Diesel Engine Noise Emission
Like many other machines, diesel engines also generate variations in air pressure. These variations disperse in the air as longitudinal vibrations. The human ear is able to perceive such pressure variations within a frequency range of approximately 16 Hz
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Diesel Engine Noise Emission Bruno M. Spessert and Hans A. Kochanowski
16.1
Fundamentals of Acoustics
Like many other machines, diesel engines also generate variations in air pressure. These variations disperse in the air as longitudinal vibrations. The human ear is able to perceive such pressure variations as noise within a frequency range of approximately 16 Hz–16 kHz. High and low frequency noise components in this frequency range are perceived to be far less loud than noise in the frequency range of 0.5–5 kHz. This frequency-dependent sensitivity of the human ear can be accommodated in a frequency-dependent evaluation curve (A-weighting). At a frequency of 1 kHz, the human ear is able to hear sound pressure amplitudes between approximately 2 105 and 20 Pa. (Pressure variations above this amplitude range are experienced as pain.) The large amplitude bandwidth of human hearing would make a specification of linear sound (pressure) values extremely unwieldy. Sound (pressure) levels are usually employed instead:
(where A is the measuring surface and Ao = 1 m2 the reference surface). The surface enveloping the sound source in which the sound pressure is measured is referred to as the measuring surface. The decibel (dB) is the unit of sound pressure levels and sound power levels. Routinely applied, A-weighting is identified by a suffix (LpA or LWA, in the unit dBA or dB(A)). A doubling of the sound energy corresponds to an increase of the sound pressure level or sound power level by 3 dB or 3 dB(A). However, humans do not perceive such a doubling of sound energy as twice as loud. A doubling of noise is not subjectively perceived until the sound energy has roughly decupled, i.e. the sound pressure level has increased by 10 dB(A). Noise can be measured by microphones that capture air pressure variations through a thin diaphragm, which forms part of a capacitor. Air pressure variations cause diaphragm vibrations, which in turn cause corresponding variations of the capacitor’s electrical charge, which are proportional to the air pressure variations and relatively easy to process further.
Lp ¼ 20logp=po (where Lp is the sound pressure level, p the sound pressure and po = 2 105 Pa the reference sound pressure). The sound pressure level specifies the level of sound for a measuring point, i.e. the noise exposure (noise pollution) at the measuring point. However, the noise radiated by a component or a machine is either described by specifying the sound pressure level and additionally the measurement distance (!) or, alternatively, the sound power level: LW ¼ 10logP=Po (where P is the sound power and Po = 1012 W the reference sound power). The reference variables po and Po are selected so that the following applies: LW ¼ Lp þ 10logA=Ao
B.M. Spessert (*) FH Jena (University of Applied Sciences), Jena, Germany e-mail: [email protected]
16.2
Development of Engine Noise Emission
Legislators have been steadily tightening noise limits for practically every vehicle and unit of equipment since the early 1970s. An
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