Noise Reduction
The human ear responds to frequencies in the range 15-– 20,000 Hz. approximately.
The response of the ear to sound is not linear being less sensitive at low frequency. The ear’s response varies with loudness level and frequency. The unit of loudness level is the phon.
A 1000 Hz note may, for example, have a sound pressure level of 66dB; a 100Hz note appears equally loud if the sound pressure level is 85dB.
When sound levels are measured the variation in the sensitivity of the ear can be taken into account by incorporating weighting networks in the meter; these are termed A, B and C; where A refers to the lowest sound pressure level.
In the Building Services Industry the dB(A) measure is normally used as the indicator which broadly relates to the subjective reactions.
The simplest and most convenient indicator of subjective response (to noise) is the dBA measure obtained using a sound level meter.
The level of noise emitted from industrial premises is currently assessed using the Corrected Noise Level (CNL in dBA) as described in BS 4124. The CNL consists of a basic level to which a correction has been added to allow for the tonal character (whine, hum) the impulsive character (bangs, clangs) the intermittency and the duration of the noise. A CNL /dBA value of 60 would represent the continuous noise from stamping machinery and handling thin metal sheet in a can making factory.
Noise rating curves are a method of assessing the back ground noise level for annoyance and speech intelligibility in a given environment, and are attempts to express equal human tolerance in each frequency band. From a series of experiments the NC and NR curves have been produced. For all practical purposes the NC and NR curves may be regarded as mutually interchangeable. Acoustic insulation of pipework may be undertaken to control either noise generated within the pipework or within equipment connected to the pipework and radiated by the pipework, flanges, valves, supports, etc.
It is not necessary for an acoustic insulation system to have high performance at all frequencies. The frequency dependent performance should be compared with the frequencies at which the noise problem exists, and an assessment undertaken against the relevant criteria for alternative forms of noise control. Thus for a particular problem one layer of acoustic insulation material may achieve a sufficient and satisfactory reduction in pipework radiated noise level when compared against another type of pipework cladding system.
An insulation system will require just sufficient acoustic performance and other factors that will influence the final choice will be :
- Total system weight
- Total system thickness
- Ease of installation
- Durability of acoustic performance with time
The acoustic performance of a system based on Armaflex should therefore be shown to be “sufficient for the purpose” with the additional benefits of Armaflex, compared with alternative systems, of ease of installation, closed cell, low weight, etc. |
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