Traffic, Mills / Factories, Aeroplanes, Rock concerts, Diwali crackers,... What do these have in common? ... well NOISE is the obvious answer ! Industrial workers, musicians, drivers.. why every one of us.. is exposed to NOISE. Noise is everywhere and we cannot escape being exposed to it. That noise is harmful is known to most of us. However to what extent is noise harmful? Is each one of us really at risk? What are the remedial measures? This article attempts to answer these question.
WHAT IS NOISE?
Noise, by definition is unwanted sound. It varies in composition in terms of frequency, intensity and duration. It is the physical effect of loud sound which is damaging.
UNDERSTANDING SOUND
The loudness of sound is related to it's intensity which is measured in decibels (dB). It is actually a ratio of intensity using zero as the softest sound the healthy human ear can hear which in wattage would be one quadrillionth of one watt of power. It is important to understand that the measurements are logarithmic. In other words a 6 dB increase in sound pressure means that the sound pressure has doubled; each 10 dB increase is ten times increase (sound pressure wise) than the lower figure. Thus 20 dB is ten times the intensity of ten decibels and 30 dB is 100 times as intense as 10 dB
The other parameter to assess sound is frequency. Frequency is measured in cycles per second, or (Hz.). In the human ear we are concerned with frequencies in the hearing range viz 20 Hz to 20,000 Hz.
Sound may be impulse (gunshot), impact (drop forge) or steady (turbine)
UNDERSTANDING THE HUMAN EAR
The ear is a remarkable hearing instrument It has the ability to hear across a wide range of frequencies. Young children generally have the best hearing. Their range as mentioned earlier is 20Hz. to 20,000 Hz. In adults, the lower limit being around 50 Hz. Intensity wise the ideal normal would be 0 db. in each frequency.
Human speech ranges from 300 to 4000 Hz.
CAN NOISE AFFECT THE EAR ?
Definitely Yes! Noise can be dangerous. If it is loud enough and lasts long enough, it can damage hearing. Noise affects the nerve endings in the inner ear which can result in
Hearing loss : The nature of hearing loss is a sensorineural hearing loss.
When hearing impairment due to noise begins, the high frequencies are often lost first which is why such people have difficulty hearing the high pitched voice of women (turning deaf to your wife?....) and of children.
Loss of frequency can also distort sound so that speech is difficult to understand even though it can be heard. Difficulty results in differentiating words that sound alike especially S, F, SH, CH, H or soft C sounds because the sound of these consonants is in a much higher frequency range than the vowels or other consonants.
Tinnitus: Noise also often results in Tinnitus. Commonly, the onset of noise induced tinnitus is gradual & intermittent. This intermittent pattern may continue for months or years and may eventually become constant.
Others: Other ear related symptoms that have been described are hyperaccusis & recruitment. Hyperaccusis is a condition in which the individual hears all sounds as abnormally &painfully
loud. Recruitment is a condition in which there is an abnormal growth in loudness of loud sounds. These phenomena can make hearing aids fitting difficult.
HOW CAN YOU TELL WHETHER A NOISE IS DANGEROUS TO THE EAR?
People differ in their sensitivity to noise.
As a general rule, noise may damage hearing if
a) you have to shout over background noise to make yourself heard at a 3 feet distance
b) the noise hurts your ears
c) It makes your ears ring
d) You are slightly deaf for a few hours after exposure to noise.
HOW LOUD IS TOO LOUD?
Most studies have established that exposure to more than 80 dB is harmful. Not only the intensity but also the length of time a person is exposed is significant. The longer one is exposed, the more damaging it is. It has been established beyond doubt that firecrackers (if they burst close enough), loud rock music and walkmans can cause noise induced hearing loss in youngsters.
The following table gives an estimate of common environmental noise :
TABLE-1
Theoretical Maximum: 194 dB
Rocket Launching pad 188 dB
Jet take off 150 dB
Gun Shot 140 dB
Pain Threshold
Air raid siren 130 dB
Rock concert / Auto horn at 3 ft 120 dB
Diesel truck/ Pneumatic drill 110 dB
Police Siren ( 100 ft ) 100 dB
Motorcycle (25 ft.) / Truck (50 ft.) 90 dB
Electric shaver 85 dB
Hearing loss risk threshold
Busy street/Garbage disposal 80dB
Restaurant/Vacuum cleaner 70dB
Air conditioner 60dB
Light auto traffic(100ft.) 50dB
Quiet Office 40dB
Whisper 30dB
Rustling leaves 20dB
Barely audible 10dB
The Occupational Safety & Health Act (OSHA) of 1970 passed regulations regarding noise in the workplace. They have been further redefined in 1981 & 1983.
The current maximum exposure for continuous noise is 90 dB for an eight hour period. For every 5 dB increase in noise, the exposure time is cut in half. 115 dB is the maximum level for continuous exposure for unprotected ears regardless of exposure time.
Permissible exposure for continuous noise based on 90db
WHAT OTHER FACTORS AFFECT SUSCEPTIBILITY TO HEARING LOSS?
High blood pressure, Heart disease: These conditions do not cause appreciable hearing loss by themselves. Studies indicate that they may increase the sensitivity of the inner ear to noise thus increasing the risk of noise induced hearing loss (NIHL).
Smoking: Smokers are at a slightly higher risk for NIHL compared to nonsmokers
Sex: Women in general demonstrate better hearing than men that continues into advanced age. This is probably due to greater exposure to noise in general for males, since in childhood there is little difference between the two sexes.
Age: Controversy still exists whether the elderly are more sensitive to hazardous noise.
WHAT ARE HEARING PROTECTORS? HOW EFFECTIVE ARE THEY?
Hearing protection devices decrease the intensity of sound that reaches the eardrum. They come in two forms:
Earplugs: they are small inserts that fit into the outer ear canal. To be effective, they must totally
block the ear with an airtight seal.
Earmuffs: These are like large headphones that fit and form an airtight seal around the entire circumference of the ear. They have an adjustable headband.
Properly fitted earplugs or earmuffs reduce noise by 15-30 dB.
What about cotton stuffed in the ear ? They are very poor protectors because they reduce noise by approximately 7 dB only.
DO HEARING PROTECTORS IMPAIR SPEECH UNDERSTANDING WHEN IN USE?
Just as sunglasses help vision in very bright light, so do hearing protectors enhance speech understanding in noisy places. Even in a quiet setting, a normal hearing person wearing hearing protectors should be able to understand a regular conversation.
Hearing protectors do slightly reduce the ability of those with damaged hearing or poor comprehension of language to understand normal conversation. All the same, it is these very people who need the hearing protectors most to prevent further inner ear damage.
DIAGNOSIS OF NOISE INDUCED HEARING LOSS: is by a detailed audiometric assessment. Pure tone Audiogram and other test like SISI (Short Increment Sensitivity Index), Speech discrimination scores are, essential to assess the total social hearing handicap.
OTHER HEALTH CONSEQUENCES OF NOISE EXPOSURE:
Noise can stress our circulatory, respiratory and digestive systems .
Continued exposure to noise may cause headaches, fatigue, and elevated blood pressure.
Noise has also been shown to interfere with children's learning and even to affect an unborn child.
