17.6 Hearing

This vocalist, his band, and his fans enjoy hearing music.

The human ear has many functions.

It can give us a lot of information.
From its input, we can detect musical quality and nuances of voiced emotion.

Neither is felt by the ear.

When we hear the sounds of a diving board, it's because there are higher-frequency sounds in each.

Humans and other animals have different hearing ranges.

bats and dolphins can hear up to 100,000-Hz sounds.
Dogs respond to the sound of a dog whistle with sound out of the range of human hearing.
Elephants are known to respond to low frequencies.

Most of us have an excellent relative pitch, which means that we can tell if a sound has a different Frequency from another.

If the frequencies of the two sounds are not the same, we can discriminate.
500.0 and 501.5 are not the same.
Pitch perception is not affected by intensity or other physical quantities.
Some people can identify musical notes by listening to them.
Perfect pitch is an uncommon ability.

The ear is sensitive to sound.

The lowest audible threshold is about 0 decibels.

Only a few measuring devices are capable of seeing over a trillion.

It is possible to see differences of 1 dB and a change of 3 dB at a given Frequency.
Loudness isn't related to intensity alone.
The amount of Frequency has a big effect on how loud a sound is.
The ear's maximum sensitivity is 2000 to 5000 hertz, so it's possible to hear sounds louder than those at 500 or 10,000 hertz, even when they all have the same intensity.
The ear is less sensitive at low frequencies than at high frequencies.
Table 17.4 shows the dependence of human hearing perception on physical quantities.

The effects are not linear and there is more detail.

There is no mistaking a violin for a piano when it plays middle C. Each instrument has its own set of frequencies and intensities.

It is not easy to correlate timbre perception to physical quantities.
Timbre is not objective.
The terms dull, brilliant, warm, cold, pure, and rich are used to describe a sound.
The realm of perceptual psychology is where higher-level processes in the brain are dominant.
Music and noise are examples of how this is true.
We will focus on the question of loudness perception.

The decibel is a unit of physical intensity whereas the phon is a unit of loudness perception.

Equal-loudness curves are what the curved lines are.
The curve is labeled in phons.
The average person will perceive a sound along a curve to be equally loud.
The curves were determined by the large number of people who listened to the sounds.
phons are taken to be the same as decibels.

People with normal hearing have a relationship of loudness in phons to intensity level.

All sounds on a given curve are perceived to be equally loud.
The decibels and phons are the same.

To find the loudness of a sound, you need to know the intensity and Frequency of the sound, as well as the point on the square grid where you can find it.

The curves marked 70 and 80 phons are halfway between 100 and 80 decibels.

The intensity level of a sound is determined by the frequencies and loudness of the sound.

The intensity level can be determined from the vertical axis once that point is found.

At that point, it is about 67 decibels.

The answers have uncertainties of several phons or decibels, partly due to difficulties in interpolation, but mostly related to uncertainties in the equal-loudness curves.

Most people don't perceive sounds below the 0-phon curve.

A sound at 40 dB is inaudible.
The threshold of normal hearing is represented by the 0-phon curve.
Some sounds can be heard at low intensities.
A 5000-hertz sound is audible because it lies above the 0-phon curve.
There are dips in the loudness curves between 2000 and 5000 Hz.
The ear is sensitive to frequencies in that range.
A sound of 15 decibels has a sound of 20 decibels, the same as a sound of 20 decibels.
The curves rise at both extremes of the range, indicating that a louder sound is needed at those frequencies to be seen as loud as at the middle frequencies.
To make a sound sound as loud as a 20 decibel sound, it must have an intensity level of 30 decibels.
The sounds above 120 phons are damaging.

We don't use our full range of hearing often.

This is true for frequencies above 8000 Hz, which are rare in the environment, and are not necessary for understanding conversation or enjoying music.
People who have lost the ability to hear high frequencies are usually unaware of their loss until they are tested.
Hearing losses of 40 and 60 phons will have an effect on the curved lines.
A 40-phon hearing loss at all frequencies allows a person to understand conversation, although it will seem very quiet.
A person with a 60-phon loss will not be able to understand speech unless it is louder than normal.
Speech may seem different because higher frequencies are not seen as well.

