THE MECHANICS OF HEARING

Transcription

1 CONTENTS The mechanics of hearing Hearing loss and the Noise at Work Regulations Loudness and the A weighting network Octave band analysis Hearing protection calculations Worked examples and self assessed questions Stan Simpson 2010

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3 THE MECHANICS OF HEARING THE EAR AND ITS PARTS The human ear can detect sound of frequencies from about 20 Hz to 20,000 Hz. It can cope with a very wide range of sound pressures, from the quietest of whispers at about 2 x 10-5 Pa to sounds that are painful to listen to at about 200Pa 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. THE EAR Hammer Bones of the Ossicles Anvil Stirrup Section of the Cochlea The Cochlea Sound waves enter the ear through the auditory canal; this provides some amplification at due to funneling of the sound towards the eardrum and resonance of the air within the canal. The movement of the eardrum is transmitted via the ossicles to another drum called the oval window. The ossicles have a transformer action of about 22:1 converting the low-pressure movement of the ear drum into a high-pressure movement of the oval window. The oval window 1

4 is the entrance to the coiled up organ of the cochlea, which is filled with saline solution and is separated into upper and lower galleries by the basilar membrane. The movement of the oval window generates a sound wave in the saline solution; this travels along the upper gallery of the spiral cochlea causing the basilar membrane to undulate. This movement activates the sensory hair cells that are embedded in the basilar membrane. The hair cells convert the movement caused by the sound wave into electrical signals. These signals are transmitted, via the auditory nerve, to the brain. THE BASILAR MEMBRANE Sensory Hair Cells in the Basila Membrane hair cell s Hairs move as the membrane flexes and create a signal Oval Window Stretched out Cochlea Basilar Membrane Separates the Cochlea into Two Galleries Movement of the membrane at various frequencies HEARING LOSS The outer ear isn t susceptible to damage due to noise, although temporary deafness may be caused by an obstruction in the ear canal (boils, wax or foreign body) CONDUCTIVE HEARING LOSS The middle ear (eardrum, ossicles and oval window) can be damaged by acoustic shock waves which, if intense enough, may burst the eardrum and also break or dislodge the small bones (ACOUSTIC TRAUMA). The middle ear is also subject to damage due to disease (scarlet fever, measles, mastoiditis, tonsillitis, catarrh, osteosclerosis, etc.). Surgical treatments are constantly improving and this type of conductive hearing loss may be repaired. Any residual deafness can be compensated for by the use of hearing aids. 2

5 SENSORY HEARING LOSS The inner ear consists of the cochlea, which contains the basilar membrane and the sensory hair cells. Some drugs may affect the hair cells (quinine and streptomycin) and result in deafness. The normal aging process also affect the hair cells as they die and are not replaced (PRESBYCUSIS). This is most noticeable at high frequencies and spreads gradually to lower frequencies. NOISE INDUCED HEARING LOSS The most common form of damage is due to prolonged exposure to loud sounds. This eventually causes the hair cells to die and, as they are part of the cortex, they are not regenerated resulting in a shift in the hearing threshold so that quiet sounds can not be heard. A pre-cursor to hearing damage is a temporary threshold shift (TTS) which is the sensation of deafness most people have experienced after being subject to loud sounds. This is caused by the nerve cells becoming fatigued making them switch off temporarily. After prolonged exposure over a number of years this becomes a permanent threshold shift (PTS) as the hair cells die. Damaged Hair Cells Unfortunately, noise induced hearing loss also introduces distortion as some frequencies are affected more than others. The ear is most sensitive at 4 khz and the hair cells that detect sound around this frequency suffer the worst damage. This is range of frequencies allows us to differentiate between the consonants c, p and t making listening to speech very difficult even if it is spoken with a loud voice. TINNITUS Sometimes the damaged hair cells can trigger off without being moved by vibration of the basilar membrane. This results in the brain receiving a signal when no sound is present, usually a ringing, buzzing or rumbling which can be heard by the affected person. When normal everyday sounds are present they tend to be more dominant than the tinnitus. When ambient noise levels are low, particularly at nighttime, the tinnitus becomes more prominent and can be distressing to the sufferer. Providing background broadband sound can help mask the tinnitus. SAQ1 a. State the two ways in which the ear can be damaged by sound and explain which part of the ear is damaged in each case. b. Describe the main features of noise induced hearing loss. Answers at the back of the booklet. 3

