Hearing to lose

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Hearing to lose
The Ear
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The Quiz

So, what is the hearing that we have got to lose?

Hearing is what we use to perceive sound.  Sound is perceived when vibrating molecules (in air, water, wood, or all manner of things) emanating from a source are processed by the ear and identified by the brain as sound.  If the brain doesn't identify something as sound, then it isn't sound.  For that reason, when a tree falls in a forest and there's no-one to hear it, it doesn't make a noise.  Sure, it makes molecules of air vibrate, but they're only vibrations, not sound.  If the vibrations are very intense, the brain doesn't perceive sound, but rather interprets it as pain.

A sound source vibrates and sets surrounding molecules vibrating.  This vibration is passed on to the neighbouring molecules, making a ripple effect, like ripples in a pond. The molecules themselves don't travel in the sound; they just pass the message on.  By analogy, think about a Mexican wave, or the final scene in Crocodile Dundee where the conversation gets passed up and down the station platform, where the message gets passed on, but the people stay still. 

As the molecules vibrate, they get closer to the next molecule, and further away from the one on the other side, so you get a ripple of some squashed together (so the pressure increases), and some pulled apart (so the pressure drops), that looks something like this:   III  I I  III  I  I  I III   I I I

For all sorts of complicated reasons that we need not go into, these changes in pressure, if you look at them over time, can be seen as a wave.

Sounds have two basic properties - Intensity and Frequency

1) Intensity is how much energy there is in a wave.  Intensity is determined by how strongly the molecules are vibrating.  The more energy there is in the wave, the higher the intensity.

Intensity is a physical thing, but it is perceived psychologically as loudness.  Usually, the greater the intensity, the louder the sound.  There are exceptions to this, though, and you can't say that intensity is loudness, because of three phenomena that occur:

fatigue - the cells in the ear get tired of passing the information on to the brain, so the next time you hear a sound of the same intensity, it doesn't seem as loud.
adaptation - as you listen to a sound of a fixed intensity, it seems to get quieter.
recruitment - this is something that occurs with many people who have hearing loss.  At low intensities they can't hear anything at all, but at higher intensities they hear just as loudly as anyone else.  This often accounts for old people who tell you not to mumble, and then when you shout say "you needn't shout, I'm not deaf, you know".  It's also important for people who wear hearing aids.  If a quiet sound is amplified to a level where it can be heard, it might be heard as a very loud sound, and this can even be painful.  Children who reject their hearing aids may be doing so because it hurts.

Loudness (the psychological perception of intensity) is measured in decibels.  (We'll come back to this).

2) Frequency is the number of waves per second.  Waves are counted in cycles from basic pressure to maximum pressure, to basic pressure, to minimum pressure and back to basic pressure.  So frequency is measured in "cycles per second" or "cps" or "Hertz" or "Hz".  1,000 cps is 1,000 Hertz or one kilohertz.

Just as intensity and loudness are linked, so frequency is psychologically related to pitch.  The higher the frequency, the higher the pitch.  Again there isn't an exact link, and the intensity and duration of the sound at a given intensity will affect the pitch that we hear it at.

Just to confuse things, wavelength is related (inversely) to frequency.

Long wavelengths have low frequencies (eg it is so long that only one cycle is completed in a second)

Short wavelengths have high frequencies (eg one cycle is very short, so you can get a lot of them into a second).

It's like a train passing when you're on a platform.  If there are long carriages, it is easy to count them, but if they are lots of short carriages, you have to count very fast to count each one.

Speech sounds, like all sounds, are made up of waves, but they aren't simple waves.  They are different waves piled on top of each other.  Some speech sounds, like vowels, are nice, regular waves (called "periodic"), and others, especially some consonants, are jagged, irregular waves (called "aperiodic".  "A" here just means "not").  Importantly, some speech sounds are louder than others and some are higher pitched than others.  Clearly, this means that some are more easily heard than others, and if there is anything to stop the brain receiving the information about the sound perfectly (eg the environment being noisy, or the ear not working properly), then some sounds may be lost or misinterpreted.

? Try and work out which of these sounds are high pitched or low pitched, and which are loud or soft.  Which might you have difficulty hearing over the telephone, or in a busy room?  z, sh, ee, th (like in "thistle"), m, or, ah, f.

Generally, vowels tend to be louder than consonants, and many consonants have a higher frequency than vowels.

What is a decibel?  It is the unit of loudness.  We have already seen that we can hear a very large range of sound, so any scale trying to show what we hear has to deal with something that might go from 1 to 100,000,000,000,000.  This is why we use a log scale, in which the changes in soft sounds are measured as big changes on the scale, and the changes in very loud sounds do not measure as large changes of the scale.

A small increase in loudness at low intensities makes a lot more decibels louder, but at high intensities, a very large increase in intensity is needed to be many more decibels.

The scale we use is arbitrary, like temperature scales. The arbitrary point of zero for the Centigrade scale is the temperature at which water freezes.  The arbitrary point of zero for decibels is the softest sound that can be heard by most eighteen year-olds with no known damage to their hearing.  Just as you can have degrees below zero, for things that are colder than ice, so you can have decibels that are below zero, for people who can hear better than your average undergraduate.  Children can often hear better than people in their late teens, so many children have minus scores when their ears are tested.

The scale is roughly

dB

What you might hear

0

Only just hear it (or maybe just can't hear it)

20-30

Whispering or a quiet voice

60

Normal speaking voice

70

Shouting

90

Heavy lorries grinding past you

110-120

Aeroplane taking off

>120

Pain

The intensity is not the only thing that influences the loudness.  Frequency also matters.  There is an optimum frequency for us to hear sounds of any intensity.

Hearing loss is usually measured in loudness, averaged over frequency. 

What is the effect of hearing loss?   You can't hear as well as if you did not have a hearing loss.  It is possible to simulate what hearing loss might sound like, so that hearing people can get some idea.

?  Hearing students should make time later to listen to the audio tape of simulated "hearing loss".

Importantly, it has some considerable effect on the acquisition of speech, which is why it is all taken so seriously.

dB loss

 

Effect on spoken language

20

mild

attention problems

40

mild

can't hear quiet conversation

50

moderate

loud voices ok, but not in crowds or noise

80

severe

loud voice is heard, but not words

>90

profound

hearing can't be used for language

 

An audiogram is made when hearing is tested.  Different audiograms have different shapes, according to the type of hearing loss that a person has.

A great deal of importance is placed on audiograms, but it must be remembered that hearing loss is only relative to everyone else, and that for many people who are born deaf, terms like "profoundly deaf" are not of great importance.

 

The course is copyright
to the Centre for Deaf Studies and the Lecturers named above
and should not be used for any other purpose than personal study.
2000

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This page was last modified January 16, 2000
jim.kyle@bris.ac.uk