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Classification of deafness


Sound is measured in terms of its frequency, caused by the rate of vibration of the sound wave and also by its loudness or intensity. The frequency of sound is measured in cycles per second or Hertz (Hz) and the intensity of sound in decibels (dB). The decibel is a logarithmic scale. Everyday sound intensity levels in decibels are as follows, Table 1 (DES, 2000).


Table 1 Everyday sound intensity levels in decibels.

Source

Power in dB

Threshold of hearing

0

Almost silence

10

Quiet room, e.g. library

20

Whisper

30

Quiet street

40

Quiet conversation

50

Normal conversation

60

Loud conversation

70

Alarm clock at 1 metre

80

Everyone talking in a classroom

90

Radio playing very loud (1m away)

95

Pneumatic drill

110

Threshold of pain

120

Aircraft at 25 m

140

Rifle close to the ear (rupture of the eardrum)

160

 

An audiologist using an instrument called an audiometer measures hearing and the results are graphed on an audiogram, which shows what one can hear at different frequencies against different intensities. The audiogram is laid out like a piano keyboard, with low to high frequencies going from left to right, and the graph is laid out from soft sounds on the top to loud sounds on the bottom (earinfo, 1996). The vertical lines on an audiogram represent pitch or frequency. For the purpose of hearing tests the following frequencies are tested: 125, 250, 1,000, 2,000, 4,000 and 8,000 Hz. The horizontal lines represent loudness or intensity. The intensity of sound is measured in the range from 10dB - 110 dB. The point at which an individual distinguishes sound from silence is called 'the absolute threshold of hearing'. On the audiogram - X represents the absolute threshold of hearing of the left ear, O represents the absolute threshold of hearing of the right ear.

How to read an audiogram

Visit the following website http://www.earinfo.com/howread1.html

 

The following classifications of hearing impairment have emerged as useful guidelines (SERC, 1993).

Minimum audible intensity Level of Impairment

20 - 30 Decibels Mildly Hard of Hearing
30 - 60 Decibels Moderately Hard of Hearing
60 - 89 Decibels Severely Hard of Hearing
90 Decibels or over Profoundly Deaf

Typical audiograms representing these categories are shown at earinfo.com

The 'speech banana' is defined as an area on the audiogram that approximately defines where typical speech sounds occur (Hazan, 2001). Click on this link for a diagram of the speech banana (page 2) The speech Banana. The shaded area on this audiogram represents the frequencies and intensity levels of sound in normal conversational speech.

The primary energy of vowels occurs more in the lower frequency range while the primary energy of consonants is in the higher frequency range. The vowels give power to the sound. The consonants are relatively weaker sounds but give intelligibility to speech (earinfo, 1996). There is great difficulty in trying to cope with language without the important information-carrying consonants (Fraser, 1996). Here are the opening lines of two familiar nursery rhymes. In the first all the vowels have been omitted and in the second, all the consonants have been omitted.

Tw-nkl- Tw-nkl- L-ttl- St-r
H-w - W-nd-r wh-r- y-- -r-


-a-- a-- -i-- -e-- u- --e -i--
-o -e--- a -ai- o- -a-e-


The sense in the first is relatively unimpaired, with the consonants giving the clues that are necessary. The second is difficult if not impossible, unless you are given the additional clue that it involves a boy and a girl going up a hill. This example illustrates the difficulties encountered by the child with a moderate, severe or profound hearing loss in a classroom situation. Hearing aids are fitted which can make speech and other sounds more accessible.

For more information on hearing aids visit the following website: RNID A user's guide to hearing aids

References

DES (2000) Teacher's Reference Handbook - Physics, The Department of Education and Science, The Stationary Office, Dublin.

earinfo (1996) earinfo.com <http://www.earinfo.com/howread1.html> accessed on 23/03/2002.

Fraser, B. (1996) Supporting Children with Hearing Impairment in Mainstream Schools, Franklin Watts, London.

Hazan, V. (2001) Introduction to acoustics and speech preception, In Ballantyne's Deafness (Eds, Graham, J. and Martin, M.) Whurr Publishers Ltd, London.

SERC (1993) Report of the Special Education Review Committee, Government Publications, Department of Education and Science, Dublin.


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