Acoustics and Psychoacoustics -
How your ears work
The Human Ear consists of three parts, which are the outer ear, the middle ear and the inner ear:
(i) The outer ear – consists of the pinna (auricle), ear canal and eardrum
(ii) The middle ear – air filled space that has 3 small bones, the malleus, incus and stapes but collectively called the ossicles.
(iii) The inner ear – is for hearing and balance, consisting of the Cochlea, a snail shell looking coil which is filled with special fluids and has thousands of sensory ‘hair cells’. The cochlea connects to the central hearing system via the auditory nerve.
The range and scope of the human ear
A description of the human hearing range in terms of frequency and amplitude (e.g. loudness curves, frequency response)
The human hearing (frequency) range is from 20hz to 20khz at birth however this reduces as we get older. Our main hearing range is in the range of the human voice. The deterioration of our hearing range is due to the damage of the sensitive hairs which detect the higher frequencies, also due to the fact that these frequencies are much more difficult to detect as they are naturally much quieter than the lower frequencies.
Humans, while not able to echolocate, can use the minute differences in time of the arrival of the vibrations to the different ears to detect the direction of sounds.
In terms of dynamics, the range between the softest and loudest sounds the ear can detect is 120 decibels.
The decibel is the log of the ratio of two quantities multiplied by 10. This means that the ear can hear sounds whose strength lies anywhere within a range of over 12 orders of magnitude. The ear is sensitive enough that it can detect sounds which are so weak that the air molecules move less than the diameter of an atom! But yet it is also able to handle much louder sounds without overloading and saturating ("maxing out") which would cause undesirable distortion. This is accomplished by means of an automatic gain control system (AGC) which attenuates the response to louder sounds.
The graph above shows the difference in amplitude required at various frequencies for the ear to detect the sound. As you can see the higher frequencies have to be much louder.
Source(iv):
Localisation focuses mostly on the dominant primary sounds and not so accurate when combined with background sounds and reverb or echoes.
The ear uses cues from the horizontal plane (azimuth) and the time differences of the sounds arriving to the two ears (binaural). The primary cues in the vertical plane are monaural and generally modified by the torso, head and outer ear.
Source(v): http://www.cochlea.eu/en/sound/psychoacoustics/localisation
Detection/Perception
Source (Vi): Figure from: van der Willigen. 2008. Auditory Perception. Nijmegen, Netherlands: Radboud University.
1) Sound wave are detected by the Tympanic membrane
2) Tympanic membrane vibrates the auditory ossicles (Malleus, Incus and Stapes)
3) Vibrations of the oval window causes pressure waves in the vestibular duct
4) Pressure waves travel to the round window of the tympanic duct, distorting the Basilar membrane
5) The vibrating basilar membrane vibrates the hair cells against the tectorial membrane
6) This sensory information is transmitted to the nervous system via the cochlea branch of the cranial nerve VIII.
Hearing damage types and causes & prevention methods
Excessive exposure to noises that are too loud, too close, or experienced for too long can cause irreversible and permanent damage to hearing. Noise-induced hearing loss, or NIHL, and tinnitus, or ringing in the ears, can occur with sudden exposure to “impulse” noise or prolonged or repeated exposure to noise levels at or above the safe threshold level of about 85 decibels.
Source: (Viii)
Age-Related Hearing Impairment (ARHI)
ARHI is the most prevalent form of hearing loss in humans. The main characteristics are reduced sensitivity and the ability to comprehend speech where there is background noise, slower processing of sounds and reduced localisation skills.
Ear Loss Prevention
To reduce the risk of noise induced hearing loss, wearing earplugs is recommended when you are being exposed to loud noise for an extended period of time in places such as live concerts and loud clubs or workplaces with loud machinery. Noise cancelling ear plugs include Medium, High and Max protection filter sets, making them ideal work earplugs for use with power tools, heavy machinery or just for mowing the lawn.
Bibliography
(v): https://www.cochlea.eu/en/sound/psychoacoustics/localisation
(Vi) Figure from: van der Willigen. 2008. Auditory Perception. Nijmegen, Netherlands: Radboud University.
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