Can you use a Studio Bulkhead as a Helmholtz Resonator?

Posted: Tue Jan 22, 2008 12:07 pm

Hello everyone,

I have a few questions concerning a bulkhead in my studio. Any assistance would be greatly appreciated.

Thank you in advance,
Jeffrey
http://www.jlstudios.ca

Here is a picture of the studio bulkhead:

Here are the dimensions of that bulkhead:

I have two klipsch Cornwall III speakers I would like to recess into the face of the bulkhead angled downward. I was then going to stuff the remaining interior of the bulkhead with Roxul AFB, about 75% full. I would make sure everything was air tight.

The room specs out pretty good except for some lower bass issues. My room Room Modes dominate: 36hz to 128hz.

Total volume of bulkhead would be 13.221 FT with the two klipsch Cornwall III speakers soffit mounted.

My questions are:

Would this bulkhead make a good Helmholtz Resonator if I was to port the enclosure?

Would it be best to use 1 port or to divide the bulkhead into three indivdual cavities and port each?

I am having a hard time figuring out the tuning....any ideas as to the port diameter and depth?

Where should the port(s) go?

Or, should I rip the bulkhead out all together and construct bass trap panels to straddle the corners?

Math Follows:

ROOM

Computed Information:
Room Dimensions: Length=15.45 ft, Width=10.3 ft, Height=8.8 ft
Room Ratio: 1 : 1.17 : 1.75
R. Walker BBC 1996:
- 1.1w / h < l / h < ((4.5w / h) - 4): Fail
- l < 3h & w < 3h: Pass
- no integer multiple within 5%: Pass
Nearest Known Ratio:
- "24) A worst case scenario calculated by RPG" 1 : 1.075 : 1.868
RT60 (IEC/AEC N 12-A standard): 219 ms
- ±50ms from 200Hz to 3.5kHz = 169 to 269ms
- ±100ms above 3.5kHz = 119 to 319ms
- <+300ms at 63hz = 519ms
- 300<600ms
RT60 (ITU/EBU Control Room Recommended): 183 ms
- ±50ms from 200Hz to 4kHz = 133 to 233ms
- <+300ms at 63hz = 483ms
- 200<400ms
Absorbtion to achieve ITU RT60: 373 sabins
Volume: 1400 ft^3
Surface Area Total: 768 ft^2
Surface Area Floor: 159 ft^2
Surface Area Ceiling+Floor: 318 ft^2
Surface Area Front Wall: 90 ft^2
Surface Area Front and Rear Wall: 180 ft^2
Surface Area Left Wall: 135 ft^2
Surface Area Left and Right Wall: 270 ft^2
Surface Area 4 Walls: 450 ft^2
Surface Area 4 Walls + floor: 609 ft^2
(sabins - front wall - carpet) / Left+Right+Rear wall: 34 %
(sabins - front wall) / Left+Right+Rear wall: 78 %
Schroeder Fc: 128hz
Frequency Regions:
- No modal boost: 1hz to 36hz
- Room Modes dominate: 36hz to 128hz
- Diffraction and Diffusion dominate: 128hz to 512hz
- Specular reflections and ray accoustics prevail: 512hz to 20000hz
Count (36.5-229hz) : Axials=13, Tangentials=54, Obliques=72
Count (36.5-100hz) : Axials=4, Tangentials=5, Obliques=1
Critical Distance (direct = reverberant field): 12.40f

BULKHEAD

23.203 FT left side
23.203 FT right side
4.25 FT centre
------------------
29.453 FT volume

Cornwall III SPEAKERS- dimensions are 35.75" H x 25.31" W x 15.5" D

SPEAKERS

8.116 FT
8.116 FT
----------------
16.232 FT volume

TOTAL VOLUME of Bulkhead (two klipsch soffit mounted)
13.221 FT

Posted: Tue Jan 22, 2008 3:37 pm

Hi Jeffrey,

I already answered your email, but for the benefit of others who may be interested here it is again:

I don't recommend tuned absorption in rooms that size. What you really need is broadband bass trapping, plus absorption at the first reflection points on the side walls and optionally the ceiling. I also don't recommend angling speakers to point down from above. You'll do much better to set things up so the tweeters are at ear level.

Quote:

Or, should I rip the bulkhead out all together and construct acousticbass trap panels to straddle the corners?

Yes, my best advice is to remove that bulkhead, and then treat the room corners as shown on the Placing RealTraps page of my company's web site site.

--Ethan

Posted: Wed Jan 23, 2008 6:52 am

I have decided to rip the bulkhead out after all. There are two many variables to contend with by trying to tune the enclosure and interface the klipsch by means of soffit mounting. Thanks to Ethan for your feedback.

The treatment for the studio will be as follows:

4 Corners:
Super chunks / Floor to ceiling triangles 12" x 12" x 24"

Front Wall:
Acoustic Panels 24" x 48" x 4"

Ceiling:
Acoustic Cloud 24" x 48" x 4"

Side Walls:
Acoustic Panels 24" x 48" x 2"

Back Wall:
Soft Diffusion / Scatter Blocks 12" x 12" x 2" and 3"

Best,
Jeffrey

Posted: Tue Feb 05, 2008 9:29 pm

Forget that mambo jambo formulas, get a mic a make some measurements in the room. Predicting the behaviour of such tuned absorber is difficult EVEN using computacional methods such as FEM/BEM

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