The "Room Acoustics" thread

[sidvee]

@Santhosh/SId

Should the first reflection points be treated with absorbtive material or diffusive material?

How effective is a decorative shelf/wall hanger (jute) as a diffuser, particularly for high frequencies?

Cheers

@asterix - Conventional knowledge is to use absorbers at first reflection points - logic being that reflected sound waves are absorbed so as not to interfere with direct waves reaching the listener, thus causing mudiness. Another school of thought is that if the rest of the room is not too reflective then diffusers at the first reflection point will diffuse secondary waves - thus reducing their energy - and also preventing them from interfering with direct waves so almost serving the same purpose as absorbers, but with a key difference that the secondary waves will be dispersed and linger on creating a slight diffusive effect ie a more lively room. I have seen both, infact one high end audio demo. room in Detroit (where I used to live before) had the first reflection points totally treated with diffusers- and it sounded pretty nice. So in conclusion I think it will be based on your preference and also the reflectivity of the rest of the room. I am using absorbers at first reflection points.
I do not think decorative shelves etc will be very effective just because they will lack either 1) density 2) size. OTOH a lot of people swear by bookshelves with odd sized books in each shelf as effective diffusers. me thinks you need to experiment. I have also see in audio shows wherein many high end manufacturers display artificial plants in the room at first reflection points, behind the speakers etc. used as diffusers.
Hope this helps.
cheers
Sid

 

[cmsajith]

Just a thought, since the triangular shaped bass traps will be an aesthetic problem for many in living room, how about including a cupboard like structure ~7ft height (not a heavy one) as a continuation of the other furniture or as a separate one in the corner like shown in the picture:
Imageshack - 123fo.png.

Note : No gurantee for its effectiveness, but it should be enough as the grey colored portion is having absorber and 1ft depth behind (can try using panels in the inner surface or chunk filling) with another niche with lights for aesthetics

 

[santhosh]

@Santhosh/SId

Should the first reflection points be treated with absorbtive material or diffusive material?

How effective is a decorative shelf/wall hanger (jute) as a diffuser, particularly for high frequencies?

Cheers

Like Sid mentioned, from what I have read absorption for first reflection points is better. Anything that is behind the listening position is best diffused. It is also important to know that not everything that hits the absorber is absorbed, the angle at which the sound waves come in contact with the absorber also matters. Most of the sound waves are absorbed and some portion diffused depending on the absorber used and whether the waves are off-axis w.r.t to the absorber.

Just a thought, since the triangular shaped bass traps will be an aesthetic problem for many in living room, how about including a cupboard like structure ~7ft height (not a heavy one) as a continuation of the other furniture or as a separate one in the corner like shown in the picture:
Imageshack - 123fo.png.

Note : No gurantee for its effectiveness, but it should be enough as the grey colored portion is having absorber and 1ft depth behind (can try using panels in the inner surface or chunk filling) with another niche with lights for aesthetics

Low frequencies can pass through non porous surfaces and will not be reflected. So for an effective bass trap, density is very important while porousity is not. So, you can still do a cupboard like structure for aesthetics but it should contain some high density absorbing material on its back wall. A workable design would be to build a wedge like structure first with high density absorbent material and use the front face of the wedge to build a cupboard. Sorry, not good at drawings othrwise would have drawn one like you did. The cupboard itself will need to be designed carefully so as to not make it a resonating cavity, which I am sure you have already considered. I will be doing the same, but mine will be an open rack to store records/cds and blu-rays..with light et all.

 

[santhosh]

Only in recording studios specifically for music will have entire walls with diffusers as the scenario is different. I have seen (pics) several recording studios with full of diffusers front/side/back.
Sample pic of a well known recording studio: http://gallery.avsforum.com/data/509/screenshot_0124.jpg. (Is this pure science or art, no idea)
Another listening room : http://img268.imageshack.us/img268/1228/palroom3.jpg
............................
Following picture shows a sample setup from GIK Acoustics: http://www.gikacoustics.com/images/room_layout_662.gif. Please note the distance from the listener to the back wall and the locations of the diffusers.

Thanks for sharing those very nice pics sajith.

That recording studio is built like an anechoic chamber. Just curious, which well known recording studio is this? Recording Engineers need to be cautious of injury from venturing too close to those stalagsides(?):lol:. Their cries for help may go unheardhyeah:

This picture speaks a thousand words. We are all on the same page. This is exactly what Sid, you and me are trying to say.

All I can do about the the listening position in the picture is envy someone who has access to such a big room:licklips: and also that it is a dedicated listening room unlike many(including me) who use the room for listening, HT and even gaming.

 

[santhosh]

Here the diffusers will act as sound breakers as well as reflectors

Question to all: All diffusors reflect (in various directions - some away and some towards the listener), but are all diffusors sound breakers?

 

[sidvee]

This picture speaks a thousand words. We are all on the same page. This is exactly what Sid, you and me are trying to say.

