HUMAN PERFORMANCES AND LIMITATIONS

1. THE BASIC FUNCTIONS OF THE EAR:

The ear performs two quite different functions.

It is used to detect sounds by receiving vibrations in the air, and

secondly, it is responsible for balance and sensing acceleration.

Of these two, the hearing aspect is more pertinent to the maintenance worker, and thus it is necessary to have a basic appreciation of how the ear works.

1.1. Description of the ear:

The ear has three divisions: outer ear, middle ear and inner ear. These act to receive vibrations from the air and turn these signals into nerve impulses that the brain can recognise as sounds.

• Outer ear:

  The outer part of the ear directs sounds down the auditory canal, and on to the eardrum (Tympanic membrane). The sound waves will cause the eardrum to vibrate.

• Middle ear:

  When the sound waves hit your eardrum, it vibrates and the brain interprets these vibrations as sound (Pitch and Intensity).

  Beyond the eardrum is the middle ear which transmits vibrations from the eardrum by way of three small bones known as the ossicles, to the fluid of the inner ear.

  The middle ear is usually filled with air which is refreshed by way of the eustachian tube which connects this part of the ear with the back of the nose and mouth. However, this tube can allow mucus to travel to the middle ear which can build up, interfering with normal hearing.

• Inner ear:

  Unlike the middle ear, the inner ear is filled with fluid.

  The last of the ossicles in the middle ear is connected to the cochlea.

  This contains a fine membrane (the basilar membrane) covered in hair-like cells which are sensitive to movement in the fluid. Any vibrations they detect cause neural impulses to be transmitted to the brain via the auditory nerve.

The amount of vibration detected in the cochlea depends on the volume and pitch of the original sound

→ Cross-section of the ear:

1.2. Performances and limitations of the ear:

The performance of the ear is associated with the range of sounds that can be heard in terms of:

• Pitch (sensation on frequency),

• Volume (sensation on intensity).

2. PITCH OF SOUND:

• Audible frequencies:

  For humans, sound is audible approximately from 20 Hz to 20 000 Hz.

  The sensation of frequencies is commonly referred to as the pitch of a sound. A high pitch sound corresponds to a high frequency sound wave and a low pitch sound corresponds to a low frequency sound wave.

• Pure sound:

  • Period:

    Although sound is a compression wave, each period of the two pure sounds can be represented by a sine wave. The period of the sound (a) is twice than the sound (b), so the pitch of the sound (b) is bigger (one octave) than the sound (a).

• Pure sound:

  • Frequency:

    By definition, frequency is the inverse of the period. The frequency of the sound (b) is twice than the sound (a), so the pitch of the sound (b) is bigger (one octave) than the sound (a).

• Quality (musical) sound:

  You have probably heard the word harmonics, or harmony before, though probably in a musical context rather than a physics one. Harmonics are indeed important in music - and we shall find out why.

  Sounds may be generally characterized by pitch, loudness, and quality. Sound "quality" or "timbre" describes those characteristics of sound which allow the ear to distinguish sounds which have the same pitch and loudness.

  Timbre is then a general term for the distinguishable characteristics of a tone.

• Noise:Noise is a random signal with a continue power spectral density (or power density). In other words, the signal contains power within a fixed bandwidth at any center frequency. The relationship between Intensity and frequency is called spectrum of noise.

Remark:

The main characteristics of a noise are its pitch and its intensity. The intensity is the sum of power density on the total frequencies.

3. POWER AND INTENSITY:

3.1. Propagation of the sound:

At normal atmospheric pressure (P = 1 atm), in the air, the speed of the sound is approximately: V = 344 m/s.

This speed corresponds at Mach = 1.

3.4. Measurement of intensity of the sound:

Since the range of intensities which the human ear can detect is very large: from 10 -12 W/m2 (threshold of hearing) to 10 +4 W/m2 (Perforation of eardrum).

The scale which is frequently used by physicists to measure intensity is a scale based on multiples of 10.

This type of scale is sometimes referred to as a logarithmic scale. The scale for measuring intensity is the decibel scale or the noise level.

Remark:

When the noise intensity doubles, the sound level expressed in decibels increases by 3 dB.

3.5. Typical sound levels for various activities:

3.6. Equivalent level – Noise dose:

The combination of duration and intensity of noise can be described as noise dose.

• Remember: When the sound intensity double, his noise level increases by 3.

• Now note that: When the sound duration double, his noise level increases by 3.

• Conclusion: the noise dose is the product of the intensity and the duration.

Exposure to any sound over 85 dB constitutes a noise dose, and can be measured over the day as an 8 hour Time Weighted Average sound level (TWA).

Permanent hearing loss may occur if the TWA is above the recommended maximum.

It is normally accepted that a TWA noise level exceeding 85 dB for 8 hours is hazardous and potentially damaging to the inner ear. Exposure to noise in excess of 115 decibels without ear protection, even for a short duration, is not recommended.

• → Equivalent levels:

3.7. Impact of noise on performance:

Noise can have various negative effects in the workplace.

It can:

• be annoying (e.g. sudden sounds, constant loud sound, etc.),

• interfere with verbal communication between individuals in the workplace,

• cause accidents by masking warning signals or messages,

• be fatiguing and affect concentration, decision making, etc,

• damage workers' hearing (either temporarily or permanently).

Notes:

• Intermittent and sudden noise are generally considered to be more disruptive than continuous noise at the same level.

• In addition, high frequency noise generally has a more adverse affect on performance than lower frequency.

• Noise tends to increase errors and variability, rather than directly affect work rate.

3.8. Hearing impairment:

Hearing loss can result from exposure to even relatively short duration noise. The degree of impairment is influenced mainly by the intensity of the noise. Such damage is known as Noise Induced Hearing Loss (NIHL).

The hearing loss can be temporary - lasting from a few seconds to a few days - or permanent.

• Temporary hearing loss may be caused by relatively short exposure to very loud sound, as the hair-like cells on the basilar membrane take time to ‘recover'.

• With additional exposure, the amount or recovery gradually decreases and hearing loss becomes permanent.

• Thus, regular exposure to high levels of noise over a long period may permanently damage the hair like cells in the cochlea, leading to irreversible hearing impairment.

• → Irreversible damage due to a regular exposure:

4. HEARING PROTECTION:

Hearing protection is available, to a certain extent, by using ear plugs or ear defenders.

Noise levels can be reduced by up to 20 decibels using ear plugs and 40 decibels using ear muffs.

However, using ear protection will tend to adversely interfere with verbal communication. Despite this, it must be used consistently and as instructed to be effective.

• It is very important that the aircraft maintenance worker understands the limited ability of the ears to protect themselves from damage due to excessive noise. Even though workers should be given appropriate hearing protection and trained in its use, it is up to individuals to ensure that they actually put this to good use. It is a misconception that the ears get used to constant noise: if this noise is too loud, it will damage the ears.

• Reduce noise at source:

  It is good practice to reduce noise levels at source, or move noise away from workers. Often this is not a practical option in the aviation maintenance environment. Hearing protection should always be used for noise, of any duration, above 115 dB. This means that the aviation maintenance worker will almost always need to use some form of hearing protection when in reasonably close proximity (about 200 - 300m) to aircraft whose engines are running.

• Presbycusis:

  Hearing deteriorates naturally as one grows older. This is known as presbycusis. This affects ability to hear high pitch sounds first, and may occur gradually from the 30's onwards. When this natural decline is exacerbated by Noise Induced Hearing Loss, it can obviously occur rather sooner.