R/E/P Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: [1] 2  All   Go Down

Author Topic: bass trap optimum depth  (Read 5942 times)

OOF!

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 366
bass trap optimum depth
« on: March 19, 2007, 03:58:54 am »

Hi,
i have a 16.25 X 12 X 8 foot control room that has a severe dip at 80 Hz.  i already have a ton of Ethan's RealTraps traps in the corners, ceiling and walls but obviously need a serious trap on the rear wall, as he has suggested to me.
the speakers are 30 " off the front wall and my mix position is another 3 feet in front of the speakers.  I've ordered a few boxes of OC 705 and will build a 6'wide X 4'high trap that will be 10" deep with 6" of 705 and a 4" air gap to the wall.  is this the most optimum design?  is 4" of 705 with a 4" gap enough?  i don't mind going with 6" if it will be more effective in the 80 Hz range.
thanks for the help,
David
Logged

Tom C

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 377
Re: bass trap optimum depth
« Reply #1 on: March 19, 2007, 09:15:16 am »

OOF! wrote on Mon, 19 March 2007 08:58

Hi,
i have a 16.25 X 12 X 8 foot control room that has a severe dip at 80 Hz.  i already have a ton of Ethan's RealTraps traps in the corners, ceiling and walls but obviously need a serious trap on the rear wall, as he has suggested to me.
the speakers are 30 " off the front wall and my mix position is another 3 feet in front of the speakers.  I've ordered a few boxes of OC 705 and will build a 6'wide X 4'high trap that will be 10" deep with 6" of 705 and a 4" air gap to the wall.  is this the most optimum design?  is 4" of 705 with a 4" gap enough?  i don't mind going with 6" if it will be more effective in the 80 Hz range.
thanks for the help,
David


In that frequency range more is always better, so I'd go 6" or even
8".
Tom

Logged
Tom

.signature failure

stuntbutt

  • Newbie
  • *
  • Offline Offline
  • Posts: 49
Re: bass trap optimum depth
« Reply #2 on: March 20, 2007, 02:06:53 am »

Well I probably know just enough to be dangerous, but I think that the optimum depth of absorption for 80Hz would be 3.5 feet.
Logged

OOF!

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 366
Re: bass trap optimum depth
« Reply #3 on: March 20, 2007, 11:49:29 am »

stuntbutt, you are indeed dangerous!
once I have the 6' x 4' x 10" box built, i'm going to load it with increasing thicknesses of 703 and test each time to find the best thickness.  maybe it will be a solid 10" of material.  i'll report back.
David
Logged

Ethan Winer

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 571
Re: bass trap optimum depth
« Reply #4 on: March 20, 2007, 04:58:25 pm »

David,

> maybe it will be a solid 10" of material. <

The thicker the better. Always.

--Ethan

stuntbutt

  • Newbie
  • *
  • Offline Offline
  • Posts: 49
Re: bass trap optimum depth
« Reply #5 on: March 21, 2007, 03:11:11 am »

I think some of Ethan's DIY panel absorbers would be more effective.  They can be built to target specific problem frquencies.  Anything LESS than 3.5 feet would be more absorptive at other frequencies than 80Hz, making your room more unbalanced.  Stop me before I advise again!
Logged

Tom C

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 377
Re: bass trap optimum depth
« Reply #6 on: March 21, 2007, 05:36:54 am »

stuntbutt wrote on Wed, 21 March 2007 08:11

Anything LESS than 3.5 feet would be more absorptive at other frequencies than 80Hz, making your room more unbalanced.  Stop me before I advise again!


That's not exactly correct: once you reach a trap thickness where
100% sound is absorbed this doesn't change anymore, so if e.g. with
1 ft everything above 100 Hz is fully absorbed, with 2 ft everything
above 50 Hz is absorbed and so on, it doesn't change the higher frequencies
anymore.
Doesn't make things above unbalanced.

Tom

PS: the frequencies above are not accurate and just an example.

PPS: of course with tuned absorbers (instead of broadband) that's
    completely different.
Logged
Tom

.signature failure

franman

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 580
Re: bass trap optimum depth
« Reply #7 on: March 21, 2007, 10:02:49 pm »

David,

If you're suffering a tight notch at 80Hz in your mix position, you might want to explore some tuned traps. You've already got a fair amount of broadband bass trapping... we've had good luck with both panel traps and Helmholtz cavity traps in this frequency range. It'll take a little homework, so I would look around your room to find areas where 80Hz is peaking.. These are locations that you could consider some tuned traps... Get out the Master Handbook of Acoustics (or get online) and do some searching for the formulas for these types of bass absorbers... Pretty easy to find and the Math's not hard.. (even I can do it!!)...

Let us know how your 'deep' porous trap works out, but I don't think 10" is deep enough to be extremely efficient down to 80Hz...Quarter Wavelength would be around 3.5ft (as Stunbutt pointed out).. At some working area in your trap it would need to approach that depth to be efficient down to 80Hz... That's just the way it is!

CHeers
Logged
Francis Manzella - President, FM Design Ltd.
                 - Managing Director, Griffin Audio
fmdesign.com
griffinaudiousa.com

OOF!

