DIY diaphragmatic bass absorbers

[john12ax7]

Foam has gotten a bad reputation  due to the prevalence of 2" wedge. If you compare a similar volume of foam to mineral or glass wool the performance is just as good,  sometimes better.

 

[abbey road d enfer]

The big danger with foam on a hard surface is that it could create a triple leaf system.

Would you care to elaborate? What is the "danger"?

That's why I use mineral fiber board. Thickness 4 to 6". Less than 4" is useless.

That's certainly fine for midrange absorption, but here the subject is diaphragmatic absorbers, that are specifically aimed at eliminating LF resonances.

But most important for me are measurements. I can't hear everything, like people with golden ears can. But I can see what's wrong in a graph and, more importantly, I can see where it's coming from.

I fully agree. I see too many people installing a standardized kit and thinking they(ve solved their acoustic issues...

And sometimes, it comes from the desk, not the walls. If it's the desk and the speakers are also on the desk, sliding them a few inches to the left or to the right could be enough to solve reflections. If it's bad, replace the desk surface with a perforated one.

You mean, drilling holes in your mixer? 

 

[DerEber]

Besides, getting foam that is actually useful is next to impossible. You would need high density open cell foam and that's not common in the marketplace. I tried to get some, but the manufacturer told me it would need to be made, or I could wait for leftovers from another order. Even those leftovers were expensive.

Open cell foam can be obtained easily at stores where they make cushions. At least in my area.
It's not a total bargain, but much cheaper than the basotect stuff.

It's also likely that they have a much better glue for you than the stuff you can buy at the average hardware store.

best,
Stephan

 

[DerEber]

You mean, drilling holes in your mixer? 

Stop saying that!
8)

 

[john12ax7]

I think it's Eric Valentine who designed a mixer with an absorbent top.  So no drilling required.

 

[cyrano]

Would you care to elaborate? What is the "danger"?

A triple leaf will typically result in less absorption when compared to a double of comparable size. It's much harder to calculate too  :

For me, it's a waste of effort and money.

That's certainly fine for midrange absorption, but here the subject is diaphragmatic absorbers, that are specifically aimed at eliminating LF resonances.

I hadn't encountered the phrase "diaphragmatic absorbers" yet, so I went by intuition as Google didn't provide anything useful. Thanks for setting me straight.

You mean, drilling holes in your mixer? 

No I mean the table under the mixer. But even the mixer might produce some reflections. In that case, cover with blankets and work under the blankets? 

 

[cyrano]

Open cell foam can be obtained easily at stores where they make cushions. At least in my area.
It's not a total bargain, but much cheaper than the basotect stuff.

It's also likely that they have a much better glue for you than the stuff you can buy at the average hardware store.

best,
Stephan

Thanks for the tips, Stephan. I tested some of that stuff too. It was more expensive than the mineral wool (for the same result), so I didn't bother.  Besides, it's harder to apply.

 

[abbey road d enfer]

A triple leaf will typically result in less absorption when compared to a double of comparable size. It's much harder to calculate too 

Again, I need some explanation; what is a double or triple-leaf? Certainly you don't mean a joint?  ;D

 

[cyrano]

Double leaf is 2 layers. Triple leaf is 3 layers of absorption.

You'd expect triple to be better than double. For beer it is

But for absorption panels, it can be less than double layer. Double layer is relatively easy to calculate and correct. Triple is not easy. You might get lucky, but sometimes 3 layers are worse than 2 layers.

That's also when using absorption as acoustic isolation material. In that case, the wall is often the third layer and screws up your calculation.

TBH, I don't calculate a lot, because I don't have exact enough values for the materials I use. These are for thermic isolation and even those specs could be embellished by the marketing dept, or the method they use to measure them. And specs for acoustic properties are usually simply guesswork, or absent.

An ideal value for density, fi is around 35 kg per cubic meter. Higher is better, but 35 kg/m3 is readily available for a good price. In practice, though, I start with just placing a rough board in the place I think is good and measuring again. That's faster than trying to work it out in numbers.

That's what works for me. I have a friend who is a professional acoustic engineer. He measures far less than I do, but he's done it often enough over 20 years to "see" where the boards need to go, or to guess what to change in the geometry of a speaker cabinet.

 

[abbey road d enfer]

Double leaf is 2 layers. Triple leaf is 3 layers of absorption.

I'm not sure I follow you. Three layers of identical material is bound to be more efficient than two layers. Now, if you mean layers of different materials, that's a whole new subject, because the motivation for doing so is either to combine materials that have different frequency range (e.g. rock fiber for mids and highs and lead for lows), or sandwiching materials in order to realize a resonant system, where the resulting absorption is not the sum of the individual parts, but rather the result of a complex (2nd-order damped resonator) interaction.

You might get lucky, but sometimes 3 layers are worse than 2 layers.

Do you have a specific example?

  I have a friend who is a professional acoustic engineer. He measures far less than I do, but he's done it often enough over 20 years to "see" where the boards need to go, or to guess what to change in the geometry of a speaker cabinet.

