John Sayers' Recording Studio Design Forum Archive

Waka wrote: Hi Stuart, that's very interesting. Can I ask which formulae you use for your 3 leaf systems? I'm struggling to find a good one.

One form of the equation is this: There's a simpler form, which is not applicable in your case as it requires that M1 and M3 are identical, and together are identical with M2, as well as D1 being identical to D3. If those conditions are met, then:

f+ = (1 / (2 x pi) ) x SQRT( (3.6 x Rho x c^2) / ( m x d ) )
f- = f+ / ( SQRT(2) )

Where:
Rho= air density
c= speed of sound in air
m=the mass of leaf 1 (or leaf3) by itself (or half of M2, if you prefer)
d=the gap between M1 and M2 (which is the same as the gap between M2 and M3)

This does NOT apply to your situation for another reason too: it assumes that the spring is air. In your case, the spring is rubber, so you have to replace that term with the spring constant for your rubber.

An interesting fact: in this configuration, the equivalent 2-leaf system (having the same total mass and total thickness), provides BETTER isolation than the 3-leaf system, up to a frequency that is FOUR TIMES the MSM resonant frequency of the 2-leaf system. Above that point, 3 leaf is better, but below that point 3 leaf is worse, and 2 leaf wins. This is why it's never a good idea for studios to have 3-leaf systems, since you need much greater mass and cavity depth, just to get the same isolation for low frequencies.

Another interesting fact: For this configuration f+ will always be exactly twice the MSM resonant frequency of the equivalent 2-leaf system. So if you can't be bothered to do the three-leaf math, then you can just figure out f0 for the 2-leaf, multiply by 2 to get f+, then work backwards from there to get f-. That's the simplest method... but it's only really valid for this specific configuration, where m1=m3= (m2*0.5) =m, and d1=d2=d. For your system, where those conditions are not true, you have to use the full equations.

The math gets a little more complicated here.

- Stuart -

I have couple of questions.

-I have this 5cm air gap (discussed in page 1) between the front wall and soffit back. Should I cover the back of the soffit with plywood? Or fabric?
-Should I put a Plywood between the soffit and the soffit "wing"? -Matt

MattHank wrote:I have couple of questions.

-I have this 5cm air gap (discussed in page 1) between the front wall and soffit back. Should I cover the back of the soffit with plywood? Or fabric?
-Should I put a Plywood between the soffit and the soffit "wing"?

soffit back.jpg

-Matt

I wouldn't put a back on the soffits. The soffit faces are designed to direct energy towards the room, they aren't sealed units. If you placed plywood on the backs it would vibrate and resonate. Fabric/strapping to keep the insulation from dropping might be a good idea.

Plywood between the soffit and soffit wing, again is unnecessary. Just build the framing and face the front with your heavy mass.

Dan

Here's my cloud design. It's hard backed with 10cm insulation. Frank drilled holes to the back, what's the reason for that? To maximize absorption and still have some reflection? Should I also do so? I did beams that sit on the top of the soffits and support the cloud, because I don't know how much load my ceiling trusses can handle. I thought it would be safer this way. The cloud's back edge is 1,10m from the midde of the soffits, just a little over mixing position. The cloud angle is 25°. Maybe I should cover more of the ceiling area between the backwall hangers and the cloud.
-Matt

I tried to fit wider hangers to the corners. This blocks half of the doorway, but it's ok. There's 2,5cm glasswool on both sides of the fiber board. Is it enough?

Any comments?

-Matt

It's quiet here!

Sorry Matt!

Yep 25mm is what John recommends(/ed).
Here's a post from 2009 where he shows a drawing:
http://www.johnlsayers.com/phpBB2/viewt ... 65&start=1
He mentions a 6 to 8 inch (150 - 200mm) gap between them.
If you look there may be a more recent thread in case he has updated his design.

Dan

Thanks waka!

