Accoustic Foam.... I think I'm crazy....

Well for the time being I've decided not to change my speakers as I absolutley love them, so I've gone for the room correction route and bought lots of studio quality accoustic foam. This includes 4 bass traps and a lot of sq. meters of the stuff for the trebble. One thing I'm concerned about is the bass traps only go down to 40hz and my speakers are supposed to do 20hz, does this mean I'll just get a big boom lower down??

Now I'm wondering where to put this stuff, e.g. would it make any difference if some was behing my sofas?

Is there a technique for getting the right frequency response? or is this all guesswork (assuming I don't have to spend £1000's on a machine to measure this!).

Lastly, anyone got any tips for making this stuff look good? It's going in my lounge!.

Cheers, Tim

 

what are you planning to buy? Foam generally doesnt go down very low, tuned traps go to 40hz, but foam isnt effective below 100hz generally.

More info please!

 

Hi Tim

Its a lot more complicated than people sometimes make it sound on a hifi forum.

You need to measure the room to find out what treatments are necessary (where in the room and at what frequency) and then use the appropriate room treatment to fix.

There are a multitude of products all designed to do a different job at different frequencies.

Just putting some foam about the place will do little IME to help your room.

To do it properly you need a sound engineer to visit with professional equipment.

A bodge job of room measurement can be done with a sound meter and a test tone disc, but cheap sound level meters dont give all of the information you need - plus of course you cant buy the pre-requisite experience at maplins either!

I'd reccomend searching in the yellow pages for a sound engineer, or getting the full version of CARA software, and trying to make sense of it yourself.

I know someone who has CARA who may be willing to sell it, send me a PM if you want and I can ask him.

Cheers
Chris

 

Several points, having done this before...

1) Foam bass traps have little effect below 120Hz or so. You need resonant traps (eg tubetraps or DIY helmholtz resonators) for a good effect below this.

2) The room is the key point to the whole thing. The problem(s) needs to be identified before buying materials to "correct" it. The dimensions of the room specify the resonant modes, not the speakers.

3) Measurement equipment.... I have a Behringer DEQ2496 and Behringer ECM8000 measurement mic that can do room measurement and equalisation. They're about £260 or so together.

4) In a typical domestic environment there should be enough HF absorbtion anyway (eg pile carpet with underlay on suspended floor, sofa, etc). Finding an underdamped living room is very rare. Your main problems are likely to be in the LF region where foam is less effective. My living room has no HF issues with only a sofa, carpeted floor and one large oil painting, but significant LF issues.

A larger number of foam corner traps can help smooth out a typical room in the mid and upper bass region. Remember that the foam bass traps also have HF absorbtion.

What are the dimensions of the room? That allows you to work out where the problems will be. The approach I would take to begin with is:

1) Calculation
2) Measurement
3) Comparison of measurement to calculation

After that you may be more confused, or you may have a better idea of what needs doing.

Given what you've bought, I would put the bass traps in the corners behind the speakers. They'll almost certainly not make anything any worse. If the room doesn't echo when you clap your hands then I'd be very wary of putting foam tiling up because you could easily make the room too dead.

 

if the foam is flat sheets you can layer it to give you different absorption levels, but as pointed out above, you really need to find out what needs treating.

Membrane traps are another type of trap that get low, also broadband traps can start at 50hz and go up to 500hz

 

Thanks all,

Ok let me be a bit more specific... The right speaker is standing in a corner made by a large fireplace and this is causing major boom. The other speaker is basically free standing.

Right hand side of the room.
- is empty space
= is a wall

---------=fp fp is the fireplace.
---------=fp
---------=== this is the problem corner.
------------=
---------sp-= sp is the speaker.
-----------== slight jutted out wall here
------------=
------------=
------------=
Sofas etc. here


Left hand side of the room.

=---------------
=---------------------
=------Sp---------------
======---------------
======---------------
======---------------
======----Sofas etc.-

The bass traps are supposed to go down to 40hz (do you think they were lying?) so I intend to pile them up in the corner to absorb as much bass as possible. Shouldn't be an issue with them absorbing trebble either as the speakers are bi-polar and a great deal more trebble gets bounced back from the right hand side. The room currently does ring when you clap. It has wooden floors, which don't help. Part of the reason I'll be using the foam is to absorb the sound so that I can play the hifi louder and not bother the neighbours. I guess most of this I can do by ear.

Thanks for the advice about stacking the foam, that's very useful!

I have a friend with an spl meter and I have test tones (I think!). What I'd like is a reduction in bass and the ablity to play louder at the moment.

"You need resonant traps (eg tubetraps or DIY helmholtz resonators) " do you know any companies that sell these??

As a rough guide do you think it'd be worth measuring the room with the assumption that the speakers are a flat frequency response and there's no furniture in it? Then asking someone to check where to put the foam and experimenting??

Thanks, Tim

 

What are the dimensions of the room?

If the room rings with all furniture and wall coverings as you want them then the tiles will be useful. A patch on the sides to cut down reflections on the side walls to listening position is a good start.

 

adding foam to walls wont soundproof the room.

Foam is not efficient below 100hz, to say it is efficient to 40hz is misleading. Have you got any data from the company? Foam type etc?

Get your room measured if you can, you will know whats wrong then, but measure it with the stuff you will have in the room there, like sofa etc.

something as simple as moving the speakers away from the walls could clean the sound up alot.

 

One simple way ......

stand in the centre of your room.....all sound off and listen to the silence for a moment. The clap your hands together once and listen to the sound decay. This will tell you a number of things but not in an empirical way.

Firstly this will tell you how absorbant /or reflective your room surfaces are. A room that looks modern sparsely furnished you will hear the clap [sound impulse] decay in a series of rapid but diminishing claps that sound like a series of pulses[ as thats what they are] all runing together forming a continuos sound, over several fractions of a second. The longer the decay the worst the situation. You will need to try this in a number of different rooms toget a feel of how it sounds.

