a swing in the playground.

If someone pushes you at the end of the backward swing they are "in phase" or "resonate" with your motion; that is they are adding forward energy just as you start swinging forward.

A problem can arise if the speaker cabinet with its mounting resonates at 20Hz and you are playing some heavy bass also at 20Hz.

Just like the swing, the heavy bass will add a little bit of energy to the shaking speaker at the same point in every cycle. Eventually the speaker cabinet will have enough energy to visibly shake, if the mounting doesn't break first!

That's why @jrmichler recommended that the natural (or resonate) frequency of the speaker with its mounting be much lower than the lowest frequency sound you expect.

Hope this helps!

Cheers,
Tom
That makes sense, thanks for the analogy. Have chosen 1/4"-20 vibration-damping sandwich mount made from neoprene rubber. Its max capacity is 31 lb per mount. The subwoofer enclosure I estimated to be 40 lb. Should I drop down to 8-32 thread size with max capacity of 20 lb per mount? My understanding of these if you want a desired amount of rubber squished. How close to max capacity should I get for optimal squish?
1695220389025.png
 
  • #13
Think this calculation is right, natural frequency shouldn't be a worry then.
fn=1/2pi sqrt(k/m)
Spring constant for compression of cylindrical elastomer:
k=AE/L
Eneoprene rubber=800 [psi]
Acontact=(OD/2)^2-(ID/2)^2=pi/4 * (OD^2-ID^2)
Acontact=pi/4 * (5/8^2-1/2^2)
Acontact=0.11 [in^2]
k=0.11 [in^2] 800[lbf/in^2]/(5/8)[in]
k=140.8 [lbf/in]

I know my mass to be:
mplywood=0.72 [slugs]

Hence,
fn=1/2pi *sqrt(140.8 [lbfin]/0.72 [slugs])
fn=2.23 [Hz]
 
  • #14
berkeman said:
Welcome to PF. :smile:Can you say why you chose #4 bolts? That seems pretty thin from my perspective...
If #4 screws are the 3mm (M3) that they look like they seem inadequate for anything other than locating very small items. I'm not familiar with this (yet another) dimensional term that these Imperial Engineers like to use.

For mounting something as heavy as this wooden box, you have to imagine the effect of dropping it on one corner from even just a few 'inches'. I would have thought M10 would be adequate but M12 would look more impressive.

coby_rumblewood said:
Can just put lock nuts on top and bottom of plates
I'd think in terms of Nyloc (or is that what you mean?).
 
  • #15
sophiecentaur said:
I'm not familiar with this (yet another) dimensional term that these Imperial Engineers like to use.
sophiecentaur said:
I would have thought M10 would be adequate but M12 would look more impressive.
What is this strange "M" thing that you speak of? :wink:
 
  • #16
berkeman said:
What is this strange "M" thing that you speak of? :wink:
I love 'em. No need to tinker with fractions to decide which of the two is bigger. There are very few common options for the M range and the standard ones are perfect for my bodgery exercises.
 
  • #17
berkeman said:
What is this strange "M" thing that you speak of? :wink:
M = Metric size.
12M is 12 millimeter major diameter, or 1/2 inch to us in the States.

Think this calculation is right, natural frequency shouldn't be a worry then.
fn=1/2pi sqrt(k/m)
Spring constant for compression of cylindrical elastomer:
k=AE/L
Eneoprene rubber=800 [psi]
Acontact=(OD/2)^2-(ID/2)^2=pi/4 * (OD^2-ID^2)
Acontact=pi/4 * (5/8^2-1/2^2)
Acontact=0.11 [in^2]
k=0.11 [in^2] 800[lbf/in^2]/(5/8)[in]
k=140.8 [lbf/in]

I know my mass to be:
mplywood=0.72 [slugs]

Hence,
fn=1/2pi *sqrt(140.8 [lbfin]/0.72 [slugs])
fn=2.23 [Hz]

Don't forget about Horizontal shaking. Again, @jrmichler is the one to get a good answer from.
 
  • #18
Tom.G said:
M = Metric size.
12M is 12 millimeter major diameter, or 1/2 inch to us in the States.
(I was joking, eh?) :wink:
 
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FAQ: Acoustics Intuition of Subwoofer Mounting

What is the optimal placement for a subwoofer in a room to achieve the best acoustics?

The optimal placement for a subwoofer often depends on the room's dimensions and acoustics. Generally, placing the subwoofer in a corner can amplify its output by creating more reflections, but this can sometimes lead to boomy or uneven bass. A common method to find the best placement is the "subwoofer crawl": place the subwoofer at the listening position and move around the room to find where the bass sounds the most balanced, then place the subwoofer in that spot.

How does wall proximity affect subwoofer performance?

Placing a subwoofer close to a wall can increase its efficiency because the wall can reflect sound waves back into the room, effectively boosting the bass. However, this can sometimes cause issues with standing waves, leading to uneven bass response. Experimenting with the distance from the wall and using acoustic treatments can help mitigate these issues.

Does the height at which a subwoofer is mounted affect its performance?

Yes, the height can affect performance. Subwoofers are generally designed to be placed on the floor because low-frequency sounds are omnidirectional and can fill a room more effectively from that position. Elevating a subwoofer can sometimes lead to a loss of bass impact and uneven distribution of sound waves.

Is it better to have one or multiple subwoofers for optimal sound quality?

Using multiple subwoofers can provide a more even bass response throughout the room. This setup helps to minimize standing waves and dead spots that can occur with a single subwoofer. Proper placement and calibration are crucial when using multiple subwoofers to ensure they work together harmoniously.

How do room dimensions and shape impact subwoofer acoustics?

Room dimensions and shape significantly impact subwoofer acoustics due to the way sound waves interact with surfaces. Rectangular rooms often have predictable standing wave patterns, while irregularly shaped rooms can cause more complex reflections and diffractions. Understanding the room's modes and using acoustic treatments can help optimize subwoofer performance.

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