Cyberphysics - loudness perception

The Ear: Loudness Perception

See also ear structure and dBA/dB scales

Loudness Perception

You need to know how to draw this graph! (and label axes and mark on values!)

To be perceived, sounds must exert a shearing force on the stereocilia of the hair cells lining the basilar membrane of the cochlea (so that they can then send electrical signals through to the brain). Different frequencies of sound produce a different response and therefore there is a range of sound intensities necessary for the threshold of hearing over the audible hearing range.

Note that the axes are BOTH logrithmic as the range is so great!

Lowest intensity detection comes at about 3 kHz
Range is 20 to 20kHz

The graph below shows how loud in dB sounds have to be at various frequency to be perceived at a particular 'loudness' by us.

How 'loud' the sound is depends on the amplitude of vibration of the basilliar membrane and hence the hairs that trigger electrical signal transmission in the cochlea and the number of them stimulated (Some are only stimulated above a certain threshold)

The 'phon': In acoustics, a unit of subjective loudness level equal to the sound pressure level in dB compared to that of an equally loud standard sound. Note: The accepted standard is a 1-kHz pure sine-wave tone or narrowband noise centered at 1 kHz.

You need to know how to draw this graph! (and label axes and mark on values!)

When excessive, this force can lead to cellular metabolic overload (compare with 'flooding' optical receptor with bright light!), and this can produce cell damage and cell death. Noise-induced hearing loss therefore represents excessive "wear and tear" on the delicate inner ear structures.

Example: A teenage girl was seen for a school physical examination.

Screening audiometry performed in the office revealed a 30-dB (mild) elevation of hearing thresholds at 4,000 Hz. A confirmatory audiogram taken by an audiologist showed a sensorineural loss in a "notch" pattern at 4,000 Hz (Figure 1). In response to questioning, the girl reported spending several hours a day listening to music through headphones. The previous night, she had spent several hours at a rock concert without wearing hearing protection. Afterward, she noticed that her ears were ringing and "felt like there was cotton in them." Several days later, her hearing had returned to normal.

This is an example of a person who has experienced a "temporary threshold shift." Temporary threshold shifts are common in persons exposed to excessive noise, and they represent transient hair cell dysfunction. Although apparent complete recovery from a given episode can occur, repeated episodes of such shifts occurring after noise exposures give way to permanent threshold shifts because hair cells in the cochlea are progressively lost.

Summary table for Differential Diagnosis of Sensorineural Hearing Loss

Condition	Causative agents	Features	Typical audiogram pattern	Mrs J's Pointers!
Age-related hearing loss (presbycusis)	Probably caused by the loss of elasticity of the ossicular chain (the malleus, incus and stapes) through arthritic type changes (otosclerosis) Debris or cerumen impaction in the external auditory meatus can cause a problem. This commonly exacerbates Presbycusis in older people but can be treated by having ears syringed.	Gradual onset	Bilateral high-frequency loss, "downsloping"	This you need to know - loss of high frequency perception (steeper sides to the audiogram pattern curve output) - for both ears (bilateral - on both sides!)
Congenital hearing loss	Genetic factors, prenatal infections/toxic exposures, birth trauma	Present at birth	Variable	Not on syllabus!
Noise-induced hearing loss	Noise exposure	Gradual onset, tinnitus common	Bilateral high-frequency loss, "notch" at 3,000 to 4,000 Hz	Loud music makes you 'age' as far as your hearing goes! The 'notch' at 3/4KHz relates to damage to the ear. It would show up as more dB needed to perceive at that pitch!
Sudden hearing loss	Viral infections, trauma, vascular, drugs	Sudden onset, otologic emergency	Variable; may be unilateral or bilateral	Not on syllabus!
Neoplastic	Acoustic neuroma (vestibular schwannoma), other tumors Usually gradual onset; tinnitus may be present Unilateral loss			Not on syllabus!
Meniere's disease	Unknown (may be endolymphatichydrops)	Fluctuating, progressive hearing loss, tinnitus, vertigo	Low-frequency loss, "upsloping" or flat	Not on syllabus!
Ototoxicity	Chemotherapeutic agents (cisplatin [Platinol], nitrogen mustard), aminoglycosides, furosemide (Lasix), salicylates, quinine	May be accompanied by tinnitus, vertigo, nystagmus	Usually bilateral, symmetric, high frequency (may be very high frequency)	Not on syllabus!
Other	Infections: herpes, meningitis, mumps, syphilis, tuberculosis Systemic disease: vasculitis, renal failure, head injury Genetic factors: idiopathic	Often gradual onset, associated disease	Variable	Not on syllabus!