LOW - MID ABSORBERS

If you look at the absorption coefficients of various materials you will notice that some of the fibreglass products absorb low-mid frequencies very efficiently as does a panel absorber with a fibreboard panel instead of a plywood panel. But the best low mid absorber (and the best looking) is the helmholtz resonator - often called a slat resonator.

THE HELMHOLTZ RESONATOR

The helmholtz resonator (named after a Mr Helmholtz who discovered it) can best be demonstrated by taking a normal soft drink bottle and blowing over the mouth of the bottle - a note is produced. Now place some cotton wool in the bottle and try again. You will notice the note has reduced- well not really, the note is produced but the wool absorbs the resonance and turn the sound energy into heat! Imagine, if you lined a whole wall with bottles of various sizes, all filled with insulation material. You would now have a low-mid (200 - 500Hz depending on the bottle size) absorbing wall that as well as absorbing the low mids would also reflect or diffuse the high frequencies. I haven't tried it yet but it would be worth trying if you are short of cash because bottles are cheap. The Romans used to do it using clay jars which they placed around their theatres.

The helmholtz resonator is often called a slat or slot resonator because you can create a helmholtz resonator by building a wall with slats of timber separated by slots as in the following diagram

The insulation in a resonator can be standard pink batts or any fibre fill such as dacron etc. this is a standard timber frame with 75mm x 35mm studs at 450mm spacing this is the depth from the wall which may vary This is the width of your timber slat This is the gap between the slats This is the thickness of the timber used The timber slats can be either finished or rough sawn. If the gaps vary say 5mm, 10mm, 15mm,20mm and the wall is angled as shown below, a broad band low mid absorber is created that still keeps the the high frequencies alive. Remember the cavity behind must be sealed to an airtight container, like the bottle.
Further more, our scientists have created a formula with which we can tune the resonator to a specific frequency. If we vary the depth from the wall, slat width, slot width (and the slat depth) we can create a wall that is a broadband low-mid frequency absorber. The beautiful thing about these absorbers is that they still reflect high frequencies, in fact they will diffuse them which is even better.
The angle to the wall here can be either horizontally, as shown, or vertically with the 100mm at the bottom of the wall and the 300mm at the top.

As you can see a slat wall like this can break up parallel walls thus stopping standing waves. Because the distance from the front to the back is varying from 300mm to 100mm or around 12 degrees, the wall becomes a broadband absorber. So simple yet so effective! I've seen some beautiful looking ones where you cut the slots out of a sheet of quality particle board with a timber veneer.

Another form of helmholtz resonator is created using perforated plywood - i.e. plywood with hundreds of holes in it. We call it pegboard in Oz, you see it in hardware stores holding up tools etc. If you place a panel of this over an air cavity like in a panel absorber not only do the little holes act like bottle necks the whole panel acts as a low frequency panel absorber!

The formula for calculating the helmholtz resonant frequency is:

2160*sqrt(r/((d*1.2*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.

You can open the helmholtz formula as an Excel file and do your own calculations.
You must have Microsoft Excel on your computer for this file to open.

Take it from the top

 

normal pink batts or any fibre fill like dacron etc. this depth is variable use the calculator to work out the frequency response required The timber slats can be either vertical or horizontal or at 45 degrees if you like This cavity must be totally sealed to act like the bottle