PROPERTIES AND MEASUREMENT OF NOISE
Physically, sound is a mechanical disturbance propagated as a wave motion in air and other elastic or mechanical media .such as water or steel.
Physiologically, sound is an auditory sensation evoked by this physical phenomenon. However, not all sound waves evoke an auditory sensation, e.g., ultrasound has a frequency too high to excite the ensation of hearing.
The physical properties and perception of sound or noise are expressed and measured in different concepts and units.
3.1 Physical Properties and Measurement
Sound waves involve a succession of compressions and rarefactions of an elastic medium such as air. These waves are characterised by the amplitude of pressurechanges, their frequency, and the velocity of propagation. The speed of sound, the frequency and the wave length are related by the equation:
Wave length = Speed of sound/frequency
The speed of sound in air. at a temperature of 20° c , is approximately 344 rn/s. Sound travels much faster in solids than in air. For example, the speed of sound is 3,962 rn/s in wood and 5,029 m/s in steel.
Frequency is defined as the number of compressions and rarefactions per unit time (sec). Unit of frequency is hertz (Hz). Human hearing is sensitive to frequencies in the range of about 20-20,000 Hz (the audio frequency range).
Sound pressure is used as the fundamental measure of sound (amplitude) as it is measurable directly by instruments. The weakest sound pressure disturbance that can be detected by an "average" person at 1,OOO Hz has been found to be 20 µNlm2 and the largest sound pressure perceived without discomfort is of the order of 107 µN/m2. Because of such a wide range, the use of a linear pressure scale has been found to be impractical. It has been found convenient to employ sound pressure level, a quantity, which is proportional to the logarithm of sound pressure. By this, the sound pressure range of interest is compressed between 0 to 130, a range convenicnt to use. The sound pressure level is expressed in the unit of decibel (dB).
Sound Pressure Level is defined as :
Lp = 10 Log10 (P/Pr)2
Where Lp = sound pressure level, dB
P = root mean square sound pressure, usually in µN/m2
Pr = reference sound pressure
Log10 = Logarithm to the base 10
The reference sound pressure, Pr, has an internationally agreed value of 20 µN/m2.
Sound is measured with a sound level meter which is usually a portable, self- contained instrument incorporating a microphone, amplifier, a voltmeter and attenuators, the whole of which is calibrated to read sound pressure levels directly.
3.2 Sound Perception and Measurement
The magnitude of a sound, as perceived by human ear is called its loudness. Ear is not equally sensitive at all frequencies and amplitudes of sound pressure. For this reason, even though, the sound pressure levels of two different noises may be same, the first may be judged to be louder than the second if the sound energy of the first is concentrated in a frequency region where the ear is more sensitive.
Fig. I shows a set of equal loudness curves for an average young person with good hearing, listening to pure tones.
To obtain levels which bear a closer relationship to loutiness judgements than sound pressure levels, so called frequency weighting networks, are incorporated in sound level meters. Such levels are termed as Sound Levels. Generally, three types of filters having frequency response curves A, B & C, as shown in figure 2, are employed to match the ear response at low, medium and high loudness, respectively. However, extensive experience has shown that A-filter usually provides the highest correlation between physical measurements and ,subjective evaluations of loudness of noise. Levels on the A-scale are commonly expressed in dB(A).
Most Sound Level Meters integrate instantaneous sound pressures over some time constant viz. Fast-, Slow- and Impulse-response times. Fast-response corresponds to a time constant of 0.125 s and is intended to approximate the time constant of human hearing system. Thus, all measurements of sound pressure levels and their variation over time should be made using the Fast response time, except where it is not possible. Sound Pressure Meters include a Slow-response time with a time constant or I s, which may be used, for convenience, in non-recording Meters. Sound Pressure Meters also include Impulse-
response time with a time constant of 0.035 s, for measurement of impulse noise, such as noise from firecrackers, gun-shot etc.
Equivalent Continuous Sound Pressure Level, Leq is the level of that steady sound which over the same interval of time, contains the same total energy (or dose) as the fluctuating sound.
Equiv.uent sound level Leq can be obtained from variable sound pressure level, L, over a time period T, by using following equation:
Equivalent continuous sound level has gained widespread acceptance as a scale for the measurement of long-term noise exposure.