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 366
Re: bass trap optimum depth
« Reply #8 on: March 22, 2007, 09:42:04 am »

thanks for the reply,
so you're saying that they'll be a spot (or spots) where there's a big rise at 80- and that's where i'd put the tuned traps?  i haven't put the measuring mic in the back of the room yet.  my rear corner traps are 4" thick straddling the corners and only cover half the height.  perhaps filling them in and making them go floor-to-ceiling would help.  i'll do some more testing and report back what helps the most.  the insulation arrives tomorrow so i'll put it in the box i've built on the back wall and see (pray) it makes a difference after all the construction i've already done!

by tuned trap you're speaking of a sealed box with a plywood membrane on the front, correct?

thanks again, everyone, for the advice.  much appreciated.
David
Logged

OOF!

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 366
Re: bass trap optimum depth
« Reply #9 on: March 22, 2007, 10:41:39 am »

one more question-  i've found the info for calculating the depth etc.  but no one mentions the dimensions of the box for either a Helmholtz or membrane resonator.  what about length and width of the box?
thanks again,
David
Logged

OOF!

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 366
Re: bass trap optimum depth
« Reply #10 on: March 22, 2007, 04:21:05 pm »

well, sadly, putting all 18 pieces of 2" 703 into the box on the rear wall (still in their 3 cardboard boxes) gives me very little improvement at the 80Hz area.  music playing out of the monitors does appear to have a much better low-end, but when i measure with a mic, i still have a huge dip at 80Hz.  I also have about an 8-10db difference in level if the mic is held up at ear level (softer) or knee level (louder) when I'm back by the rear wall.  
Maybe as you suggest, a perforated sealed trap would be more effective on the rear wall.  I'd like to make the box fit into the one i've already built, so it would be 2' x 4' and could have a depth as great as 10".  according to the charts, if I use 1/4" ply as the front with 150 5mm holes and a 9" gap, that should be effective at 75Hz.  I will fill it with the 703.  
my question is:  how many of these 2 x 4 boxes would i need if my rear wall is 12' across and 8 ' high.
Thanks again for the advice,- i'll keep experimenting.
David
Logged

Tom C

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 377
Re: bass trap optimum depth
« Reply #11 on: March 22, 2007, 04:36:12 pm »

OOF! wrote on Thu, 22 March 2007 21:21

I also have about an 8-10db difference in level if the mic is held up at ear level (softer) or knee level (louder) when I'm back by the rear wall.  



If that's not your listening position I wouldn't care too much
about it, you can't remove dips and peaks everywhere in your room.

But IF it's your listening position and the level changes when you
go up or down you need some more trapping of your ceiling.

Can of worms, I know.

Tom

Logged
Tom

.signature failure

OOF!

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 366
Re: bass trap optimum depth
« Reply #12 on: March 22, 2007, 11:07:37 pm »

can of worms is right!  no the listening position is quite stable L/R and up/down.  just that one spot at 80Hz.  I'll try a bunch of different stuff over the next week or so.  definitely picking up the Everest Book.  Mountains to climb....
david
Logged

matucha

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 304
Re: bass trap optimum depth
« Reply #13 on: March 24, 2007, 06:02:27 am »

Move the damn monitors and listening position around util you find your spot. This is the most important! Every ~4" makes difference.
Logged

jfrigo

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1029
Re: bass trap optimum depth
« Reply #14 on: March 24, 2007, 06:36:27 pm »

OOF! wrote on Thu, 22 March 2007 10:41

one more question-  i've found the info for calculating the depth etc.  but no one mentions the dimensions of the box for either a Helmholtz or membrane resonator.  what about length and width of the box?
thanks again,
David


There are several kinds of traps based on Helmholtz resonance. These can include slat absorbers, perforated panel absorbers (a personal favorite of mine), masonry slot absorbers etc. At its most basic, Helmholtz resonance doesn't care about the dimensions of the box as much as it cares about the volume of the cavity. Of course the other variables are the surface area of the opening, and length (or depth, like the length of the neck of a bottle) of opening, adjusted for effective length rather than physical length (having to do with the air column's interaction with air on either side of the opening since the air column is a gas and not a solid).

To figure out the dimensions, you must do the calculations, and then take other things into consideration. You could make the cavity shallow, but you would need more area to attain the proper volume, thus limiting how many resonators you could fit in a space, ultimately affecting how much bass you could trap. You can't just put up a single Helmholtz absorber. You need to cover some area with them to make a significant difference. If you have plenty of depth, you can fit more per sq. ft. of surface area, and have more absorption.

In the case of slat absorbers and perforated panel absorbers, there are formulaic examples for reasonable depths, such as 8", where you can figure spacing of openings, size (area) of openings, and depth of openings for the box depth (hence volume). The ultimate dimension depends on how large an area you wish to cover with this (for example) 8" deep box with the suface openings as calculated for your target frequency.

Other things to keep in mind with Helmholtz are that you want your box to be air tight (apart from the required openings in front of course) and rigid. If it flexes easily, or air can escape from the sides or back, the spring of the cavity volume will change and the as-built response will not match your calculations.
Logged
Pages: [1] 2  All   Go Up