The Human Ear and the Hearing ProcessNoise Induced Hearing Loss

Hearing ProtectionOH&S Principles

AUD202Audio and Acoustics Theory

Musical Instruments and SoundStanding Waves in String and Pipes

Envelope of soundNIHL report

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Comb filtering is caused by a wave combining with a delayed version of itself

Comb Filters

The mathematical relationship between the nulls is they are odd harmonics of the fundamental

The mathematical relationship between the nulls of a comb filter is: they are odd

harmonics of the fundamental

The peaks are even harmonics of the fundamental

ADSR (Attack, Decay, Sustain, Release)

Standing Waves in String Instruments

The standing wave constraint of string instruments is that at each end of the medium there must be a node.

String instruments produce a fundamental plus odd and even harmonics

Standing Wave Constraints

String instruments have a node at each end of the string.

Open wind instruments have an anti node at each end of the pipe.

Open closed wind instruments have a node and an anti node.

Hearing and the ear

Fields Related to Hearing

• Artistic (Music, Sound Art)• Audio Engineering (Live, Studio)• Media (TV, Radio, Film, Games etc)• Physics (Properties of Sound)• Acoustics (Architecture, Sound Engineering)• Psychoacoustics (Research, Audio Codecs,

Localisation, Perception)• Medical (Audiology)• Communication• Academic (Research, Teaching, Learning)

Absolute Threshold of Hearing

The threshold of hearing is the minimum sound level of a pure tone that an average ear with normal hearing can hear with no other sound present.

The reference for 0dB SPL is defined as the ‘threshold of hearing’ of a young undamaged ear in the ears most sensitive range - between 1kHz and 4kHz.

Threshold of Pain

The Threshold of Pain is the pressure at which sound becomes painful for a listener

120dB is generally the threshold of pain

Anatomy of the Human Ear

• The three main sections of the ear are: the Outer Ear, Middle Ear and Inner Ear

• The ear changes sound pressure waves from the outside world into nerve impulses sent to the brain

Stereocilia

Stereocilia

Figure: Dead receptor cells (n.d.)

Stereocilia

Stereocilia

MalleusHammer

IncusAnvil

StapesStirrup

Tympanic Membrane

Eardrum

Anatomy of the Human Ear

The outer ear collects sound. The sound pressure is amplified through the middle ear and passed from air into liquid. The inner ear changes soundwaves into nerve impulses. The nerve impulses travel along the auditory nerve to the brain.

It also helps us with balance and positioning

Figure: The ear (Palmer 2003)

Outer Ear

The pinna and the external auditory canal are part of the outer ear

Sound Waves travel through the external auditory canal, strike the tympanic membrane (eardrum) and causes it to vibrate

The external ear selectively boosts frequencies around 3 kHz. This makes humans most sensitive in this range and makes us prone to acoustical injury and hearing loss near this frequency

Middle Ear

Sound waves travelling through the external auditory canal will:

(1) Hit the eardrum causing the (2) hammer, anvil and stirrup bones to move(3) the stirrup bone shakes part of the cochlea

changing the pressure in the air to pressure in liquid (inside the cochlea)

Inner Ear

The cochlea converts sound pressure impulses from the outer ear into electrical impulses which are passed on to the brain via the auditory nerve.

The vestibular system is dedicated to balance

EQUal loudness curves

Equal Loudness Curves

Equal-loudness Curves are a measure of sound pressure (dB SPL), over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones.

Equal Loudness Curves

The unit of measurement for loudness levels is the phon and is arrived at by reference to equal-loudness contours

NOISE INDUCED HEARING LOSS

Typical progression of NIHL (over 40 years) (n.d.)

In Australia it is estimated that 37% of all hearing loss is a result of noise exposure (Access Economics, 2006), and The Australian Safety and Compensation Council estimates that 1 million Australian workers are potentially exposed to dangerous noise levels each year and that compensation claims in 2001/2 for occupational noise induced deafness accounted for direct costs of $30 million, which it further estimates to be less than 10% of the total cost of noise. (Australian Safety and Compensation Council, 2006).