I've met many of these guys who walk in the room, clap their hands and draw the acoustic treatment on the back of an envelope. Half of the times it works, half of the times it doesn't and the guy doesn't know what to do.
I'm not saying your friend is like that, but a good measurement kit is definitely a must for someone who wants to make a living out of it.

 

[john12ax7]

For soundproofing the leafs refer to mass separated by air cavity. Double leaf is mass-air cavity-mass while triple leaf is mass-air cavity-mass-air cavity-mass. The danger of a triple leaf is that you raise the resonant frequency of the wall system which decreases isolation at low frequencies.

Note: A mass layer doesn't need to be a single piece of material, multiple drywall screwed together is still one mass layer. Also the air cavity is typically filled with light insulation.

What is the reasoning that foam would foam create a triple leaf system?

 

[abbey road d enfer]

For soundproofing the leafs refer to mass separated by air cavity.

Thanks for the clarification.

The danger of a triple leaf is that you raise the resonant frequency of the wall system which decreases isolation at low frequencies.

How's that?

 

[john12ax7]

How's that?

I have some papers I would need to find that shows the experimental data. Essentially a double leaf mass-air space-mass system behaves as a lossy resonant LPF. The resonant frequency is dependent on the mass and depth of the air space. If you have a 12" air gap and decide to add mass in the middle you now have two 6" gaps and have created a higher order system with a higher resonance. It works out that extra mass is, in general, better served adding on to an extra leaf, instead of creating a new one.

 

[john12ax7]

Regarding the original topic here is a design published by Tim of Buzz Audio. If anyone else has some DIY plans please share them.

 

[abbey road d enfer]

If you have a 12" air gap and decide to add mass in the middle you now have two 6" gaps and have created a higher order system with a higher resonance.

Indeed. That's because there is a space constraint, not because it's a triple resonator. If space was not constraint, a triple system would be more efficient than a double.
The problem when asserting such claims is that one must specify which parameters are allowed to change or not.
If the original wording were "given a certain available depth, a 3-leaf is less efficient than a 2-leaf", I would not have challenged...

 

[john12ax7]

Indeed. That's because there is a space constraint, not because it's a triple resonator. If space was not constraint, a triple system would be more efficient than a double.

Not necessarily.  The issue tends to come up when modifying existing structures.  This link shows an example.  By adding spaced drywall to an existing concrete  wall you have both added mass and increased the depth of the wall,  in general good things,  yet low frequency performance has now been destroyed.

https://www.soundproofingcompany.com/soundproofing101/triple-leaf-effect/

 

[cyrano]

I'm not sure I follow you. Three layers of identical material is bound to be more efficient than two layers. Now, if you mean layers of different materials, that's a whole new subject, because the motivation for doing so is either to combine materials that have different frequency range (e.g. rock fiber for mids and highs and lead for lows), or sandwiching materials in order to realize a resonant system, where the resulting absorption is not the sum of the individual parts, but rather the result of a complex (2nd-order damped resonator) interaction.
Do you have a specific example?

Yes, 3 different materials. Which also happens if you have two materials mounted a bit off the wall.

Not really an example. It's just a personal rule. KISS, you know.

I've met many of these guys who walk in the room, clap their hands and draw the acoustic treatment on the back of an envelope. Half of the times it works, half of the times it doesn't and the guy doesn't know what to do.
I'm not saying your friend is like that, but a good measurement kit is definitely a must for someone who wants to make a living out of it.

The last treatment I did, the studio owner had three offers from "professionals", going from 7.500 to 17.000 €. None of those cared enough to even visit the studio, let alone clap hands. We did it for just over 3.000 € ex VAT. 1/3 of the price was for measurements before, during and after.

It was a difficult home studio. Small, stuffed with gear and two walls in glass. We ended up covering ceiling and the one wall each with one large absorber. No corner treatment needed. The first producer who tried it after treatment, immediately booked for three CD productions in the future, date to be determined.

 

[abbey road d enfer]

Not necessarily.  The issue tends to come up when modifying existing structures.  This link shows an example.  By adding spaced drywall to an existing concrete  wall you have both added mass and increased the depth of the wall,  in general good things,  yet low frequency performance has now been destroyed.

https://www.soundproofingcompany.com/soundproofing101/triple-leaf-effect/

What I see is that, given a certain depth, going triple is worse than double, which I totally agree with. Now, if the triple-leaf was designed with the constraint of maintaining the lower resonance, the performance would be better; indeed, that would involve increasing depth.

 

[abbey road d enfer]

None of those cared enough to even visit the studio,

I would guess they had plans of the room? Strictly speaking, one does not need to "visit" a room to design acoustic treatment, as long as plans are established and verified. For soundproofing, preliminary measurements must be done, though.

let alone clap hands.

We all know (do we?) it's utterly useless, akin to kicking tyres when assessing a used car.

  1/3 of the price was for measurements before, during and after.

That's right. Acoustic treatment materials are inexpensive, it's the labour, particularly in terms of making it look nice, that is costly. And final tuning.

 

[boji]

You mean, drilling holes in your mixer? 

I think it's Eric Valentine who designed a mixer with an absorbent top.  So no drilling required.

Yeah isn't it the same beady screen found on AKG mics from the 80's? D1000? I imagine it does HF decently.