Author Topic: air cavity increase and sound attenuation properties  (Read 7362 times)


  • Guest
air cavity increase and sound attenuation properties
« on: March 20, 2008, 01:58:13 PM »
hello all - my first post on this forum

ive been looking around a bit into room acoustics and soundproofing as im an architecture student currently designing a music academy. the site is by a busy road and the building facade (the one facing the road - which will deal with the traffic noise) is currently projected to be responsive - that is, the facade alters according to the ambient acoustic environment as well as the internal sound generated. thats the fun part - the difficult bit is trying to get this to work.

so far - i've been thinking along the lines of an inflatable/expandable cell with some sort of translucent/clear sound barrier on either side. the facade would be a sandwich of MLV > Expandable Air Cavity> MLV again on the other side.

my question is whether this idea is a) completely ridiculous in terms of its effectiveness at absorbing traffic noise (i know low frequency rumble is a problem) but also whether increasing the air cavity by inflating the cell has any impact on sound proofing - ie. does a bigger air cavity equal a reduction in sound transmission?

ive also looked around for other types of "expandable" sound insulation - but havent really had much luck - im not sure if this concept will hold yet.

im willing to sketch/scan/upload basic facade sections if anyone's interested in seeing them.

thanks in advance



  • Guest
Re: air cavity increase and sound attenuation properties
« Reply #1 on: March 20, 2008, 09:25:52 PM »
Sounds like an interesting concept, but I doubt you'll get the kind of noise reduction that's required. 

To get good isolation, especially from low-frequency noise, you need inoperable window units with heavy, thick glass (preferably laminated) and a large airspace.  When it comes to bass, there's no getting around the mass law...


  • Guest
Re: air cavity increase and sound attenuation properties
« Reply #2 on: March 21, 2008, 05:59:01 PM »

yes ive been worried about the lower frequencies - traffic rumbling would be a nightmare

does the idea that if you expand the airspace you get a reduction in sound being able to penetrate still hold though?

Mark Daveis

  • Guest
Re: air cavity increase and sound attenuation properties
« Reply #3 on: March 22, 2008, 08:42:15 PM »
How would you inflate that much mlv mass as it would be very heavy?
If you just do one wall then something as loud and with as much energy as traffic noise will just go around and come through other walls or roof.
The limpness and mass of a material is proportional to its sound blocking power. And mlv is limper than lead and both have lots of mass.
You are exactly right the larger the cavity the greater the sound reduction including bass sounds. Stand one side of a farmers field and play a stereo at loud 100+ db then have someone move it far away to the other side of the field and notice the difference. The sound will reduce following a inverse square law.
In real terms though you can reduce all or nearly all of the noise like LF traffic noise but you will need a very large cavity which is not practical. So instead we use say 4 inches or more cavity with two heavy material layers to get to about 60 stc and if you want more stc then you will need to make another cavity or triple leaf or even a quad leaf design.
I have made a triple leaf design a bit like a sound studio which is over 90 stc! This used only 30 kg/msq panels on a double walled structure inside a normal room with lots of GG. But note the outer room was quite large and had about 1 foot thick concrete walls. But the readings I got using a sound meter were excellent and even on fireworks night here in the UK you can hardly hear the loud explosions which are like a shot gun so triple leaf design can be made to work very well.
You will get more resonance problems with multiple leaf designs but using more limp materials like rubber or mlv will help stop these depending on how much you want to spend. :)


  • Guest
Re: air cavity increase and sound attenuation properties
« Reply #4 on: March 23, 2008, 01:42:51 PM »
thanks for the reply

i have been studying some acoustics books and it seems that most of the low frequency sound from traffic is actually impact sound - ie. sound that travels through the structure and vibrates the inside of the room.

i am thinking of employing an expandable air cavity with 2 or 3 layers of mlv at the same time as decoupling the entire room from the supporting structure by means of "floating" it on springs, or fluid dampers. this way i reckon that the mlv>air cavity>mlv sandwich will nullify mid range and high frequency sounds whilst the decoupling will remove the low frequency transmission.

i think this solution may have the potential to work

my next step will be a sound survey of the site - hopefully i can borrow a sound meter from uni, to see how bad the traffic noise actually is

one question remains however - that is, could i use a denser gas instead of air to provide more effective attenuation in the expandable cavity?

ive read that fog and smoke both attenuate sound to a higher degree than air (the optimum attenuation for air being at 0.3-0.4% humidity) - so does it hold that a denser gas might be better still>?

something to look into...


