EHG Acoustic Solutions including Duct Silencers
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General information on sound attenuation
It is desirable and often required that noise be attenuated or removed. In principle, the noise sources in a ventilation system can be roughly divided into four groups: fan noise, duct noise, noise from dampers/leaks, and building noise. Silencers are specially designed for attenuating noise originating from the sources specified from fan and duct noise.
Sound attenuation is usually required in order to comply with project specifications or to fulfill a wish for a better environment. These acceptable noise levels are the result of all sound and noise sources (ventilation, electrical fittings, water pipes, fans in computer equipment, etc.).
Calculation of sound in ventilation systems
A precondition for sizing sound attenuators is to obtain an accurate sound calculation of the system. The sound calculation is based on the sound power level, which is then converted to the sound pressure level.
In general, the sound power level of the fan must be known. The following must also be available before the sound attenuation calculation can be made:
1. Octave band analysis (frequency analysis)
2. Definition of desired end result
The A-weighted sound level
It has been found that humans do not perceive two sound pressures of the same magnitude but with different frequencies identically. However, it is possible to make the individual frequencies comparable through appropriate correction (A-filter) of the linear sound pressure level. The value achieved is called the A-weighted sound pressure level, LpA, which is the sum of the corrected sound pressure levels in the individual octave bands. Instruments are available in which the A-filter can be inserted when measuring the individual octave bands. The measurement results are designated: Whole octave A-weighted values, measured in dB.
Specification of requirements
There are 2 methods for specifying the noise requirement:
1. by means of NC
2. by means of LpA
Recommended maximum values for sound pressure levels
Below is a table which indicates the recommended maximum values for sound pressure levels in N-value and LpA. There is no fixed ratio between the various sound specifications, but as long as the noise in question is ventilation noise without pure tones, the following rule of thumb applies: LpA = NC + 5
|Building type||Room type||NC|
For a full listing of building types, please reference page 4 in the Silencer Catalog.