1. Mechanics of the Ear Loudness and other Features Hearing Risk
THE EAR AND HEARING
Mechanics of the Ear
Loudness and other Features
Hearing Risk

2. THE EAR AND ITS PARTS The ear consists of three parts.
Outer ear, which funnels the sound towards, and includes the eardrum.
Middle ear, where the vibration of the eardrum is transmitted to the inner ear by three small bones called the ossicles.
Inner ear, consists of the fluid filled organ called the cochlea. This is where sound is converted from mechanical vibration into an electrical signal for transmission via the auditory nerve, to the brain.

3. THE MECHANISM OF THE EAR
testing

4. Outer Ear
Channels sound towards the ear drum, provides some amplification and helps in determining the location
of a sound.
This is achieved by:
• Inter-aural time delays
• Inter-aural intensity differences
• Spectral Cues
- sound is changed by reflections
from the outer ear (pinna) and
upper torso (shoulders)

5. Localisation Simple mono & stereo
Use of time delay to improve stereo effect
Use of intensity difference to improve effect
Both together

6. Middle Ear(The Ossicles) (Hammer, Anvil and Stirrup)
pivots
Movement
transferred
to the inner-ear
via the Oval window
Movement of eardrum
Mechanical advantage about 22:1
Aural Reflex -- prevents damage

7. The Inner Ear

8. The Cochlea Oval window The Cochlea --- Uncurled
Cochlea is filled with liquid (saline solution)
Oval window
The Cochlea --- Uncurled
Round window
The Ossicles vibrate the Oval window and a wave travels along the upper gallery, through a small hole at the end and back along the lower gallery ---
The Round window prevents reflections back along the lower gallery

9. Section of the Cochlea
Basilar
Membrane

10. Basilar Membrane
The membrane between the upper and lower galleries flexes as the sound wave pass.
Different areas along the membrane resonate at various frequencies.
The hair cells are stretched and send signals to the brain.
Quiet sounds move floppy hair cells
Loud sounds move stiff hair cells
Movement of the membrane at various frequencies

11. Perception of Echoes
When a sound enters the inner ear and the basilar membrane is set into motion and signals from the hair cells are sent to the brain.
If delayed sound reflected from room surfaces enters the inner ear soon after the first sound it is integrated with the first sound.
If the delay is too long the basilar membrane is reactivated and this is heard as a separate sound (an echo)

12. Section of the Basilar Membrane showing the hair cells

14. Animations http://www.youtube.com/watch?v=s48QZL37jW8&feature=related

15. HEARING LOSS Two main types are:- CONDUCTIVE HEARING LOSS
SENSORY HEARING LOSS

16. Causes of Conductive Hearing Loss
Wax or other obstruction
Acoustic Trauma (Acoustic Reflex Action)
Ruptured eardrum
Dislodged or broken ossicle bones
Disease
Scarlet fever, measles, mastoiditis, tonsillitis, catarrh, ostosclerosis

17. Causes of Sensory Hearing loss
Damage to the sensory hair cells
Drugs such as quinine and streptomycin
Age related (Presbycusis)
Affects higher frequencies
Why the ‘mosquito’ works
Noise Induced

18. Noise Induced Hearing Loss
Temporary Threshold Shift (TTS)
Hair cells switch off -- worst at 4 kHz

19. Permanent Threshold Shift (PTS)
Eventually after prolonged exposure the damage becomes permanent - 4kHz “dip”
90 dBA
100 dBA

20. PTS is often accompanied by Tinnitus
Ringing in the ears due to the nerve cells triggering off randomly.
Can be very disturbing to the sufferer, particularly at night when the background (masking) noise levels are low
Some relief can be had by providing ‘background’ broadband sound (white noise) which can help to mask the tinnitus.

21. The Control of Noise at Work Regulations
Introduced in 1990 and amended in 2005 to protect people from suffering hearing damage due to noise.
Based on the LEP,d (daily dose) --- COVERED IN TUTORIALS
Action values
80dBA for 8 hours &/or peak of 135dB – voluntary
85dBA for 8 hours &/or peak of 137dB – mandatory
Limit values (at the ear with hearing protection)
87dBA for 8 hours & peak of 140dB
Duties of employers and employees
Based on assessing the risk and reducing the risk by engineering methods (not simply by issuing ear muffs)

22. LOUDNESS Sound level is not the same as loudness
loudness is subjective
Research by Munson and Fletcher in 1933
used tones to establish the threshold of hearing

23. Loudness .v. frequency 1 kHz tone 500 Hz tone 100 Hz tone 4kHz tone
After the threshold of hearing had been determined they then developed loudness contours by comparing different tones to a 1 kHz tone.
1 kHz tone
500 Hz tone
100 Hz tone
4kHz tone

24. Equal Loudness Contours (Munson & Fletcher)

25. Research Conclusions Ear less sensitive at low and high frequencies
Ear becomes much more linear at higher sound levels
This is why music sounds different (better) when it is played loudly.
A dB change is perceived as being twice as loud (-10 dB change is half as loud)
Need filters to make sound level meters mimic the sensitivity of the human ear at different frequencies.
Three filters (A, B, C) developed
intended for use with low / medium / loud sounds
Only the “A” filter is still in use.

26. “A” weighting Curve Used in SLM’s to mimic the human ear

27. Octave Band Analysis
The frequency spectrum can be split into narrow frequency bands.
1/1 octave
1/3 octave
Used in specifying
acoustic conditions
Used in noise control
Used in hearing protection calculations

28. ACOUSTIC DESIGN CRITERIA
Noise Rating, NR is used in the UK to specify the acoustic criteria inside offices
Based on research in the 1960’s
on the affect of road traffic noise
on the occupants of dwelling houses.
The shape of these curves is similar to the “A” weighting but inversed
For a particular NR the levels in each octave band must be below the specified curve
Other rating curves are Noise Criterion (NC) and Room Criterion (RC)

29. Hearing Protection Assessments
Based on octave band analysis of the noise at the ear position.
This is corrected for the sensitivity of the ear (“A” weighted)
The performance (assumed protection) of the selected hearing protection (muffs, plugs) is then subtracted from each band.
Finally, all the bands are added (logarithmically) to find the overall “A” weighted protected level at the ear.
+4 dB is added to account for the difference between the measured performance of hearing protection and that expected in “the real world”

30. Ear Muff Performance Testing
Hearing threshold is measured using octave band noise from 63 Hz to 8 kHz
Firstly, without ear muffs
and then again
with ear muffs.
The difference is the attenuation provided
A group of subjects are tested to obtain the mean & SD

31. Laboratory tests of hearing protection
Assumed Protection = mean – SD
Assumed protection = 12 – 3 = 9 dB
Attenuation (dB) 12
number of
people
tested
SD=3 dB
mean
500 Hz octave band

32. Typical Octave Band Analysis used in Calculating the Effect of Ear-Muffs
We also need to add a ‘real world’ correction to account for the difference between manufacturers data (measured in perfect conditions) of +4 dB so the result is 77.3 dB(A)

33. Refs / links Physiology of the ear.