low boomy sound

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low boomy sound

Postby steve w » Sun Mar 21, 2004 8:35 am

Hi, I run a very small studio in Tauranga New Zealand. I have had a lot of success and enjoy what I do a lot but one thing has been bugging me for a while now.
In my sound room (booth) I have a very slight low boomy sound. With vocal tracking its not a problem because 99.9% of the time reverb is applied to the track and there is no trace of the sound but with voice only, dry I get this boxy type of sound.
I can remove it with the eq but its still not good enough. I am getting more and more work and it needs to go.I can record in the control room which is a lot bigger and has great acoustics but it defeats the purpose of having a sound proof room.
The rooms dimensions are, 2.4 metres x 3 metres. I have drywall on the window side and various sound diffusers around the room, curtains and carpet on the floor.
It is a square room which is not good I know but I thought with diffusers I could elminate most of the echo. Any help would be appreciated.
thanks
Steve
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Postby Eric.Desart » Sun Mar 21, 2004 9:20 am

Hello Steve,

Welcome to the group.
It's a good idea to read Scott's FAQ.

The problem with diffusers is:
1) They must be very large to influence those low frequencies.
2) Diffusers by definition only influence traveling waves.

You can divide small room acoustics in 2 main different principles (that's a very rough approach).
    a) Standing waves = modal frequencies: this are the frequencies which fractional wavelengths match the room measures causing resonances better known as room modes.
    They are defined by the room geometry and cause static patterns of nodes (theoretical 0 dB spots) and antinodes (maximum pressure spots).

    b) Traveling waves = non-modal frequencies (which doesn't match room measures) which propagation is defined by geometrical acoustics.
    Those ones propagate in a spherical manner (but of course limited by directivity of speakers in function of frequency) and show a directivity in function of the source and reflection angles on the boundaries.
    This are the ones you can influence with diffusers.
    However, there is a relation between the wavelength of those frequencies and the size of those diffusers.
    So to scatter/diffract to those low frequencies isn't often very easy.
    Also here one can (but doesn't have to) have trouble caused by boundary interference, mainly related to the early reflections (pathlength differences superposed waves between direct and all reflection paths causing phase shift which can vary between cancelation, via nothing, to amplification on specific spots).
    Changing position of speakers or will influence this phenomenon (unlike the modal frequencies, where the speaker position only influences the strength of the coupling with the modes)

Look also at the docs in the links section:
Room acoustics & Interaction with speakers

The easiest way to solve/improve this is broadband absorption in the corners.
And if necessary (to check afterwards) one can add additional absorption or diffusion on the mirror source spots.
Carpet and curtains (if no good for low frequencies) will enhance those problems.
Reason: they mainly absorb mid and highs and leave the lows untouched.
One should aim to broadband absorbers which cover the whole frequency range. Relative over absorption in mids and highs will mostly cause a poor listening environment.

The corner absorption as described in Scott's FAQ is a really good thing to start with.

Best regards
Eric
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Postby steve w » Sun Mar 21, 2004 8:33 pm

thanks for the reply eric. I will look into this more as you have said
Steve
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Postby Dan Nelson » Sun Mar 21, 2004 11:35 pm

The room is 2.4m by 2.4m by 3m ? This is the recording room correct? If this is the case you may need to build something tuned to the major modes of the room. Maybe build out 1 wall to the narrow the 2.4 some. Narrowing to 2235mm looks to spread some modes you would need to play with numbers some to see what would work best. I hate to make a small room smaller but it might be a good option. I'm sure you will have questions after you read the FAQ. There are some calculation tools on http://www.studiotips.com under calculation tools

Dan
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Postby Eric.Desart » Mon Mar 22, 2004 7:20 am

Hello Steve,

I'm sorry,
I forgot to comment on the square shape.

Dan's comment is VERY important.
It relates to those room modes. A room with equal measures will show also coinciding similar modes related to those equal measures. This enhances this modal problem.

Thanks Dan

Regards
Eric
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Postby steve w » Mon Mar 22, 2004 7:55 pm

Thanks for the replies guys.
Yes Dan at this stage this is my recording room. Its only used for voice overs,acoustic guitars etc and I use the control room a lot to record electronic instruments.
I dont want to sound stupid but sometimes I find it hard to get my head around all the room dimentions, calculations stuff. I work a lot with my ears but I know now I need to do some maths to get this right. A lot of the acoustic terminology hurts my brain also and again I see I'll have to sit down and concentrate on these calculations.
Does having carpet on the recording room floor make a difference, I have heard through some sources it does, is this correct.
I'll keep at it
Thanks
Steve
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Postby Dan Nelson » Tue Mar 23, 2004 4:20 am

There are Excel spreadsheets from both Jeff and Scott at the link I posted. Spend a few hours playing with the numbers and I think you will start to see how it works.
In my first studio that I tried to really work out the acoustics, I started the calculations on paper (very slow to say the least) then I friend brought over 2 programable calculators (much faster after 4 hours of entering numbers in as fast as we could, we have one tenth the calculations done) So then we entered the formula in, I think the program was called TK solver on a state of art IBM XT and it did it in seconds.

Carpet will make a difference weather it is a good or bad difference depends the size shape, construction, and other materials in the room. Carpet absorbs mostly high frequencies so there is a chance it makes the low frequency problems more noticable. On the other hand it might be taking out a ceiling to floor flutter echo. There is no magic bullet in room acoustics, it how you put it all together that makes it sucessful or not. Small rooms are very tough but not impossible to get them to sound acceptable.

What is the construction on the room?

Dan
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Postby steve w » Tue Mar 23, 2004 7:53 pm

Hi Dan,
The room before it was coverted into a recording area had block walls on two sides and framing and insulated drywall was constructed on the other two walls.The ceiling has insulation then drywall coved in a parachute type of material creating a padded effect.
The room is 3metres wide by 2.4 metres long. The wall you face towards the control room has a 1.2metre x 600mm double glazed panel roughly in the centre.This wall is a combination of painted drywall , carpet and small diffuser panels.
the other 3 sides have curtaining around 80% of the walls, a studio door and bits of small paneling in various places.The floor is carpeted. The low sound is very slight but enough to get that cardboard box sound on recorded speech.
None of my clients have ever noticed but its definitely there.
I have no true acoustic absorbtion panels in this area, you know, specifically for studios,
this may be also part of my problem.Would this absorb the very low frequencies?
By the way there is little or no block exposed in the room.
Thanks
Steve
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