Over Trapping a Dead Room

**Prof.** · **11-07-07, 07:36 PM**

This is my question...to all the acoustic experts here...

If one already has a room that is very acoustically dead,(in this case a very small room) is not further trapping just going to make it even deader?

It seems to me that a very dead room is going to adversely affect the sound quality, virtually taking the life out of the sound so to speak..

I realise that a lot of trapping is going to help to improve the bass side of things, but isn't there going to be some further constraint on the rest of the frequencies?

**Kal Rubinson** · **11-07-07, 07:39 PM**

Quote:

Prof. wrote:

I realise that a lot of trapping is going to help to improve the bass side of things, but isn't there going to be some further constraint on the rest of the frequencies?

There are bass traps that have minimal effect at the higher frequencies.

Kal

**bpape** · **11-07-07, 07:42 PM**

It depends on how, where, and with what you've treated the room.

You can in theory completely cover every surface of the room in narrow band bass absorbtion and have almost zero effect above say 500Hz. Not that this is desirable but it's possible.

Now, realistically, in a small room, one usually needs to use a more broadband solution simply because there is limited amount of space to deal with. On the positive side, modal frequencies in smaller rooms are higher in frequency.

The trick is to determine what the target decay curve should be based on size, usage, construction, furnishings, etc. and then come up with a treatment plan that will address SBIR, modal issues, early reflections, etc. but do so in a way to stay within the target range.

More to your question - if one simply takes some of the say 1 and 2" material in the room and converts it into 4 or 6" material, you're not changing the mid and high frequency absorbtion but you are adding additional low end control in the lowest few octaves.

Bryan

**Prof.** · **11-07-07, 08:09 PM**

Thanks for the replies...

I was planning on additional floor to ceiling bass traps to the rear corners, but since my room is already dead, I was afraid that it would maker it even deader and start to suck the life out of the overall sound..
It would appear by all accounts, that isn't going to happen.
I will be using 6" total thickness of stacked 703 equivalent..

**Josuah** · **11-07-07, 08:19 PM**

Plus, a totally dead room means that the only thing you hear is what's coming out of the speakers and should be the most accurate reproduction of the audio. Being in a wide open plain should not have bird chirps reflecting even a little. And if there was supposed to be a reflection, it should come out of the opposite speaker with a delay anyway.

On the other hand, you might think you're the only person left on the planet.

**Prof.** · **11-07-07, 08:27 PM**

Quote:

Josuah wrote:

On the other hand, you might think you're the only person left on the planet.

That's what I feel now every time I watch a movie..
Living in an isolated rural area where there is not a sound at night, only adds to the illusion

**Ethan Winer** · **11-08-07, 11:42 AM**

Quote:

Josuah wrote:

Plus, a totally dead room means that the only thing you hear is what's coming out of the speakers and should be the most accurate reproduction of the audio.

I agree with that. The smaller the room, the more dead you need to make it for good reproduction. A dead sounding room is not the evil many would have you believe. I'm not saying totally dead, but mostly dead. Especially at bass frequencies.

BTW, nice to see you here Kal!

--Ethan

**bpape** · **11-08-07, 12:29 PM**

If you're going to do floor to ceiling 6 inchers, straddle the corner and bond an FSK facing onto the side facing the room. That'll minimize the absorbtion above about 350Hz.

Bryan

**Prof.** · **11-08-07, 06:17 PM**

Quote:

Ethan Winer wrote:

The smaller the room, the more dead you need to make it for good reproduction.

Is that because the intensity of the waves hitting the walls is greater than in a larger room?

**Prof.** · **11-08-07, 06:21 PM**

Bryan...If my memory serves me, FSK is some kind of paper facing on the 703 isn't it?
Where would i likely get this paper and how would you bond it to the fibreglass?
I doubt if they've ever heard of it here in Australia..

**bpape** · **11-08-07, 10:59 PM**

Smaller rooms require proprotionately more bottom end control because the modes are by default closer together and there's also less space for them to spread out and work in the air with other things in the room.

The FSK is a combo of a heavy paper, a mesh, and a foil covering all bonded together. If you can't get it there, look for a heavy 'butcher paper' - that will be similar. Bond with a medium grade spray adhesive like Scotch 77

Bryan

**Ethan Winer** · **11-09-07, 01:22 PM**

Quote:

Prof. wrote:

Is that because the intensity of the waves hitting the walls is greater than in a larger room?

In a small room the walls are closer so the reflections are stronger. Also, as Bryan tried to say

the modes in a small room are farther apart, which makes them more damaging.