6 THE CONTROL OF NOISE AT WORK REGULATIONS 2005 The EU Physical Agents (Noise) Directive that was adopted in 2003 is being implemented by The Control of Noise at Work Regulations The Regulations set maximum levels of exposure for employees. These are called EXPOSURE ACTION VALUES above which the employer must firstly endeavour to reduce the noise levels at source and as well as other measures to ensure that employees are protected. (1) The lower exposure action values are (a) a daily or weekly personal noise exposure of 80 db (A-weighted); and (b) a peak sound pressure of 135 db (C-weighted). (2) The upper exposure action values are (a) a daily or weekly personal noise exposure of 85 db (A-weighted); and (b) a peak sound pressure of 137 db (C-weighted). (3) The exposure limit values are (a) a daily or weekly personal noise exposure of 87 db (A-weighted); and (b) a peak sound pressure of 140 db (C-weighted). If the UPPER EXPOSURE ACTION VALUE is exceeded then the level at the ear, including the attenuation provided by hearing protection, must not exceed the UPPER EXPOSURE ACTION LIMIT. See the booklet on Noise Indices for details on how the daily personal noise exposure is calculated. The Regulations make provision for:- (a) lower exposure action values, upper exposure action values, and exposure limit values for daily or weekly personal noise exposure and for peak sound pressure (regulation 4); (b) risk assessment (regulation 5); (c) elimination or, where elimination is not reasonably practicable, reduction of exposure to noise to as low a level as is reasonably practicable (regulation 6(1)); (d) a programme of measures, excluding the provision of personal hearing protectors, to be taken at the upper exposure action values to reduce exposure to noise to as low a level as is reasonably practicable (regulation 6(2)); (e) actions to be taken at the exposure limit values and prohibition on exceeding the exposure limit values (regulation 6(4)); (f) the provision of personal hearing protectors upon request at the lower exposure action values and compulsorily at the upper exposure action values (regulation 7(1) and (2)); (g) the designation in the workplace of Hearing Protection Zones (regulation 7(3)); (h) employers'; and employees' duties concerning the use of equipment, including personal hearing protectors, provided under the Regulations (regulation 8); (i) health surveillance (regulation 9); (j) information, instruction and training (regulation 10); 4

7 LOUDNESS Loudness is a subjective response to sound and varies from person to person. Tests also show that the human ear is not equally sensitive at all frequencies. This can be demonstrated by asking people to adjust the sound pressure level of pure tones of different frequencies so that they all sound equally as loud. The resulting equal loudness contours show that we would judge a pure tone of 60 db at 31.5 Hz to be equally as loud as a pure tone of 20 db at 250 Hz or the same as 10 db at 1 khz. Loudness contours are measured in phons; the units are in db based on the sound pressure level of the contour as it passes through the frequency of 1 khz. That is the phon level for a particular frequency is equally as loud as the same sound pressure level in db at 1kHz. As the loudness of sound increases the ear becomes more linear, that is, the variation in loudness with frequency becomes less pronounced and at 100 phons the equal loudness contour is relatively flat over most of the frequency range. This is the reason why music sounds different when played loudly and why, in recording studios, sound engineers who mix music can suffer hearing damage due to listening to music at very high levels in order to obtain a flat response from their ears. EQUAL LOUDNESS CONTOURS A Weighting When measuring environmental noise we require the sound level meter to give a reading (in decibels) which reflects the loudness perceived by the human ear. To account for this all sound level meters have weighting filters that correct the levels at different frequencies to make them all equal. Originally a number of weighting filters A, B and C were used depending on the loudness of the sound being measured but gradually the B and C weightings fallen into disuse. The A weighting is approximately equal to the 40 phon contour which is accepted as the level of most environmental noise.. Since it is the sensory hair cells that receive quiet sounds that are the first to be damaged by noise the A weighting is also deemed suitable for assessing workplace noise. 5

