30 hz and 100 Hz standing wave - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 02-18-09, 11:12 AM Thread Starter
Elite Shackster
HTS Moderator Emeritus

Anthony

Join Date: Oct 2006
Location: Virginia
Posts: 2,213
Re: 30 hz and 100 Hz standing wave

No, I was thinking along the lines of "cut ports in the front or along the back edge". I could use subwoofer ports to get what I needed. It's a huge volume, though, so it might look comical to have all those holes.

Since the front of the riser is out into the room (i.e. in the null region) would that be the best place for the ports? Or would it be better to have the ports at the high pressure region, i.e. along the back wall?

Hmm. First idea should probably be testing new subwoofer locations. The ideal velocity source location (dipole sub) would be smack in the middle of the room. The wife may not like it, but at worst it's a coffee table If that doesn't work, only then will I consider cutting holes in the riser.

If you don't mind, please check my math here, though, in case that's the path forward for me:

2*pi*f_rez = v*Sqrt(A/(V*L)) Where:
f_rez is tuning frequency
v is the speed of sound
A is the overall port area ( can that also be N*a for N smaller ports of area a?)
V is the volume of the cavity (how does stuffing affect? 30% more perceived volume?)
L is the port length

If all that's right, and I look for a tuning of 35 hz or so, I would need 4, 4" holes or 15 2" holes of just the thickness of the joist beam (1.5"). With increased perceived volume, I would need a 5th hole, if the 30% number is correct.

thanks again for the help.
Anthony is offline

Old 02-18-09, 11:30 AM
HTS Senior Moderator

Bryan Pape

Join Date: Sep 2006
Location: Wildwood, MO (St. Loui
Posts: 5,209
Re: 30 hz and 100 Hz standing wave

Here is the fomula to calculate slatted Helmholz resonators

fo = 2160*sqrt(r/((d*1.2*D)*(r+w)))

r = slot width
d = slat thickness
1.2 = mouth correction
D = cavity depth,
w = slat width
2160 = c/(2*PI) but rounded
c = speed of sound in inch/sec.

If you want to play what if games with different types, check here for a download

http://www.whealy.com/acoustics/Porous.html

I am serious... and don't call me Shirley.

Bryan Pape
GIK Acoustics
bpape is offline

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