The Science of Sound: How It Works
How Sound is Transmitted
Properties and Criteria for Sound Absorption
When sound is absorbed, the wave is converted into thermal energy. Absorption of a given material is frequency-dependent and is affected by the size, shape, location, and mounting method used. A good sound absorber is normally a porous material. The ability to absorb a given sound is expressed by a Sound Absorption Coefficient. This coefficient varies from zero to one – the higher the coefficient the more effective the absorption.
Human perception of Sound
For humans, hearing is limited to frequencies between about 20 Hz and 20,000 Hz (20 kHz). Higher frequencies are generally perceived with greater intensity up to about 8,000 Hz (8 kHz) and then decrease again. In other words, a 60 dB sound at 40 Hz may be barely noticeable, while a 60 dB sound at 2,000 Hz would be very noticeable.
In automotive applications, common unwanted noise have a variety of frequencies, depending on the source. The chart at right shows general frequency ranges of noise from various sources.
Fiber Diameter Effects
A smaller fiber diameter results in better absorption, especially in the middle and higher range frequencies. A 50% reduction of fiber diameter results in a four fold increase in flow resistivity. Sonozorb has a fiber diameter generally 3 to 30 times smaller than comparable synthetic acoustic insulations.
Material Thickness and Density Effects
Generally the thicker the insulating material, the more effective the absorption. Material that is more dense will also achieve better absorption performance.