  • Guest
Re: air cavity increase and sound attenuation properties
« Reply #5 on: March 23, 2008, 04:33:09 PM »
ive been digging around an old acoustics book and i think ive come up with a workable solution (at least in theory)

traffic noise - in its low frequency form is mostly inaudible and is usually felt through vibrations. in this case - mass wont do anything to stop the rumbling and decoupling of the structure is what's needed.

outlne of impact sound transmission re traffic noise

proof that the mass law must be taken into account with coupled walls and slabs - or the results become innacurate

in that book it also goes on to say that the air cavity in a cavity wall should ideally be as large as possible in order to achieve maximum attenuation

so - my suggestion is to decouple the rooms requiring isolation from the supporting structure to reduce traffic impact sound transmission (either on fluid dampers as mentioned above, or on some sort of rubber or springs)

this then combined with the expandable sound isolation system (which should sort out higher and mid range frequencies by a mass loaded vinyl > expandable air cavity > mass loaded vinyl sandwich) would seem to deal with the vast majority of sound being generated.

just to clarify - this isnt a spec for a top notch recording facility, just a music production studio, and as such it doesnt need to completely sound tight.

ive also had an idea regarding an adjustable volume setting for the room. if the room was made to contract (the roof would descend powered by an inflatable air pocket) this would alter the acoustics of the room, making the sound generated inside seem louder and in effect cancelling some of the outside noise that still gets through whilst at the same time also increasing the sound proofing of the roof level.

the problems with this to me so far seem to be;

a) mass loaded vinyl (based on my crude calculations and best information on the internet) seems to be only available at about a sixth of the density of conrete. if concrete is 2300kg/metre cubed, mlv is only about 330kg/metre cubed - im not sure if its possible to get a denser vinyl system - currently this is what im counting on

b)air in the air cavity might not be the best sound attenuator - from this table here

fog and smoke both attenuate sound better than air - (with air's optimum humidity for attenuation being about 0.3-0.4%)

im wondering if there's denser gases (which are stable) that could be used to inflate the expandable system and so provide a better level of attenuation - any suggestions? (coloured gases could also be quite nice - aesthetically speaking)

this is more or less where im at - at the moment

i think the next step will be an actual sound survey of the site (deptford creek) so here's hoping i can blag a sound meter off uni - to see whether the traffic is actually that bad - i also have some scope of re-siting the building further away from the road.

thanks for all your suggestions so far

Mark Daveis

  • Guest
Re: air cavity increase and sound attenuation properties
« Reply #6 on: March 23, 2008, 11:41:12 PM »
A sharp change in density is what the cavity uses between two high mass layers so a denser gass may not improve performance and even so I dont think you will get much difference.
HF can be reduced with just a fair amount of mass and the cavity should take care of the mid range as well.
Liquid or air damping may be expensive to set up but work well. Also Neoprene has the highest natural frequency but still below our hearing range and Helical springs are better so all of the choices will work.
Are you talking about infra sound which is so LF that we can feel it but not hear it?
I could not get the pictures to work but the book sounds great  - could you tell me the title of the book.
Mlv damps so well that it is as effective as say solid materials of 2 to 3 times its mass. :)


  • Guest
Re: air cavity increase and sound attenuation properties
« Reply #7 on: March 24, 2008, 12:27:40 PM »
thanks for the reply mark

its looking positive now -

for some reason this forum doesnt let me post the large version of the picture but if you go over to this forum then you'll see the larger versions

the book is called Room and Building Acoustics and Noise Abatement - by Willi Furrer

the publisher is Butterworths - and the year is 1964

by impact sound transmission i meant the sound that we feel but dont hear yes - traffic rumble etc...

Mark Daveis

  • Guest
Re: air cavity increase and sound attenuation properties
« Reply #8 on: March 24, 2008, 10:45:00 PM »
Considering the very low end of the frequency scale will make a good design because if you can significantly reduce the infra sound which we can notice but not really hear then all the higher frequencies will be dramatically decreased depending on the amount of mass and cavity you used.
If you are looking for a rough performance guide then I can say that in one of the triple leaf rooms that I built the infra sound or shaking you can feel in body parts from this was reduced by about 70% or more and other hear able LF sound like rumbling was reduced by less than half and high pitched sounds were almost all gone unless very loud. Note I built a isolated doubled wall room within a room which was set up in a concrete walled large room. But the doubled setup I built inside had low mass of about 20 to 30 kg/msq and plenty of GG and multi layers of mlv on the floor because it was easier. Its best to totally decouple the walls/roof and not use clips or spring hangers to get the best effect and only connections should be on the floating floor.

Download these to get more info on tested and built designs:

There are triple leaf and better designs in there and the test data so should give you some ideas of what to expect.

When we design a cavity between 2 heavy/dense layers we want a low density medium in the cavity like air because the sharp density change which is what causes the transmission loss. Remember when sound passes in a different density material it changes its direction which causes a loss of sound energy. Of course fog/smoke work better but you wont get any real change for LF sound and Hf sound will be decreased enough just by using a decoupled cavity design. :)

Thanks for the link I can see the book pictures now - very interesting.