Module 1: Occupational Hygiene - Section 5: Instrumentation

OH5.4: Occupational Noise Exposure - What is noise?

What is noise?

Hearing is a series of events in which the ear converts sound waves into electrical signals that are sent to the brain and interpreted as sound. Sound can either have a soothing or annoying effect, depending on the listener and circumstances. Rock and rap concerts are generally regarded by the older generation as nothing but noise. On the other hand many younger people consider classical music boring and old-fashioned.

Sound that is generally unwanted, unpleasant to the ear or damaging sound is referred to as noise. In other words, noise is a sound that interferes with what people are trying to do (communicate, send warning signals) or which has a damaging effect on health (increase in stress level, loss of hearing). ¹

There is no known instrument, apart from the ear, that can distinguish between sound and noise. In what follows, the words noise and sound will be used interchangeably.

Characteristics of sound:

Sound can be described in terms of intensity (perceived as loudness) and frequency (perceived as pitch). ⁴ The extent of damage that can be cause by noise depends mainly on how loud it is and the duration of exposure. The frequency or pitch can also have some effect since high-pitched sounds are more damaging than low-pitched ones.

The frequency of a sound wave refers to the number of sound waves produced per second, i.e., the number of times per second a vibrating body completes one cycle of motion. The unit for frequency is the Hertz (Hz). Audible sound is found within a wide frequency range. A healthy, young person can hear sounds with frequencies from as low as 20 Hz and as high as 20 000 Hz.

The sounds of human speech are mainly in the range 300 to 3000 Hz. Sound consisting of a single frequency is referred to as a pure tone. Most sounds encountered in industrial environments are made up of several tones of varying intensities. These sounds constitute a broad band spectrum.

The boundary between high and low frequencies is generally established at 1000 Hz. The human ear is most sensitive to sounds with frequencies in the 1000 to 4000 Hz range.

Intensity:

Sound pressure or intensity refers to the amount of air pressure fluctuations or sound waves a noise source creates. We "hear" or perceive the amplitude of sound waves or air pressure as loudness.

Sound pressure is usually expressed in units called Pascals (Pa). A healthy, young person can hear sound pressures as low as 0.00002 Pa. A normal conversation produces a sound pressure of 0.02 Pa. The sound is painfully loud at levels around 20 Pa. Thus the common sounds we hear can have sound pressures that vary over a wide range (0.00002 Pa to 20 Pa).

Decibels:

The human ear is conseuently sensitive to an astonishing range of sound pressures or intensities. It is difficult to work with such a broad range. To overcome this difficulty a log (base 10) scale with units called decibels (dB) has been introduced. The decibel scale is more convenient because it compresses the scale of numbers to a manageable range.

The decibel scale is set with a reference point of sound intensity of 10-12 Watts per square meter (W.m^-2) equaling 0 dB. All other sounds are then measured and given a value in decibels relative to this reference point. For example the pain threshold becomes 120 dB.

(These diagrams show that an increase in the decibel level corresponds with a much greater increase in the sound pressure measured in Pascals and an even greater increase in the power measured in Watts).

Adding Noise Levels:

The arithmetic of decibels deals with ratios, not ordinary numbers. Therefore, decibel levels of two or more sound sources cannot simply be added. Table A is used to determine the combined effect of two or more sound sources. The number of decibels to be added to the higher level depends on the difference in their levels.

In general any doubling of sound intensity gives an extra 3 dB, which is consequently twice as damaging to health (even though it appears to be only a small increase as in the example above. This principle is used in noise control as will be discussed later in the context of legislation.

The sensitivity of the human ear to sound also depends on the frequency or pitch of the sound. People hear some frequencies better than others. If a person hears two sounds of the same sound pressure but different frequencies, one sound may appear louder than the other. This occurs because people hear high frequency noise much better than low frequencies.

In measuring sound levels, instruments which resemble the human ear in sensitivity to noise composed of varying frequencies are used. The instrument measures the A-weighted sound level in units called dB (A). An A-weighted filter which is built into an instrument integrates intensities of sound at different frequencies while de-emphasizing low frequencies by assigning these lower weights than the higher frequencies in the calculation of summary dB(A) units.

A-weighting serves two important purposes:

• gives a single number measure of noise level by integrating sound levels at all frequencies present in the broad band sound; and
• gives a scale for noise level as experienced or perceived by the human ear by weighting the frequencies so that the dB(A) units reflect the range of audible frequencies.

REFERENCES:

General Introduction to Occupational Health: Occupational Hygiene, Epidemiology & Biostatistics by Prof Jonny Myers is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.5 South Africa License