Sound – Part 2 Year 7 Science

Review In our last lesson we learnt about what sound is. We found that sound is a form of energy caused by vibrations. We also discovered that sound needs a medium to travel in that contains particles (air, water, steel etc.) and hence will NOT travel in a vacuum. We found out that sound travels at about 330 metres per second (or 1225 km per hour) and travels at different speeds in different materials

Speed of Sound

Review We also learnt what the meaning of pitch and frequency is and how they are related. We learned about hertz – the unit used in measuring frequency. Finally, we learnt that humans having a hearing range from approximately 25 hertz to 20,000 hertz. However, animals have a greater hearing range with bats able to hear sounds as high as 150,000 hertz.

A Sound Spectrum

Reflections and Echoes Sound energy can also be made to reflect. An echo is hearing sound for a second time because the sound is reflected. Because sound takes about a second to travel 300 m, shouting loudly some distance in front of a cliff or large building will produce an echo seconds later. We can estimate the distance of sound reflecting surfaces by counting how long it takes to hear the echo. Try this for yourselves !!!

In the remainder of today's lesson we are going to learn about human hearing and noise pollution.

Hearing sounds - The human ear Sound waves cause small but rapid pressure changes on anything they strike due to the vibrations of particles. The human ear turns these pressure changes into nerve impulses which can be sensed by the brain. The human ear contains three distinct parts The outer ear The Middle ear The Inner ear

The Human Ear Let us look at each of these parts in detail…

The Outer Ear The fleshy outer part of the ear is called the auricle. Some animals have large auricles (such as cats) and hence are able to detect very small levels of sound. They can also twist their auricles to collect sounds coming from a variety of directions. However, we have to turn our head to collect sounds from different directions.

The Outer Ear The auricle collects sounds in the outer ear and funnels the air vibrations through a passage called the ear canal. At the end of the ear canal there is a thin sheet of muscle and skin called the ear drum. This ear drum membrane vibrates in response to the vibrating air particles.

The Middle Ear Behind the ear drum lies an air-filled space containing three small bones or ossicles. These three bones are called the hammer, anvil and stirrup.

The Middle Ear As the ear drum vibrates, these bones act as levers and carry the vibrations to another sheet of skin called the oval window. The middle ear also contains the eustachian tube. This tube is used to keep the air pressure in the middle ear the same as atmospheric pressure.

The Inner Ear Behind the oval window is a fluid filled ‘snail-shaped’ tube called the cochlea. When vibrations are passed through the oval window by the stirrup, the fluid moves tiny hairs inside the cochlea. These hairs are attached to the nerve receptor cells that send messages to your brain through the auditory nerve.

The Inner Ear The inner ear also contains the semicircular canals. These fluid-filled organs are not involved in hearing sound. They are used to provide your brain with information associated with balance.

Sound Intensity Now, we found the rate at which particles vibrate affects the pitch of the sound and frequency. The magnitude or amplitude at which the particles vibrate affects the loudness of sound. We hear different intensities of sound as different levels of loudness. High intensity sounds are very loud. Similarly low intensity sounds are often quiet.

Sound Intensity Sounds are all around us and we live in a noisy world !! Our hearing organs are very delicate and can be easily damaged by high intensity sounds. Sometimes the damage is temporary and at other times it can be permanent - resulting in deafness. For those whose jobs are at airports there is a real risk of becoming deaf. So we need to be able to measure sound intensity so we know when to protect ourselves in dangerously noisy environments. How do we do this???

Measuring Sound Scientists measure sound levels with a special unit called a decibel – pronounced “dess-i-bell”. The decibel is written as dB. The softest sound that an average ear can hear is called 0 dB. This is called the threshold of hearing.

Decibel (dB) levels of various sounds

A safe work environment In the workplace it is important to create a safe and comfortable environment in terms of sound. The law limits both the intensity and amount of time we should be subjected to sound for a safe environment.

Breaking the sound barrier When an aeroplane moves faster than the speed of sound, it pushes aside air particles with great force – creating a shock wave. It forms a cone of highly pressurised air particles. A sonic boom is heard when waves at the base of this pressurised cone strike someone's ear. Sonic booms are so loud they can shatter windows and burst ear drums.

Sonic Boom due to supersonic jet