All I can do about the the listening position in the picture is envy someone who has access to such a big room:licklips: and also that it is a dedicated listening room unlike many(including me) who use the room for listening, HT and even gaming.

BTW I use 6 GIK 244 panels in my room in a manner similar to the picture (of-course I do not have such a large room) and 6 diffusers are on order for the side walls at my listening position. I also was planning to use tri traps in the corners but again not enough room. Regardless the 244's with 4" thickness perform quite well in trapping bass frequencies.
Cheers
Sid

 

[sidvee]

Question to all: All diffusors reflect (in various directions - some away and some towards the listener), but are all diffusors sound breakers?

Santhosh - AFAIK all the diffusers that I have seen were made of hard reflective surfaces such as hard plastic or wood, so if I understand your question right, yes these will definitely break the sound wave or act as sound breakers.
Cheers
Sid

 

[jitubisht]

Guys,

Just wondering, has anyone checked the stuff these guys are selling:

Acoustic Foams - Acoustic Foams, Manufacturers Acoustic Foams, Acoustic Enclosures, Acoustic Foams India, Acoustic Enclosures, Manufacturers Acoustic Canopy, Anechoic Chamber

Anyone heard any reviews?

Cheers

 

[jitubisht]

Where are the doors?

(Sorry bad joke)

 

[cmsajith]

Low frequencies can pass through non porous surfaces and will not be reflected. So for an effective bass trap, density is very important while porousity is not. So, you can still do a cupboard like structure for aesthetics but it should contain some high density absorbing material on its back wall.

Yes, eventhough purosity not required for passing low freq, there is another reason for making the face porus, to avoid any resonances due to a closed box structure. Moreover, it is open/porus at the top and bottom for the same reason. This can be made as either a trap functioning similar to this conventional bass trap : http://www.realtraps.com/cust_schmidt.jpg with open space behind and open at the top/bottom or as a chunk trap by filling the structure with acoustic absorbers.

Note : For Low freq high density board is a must.

I will be doing the same, but mine will be an open rack to store records/cds and blu-rays..with light et all.

You can have an open rack attached to this keeping the basstrap portion at the corner.

 

[cmsajith]

Question to all: All diffusors reflect (in various directions - some away and some towards the listener), but are all diffusors sound breakers?

Every diffusers work different. The angle of reflection depends on the shape of the diffusing surface. Eg. QRD panels are based on the depth and the area of depth (based on some mathematical formula). For GIK, the surface is with curves and lines like structure where it diffuses sound in a particular way. So the position of placing the diffusers depends on the design. In the studio eg. above is effectively (assuming, since i didnt have any experience) using diffusers: see the difference in diffuser type used on the side walls and the corners of the front wall.

There is another kind of diffuser cum absorber which is cylindrical in shape with reflective surface for the mid and high but transparent enough to pass low freq. and absorbers inside to trap.

 

[cmsajith]

Thanks for sharing those very nice pics sajith.

That recording studio is built like an anechoic chamber. Just curious, which well known recording studio is this? Recording Engineers need to be cautious of injury from venturing too close to those stalagsides(?):lol:. Their cries for help may go unheardhyeah:

This picture speaks a thousand words. We are all on the same page. This is exactly what Sid, you and me are trying to say.

All I can do about the the listening position in the picture is envy someone who has access to such a big room:licklips: and also that it is a dedicated listening room unlike many(including me) who use the room for listening, HT and even gaming.

Even the same can be done in a small room, but by increasing the size of the back absorber panel and reducing the amount of diffusers as they will not function with a small distance from the listening position.

 

[smedhavi]

Yes, eventhough purosity not required for passing low freq, there is another reason for making the face porus, to avoid any resonances due to a closed box structure. Moreover, it is open/porus at the top and bottom for the same reason. This can be made as either a trap functioning similar to this conventional bass trap : http://www.realtraps.com/cust_schmidt.jpg with open space behind and open at the top/bottom or as a chunk trap by filling the structure with acoustic absorbers.
Note : For Low freq high density board is a must.
You can have an open rack attached to this keeping the basstrap portion at the corner.

You are right, but it is extremely difficult to absorb low frequencies due to their high wavelengths. You would need several feet of absorbent material to work with low frequencies. For instance, at 100 Hz, the wavelength is 11.3 ft!

So one of the viable options is absorption through 'dissipation of sound energy' by using 'Diaphragmatic panels' resonating at low frequencies. In simple words using panels (boards, plywood etc.) that would resonate at a particular frequency based on the mass of the panel and the springiness of the air in the sealed cavity behind. Filling the cavity with absorbent material, that does not press the panel too hard to stop it from resonating, multiplies the effectiveness of such a system. A 1/2 inch plywood with 2 inches cavity behind it filled with glass/synthetic wool that does not touch the ply can do wonders. So resonance is not always bad

Reference for the above comment:

• Master Handbook of Acoustics - By F. Alton Everest & Ken C. Pohlmann (Chapter 12)

 

[cmsajith]

You are right, but it is extremely difficult to absorb low frequencies due to their high wavelengths. You would need several feet of absorbent material to work with low frequencies. For instance, at 100 Hz, the wavelength is 11.3 ft!

