Helmholtz resonator dimensions

[longsoughtfor]

Hi - I tagged this question on the end of another thread but got no response... I figure its buried.

In all the references I've seen on helmholtz resonators (slat type resonators) I see equations for slat with and depth and solt width only. Is there a rule of thumb for how high and wide these should be? floor to celing? chair hight to celing?

Can I make a floor to celing box with a graduated slot width all the way up to try and trap multiple frequencies?

Thanks

 

[Tom Hicks]

Hi
In college I worked for a custom speaker mfg.The Helmholtz equation produces speaker boxes that are long and narrow.I wired and installed the crossovers and the raw speakers. The idea is to produce a frequency responce similar to the log of the human ear.That is to say weak on bass and treble at low levels (loudness control on your stereo).
Your question about slat types related to high frequency dispersion.On axis or off axis,that is the key for high frequency waves.Most of these slat-type devices are designed to spread the high freqs to the room bettter.More important is speaker height.Put the high freqs at ear level.

Tom

 

[longsoughtfor]

Tom - Thanks for your response! Maybe I didn't ask the right question. I am thinking of the slat/slot type resonators also called Bass Traps.

The equation for calculating the resonant frequency of the absorber is stated as:

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

Where:


f = resonant frequency of the absorber in Hertz (Hz)

r = slot width in inches

w = slat width in inches

d = effective depth of slot in inches (1.2 x the actual thickness of the slat)

D = airspace depth (depth of box behind the slots) in inches

I can calculate that fine. What I never see included in discussions of these beasts is outside box dimensions. In particular, minimum sizes.

If I do the math in a spreadsheet and vary three of the dimensions, slot width, slat width and box depth, I can design a box, lets say 4 feet wide and 8 feet tall which has a triangular foot print tapering from 8 in to 30 in. This box would have different slot/slat dimensions top to bottom getting bigger closer to the bottom (though I think it is irrelavent).

This would create an absorber with a theoretical frequency range of roughly 140Hz to 1100Hz.

So, my question is: Would this work in the real world? Has anyone done anything similar?

Thanks.

 

[Tom Hicks]

I'm sorry I initially misread your post!
Your math sounds right.But wouldn't it be a big sucker!Any why such a broad range of frequencies to trap?Looks like about an octave.Mostly rooms have just a few standing waves to deal with.I'm used to seeing guys with the barrel-type bass traps,each tuned to a specific freq.It looks like you want to analog EQ the room!

Tom

 

[longsoughtfor]

Well, I was thinking of using it as a method of getting rid of a 90 deg corner... and the varied spacing seems both practical and pleasing to look at. But your right - the range I came up with is pretty big, I can certainly scale it down.

Thanks.