There are two different types: dissipative (or absorptive) silencers and reactive silencers.
In absorptive silencers acoustic energy is converted to heat by the sound-absorbing processes which take place in the small interconnected air passages of fibrous or open-celled foam plastic materials. They are used to provide attenuation, of fan noise for example, over a broad band of frequencies. Because of the frequency characteristics of the absorbing materials they employ, this type of silencer is much more effective at medium and high than at low frequencies.
Reactive silencers work by providing an impedance mismatch to the sound waves, causing reflection back towards the source, and by using destructive interference to 'tune out' particular frequencies. The attenuation produced depends on the dimensions of the pipes and chambers of the silencer. They are usually used to reduce noise from pulsating gas flows such as the air inlet and exhaust systems of internal combustion engines. Reactive silencers can be very effective at reducing the amplitude of pure tones of fixed frequency, particularly if these are at low frequencies, where the absorptive type of silencer is ineffective. However, there can also be frequencies at which they allow sound to be transmitted with very little attenuation.
Many silencers use both reactive and absorptive mechanisms to achieve their effect.
The simplest type of absorptive silencer is a duct with walls lined with sound-absorbing material. The attenuation produced, in dB per metre run of duct, depends on a, the sound absorption coefficient of the lining material, and the ratio of perimeter to cross-sectional area of the duct (P/S) (see Example 1.0).
In order to achieve maximum attenuation the shape of the duct should be designed to give the highest possible P/S ratio, which in effect means that for a given cross-sectional area of duct the sound is exposed to the greatest possible surface area of sound-absorbing material. It follows that the optimum shape for the cross-section of a rectangular duct is a long thin one. In commercial splitter silencers a rectangular duct is split into several such sections. each lined with sound-absorbing materials.
The attenuation produced by such a narrow lined section of duct is at greatest at medium frequencies, dropping (at the high and low frequency ends of the spectrum as shown below.
The poor low frequency performance is simply because the sound absorption coefficient of the lining material is poor at low frequencies. The poor high frequency performance occurs because much of the high frequency sound energy tends to be 'beamed' down the centre of the duct airways, and is unaffected by the sound absorbent duct linings. This occurs at frequencies where the sound wavelength is comparable with or smaller than the airway width. Therefore it can be seen that reducing the airway width will improve the silencer performance considerably particularly at high frequencies.
The thickness of the sound-absorbing lining is also important factor in determining the silencer performance an increase in thickness producing improved attenuation particularly at low frequencies.
Another very important factor which must be consider is the extra resistance to the flow of air which the silencer provides, which can be measured as a pressure drop across the silencer. Reducing the airway width too much will obviously increase this resistance to an unacceptable limit.
Excessive restriction of air flow will also have an effect on another important silencer parameter, the noise generated by the flow of air through the silencer. Forcing the air through narrow airways will obviously cause an increase in flow velocity, and therefore in the amount of this self- generated noise.
Thus there is a conflict between the requirements of good high frequency sound attenuation (i.e. narrow airways) and minimum flow resistance and silencer self-generated noise (requiring broad airways).
Other factors which can affect the self-generated noise are changes of cross-section occurring within and at the ends of the silencer. It is also important that the sound-absorbent linings are kept as smooth as possible.
One way of obtaining the benefits of narrow airways without incurring the penalties of high flow resistance is to increase the number of airways, which increases the overall width of the silencer. Increasing the height has a similar effect. The alternative is to use a larger airway width, and increase the length of the silencer.
The acoustic performance of an absorptive silencer depends on four factors:
1. sound absorption coefficient of the duct lining material
2. thickness of the absorbing lining
3. width of airway
4. length of the silencer.
Silencer design is a compromise between acoustic performance, silencer size (height, width and length), acceptable flow resistance and material costs.
The position of the silencer in the duct system can be very important in determining its effectiveness in reducing the noise at the reception point. The optimum position can be governed by the possibility of noise breaking into the duct after (downstream as far as noise is concerned) the silencer, e.g. from noisy plant-rooms, or by possible break-out of noise from the duct before (upstream of) the silencer.
The performance will depend on the flow conditions into the silencer section.
Sound entering the silencer at random incidence, e.g. when the silencer is fitted close to a noise source such as a fan, will be attenuated more than sound moving parallel to the duct walls. Positioning the silencer close to bends can cause increases in pressure drop and self-generated noise.
Other practical considerations are that the sound-absorbing materials used in the silencer should be able to withstand any adverse environmental conditions to which they might be subjected, such as oil or grease, mechanical erosion, high temperatures, and attack by insects, bacteria, fungi and vermin. The sound-absorbing materials may be covered with protective layers; for example by thin perforated sheet metal to protect against wear and tear, or by thin plastic films to protect against moisture or for reasons of hygiene.