So one of the viable options is absorption through 'dissipation of sound energy' by using 'Diaphragmatic panels' resonating at low frequencies. In simple words using panels (boards, plywood etc.) that would resonate at a particular frequency based on the mass of the panel and the springiness of the air in the sealed cavity behind. Filling the cavity with absorbent material, that does not press the panel too hard to stop it from resonating, multiplies the effectiveness of such a system. A 1/2 inch plywood with 2 inches cavity behind it filled with glass/synthetic wool that does not touch the ply can do wonders. So resonance is not always bad


Reference for the above comment:

• Master Handbook of Acoustics - By F. Alton Everest & Ken C. Pohlmann (Chapter 12)

But there is a problem with "PanelTraps", one panel can address only a particular frequency or in other ways its a resonating panel tuned to a particular freq so that it will absorb only that frequency or its working range is very limited. Practically we cannot have different panels for each set of low freqs. Instead, use either normal \ chunk broadband basstraps to absorb low freqs in general and use "panel" traps to address any peak in a particular low freq. The panel \ ply thickness determines the resonating frequency at which the panel absorbs.


Adding another option to the design which I gave before. Use one face of the box structure for normal / chunk bass trap and the other faces (including the top and bottom) for functioning as a "Panel bass trap" with a separate sealed division/chamber inside to tune down some of the low freq peaks. The resonance frequency tuning for the top and bottom faces will also get affected by the gravity force along with the panel thickness, so some trial and error required for those two faces to use them as panel traps. Donno whether I am demanding more by having multiple traps in a single trap.

 

[suri]

A 1/2 inch plywood with 2 inches cavity behind it filled with glass/synthetic wool that does not touch the ply can do wonders. So resonance is not always bad

that can do magic!

the only way to try to mitigate the (bad effects) of the energy of bass frequencies below 80 hz is to -
use all methods to cause resonance in remote materials (away from the listener) and dissipate the energy.

bass power is very difficult to produce (accurately) - and also very, very difficult to control -

for maximum liberty (allowed to the designer) the room size should be

40 feet x 35 feet (and that is the minimum size of an ideal listening room - or a theater in a multiplex)

the use of corner bass traps in small rooms is an exercise in futility - they only serve to reduce the high midbass frequencies and make inferior loudpspeakers sound more refined and acceptable.

when the listener needs to tame bass frequencies - a large room and resonators might do the trick.

 

[sidvee]

Guys theories are all good but has anybody really tried bass traps/diffusers/absorbers etc in their respective listening rooms and actually measured the response from their listening position? It would be interesting to know. Beyond that everything else is subjective IMO. I know quite a few of the members have treated their rooms but has there been any measurement done before and after.
Cheers
Sid

 

[vinay]

5.1 is cheaper on assorted counts and is any day fine compared to 7.1

5.1 is all the more applicable who are new to watching movies in a dynamic way while that is a another story the best of brands will often try to sell you 7.1

V.

Good thread,

Now please explain me the difference in the feel of a 5:1 set-up and a 7:1 setup.:sad:

I am a novice into this part of the world. will it be expensive to have a 7:1 set-up compared to the latter.:sad:

 

[vinay]

One of the ways out is not to use floor springs for doors, use only hinges, keep the door thicker then the normal one, have glass wool on the door, use excellent rubber sealings, for windows use acoustic glasses, get it installed only from the people who specialize in this and have a proven track record of installing with leading studios and hospitals. If possibilities exist then use double windows or doors, Santosh has used double door.

V.

My first contribution to this thread - unfortunately in the form of a question.
What are the methods of preventing sound leakage from doors and windows of HT or media rooms? I have seen rubber seals (similar to ones used on car doors) that can be placed at the gaps of door and window sills in the US in places like home depot etc, but am not able find them here. Note these are for HT's that used carpenter fabricated doors and windows as opposed the better method of using pre fab doors or windows (like fennesta).
Cheers,
Sid

 

[vinay]

Very well said windowless room is the best of which i have been saying since the longest time i know, please do not worry about ventilation, look at the benefits of no window, someone like you who has such a great knowledge and talking about window, it is like you know even a elephant slips, so for god's sake please do not humiliate a home theatre by even thinking of having a window.

Our great Santosh used to have a window, and finally he did remove it the in-direct way.

The simple way out for ventilation is to keep the theatre door open once or twice during the day for a brief time.

V.

Any idea where these rubber gaskets could be sourced? Also this a good option for doors but what about windows, I know that a windowless room is best, but for ventilation purpose I prefer to have at-least 1 window.
Cheers
Sid

 

[vinay]

In my case i have used acoustic ceiling tiles which are made by the great brand Armstrong and i have a fully carpeted room, below the carpet i have put some white sheet which looks like a thermocal.

V.

the ceiling should be provide with sound absorbers.
the floor should be carpeted.