Helmholtz Resonator Question

What material? What frequency?

Depends on the permeability of the 'absorbant material' (could it be a sponge?) As well as the amount and shape etc...

The carbon loaded sponge that you get intergrated circuits packed in will attenuate well into the microwave frequecies. Upto 10Ghz the amount of output power is the limit because if you are thinking of several Kilowatts out then fire is a real risk.

The greater the carbon content then the greater the power loss.

Carbon dust mixed into epoxy resin, or silicon rubber could be tried.

The carbon you get as graphite lubricant for locks will do.

Helmholtz resonators are acoustic devices. They're often used in in sound studios to deaden recording booths, where a group of resonators are built into the walls with only the entrance ports visible. Each resonator in the group (in this application they are sometimes colloquially called "jugs") is tuned to a particular center frequency so that collectively the group covers a desired range of frequencies.

I've also seen HRs used as the basis for subwoofer enclosures to augment the bass response of the driver.

-e

Shown here are a collection of number of HR "jugs".

Here are some more Helmholz resonators having a different configuration...

This link provides some design information for HR-based absorbers. Click also on the link at the bottom of this link's page:

http://www.audioholics.com/techtips/roomacoustics/Helmholtzresonatorabsorber.php

-e

Why resonaters if they are deaden sound? Sorry I was thinking of micrwave application, just a case of mistaken identity.

Resonantors because they resonate - think about blowing across the nexk of a beer bottle - if you do that you've made an HR.

Attenuators because if you fit one in a moving flow - like a vehicle's intake or exhaust, the tone they produce is 180 deg out of phase with the noise in the main channel, so cancellation effects mean the remove that tone from the main flow.

The tuning is down to neck length & x-section vs body length & x-section.

To the original poster:

I'm not sure why you'd want to add absorbing material inside an HR. I would have thought that this would reduce the attenuation...although you may find that you spread the attenuation, so you get (say) 2 - 3 dB attenuation over 100-200 Hz range rather than 20-30 dB over a 10 Hz range (all figures arbitary, but you get the gist).

Far better to use the HR for what it's good at - taking out (mostly low freq) tones, and aborbing material somewhere else to take out ranges of high freq.

Ohhh.... I don't think I've heard of them called Helmholtz resonators before....

So, does that mean if you blow across the top of a bottle (Helmholtz resonator) whistling or humming at the resonant frequency, you wont hear the tone from the bottle?

John.

Hi English Rose,

Adding acoustic wadding to a resonator does make sense in certain applications. In a sense, it is analagous to adding resistance to a high-Q LC tank circuit. The dampening that results increases the bandwidth of the resonator at the expense of the Q (which follows, as BW = f_c/Q). This may be desirable, for example, to smooth the net frequency response of a group of resonators; or where the "problem frequency" in question is not sharply defined. If you had a case where a resonator tended to "ring" (not likely, given other losses), the addition of a bit acoustic wadding would cure that problem in short order.

-e

PS: Is not Japan east of England, Rose-san?

ok, i think i should had probably explained more when i wrote the question. Now that its clear whats a helholtz resonator (a low frecuency acoustic absorbent device) and what its the finality of inserting a resistance in the cavity of it (broadening the peak of absortion), i should ask a more consice and fundamented question.

When you calculate the dimentions of a resonator in order to determine an especific frenquency of resonance (the freq of absorption) you dont consider the use of an absorbent material in the cavity (fiberglass for ex.). When you introduce this resistence the freq of resonance falls, some say its because the sound speed in the absorbent material is lower making a "virtually larger" cavity hence changing the fo.

So i thought: if i know the constants of the material i can calculate the sound speed in it,

AND HERE ITS THE PROBLEM , absorbent materials in general are porous, conformed by an estructure of fibres SO THE QUESTION IS, its possible to calculate the speed of sound in an absorbent material?

So we're not talking about HRs now? Is the question just about the speed of sound through a porous material, irrespective of its surroundings?

-e

Look at Biot's model of acoustic propagation in porous media.

-e