A person with a hearing impediment might not be able to understand a woman's conversation.

The shaded region shows the frequencies and intensity levels found in speech.

The thresholds for people with 40- and 60-phon hearing losses are represented by the 0-phon line.

The hearing threshold is measured in decibels, so that normal hearing doesn't register at all.

Hearing loss caused by noise typically shows a dip near the 4000 Hz frequency, regardless of the frequency that caused the loss and affects both ears.
The most common form of hearing loss is called presbycusis.
Music appreciation and speech recognition are affected by such loss.

Audiograms show the threshold in intensity level for three different people.

The normal threshold is measured relative to the intensity level.
A person with normal hearing is depicted in the top left graph.

A child suffered hearing loss due to a cap gun.

Presbycusis is a progressive loss of hearing with age.
Nerve damage and middle ear damage can be determined by bone conduction tests.

Some interesting physics are involved in the hearing mechanism.

A pressure wave is a sound wave.

The ear is similar to a microphone in that it converts sound waves into electrical nerve impulses.

The ear is referred to as the pinna.

The ear is shown in the illustration.

The ear canal carries sound to the eardrum.

The air column in the ear canal is partially responsible for the ear's sensitivity to sounds in the 2000 to 5000 Hz range.
The middle ear converts sound into mechanical waves.
The lever system of the middle ear creates pressure waves in the cochlea that are 40 times greater than the pressure on the eardrum.
The middle ear protects the inner ear from intense sounds.
They can reduce the force transmitted to the cochlea by reacting to sound.
This protective reaction can be triggered by your own voice, so that humming while shooting a gun, for example, can reduce noise damage.

This schematic shows the middle ear's system for converting sound pressure into force, increasing that force through a lever system, and applying the increased force to a small area of the cochlea, thereby creating a pressure about 40 times that in the original sound wave.

The mechanical advantage of the lever system is reduced by a protective muscle reaction.

Figure 17.40 shows the middle and inner ear.

Nerves that send electrical signals to the brain are stimulated by pressure waves moving through the cochlea.
Nerves are stimulated at the near end and the far end by high and low frequencies.
Several mechanisms for sending information to the brain are involved in the operation of the cochlea.
The nerves send signals at the same frequencies as the sound.
There are connections between nerve cells that process signals before they reach the brain.
The number of nerve signals and volleys of signals indicate intensity information.
The source direction is provided by the brain by using time and intensity comparisons of sounds from both ears.
Music appreciation is one of the many nuances produced by higher-level processing.

If uncoiled, the inner ear is a coiled tube about 3 cm in diameter and 3 cm in length.

When the window is forced inward, a pressure wave travels through the air in the direction of the arrows, stimulating nerves in the organ of Corti.

Problems in the middle or inner ear can cause hearing losses.

Conductive losses in the middle ear can be partially overcome by sending sound waves through the skull.
Hearing aids for this purpose usually press against the bone behind the ear, rather than amplify the sound sent into the ear canal as many hearing aids do.
amplification can partially compensate for damage to the nerves in the cochlea.
There is a chance that amplification will cause more damage.
Damage or loss of the cilia is a common failure in the cochlea.

Cochlear implants are now widely accepted.

Over 100,000 implants are used by both adults and children.

The cochlear implant was invented in Australia in the 70s for a father who was blind.

The internal components are a microphone for picking up sound, a speech processor to select frequencies, and a transmitter to transfer the signal to the internal components.
The internal components consist of a receiver/ transmitter secured in the bone beneath the skin, which converts the signals into electric impulses and sends them through an internal cable to the cochlea, and an array of about 24 electrodes wound through the cochlea.
The impulses are sent directly into the brain.
The cilia are mimicked by the electrodes.

The range of sound is determined by the range of human hearing.

Many other organisms can see either sound or light.