After reading this essay you will learn about:- 1. Meaning of Noise 2. Measurement of Noise 3. Hearing Mechanism 4. Sources.
Essay on the Meaning of Noise:
“Sound” is a form of energy, consisting of wave motion. It requires medium like gas, liquid or solid for propagation. Sound waves travel (at 330 m/s = 1,070 ft./sec = 1,220 kph or 760 mph) through the medium from the source where the sound is produced to the recipient or listener. Sound waves consist of variations in pressure or oscillations of the medium in which they travel.
The rate of the oscillation is called the frequency of the sound and is measured in cycles per second or hertz (Hz). The frequency determines the pitch of the sound received by the listener. High pitched sounds have high frequencies and these are more disturbing to the individual than low frequencies, because of the varying sensitivity of the human ear.
The term “noise” is applied to the sound that causes irritation on hearing of healthy human being. In fact, there is very little demarcation of “sound” and “noise“. For a long time many individuals have accepted noise as a part of their environment.
However, over the years, the general incidence of noise has been increasing. The development of various engines, technological machinery in industries, jet planes etc. — all contributed to an increasing noisy environment. Noise is now regarded by many people as a pollution component that contributes to a deterioration of the environment.
The human ear receives sound waves and these set up oscillations in the tympanic membrane or ear drum. These oscillations cause sympathetic movements of the three ossicles or small bones in the middle ear behind the ear drum. The oscillations then pass through the fluid in the inner ear to the auditory nerve and on to the brain. In the brain, the oscillations or sounds are identified and interpreted.
The brain is able to select mixed sounds into different categories, for example, it can distinguish speech from background noises and, if desired, it can consciously suppress unwanted sounds depending upon their intensity.
The ear is able to analyse sounds into frequency components and the range of an 18-year old person with perfect hearing is between 20 Hz and 20,000 Hz. Normally hearing is more acute in the frequency range 2,000 to 5,500 Hz. but it falls off rapidly below 200 Hz and above 10,000 Hz.
However, the ear’s sensitivity ranges vary considerably from person to person and also with ages of the person concerned. As such, aged people experience a progressive hearing loss of high frequency sounds which is called “presbycusis“.
Essay on the Measurement of Noise:
There are two most important measurements of the noise, viz., sound pressure and sound intensity. The scientific acoustic unit in common use is the decibel (dB). It is not an absolute physical measurement unit comparable to the gram, volt or meter, but it is a ratio expressed as a logarithmic scale relative to a reference sound pressure level.
The decibel is defined as:
Sound intensity level = 10 log10 intensity measured/reference intensity
The reference intensity used is the threshold of hearing—which is a sound that can be first heard at a sound pressure of 2 x 10-5 N/m2 or a sound intensity of ICH2 W/m2.
The relationship between the three measurements is shown in Table 14.1:
There are several instruments for the measurement of sound:
1. Sound level meter
2. Octave band analyser
3. Magnetic tape recorder
4. Statistical analyser
5. Noise analyser etc.
The sound level meter (Fig. 14.1) evaluates sound pressure linearly or on weighted scale while octave analyser determines average sound pressure of standard octave pressure bands. Magnetic tape recorder is used for storage of sound for latter analysis.
A single measurement of sound level is obviously not adequate for rating noise exposure for possible hearing damage. Instead, special integrating sound level meters are preferred instruments for this task. They are often called “noise dosimeters“. Each dosimeter includes a small microphone that is connected to an electronic monitor by means of a connecting cable.
The small microphone can be worn at the ear, on a collar, or on a shoulder. The electronic monitor is small enough to be carried in a shirt pocket. The monitor accumulates the dose according to the 5 dB exchange rate. After the full working day, the monitor is plugged into an indicator section. The noise dose can then be displayed on a 4-digit indicator.
A sound level meter, on the other hand, has a microphone, an attenuator, a preamplifier, an indicating meter and monitoring outputs, etc. In addition, octave band analysers possess pass filters, selected by means of a rotary switch.
The dBA scale is in general use, but it is not sufficiently refined to take account of peak noise levels, the duration of noise exposure and the quality of noise that are features of specific environmental noise situations.
Equivalent Continuous Sound Pressure Level (Leq):
The equivalent continuous sound pressure level is the level which, if generated continuously, would give the same energy content as some fluctuating signal over a given time period:
Thus 60.7 dB (A) had been generated continuously over the 8-hour-period.
The energy content of the noise would have been equivalent to that of the fluctuating noise pattern given:
Noise Pollution Level (LNP):
This is an expression of noise pollution in a given environment:
The ear acts as a transducer converting sound energy into electrical energy which is sent to the receptive centres of the brain via electrical impulses (Fig. 14.2).
The ear comprises three main regions:
(i) The outer ear (air-filled),
(ii) The middle ear (air-filled), and
(iii) The inner ear (fluid filled).
Sound is directed into the ear via the pinna and the auditory canal and strikes the ear drum (tympanic membrane) which is set into vibration. The vibration is transferred mechanically across the middle ear by three small bones (Ossicles) viz., malleus, incus and stapes.