Sorry Bryan!

--Ethan

**Prof.** · **11-09-07, 04:31 PM**

Quote:

bpape wrote:

Smaller rooms require proprotionately more bottom end control because the modes are by default closer together and there's also less space for them to spread out and work in the air with other things in the room.

Thanks for that Bryan...It makes perfect sense now.

.

Quote:

bpape wrote:

The FSK is a combo of a heavy paper, a mesh, and a foil covering all bonded together. If you can't get it there, look for a heavy 'butcher paper' - that will be similar. Bond with a medium grade spray adhesive like Scotch 77

I have seen a 2" thick fibreglass sheet with what looks like an aluminium facing..
Could that be the FSK material?

**Prof.** · **11-09-07, 04:42 PM**

Thanks Ethan..

**Prof.** · **11-09-07, 05:10 PM**

I have another acoustic question for you guys if you would be so kind...

There is a ton of information available on the placing of acoustic panels at the first reflection point..but it seems to all refer to the standard setup, with your L&R speakers being either floor-standing or raised just above the bottom of the screen, as is the situation in most theatres...
It is generally recommended that the acoustic panel be placed on the side walls at the reflection points, from floor to a few inches above the height of the tweeter..
But what about speakers that are mounted above the screen?
My tweeters are about 18" below the ceiling!..Surely a floor positioned panel isn't go to be of much help down there..
Should my treatments be more in the wall ceiling corner at the first reflection point?

At the moment I have floor to ceiling suede curtains from the front of the screen, extending about 6' along the side walls, but no panels at the reflection point..
What would you advise?..

**SierraMikeBravo** · **11-09-07, 06:16 PM**

HI!

You first need to determine if you even need panels at the reflection points. If you have poor off axis response by determination through an RTA, then you should treat the reflection points. Generally, diffusion is the preferred method as absorption can reduce the soundstage. Also, it is more desirable to leave untreated space above the panel so you don't lose spatial imaging.

However, I guess we should ask these questions. Why is your room acoustically dead? Have you over treated? Too many curtains? You may want to consider removing some of the deadening material if that is the case. The other question is why is your center channel (I am assuming that this is the case) above the screen? I am not sure of your setup, but it is usually undesirable to have such a seperation between your center and your L/R speakers. This will cause panning to no longer be in the same plane.

**bpape** · **11-09-07, 06:25 PM**

Yes - you'd have to do the 3D trig (or just use the mirror) to determine the reflection location.

Curtains will help absorbing high frequencies but will not reach down into the vocal range. I'd recommend some panels behind the curtain to extend things in general.

Yes - the foil on the insulation is likely either FRK or FSK (either is similar enough acoustically to do the job).

Bryan

**Prof.** · **11-09-07, 07:39 PM**

Quote:

SierraMikeBravo wrote:

However, I guess we should ask these questions. Why is your room acoustically dead? Have you over treated? Too many curtains?

That's a good question..I suppose that considering the room is small, then there is a fair amount of square feet of curtaining..
The front wall is covered with 1" acoustic material with floor to ceiling bass traps..
The screenwall is covered with acoustically transparent cloth..
The theatre recliners are covered with micro fibre material and the floor is covered with a fair thickness of underlay and carpet...Oh and the walls and ceiling are plasterboard with insulation behind..
All in all, adding up to a very dead room.

Quote:

SierraMikeBravo wrote:

The other question is why is your center channel (I am assuming that this is the case) above the screen? I am not sure of your setup, but it is usually undesirable to have such a seperation between your center and your L/R speakers. This will cause panning to no longer be in the same plane.

The L&R speakers are above the screen, with centre speakers above and below the screen..
I was not able to place the L&R speakers below the screen because on the left hand side behind the screenwall is the equipment rack..
This is what it looks like..

The speakers sit just above the top of the screen, angled down and within the screen boundaries..

**Josuah** · **11-09-07, 07:53 PM**

Even with good off-axis response, you would want to prevent first-order reflections if you don't want the sound to actually bounce off the other surfaces. Sure, it "expands the soundstage", because the sound is echoing off. And that's going to be really bad if the off-axis response is not good (and tweeters are never good off-axis), but it will still be bad if you're going for accuracy because there's a delay between when the sound first gets to you and when the reflected sound does. And the reflected sound isn't going to be a "mirror" reflection.

Anyway, in case it's not blatantly clear, I prefer no reflections and no expanded soundstage due to reflections.

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