Thurston (2012) states that NIHL came about with the invention of gunpowder and the arrival of the Industrial Revolution in which introduced new sounds of greater intensity than ever before.

According to Safe Work Australia (2004), the national standard foroccupational noise exposure is eight continuous hours at 85db at an Aweightedsound pressure level, anything above this is deemed to be a high riskcause of NIHL. Whilst not the number one work related health condition, NIHL isstill a fairly common occurrence amongst Australian workers with approximatelyfour hundred and sixty nine out of every one million employees having made acompensation claim in the period between 2007 and 2008 (Australian Bureau ofStatistics (ABS) 2011). This makes NIHL the third highest work related healthcondition in Australia and shows the extent of the problem within the workforce.

First signs of hearing loss are a notch or reduction of hearing frequencies at around three, four and six kilohertz (KHz) and are most commonly bilateral.

Tinnitus

Tinnitus is the condition of ringing in the ears when no other noise is present.

Noise Induced Hearing Loss

• Hearing loss can be described as congenital (from birth) or acquired. NIHL is acquired.

• Exposure to loud sound can cause the hair cells in our inner ear to be damaged, resulting in noise-induced hearing loss

• Hair cells are small sensory cells that convert sound energy into electrical signals that travel to the brain. Once damaged, hair cells cannot grow back.

Hearing Loss

• Conductive Hearing Loss is caused by blockage or damage in the outer and/or middle ear (i.e. middle ear infection)

• Sensorineural Hearing Loss is a result of damage to, or malfunction of the cochlea or the hearing nerve

Noise Damage Indicators

If sounds seem muffled or softer after noise exposure, your hearing has been affected by a temporary threshold shift, which warns that your hearing has been overexposed.

If you repeatedly do this without protection, the shift can become permanent and untreatable.

Incidence of hearing loss by profession (n.d.)

Hearing Protection

Earplugs

Earmuffs

Noise Isolating headphones

Noise-cancelling headphones

Preventing NIHL

Restrict exposure to less than 90dB for a maximum of eight hours per day (Palmer 2003, p. 43).

Set volume levels on devices paired with headphones (preferably over-ear type) up to 70% volume, for a maximum session duration of around 4.5 hours (Levey et al. 2013, p. 300).

Use ear protection whilst attending loud events or doing work in industrial environments (Reid 2005, p. 54).

OH&S Principles• Understand the noise level exposure time chart and

recognise when you are damaging your hearing

• Limit your exposure to loud noise, otherwise protect your ears with hearing protection

• Use your knowledge to help protect others, e.g. young children, musicians etc

Audio Engineering Society

www.aes.org

Username: jmcacademyPassword: student1

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The DecibeldBSPL, dBV, dBu, dBm

The Inverse Square LawSPL Meters

ReferencesDead receptor cells n.d., PNW Audiology, United States, viewed 15 November 2013, <http://www.pnwaudiology.com/noise-induced-hearing-loss>.

Incidence of hearing loss by profession n.d., Audicus, United States, viewed 7 November 2013, <http://www.audicus.com/blogs/hearing-aids-blog/6071738-the-shocking-data-about-noise-induced-hearing-loss-in-the-workforce>.

Levey, S, Fligor, B, Cutler, C & Harushimana, I 2013, ‘Portable music player users: cultural differences and potential dangers’, Noise & Health, vol. 15, no. 66, pp. 296-300.

Typical progression of NIHL (over 40 years) n.d., Better Hearing, United States, viewed 21 November 2013, <http://www.betterhearing.org/hearing_loss_prevention/noise_induced_hearing_loss/>.

Palmer, AR 2003, ‘How the ear works and why loud sounds cause hearing loss’, paper presented at the AES UK 18th Conference: Live Sound, April 2003.

Reid, AW 2005, ‘Notes of caution’, The Safety & Health Practitioner, vol. 23, no. 9, pp. 51-55.