Improving Room Acoustics through Passive Acoustic Treatment – part 2

3. Resonance Absorber

Resonance absorbers, also referred to as Helmholtz absorbers, are also based on the spring-mass principle. These devices consist of a tube with volume behind. Helmholtz absorbers can be tuned to a particular resonant frequency by changing the volume of the air cavity or the dimensions of the port. Absorption is caused through the friction of the vibrating air inside the tube. Friction loss can be increased by placing acoustical damping material into the tube. Again, absorption is maximal at the frequency at which the absorber is tuned and at locations where the sound pressure shows maxima.

The resonance frequency for Helmholtz resonators with square opening can be given by:

and for Helmholtz resonators with round openings by:

c…………………….speed of sound [m/s] S……………………cross-sectional area of the resonator opening [m2] 2∆ = 0.9a……….resonator mouth correction factor a…………………….edge length of the square opening [m] R……………………radius of the opening [m] V ……………………volume of the resonator [m3] l……………………..length of the resonator opening [m]

Perforated Panel Absorbers

Perforated panels are composed of several holes representing the tubes of a Helmholtz resonator with a the associated air cavity behind the holes. Sound that hits the panel perpendicularly causes the resonators being in phase with each other leading to maximum performance of the perforated panel. Absorption decreases if sound strikes the panel at an angle.

f0…………………….frequency of resonance [Hz] p……………………..perforation percentage [%] t……………………..effective hole length (correction factor for t = panel thickness + 0.8 × hole diameter) [m] d……………………..depth of airspace [m]

The formula for calculating the perforation percentage is shown in figure 2.:

Figure 2: Panel absorber stuffed with absorbent material.

In this post, we explored two types of sound absorbers that are effective in controlling sound reflections; Other types of these absorbers and the things that need to be mentioned to improve the sound will be published in future posts. These will pave the way for creating a home theater of the highest quality.

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