I have another idea how to attach Genelecs to the speaker enclosure box without sorbothane. Maybe I could take advantage of Genelec's Iso-Pod Isolation Positioner/Decoupler that is attached to the speaker. They say it's made from special lossy rubber-like material and it's vibration isolation and damping properties reduce midrange coloration caused by unwanted vibration transmitted to supporting surfaces. The Iso-Pod has four shallow feet. Genelec has a metal stand plate, like in picture 2, to securely fit the Iso-Pod over the pins, which provides a secure mount. I could use a similar way, four bolts instead at the bottom of the speaker enclosure box to keep the speaker in place. This would be a very simple solution. Of course this isn't carefully tuned system like with sorbothane, but it works with the speakers on the meter bridge, could it work when the speaker is in the soffit?

Do they say what frequency it is tuned to decouple at? Since it is only very thin, the frequency must be pretty high...

Same rule as for MSM applies here: it will only decouple at twice the resonant frequency and above. So ask them to provide the lab test results that show how will it performs at decoupling, and check that it does, in fact, decouple down to an octave below the cut-off frequency of the speaker you pan to use.

but it works with the speakers on the meter bridge,

ANYTHING would work for a speaker on a meter bridge! That's the worst possible place to mount a speaker in any case (which is exactly why engineers put them there....), so it doesn't really matter WHAT you do to a speaker on the meter bridge, it can only improve things! Even a slice of three-day-old pizza would help considerably....

could it work when the speaker is in the soffit?

I guess the real question here is: Which soffit mounting method are you trying to follow? There are two basic approaches: one is to fully "float" the speaker so that it cannot transmit any vibration into the soffit, and the other is to mount it extremely rigidly, held very tightly in place, so that it cannot transmit any vibration into the soffit. Which of these two are you aiming at? Floating requires thick isolation pads and careful tuning, while "solid mount" requires high mass and rigidity in the structure, and very careful construction of the enclosure box, because it has to be a very tight fit.

- Stuart -

Do they say what frequency it is tuned to decouple at? Since it is only very thin, the frequency must be pretty high...

Same rule as for MSM applies here: it will only decouple at twice the resonant frequency and above. So ask them to provide the lab test results that show how will it performs at decoupling, and check that it does, in fact, decouple down to an octave below the cut-off frequency of the speaker you pan to use.

They don't say it. But I can ask.

I guess the real question here is: Which soffit mounting method are you trying to follow?

Well, I know the best way to do it is to fully float, but calculating sorbothane is too complicated for me... Maybe I need to hire you to do it.

Stuart, what do you think about my backwall hanger order? Should I change anything? Also should I make my center part between the soffits fully absorptive?

MattHank wrote:

Do they say what frequency it is tuned to decouple at? Since it is only very thin, the frequency must be pretty high...

Same rule as for MSM applies here: it will only decouple at twice the resonant frequency and above. So ask them to provide the lab test results that show how will it performs at decoupling, and check that it does, in fact, decouple down to an octave below the cut-off frequency of the speaker you pan to use.

They don't say it. But I can ask.

I guess the real question here is: Which soffit mounting method are you trying to follow?

Well, I know the best way to do it is to fully float, but calculating sorbothane is too complicated for me... Maybe I need to hire you to do it.

Stuart, what do you think about my backwall hanger order? Should I change anything? Also should I make my center part between the soffits fully absorptive?

If you're interested in floating your speakers I'm sure Stuart will be able to quote you for his designs, but if you choose to diy you always have the option of using bolts to get the required deflection, this way you don't need to calculate it as the bolts can apply the many kgs of pressure to get down to low resonant frequencies.
These bolts need to be decoupled from the frame also mind you, so look for sorbathane washers and bearings.

The other option is to use the sorbothane vibration calculator:
https://www.sorbothane.com/Data/Sites/3 ... lator.html

You want to get the natural frequency down low as possible, by increasing the weight on the sorbathane (concrete slab?) Adjusting size and shape of sorbothane and adjusting durometer until there is 15 - 20% deflection at a low frequency.

Dan