Helmholtz resonator Question ?

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Doing the calculations for a V-shape resonator using the Helmholtz calculator at the SAE site with the dimensions below gives the absorption frequencies, 201 Hz and 377 Hz. Since the depth to the wall will change due to the V-shape, does this mean it only absorves those 2 frequencies or for the entire range 201 to 377??

Maximun Depth 400
Mininum Depth 50
Slot Width 5
Slat Width 100
Slat Depth 25


Regards,

Luis.
 
The variable depth spreads out the response, so you'll have more of a broadband absorber, that will actually reach above and below the two calculated freq's by about 1/2 octave or so... Steve
 
So, about this helmholtz stuff- (I'm trying to understand here too)

I know the Helmholtz calculators use depth of the box in it's measurements and probably assumes it is for a square box, but isn't how an HR works based rather on the VOLUME of the container in relation to the size of the slots and depth /thickness of the opening (s)?

I'm picturing a 16 oz pepsi bottle and a 16 oz peanut butter jar. Even though one is taller than the other, if they had the same throat opening on top, I'm thinking they would resonate at the same frequency?

From what I've read, and this is the point I'd like to bring up for further discussion, it appears to me that a helmholtz resonator can only inherently resonate at only ONE fairly narrow frequency range, and the widening of it's frequency response is based on/ caused by; adding insulation to the cavity and/or increasing the slot sizes.

Comments or corrections?

DAN
 
Interesting question Dan - BUT - in the calulation formula there is no reference to volume, only depth.

The formula for calculating the helmholtz resonant frequency is:


f = 2160 x sqrt ( r / (( d x D ) + ( r + w )))
Where:


f = resonant frequency in Hertz (Hz)
r = slot width.
w = slat width.
d = effective depth of slot. (1.2 x the actual thickness of the slat)
D = depth of box.

If you angle the face of a resonator you vary the depth which varies the frequency of resonance.

If you look at the figure here using another resonator calculator I have it gives the figures like this: (sorry for the metric)

Slat Width 200 mm
Slot Width 5 mm
Perforation % 2.44 %

Cavity Depth 100 mm
Thickness of slats 10 mm
Center Frequency 271 Hz
Hiband (-3 dB) 542 Hz
LOband (-3 dB) 135 Hz

Now look at the hi/low bands where the curve is down -3db. As you can see the Q of the absorption is quite wide.

cheers
john
 
John Sayers said:
If you look at the figure here using another resonator calculator I have it gives the figures like this: (sorry for the metric)

Slat Width 200 mm
Slot Width 5 mm
Perforation % 2.44 %

Cavity Depth 100 mm
Thickness of slats 10 mm
Center Frequency 271 Hz
Hiband (-3 dB) 542 Hz
LOband (-3 dB) 135 Hz

Now look at the hi/low bands where the curve is down -3db. As you can see the Q of the absorption is quite wide.
Click to expand...

Thanks for the illustration John. I'm fairly new to this helmholtz thing. I have yet to build one.

For the sake of discourse, my first net search came up with loads of stuff about Mr Helmholtz, and it often refers to volume. His early resonators appear to have been used for identifying frequencies, as they exhibited a tight Q and could be made to identify specific frequencies. The metal ones at this web page could be tuned to other frequencies by changing the volume of the resonator. Pehaps like a trombone slide?

http://www2.kenyon.edu/depts/physics/EarlyApparatus/Acoustics/Helmholtz/Helmholtz.html.

This curious ratio of volume ( or depth if you will) to perforation is what leads me to still purport that an (empty) resonator can only operate at a narrow frequency regardless of variances in the box depth. The box volume remains consistent. Admittedly, I'm just hypothesizing aloud here...

John, where do you hide that other helmholtz calculator that figures in the low and high bands? :-)

Interesting stuff, I gotta build a few of these soon!

DAN
 
Sorry Dan - that link was a dead one. ;)

I've attached the .xls file.
no I haven't it won't accept an .xls file.

TRY THIS

cheers
john
 
John Sayers said:

If you look at the figure here using another resonator calculator I have it gives the figures like this: (sorry for the metric)

Slat Width 200 mm
Slot Width 5 mm
Perforation % 2.44 %

Cavity Depth 100 mm
Thickness of slats 10 mm
Center Frequency 271 Hz
Hiband (-3 dB) 542 Hz
LOband (-3 dB) 135 Hz

Now look at the hi/low bands where the curve is down -3db. As you can see the Q of the absorption is quite wide.

cheers
john
Click to expand...

John,

What is Perforation %. how do you figure out the %?

Regards,

Luis.
 
Perforation % means, what percentage of the total surface is perforated - so, if the total surface is 100 square feet (example for simple math) and the total surface that is slots (or holes) is 1 square foot, then the % perforation is 1% - this gets more difficult to calculate if using perf board for a cover, since you have to calc the area of each hole, then multiply the # of holes, then divide that total area into the total wall area.

John, is that the .XLS file from the Studiotips.com site? same name... Reason I'm asking, I tried using that one and yours on the same absorber (@ RO.org) and came up with WILDLY different values - not sure yet if I goofed on data entry, or what. Have you tried the same #'s in both sheets to see? Hoping I didn't steer "Nightmusic" into a tree without an airbag... Steve
 
knightfly said:
The variable depth spreads out the response, so you'll have more of a broadband absorber, that will actually reach above and below the two calculated freq's by about 1/2 octave or so... Steve
Click to expand...

Hi Knightfly,

Can you point me to where I can find some info on this 1/2 octave above and below the calculated freq.

Thanks,

Luis.
 
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