Acoustics – Sound-modifying means – Muffler – fluid conducting type
Patent
1985-12-03
1988-11-22
Fuller, Benjamin R.
Acoustics
Sound-modifying means
Muffler, fluid conducting type
181249, 181255, 181258, F01N 110
Patent
active
047859091
DESCRIPTION:
BRIEF SUMMARY
This invention relates to noise silencing systems for silencing noise developed in a fluid. The fluid may be a gas as in the case of the exhaust of a jet engine or a liquid in the case of a hydraulic power transmission system.
In the main existing silencers have fallen into two categories, namely the filter type and the shunt chamber type. The filter type generally comprise mechanical filter sections formed by dividing up some kind of cylindrical enlargement of the transmission pipe by means of transverse baffles incorporating restriction pipes. For example a pi-type filter may be formed by placing a baffle roughly midway along the cylindrical enlargement. Such a filter must be made part of the main transmission path and represents a substantial insertion loss. Additionally, the attenuation/frequency characteristic consists of a number of narrow frequency-band lobes with which are associated variations of impedance with frequency, which may theoretically be met by using a suitably designed series of filters. Generally the filter type silencer is regarded as being effective only at low frequencies, a typical upper limit being 300 cycles/second.
The shunt chamber type of silencer, as the name implies, comprises a chamber which generally surrounds a through-going transmission path and the present invention is directed towards this type of silencer. The chamber may have a dividing diaphragm separating the chamber from the transmission path, so that a different fluid may be used in the chamber but where the same fluid is used no separating diaphragm is necessary, as shown in FIG. 1, and where the paths of entry to, and exit from, the chamber are substantially perpendicular to the transmission path. The type of silencer where the duct is lined with a sound absorbent material is also placed in the shunt chamber category, the construction often comprising a chamber completely filled with, for example, glass fibre as the acoustic absorbent, the transmission pipe being coupled to the glass fibre in the chamber through a series of holes usually covered with a diaphragm-like fine wire mesh or glass fibre cloth. Known shunt type silencers are more effective at higher frequencies and the noise attenuation they provide is not great. Where it is desired to provide attenuation over a wide band of noise frequencies installations often consist of a combination of filter type and shunt chamber type silencers.
An object of the present invention is to provide an improved silencer of the shunt chamber type.
According to the invention, there is provided a silencer of the shunt chamber type wherein the chamber volume is principally occupied by a fluid but includes energy absorbing means, the chamber being so shaped as to provide good acoustic coupling with the transmission path and the energy absorbing means forming a characteristic impedance type termination.
The chamber may be separated from the transmission path by a diaphragm where the fluid in the chamber is different from the transmission fluid but of course this is not necessary if the two fluids are the same.
Preferably the fluid in the chamber is a gas because of its low density which gives a low characteristic impedance in the chamber, air being the most convenient gas since more data is available on the characteristics and behaviour of air. Additionally air leads to ease of topping-up and is relatively safe in leakage conditions.
Preferably the energy absorbent material comprises a metallic filament which is knitted, woven or crumpled together to form a pad or the like. Such material has high sound absorption properties which appear to be due to mutual chafing of the filaments within the pad, such energy absorption being largely aperiodic. A metallic pan-scrubber is such a material and although the term "pan-scrubber" is used hereinafter it is intended to cover other like material such as that available under the trade name "Vibrashock" which consists of very fine stainless steel wire knitted into open meshed sleeve form, which is crimped and then compacted to give a su
REFERENCES:
patent: 3960239 (1976-01-01), Frochaux et al.
patent: 4109754 (1978-08-01), Purhoen
patent: 4314621 (1982-02-01), Hansen
patent: 4346781 (1982-08-01), Ingard et al.
patent: 4487290 (1984-12-01), Flaherty
patent: 4530417 (1985-07-01), Daniel
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