Low Frequency Absorbers
This type of absorption relies on the principle of the Quarter Wavelength trapping. Otherwise the size of dedicated low frequency absorbers would exceed accommodatable dimensions.
If you would imagine an enclosure to trap low frequencies, based on the principle of the quarter wavelength approach, what would happen?
- Sound pressure equals zero at the mouth and particle velocity is maximum
- At the bottom of the cavity, sound pressure is maximum and particle velocity is minimum.
This lays out the foundation for resonators with absorptive capabilities based on friction.
How does such a resonator work?
- Glass fiber board is placed across the front of the cavity to generate friction resulting in maximum absorption.
- Zero pressure at the mouth/opening constitutes a vacuum that sucks energy toward the cavity like a ‘sound sink’.
- Trap therefore has greater absorption than the cavity itself due to vacuum effect.
- Usability of such absorption resonators is dependant on space (i.e. low frequency wavelengths aren’t small!)
The Helmholtz Resonator
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Do it yourself: Panel Absorber
Panel Absorbers
- These types of absorbers are easily made with a little handy work
- Check out “Everest, Master Handbook of Acoustics, Chapters 9 & 13” for more information on that topic
Definition: A panel of plywood or particle board is placed over an air cavity with insulation glued to the back of the panel. The panel has a resonate frequency and when it occurs in the room it resonates and the insulation absorbs the energy.
- These types of absorbers are easily made with a little handy work
- Check out “Everest, Master Handbook of Acoustics, Chapters 9 & 13” for more information on that topic
The other great advantage of panel absorbers is