8 SAQ2 a. Explain why the A weighing is used for environmental noise assessments. b. Find the A weighting correction for the frequencies 500 Hz, 2 khz and 8 khz Answers at the back of the booklet. Octave Bands Band pass filters are used in frequency analysis. These are filters that only allow sound in a band of frequencies to pass through the meter, so that the sound pressure level of the band can be measured. The most common are OCTAVE BANDS although analysis of narrower bands 1/3, 1/6, 1/12 octave bands may be necessary for detailed investigations. An OCTAVE is a doubling of frequency; the accepted octave bands are given below together with the A weighting correction for the band. Band minimum Centre Band upper A frequency frequency frequency Correction 22 Hz 31.5 Hz 44 Hz -39 db 44 Hz 63 Hz 88 Hz -26 db 88 Hz 125 Hz 176 Hz -16 db 176 Hz 250 Hz 353 Hz -8.5 db (sometimes given as 9 db) 353 Hz 500 Hz 707 Hz -3 db 707 Hz 1000 Hz 1414 Hz 0 db 1414 Hz 2000 Hz 2852 Hz +1 db 2852 Hz 4000 Hz 5650 Hz +1 db 5650 Hz 8000 Hz Hz -1 db Hz Hz Hz -6.5 db Worked Example (If you have a calculator where the whole equation has to be input first) An octave band measurement of an extract fan gives the following sound pressure levels, Octave Band Centre frequency 63 Hz 125 Hz 250 Hz 500 Hz 1 Hz 2 khz 4 khz 8 khz Sound Pressure Level 67 db 63 db 59 db 53 db 48 db 45 db 43 db 40 db Question: Find the overall sound pressure level A weighted sound pressure level. Answer: Since the measured levels at in db we must first apply the A weighting correction to each octave band and then we can add them (logarithmically) together to find the overall A weighted sound pressure level. Applying the A weighting correction we get. Octave Band Centre frequency 63 Hz 125 Hz 250 Hz 500 Hz Sound Pressure Level 1 khz 2 khz 4 khz 8 khz 67 db 63 db 59 db 53 db 48 db 45 db 43 db 40 db A weighting correction -26 db -16 db -8.5 db -3 db 0 db +1 db +1 db -1 db A weighted octave band levels 41 db 47 db 50.5 db 50 db 48 db 46 db 44 db 39 db These corrected bands can now be added together. If you have an equation-entering calculator you will have to input the following; 10 x log(10 x (41 10)+10 x (47 10)+10 x ( )+10 x (50 10) +10 x (48 10)+10 x (46 10) +10 x (44 10)+ 0 x (39 10)) --- make sure you don t miss out any brackets! 6

9 now press = to get 56.1 db (to 1 decimal place) Worked Example (for calculators that process the calculation as it is entered) An octave band measurement of an extract fan gives the following sound pressure levels, Octave Band Centre frequency 63 Hz 125 Hz 250 Hz 500 Hz 1 khz 2 khz 4 khz 8 khz Sound Pressure Level 67 db 63 db 59 db 53 db 48 db 45 db 43 db 40 db Find the overall sound pressure level A weighted sound pressure level. Since the measured levels at in db we must first apply the A weighting correction to each octave band and then we can add them (logarithmically) together to find the overall A weighted sound pressure level. Applying the A weighting correction we get. Octave Band Centre frequency 63 Hz 125 Hz 250 Hz 500 Hz 1 khz 2 khz 4 khz 8 khz Sound Pressure Level 67 db 63 db 59 db 53 db 48 db 45 db 43 db 40 db A weighting correction -26 db -16 db -8.5 db -3 db 0 db +1 db +1 db -1 db A weighted octave band levels 41 db 47 db 50.5 db 50 db 48 db 46 db 44 db 39 db These corrected bands can now be added together using the process for adding decibels covered earlier. The answer is 56.1 db (to 1 decimal place) 7