The ossicles acts as a system of levels transferring the sound pressure acting on the ear drum to the oval window at the footplate of the stapes.
The amplified pressure acting on the oval window is sufficient to cause the fluid in the cochlea to move backwards and forwards. Then the hair cells of the basilar membrane of cochlea deflect the frequency content of the sound acting on the ear. Finally, the impulse is transferred to the respective centre of the brain via the auditory nerve.
There may be defect in hearing due to transmission of sound wave/impulse to the sensory centre.
There are three kinds of deafness viz., conductive deafness (i.e. defects in pathways of sound transmission due to perforated drum, otoclenosis, blockage of air passage etc.), sensory neural deafness (defects in the cochlea or auditory nerve due to hair cell degeneration in high frequency sound), or central deafness (defects in hearing pathways in brain).
It is well-known that there is a particular sound pressure that is needed for hearing. But due to certain constrain, there is need for change of higher sound pressure level for hearing. This is threshold shift (TS).
Threshold shifts can be classified as:
(i) Temporary Threshold Shifts (TTS), resulting from exposure to high level sound for comparatively short period;
(ii) Permanent Threshold Shifts (PTS), due to prolonged noise exposure, or age.
Permanent threshold shifts which are due to years of exposure to high level of industrial noise are termed Noise Included Permanent Threshold Shifts (NIPTS).
Sources of Noise:
The sound or noise could be generated from various sources like:
(b) Industrial operation,
(c) Domestic practices etc.
(a) Transport Noise:
Transport noise originates from road traffic (i.e. vehicular), aircraft and rail traffic etc. Noise from road vehicles produces disturbance to more people than any other noise source and this has been increasing over the last decades for a number of reasons.
The total number of road vehicles and hence the density of road traffic is steadily increasing. One of the most important causes of noise on the roads is the traffic speed. The faster the traffic travels, the greater the noise volume and modern road development policy is encouraging higher speeds.
Road traffic noise fluctuates according to a number of operating factors. Noise is produced by all vehicles from the gear box and exhaust system. Heavy vehicles also produce rattles, squeaks and vibrations according to the degree of loading and age.
All vehicle produce more engine noise at faster speeds and a doubling of engine speed can increase the noise level by 13 dBA. Also tyre noise increases with speed and a wet road can increase the noise by 10 dBA.
Generally, a heavy goods vehicle produces twice as much noise as a private car, or, to a motorway observer, the noise from one heavy lorry equals that from 10 passing cars. The actual pattern of traffic noise on a main road is complex. There is a general noise level as long as any traffic is moving and this varies with the traffic density and the time of day.
The traffic noise can be accurately measured in dBA and noise levels are often expressed as on the L10 (18 hrs) index. The residential road having vehicle passing at a speed of 30 mph bears the noise level 60 dBA, while the busy main road having traffic speed of 48 kph generate noise at 70 dBA.
In addition, transport noise is also generated by the air-horn use.
In fact, there is no standard of vehicle generated noise in many countries. In UK there is a noise generation standard by the vehicle (BS, 3539: 1962) —Table 14.2:
i. Aircraft noise:
Over the last decades there has been an increase in the noise nuisance from subsonic aircraft for several reasons. This noise is variable and intermittent and it is not continuous as in the case of road traffic noise.
There are peak noise levels when aircraft are flying overhead or are taking-off and landing at airports and the peak frequency varies with the number and the type of aircraft and the operational height. The noise is mainly produced from aircraft engines.
The noise generated K different types of aircraft is given in Table 14.3:
ii. Rail traffic noise:
The noise from trains is not generally regarded as a serious nuisance by the majority of people. This may be because the noise is generally of a lower frequency than that of road vehicles and also most railway tracks run through rural areas. But where buildings are sited beside railway tracks, and specially where engine testing and shunting is carried out, there is noise disturbance.
(b) Industrial Noise:
Industrial noise is complex and varies with the design, direction of movement of working parts and the method of mounting of machines. The noise is often produced in three stages. There is an initial disturbance at the point of origin of the sound; followed by amplification, often caused by the resonance of the machine parts, the work piece or the floor and finally radiation of the sound to the surrounding environment.
Industrial noise can be classified into three types:
(i) Impact and percussive noise is produced by presses; punch and stamp machines, pneumatic drills, milling machines, cutters and routers,
(ii) The impact noise is caused when two surfaces meet each other, sometimes at high speed and vibration occurs at the point of contact followed by amplification and resonance,
(iii) The third type of industrial noise is aerodynamic noise. Aerodynamic noise is produced by a blow lamp or torch, fans and dust extractors.
(c) Domestic Noise:
Noise is also generated from domestic appliances like washing machines, spin dryers, food mixers, sink waste grinders and vacuum cleaners. Both industrial and domestic noise sources are collectively referred as occupational noise.
Some common occupational noise levels are given in Table 14.4: