500 or longer
The term interrupted usually has approximately the same meaning except that it implies that the off periods are shorter than the on periods. Intermittent or interrupted noises can be measured with a standard sound level meter and a clock or stopwatch.
Sounds whose duration is only a fraction of a second are called impulsive, explosive, or impact sounds. The terms are often used interchangeably for pulses of differing character, alike only in that they are short. They must be measured with instruments capable of following rapid changes or with instruments which sample and hold peak values.
The wave form of the noise can be modified appreciably by reflection before it reaches the ear, but it is usually described as either single-spike pulses or rapidly damped sinusoidal wave forms. Such wave forms can be evaluated fairly accurately by converting the time-pressure pattern into an energy spectrum and then performing a spectral analysis. A more accurate evaluation of the effect of intermittent but steady-level noise is possible through computation based on the ratio of on-to-off times.
The ear cannot judge the intensity of extremely short noise pulses or impact noises since it seems to respond more to the energy contained in the pulse than to its maximum amplitude. Pulses shorter than Ate second, therefore, do not sound as loud as continuous noise having the same sound pressure level; the difference is as much as 20 dB for a pulse 20 ms long. (See Table 6.2.2.) Thus, the ear can be exposed to higher sound pressures than the subject realizes from sensation alone; a short pulse with an actual sound pressure level of 155 to 160 dB might seem only at the threshold of discomfort, 130 to 135 dB for continuous noise. Yet this momentary pressure is dangerously near that at which eardrum rupture or middle-ear damage can occur.
Interruptions in continuous noise provide brief rest periods which reduce fatigue and the danger of permanent hearing impairment. Conversely, intermittent periods of high noise during otherwise comfortable work sessions are annoying and tend to cause carelessness and accidents.
Industrial noises also vary in their frequency characteristics. Large, slow-moving machines generally produce low-frequency noises; high-speed machines usually produce noise of higher frequency. A machine such as a large motor-generator produces noise over the entire audible frequency range; the rotational frequency is the lowest (1800 RPM produces 30 Hz) but higher frequencies from bearing noise (perhaps brush noise too), slot or tooth noise, wind noise, and the like are also present.
A few noise spectra are shown in Figure 6.2.1, in octave-band form. Curve No. 1 of a motor-generator set shows a nearly flat frequency response; it is a mixture of many frequencies from different parts of the machine. Curve No. 2, for a large blower, shows a predominantly low-frequency noise pattern; its maximum is around 100 or 120 Hz and can be caused by the mechanical vibration of large surfaces excited by magnetic forces. Curve No. 3 is for a jet plane approaching land; it contains much high-frequency energy and sounds like a howl or scream, while the blower noise is a rumble. Curve No. 4 describes the high-pitched noise caused by turbulence in a gas-reducing valve; it is mechanically connected to pipes which readily radiate in the range of their natural frequencies of vibration. Octave-band analyses have only rather broad resolution and are suited to investigate the audible sound characteristics; the mechanical vibrations causing the noise are best analyzed by a continuously variable instrument.
The radiating area of a source affects the amount of sound emitted; not only does the total amount of acoustic energy radiated increase roughly in proportion to the area in vibration, but a pipe or duct passing through a wall emits sound on both sides of the wall. The vibration amplitude can be only a few microinches yet produce loud sounds. If the natural frequency of an elastic member is near the frequency of the vibration, its amplitude can become large unless the member is damped or the driving force isolated.
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