10 HEARING PROTECTION CRUNCH! CLATTER!!! CRASH! BANG! Splitting the sound into octave bands is the only way in which to predict the effectiveness of any noise control. The performance of noise enclosures, barriers etc. and also hearing protection depends on the frequency of the noise that needs to be controlled. Ear muffs and plugs are rated by a method given in British Standard 5108, Sound Attenuation of Hearing Protectors, this requires the manufacturers to supply the attenuation, provided by the muff or plug, for each octave band. The overall attenuation to the A weighted sound pressure level at the ear is calculated as follows. Starting with the octave band sound pressure levels measured at the operator s ear position we have. Octave band centre frequencies 63 Hz 125 Hz 250 Hz 500 Hz 1 khz 2 khz 4 khz 8 khz Measured by Noise level a competent at the Ear 94 db 92 db 96 db 99 db 94 db 86 db 78 db 72 db person Assumed Protection of the Ear Muff 12 db 16 db 18 db 26 db 34 db 35 db 32 db 28 db Provided by manufacturer Level at the Ear with Muffs on. 82 db 76 db 78 db 73 db 60 db 51 db 46 db 44 db This now needs to be A weighted so that it represents the response of the ear. Level at the Ear with Muffs on. 82 db 76 db 78 db 73 db 60 db 51 db 46 db 44 db A Weighting Correction -26 db -16 db -8.5 db -3 db 0 db +1 db +1 db -1 db The corrected level at the Ear 54 db 60 db 69.5 db 70 db 60 db 52 db 47 db 43 db These corrected octave band levels can now be added (correctly by decibel addition) to give the assumed protected level at the ear which is 73.3 db(a). This must adjust this for the real world since the attenuation data was measured in ideal conditions. We add 4 db to the result to account for this, so the assumed protected level is 77.3 dba. 8

11 SAQ3 The octave band levels of a printing press measured at the operator s ear are as follows. Octave Band 63 Hz 125 Hz 250 Hz 500 Hz 1 khz 2 khz 4 khz 8 khz Sound Pressure Level 88 db 95 db 97 db 102 db 104 db 96 db 90 db 80 db a. Find the overall A weighted level at the operator s ear without hearing protection. b. Find the overall A weighted level at the operator s ear if the following ear muffs are worn. Octave Band 63 Hz 125 Hz 250 Hz 500 Hz 1 khz 2 khz 4 khz 8 khz Ear Muff Assumed Protection 14 db 18 db 24 db 28 db 34 db 34 db 30 db 29 db Answers at the back of the booklet. 9

12 SAQ1 - Solution a. The ear may be damaged by ACOUSTIC TRAUMA when a large amplitude sound pressure wave ruptures the ear drum and/or breaks or dislocates the small bones of the ossicles. Very severe pressures can also rupture the basilar membrane in the cochlea. Prolonged exposure to loud sounds produced NOISE INDUCED HEARING LOSS when the sensory hair cells in the basilar membrane are damaged. b. The characteristic 4 khz dip in the ears sensitivity after prolonged exposure to loud sounds is the most noticeable feature of noise induced hearing loss. This distorts speech as this frequency is important in differentiating between the consonants t, c, p, and s. In addition the damaged nerve cells may trigger off sending spurious signals to the brain which translates them into the humming, buzzing or whining sounds known as TINNITUS. Sufferers of noise induced hearing loss can become withdrawn and depressed. SAQ2 - Solution a. The A weighting correction adjusts the readings taken by a sound level meter so that it mimics the response of the human ear. As the ear is loss sensitive at low and high frequencies the meter readings are reduced so that sounds at all frequencies are weighted equally. The correction is based on the 40 phon equal loudness contour. b. From the chart on page 12 or the table on page 13 you will find that; 500 Hz correction -3 db 2 khz correction +1 db 8 khz correction -1 db SAQ3 - Solution a. The A weighted level is found from correcting the octave bands and then adding them. Noise level at the Ear 88 db 95 db 97 db 102 db 104 db 96 db 90 db 80 db A Weighting Correction -26 db -16 db -8.5 db 3 db 0 db +1 db +1 db -1 db The corrected level at the Ear is 62 db 79 db 88.5 db 99 db 104 db 97 db 91 db 79 db The A weighted level at the ear without ear muffs is found by adding the bands together --- see the answer to SAQ4 a. for the method but use the levels given above. You should get 106 db(a) notice that the (A) means that the level is A weighted. b. To find the level at the ear with ear muffs we need to subtract the assumed protection from each band before adding. We get; Assumed Protection of Ear Muff 14 db 18 db 24 db 28 db 34 db 34 db 30 db 29 db Level at the Ear with Muffs on. 48 db 61 db 64.5 db 71 db 70 db 63 db 61 db 50 db Adding these gives the assumed protected level at the ear which is db(a) or 75 db(a). Plus, of course, the real world correction of +4 db, giving 79 dba. 10