Secondly you sould be able to hear if its the start of the clap or the end of the clap...yes sounds odd but ......This should be a guide as to the first reflections in the room. There is not enough space to try to describe this here but if you google it you should get heaps of info. First reflections are bad ! these are out of phase and will seriously mess with your imaging and treble performance.

Thirdly the volume of the clap decay, again like the first one has bearing on the reflectiveness of the room.

Claps are also mid to high frequency.....therefore after applying furniture or sound deadening you can do an instant appraisal.

My room is such that virtually no decay can be heard...thats not to say there is no echo. So I can hear just the first of the echos, This in my opinion and 30years experience is about the right ammount of decay, toget good music reproduction. But be warned a dead room can sound dreadful in the same way a live one can.

I'm certain there are many more opinions and a vast ammount of technology you can use to acvhieve the same thing.

 

Just to say I'm busy reading. Lots of good suggestions, thanks all. I'll get dimensions of the room tonight. The speakers cannot be moved hence the need for correction. I have a large projector screen in the middle and everything just about fits (not ideal!).

The Mirages like to be at least 1 metre from any rear wall and as far away from sides as possible, sadly this isn't possible. If all else fails I'll have to sell these, but they're my favourite speaker so I'll be gutted. I have had these sounding superb in my old lounge.

For me, getting rid of the booming bass is of highest importance, any other improvements are a bonus. I must point out I'm on a budget since my CRT projector broke and I've just bought a house but I will shell out cash if I must!.

Cheers, Tim

 

no need to shell out cash if you've already got some stuff, its getting the best out of it thats important...

 

Hi All,

Got the foam bass traps and foam tiles last night. The tiles are next to useless and only take a little glare out of the trebble, I actually put these in front of the tweeters and they only cut out a little of the sound.

However the bass traps really worked well, I used 3 behind the right hand side speaker and it tightenned the bass a great deal. I think I still have an issue with very low bass as there is still a boom at this frequency. The speakers now have far more punch and the imaging is sounding better. I have a suspicion that they're just standard foam though!.

So round one is a success, I will have a word with the company about their tiles.

I think I'll still be looking for a bass trap that gets the really low frequencies. This is the problem with running such big speakers.

Cheers, Tim

 

Try putting the tiles behind your listening position on the back wall.

You wont find a bass trap that gets down to very low frequencies very cheaply I'm afraid. Once it gets that low you have to implement 'mechanical' methods of absorption rather than just relying on the absorptive material itself.

Look at Auralex MEGA LENRD, ASC's 20" diameter Tube Trap and I think RPG have a membrane absorber.

 

I've been following this thread with interest, along with the various threads recently, here and over at PFM concerning digital eq and room correction.

I've heard it here and elsewhere that the ideal is to treat the room first, but also that a completely dead room (ie one where a flat response speaker would have truly flat in room response) also sounds awful. It begs the question as to where in the frequency spectrum you ideally want a bit of liveliness ... and how much.

I'm assuming that ideally dealing with major room modes at the bottom end really does need sorting out, but higher up the frequency spectrum I'm a little more confused. How lively is too lively and how high up the spectrum is it an issue?

It also occured to me,that reflections in the mid to high frequencies are the major culprits for harsh sound and a confused soundstage (all that out of phase stuff is gonna play havoc).

It might be crazy idea but at least one person whose opinion I respect says it probably isn't, but if that's the case then would developing a speaker with blinkers (kind of like horse blinkers) on the wall side, and maybe the ceiling and floor sides, make more sense than treating whole walls? catch the reflections before they hit the walls/ceiling/floor so to speak.

 

And people wonder why some of us don't bother with room acoustics.

What a nightmare

DT

 

I don't think its rocket science - and per pound spent can be far and away the biggest upgrade you can make.

 

I did our listening room at work with tiles and traps from Goldring (not the same goldring that does tt carts). After research I quickly realised that it doesn't matter who makes a foam corner trap... they all behave the same because the important thing is the depth and density of the foam, and they're all about the same. They'll make a big song and dance about the shape of the front panel but it makes very little difference at all (and only at HF anyway).

As for positioning the tiles.... In general it is best to have the wall behind the speakers as dead as possible (so that there are no back reflections to muddle the soundstage), and patches on the sidewall to prevent reflections there that lead to a comb filter effect. Some people like to have the wall behind the listening position live, some prefer it dead. Experiment...

 

the only reason our foam is shaped at the front is so that it looks the same as the wall panels, nothing technical....though the back of the corners are cut...

 

A refreshingly honest response! Thank you!

 

The type of foam makes a difference though because some absorb sound better than others. Melamine is probably the best but its also expensive.

I dont really see why its so important to have the wall behind the speakers dead, only bass trapped, because high frequencies are not going to get to the wall without bouncing round the room first. Bass on the other hand is omni-directional so its very important to have the wall behind the speakers absorb bass.

If the wall behind the listening position is close to you then I'd recommend dead, if its 2m or more away from you then its nice to have diffusion. Diffusion or absorption at the first reflection points, its your choice, personally I like abfusors (absorbs and then diffuses what is left) for first reflections.

What you are suggesting Uncle Ants, is highly directional speakers. These have some good points if you only sit in one position and its only you listening, otherwise a wide dispersion might be better to accommodate a wider sweet-spot. Directional speakers may be okay for more than one listening position if the off-axis response is still linear as you can position them so that at an off centre position you are more in line with the further away speaker and off-axis from the closer one which kind of counteracts the fact you are in a different position. However, a lot of people prefer the sound of a system which has reflections, but which are controlled; it can create a nicer atmosphere.