Sound Waves

Human Ear

Sound waves are longitudinal waves composed of areas of compressed air. This compression wave passes through the air until it interacts with a material and transfers that compression into a vibration of the material.

To make a sound an object needs to be vibrating and pushing on the air around it to generate the compression wave that is transmitted through the air (or water).

Sound needs a medium to travel in, so it can not pass through a vacuum, literally, no one can hear you scream in space.

Within your ear, you have a small membrane called the eardrum, or the tympanic membrane. When the sound wave hits this membrane it causes it to vibrate. This vibration is passed on to a small series of bones that link to a fluid-filled tube called the cochlea. These vibrations are detected by tiny hairs linked to nerves. The vibration of these hairs allow us to hear.

Sound Waves, figure 1

The hairs in your cochlea are very delicate and loud noises can snap them off. Once this happens they never grow back and you lose some of your ability to hear.

Ultra and Infra Sound

Although sound waves can travel at many different frequencies the human ear is only capable of detecting them over a certain range of frequencies.

The maximum range is between 20 to 20,000 Hz, this range reduces as we age and it is the high frequency, high pitch sounds that we lose first.

Sound waves below 20 Hz are called infrasound. Those above 20,000 Hz are ultrasound. Both infra and ultrasound have applications in nature.

Many animals use infrasound to communicate over long distances, we are unaware of this because our ears cannot detect them. Whales, dolphins and many fish can generate infrasounds to communicate across the oceans. Elephants also use infrasound to send messages across the vast spaces of the African savanna.

Ultrasound is used by bats for navigation and hunting, but so do dolphins in the water. Baby rats produce ultrasound to alert their parents when they are in danger.

Applications of Sound Waves

Sound waves within the normal range of human hearing are used for communication entertainment etc, but the ultra and infrasound range also have useful applications for humans.

Ultrasound can pass through the tissues of the body without any harm. They reflect off different densities of tissues and these can be detected and used to produce an image of the inside of a living person. This is used to examine developing babies in the womb, but it is also used to look at joints and other organs of the body by doctors.

We also use ultrasound to scan structures like bridges and planes for any signs of damage and cracks. This works in the same way as the ultrasound scan used in medical examinations, by collecting and analysing the echo patterns of the reflected waves. A crack creates a boundary that reflects the sound waves and so it can be detected.

Nature produces infrasounds in the form of seismic waves. By analysing how these are absorbed and reflected by the inside of the planet, scientists have been able to work out the internal structure of the earth.

Infrasounds are good at travelling over long distance, this is one of its properties that is used by water companies and the gas and oil industry to check for leaks in their pipelines. Some of which run for thousands of kilometers across inhospitable terrain.

Explain why ultrasound is used for medical examinations.
Your answer should include: high-frequency / sound / waves
Explanation: The high-frequency sound waves can pass through the body without any harm, They allow the doctors etc to see inside the living body without any harm to the patient.
State one use of infra and ultrasound in nature.
Your answer should include: echolocation / communications
Explanation: Ultrasound as echolocation by bats and dolphins. Infrasound for communications by whales or elephants.
What is the range of human hearing and how is this affected by our environment?
Your answer should include: 20 / 20Hz / 20000 / 20000Hz
Explanation: The maximum range is from 20 to 20,000 Hz, loud sounds reduces this by damaging the hairs that detect sound in the cochlea of the inner ear.