Sound Data Required To Calculate Engine Attenuation
Description
The purpose of to this bulletin is to guide technical staff in compiling the necessary technical information required for the accurate design of acoustic control equipment for the attenuation of engine surface and exhaust noise.
Background
The science of acoustics, and “noise reduction” is an important aspect in terms of engine design. The major sources of engine noise emission are via the surface of the engine and the exhaust system of the engine. Note that for an open generator set, alternator sound data is not considered as the emissions from this equipment are deemed as negligent. If the genset has an engine-mounted radiator, then fan sound data will be required for calculation room acoustics.
Other sound sources are ventilation ducts, room reverberation,
engine vibration ‘noise,’ etc. These will not be considered in this bulletin
due to the potential complexity associated with calculating sound attenuation
due to these items!
Sound Power Level, (SWL) Lw, is defined as the speed at which sound energy is radiated or transmitted for a given period of time. The SI unit of sound power is the watt(W). It is the power of the sound force on a surface of the medium of propagation of the sound wave.
Sound Pressure Level, (SPL) Lq, is the sound we hear and is defined as the atmospheric pressure disturbance that can vary by the conditions that the sound waves encounter such as furnishings in a room or if outdoors trees, buildings, etc. Sound pressure is usually measured at a distance from the source. The unit of measurement for Sound Pressure is the decibel (dB).
A simple analogy explaining the difference between SPL and SPL is that
of a light bulb. A light bulb uses
electricity to make light so the power required (stated in Watts) to light the
bulb would be the “Sound Power” and the light generated or more specific the
brightness is the “Sound Pressure”.
Sound, just as with the light emitting from the bulb, diminishes as the
distance increases from the source.
Most sound data pertaining to engines, is measured in what is called the “A-Scale.” For example, Sound Pressure values are expressed in dB(A). The ‘A-scale’ is a weighting scale that is closest to what a human ear can process.
Another factor to recognise is that SPL and SWL are to be measured in Octave band levels. These band levels, usually in 1:1, or 1:3 scale, allow sound waves to be bunched up in frequency levels where data is easier to interpret.
In most cases, when referring to Sound data from engine manufacturers such as MTU, published data for surface sound and exhaust sound emissions, is presented as SPL in “dB(A) at 1m” at standard Octave band levels. These values are derived in laboratory conditions where sound interference associated with background noise, reverberation effects, etc., can be minimised. Typical figures are per below. With these values an acoustics contractor can design a silencer (Based on exhaust sound values) and/or a sound enclosure for the machinery.
A lot of times, Sound pressure levels are presented as one overall
figure, such as figures for enclosed gensets per the below Deutz brochure.
Although this is an accepted practice for generalising a sound attenuation value for machinery, when sound emission levels become a strict contractual requirement where sound performance is a KPI, and will ultimately have to be measured on site, presenting an overall figure for the machinery, rather than values that are broken down into octave band ratings, may not be an acceptable commercial risk. Please consult the Engineering and Technical Support group for any support or clarifications.
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