Hearing experiments

Introduction to deafness

Deafness - either total or partial - is a significant health issue today. A major source of this problem is due to loud sound. The likelihood of going deaf due to loud sound is increasing because of occupational and recreational sources of loud sound.

Now you may think deafness isn't that problematic. One of the major problems with deafness is social isolation. You stop attending parties or going out because understanding people, especially in even moderately noisy environments, becomes a problem. Something you have to strain at. And that's demanding – it makes you tired even after only a few minutes. There are also all the other problems – you mishear what the service attendant says you owe, and find you've underpaid – oh the embarrassment of it. Or even worse, when you think someone said something rather different from what they actually did and you respond in an inappropriate way...

Let me remind you of the statement of Helen Keller, who was both deaf and blind:

"Blindness separates people from things; deafness separates people from people."

So look after your hearing – it can only get worse from birth.

Ageing reduces our hearing sensitivity, especially at higher frequencies

As we get older our hearing deteriorates. This occurs even if we have led a blameless life of pure sanctity, never listening to anything but the melodious and mellifluous songs of Perry Como, Andy Williams and Richard Tauber (who??!! alright alright, they were big in my time and songs have never been the same since the 1850s I tell you….) and never taken any drugs stronger than the chamomile tea made by great aunt Maureen.

Most – if not all - of this hearing loss is due to damage at the cochlea or inner ear. As we noted earlier, the cochlea is responsible for converting the waves of pressure that make up a sound wave into biological energy. This is done by cells known as hair cells, and then the information is signalled to nerve cells that carry the information to the brain which interprets what we have heard. Damage to the hair cells or to the nerve cells will therefore result in a reduction in our ability to hear sounds and if the damage is serious, then will result in permanent deafness.

Effect of ageing on hearing sensitivity

In this simulation you will explore how ageing reduces hearing sensitivity. Recall hearing sensitivity is plotted in an audiogram which plots the softest hearing level you can hear (the threshold) for each test frequency. By convention, when testing human hearing we generally test only at frequencies from 250 Hz – 8000 Hz (these are the frequencies where our hearing is best, and these are the base frequencies that make up the complex mixtures of frequencies that make up speech). The simulation allows you to get data for the hearing sensitivity across this range of frequencies, as a function of age, in males and females.

Note two things:

1. For both males and females, the greatest hearing losses with age, occur at the high frequencies. These higher frequencies are particularly important for understanding speech and so the worsening thresholds with age mean greater difficulty in understanding speech.
2. Notice that the hearing of females does deteriorate like that of males but never as badly as males. This isn't likely due to any protective effects of female hormones (or at least any that have been established yet) on hearing, but more likely related to the fact that for the era, more males than females were likely to work in noisy occupations.

Click on the ages in the legend to view them on the graph.

Test the impact of hearing loss

Finally here you will be able to examine what the effect of the age-related hearing loss is in practical terms, in a series of audio files where we have tried to simulate the effects of the hearing levels on (a) hearing and understanding speech, and (b) hearing and understanding speech when there is noise in the background.

Essentially all we did was filter the different types of sounds through filters set to the hearing levels for those ages. For the speech-in-noise signal, we played the speech through filters set in this way, and for the speech-in-noise signal we then mixed the speech with a noise background. The last is not a perfect simulation since we should also have filtered the noise background, but it'll give you an idea of the effect of the hearing loss on speech intelligibility under everyday conditions (which are not quiet conditions, unless you're a Trappist monk living in a Himalayan monastery blanketed by a vow of silence).