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What are sound waves?

Vibration

Sound waves are produced by a vibrating object. Everything that makes a sound must have a part that vibrates.

A sound wave is a longitudinal wave. When an object vibrates it produces a longitudinal wave which travels through air to your ear.

For a sound to be heard you need three things:

• A vibrating object
• A material for the sound wave to travel through, such as air (we call this a medium)
• Something to detect the sound e.g. your ear

Sound can pass through:

• Solids e.g. sounds will pass through a wall or a glass window
• Liquids e.g. you can hear sounds in a swimming pool when your head is beneath the water
• Gases e.g. sound passes through air

Sound waves are produced by all vibrating objects. Loudspeakers work by converting electrical energy into kinetic energy. This vibrates the cone which creates the sound waves.

Sound waves can reflect off surfaces. We hear reflected sound as an echo. Hard, smooth surfaces are particularly good at reflecting sound. This is why large, empty rooms produce lots of echoes.

Soft, rough surfaces are good at absorbing sound. This is why rooms with carpets and curtains do not usually produce lots of echoes. People in a room will help to absorb sound too. The sound made by a band is very different in an empty concert arena than when it is full.

Our ears have two important functions:

• To collect sounds, convert them to electrical signals and send them to the brain
• To maintain our sense of balance

The ear is made up of three different sections that work together: the outer ear, the middle ear, and the inner ear.

Outer ear

• Parts - pinna and ear canal
• Job - collecting sounds

The eardrum separates the outer ear from the air-filled middle ear. Sound waves make it vibrate.

Middle ear

• Parts - hammer, anvil and stirrup
• Job - to pass vibrations from the eardrum to the inner ear

Inner ear

• Parts - cochlea and semi-circular canals
• Job - to convert the vibrations from the middle ear into nerve signals and to help you balance

A narrow tube called the eustachian tube runs from the middle ear to the back of the nose and makes sure that the pressure is the same on both sides of the eardrum.

When your ears pop the eardrum moves back into place, easing any pressure difference, and making you feel more comfortable. The eustachian tube can open automatically when you swallow, blow your nose or yawn. When you do these motions, you'll often hear a clicking or popping sound.

How we hear:

• a sound wave is funnelled into the ear canal by the pinna
• the vibrations in the air make the eardrum vibrate
• these vibrations are passed through the three small bones (called ossicles) to a spiral structure called the cochlea
• electrical nerve signals are passed from the cochlea to the brain through the auditory nerve
• our brain interprets these signals as sound

The energy changes in the ear are:

The frequency of a sound wave is related to the pitch that is heard:

• a high frequency sound wave has a high pitch;
• a low frequency sound wave has a low pitch.

The amplitude of a sound wave is related to the volume of the sound:

• large amplitude sound waves are loud;
• small amplitude sound waves are quiet.

Amplitude and frequency can be displayed on an oscilloscope connected to a microphone

Oscilloscope traces showing the following sounds:

• Trace 1. small amplitude, low frequency → a quiet, low pitch sound.
• Trace 2. large amplitude, low frequency → a loud, low pitch sound.
• Trace 3. large amplitude, high frequency → a loud, high pitch sound.

To measure the speed of sound in the lab, a much more accurate timing method is required because the distance travelled by the sound is much shorter. This can be achieved using a data logger

The data logger can measure and record the time taken for sound to travel between two microphones. Unlike the wooden block method, these can be quite close together.

This is because the particles of gases are further apart than liquids or solids. Sound waves travel more slowly when particles are further apart.

Sound can travel through solids, liquids and gases, but can it travel through a vacuum?A simple experiment will give us the answer.

Connect an electric bell to a battery and switch. Close the switch and listen to it ring.

Now place the bell inside a glass jar, called a bell jar, as shown above. Make sure it is tightly sealed at the bottom and connect to a vacuum pump. Stand behind a safety screen.

Close the switch again and listen to the ring. You should notice that it is not as loud, but you are still able to hear it.

Keep the bell ringing and turn on the vacuum pump. Make sure the safety screen remains between you and the bell jar. Listen carefully as the vacuum pump removes the air from inside the jar.

You will notice that as air is removed, the ringing sound gets quieter and quieter, until you cannot hear it at all. You can still see the bell hammer hitting the gong, so you know the bell is ringing, but the sound is not reaching you.

Let the air slowly back into the jar. You will notice that you start to hear the bell again.

Conclusion

• when the jar is placed over the bell it is quieter because the jar muffles some of the sound
• when the air is removed from the jar you cannot hear the sound because sound needs something to travel through
• we can still see the bell in the jar, which tells us that light passes through a vacuum

The experiment shows that sound can travel through gases (air), through a solid (glass), but it cannot travel through a vacuum. Light however, does pass through a vacuum

We say that:

• sound cannot travel through a vacuum; it needs a medium to travel through.

Sound does not travel through space because space is a vacuum. If you shouted in space nobody would hear you.

As we saw earlier in this topic human beings can generally hear sounds as low as 20 Hz and as high as 20 000 Hz (20 kHz).

Any sound above 20 000 Hz is called ultrasound. It is too high pitched for humans to hear, but other animals (such as dogs, cats, dolphins and bats) can hear ultrasound.

Ultrasound waves are longitudinal waves just like sound waves, and they cannot pass through a vacuum. Ultrasound waves have a frequency above 20 000 Hz, other than that they are like sound waves in every other way.

Ultrasound has many applications in medicine, including ultrasound scans to check on the health of unborn babies.

Scans of foetuses (unborn babies developing in the womb) are made this way and are used, for example, to measure the diameter of the head of a foetus so that growth can be monitored.

Ultrasound images can also be used to help find problems with:

• heart
• kidneys
• blood vessels
• bladder

Doctors like using ultrasound images because:

• ultrasound does not harm the patient, mother or foetus in any way
• images of internal organs like the heart can be seen without having to operate on patients
• the equipment is easy to use and relatively cheap

High frequency ultrasound waves can be used to detect objects in deep water and to measure the depth of the sea. Ultrasound is reflected off an object in the sea or from the seabed, and the echo is detected.

This technique is applied in sonar systems used to measure the depth of the seabed and to find shipwrecks, submarines and shoals of fish. The time between a pulse of sound being transmitted and detected (its echo), and the speed of sound in water are used in the calculation.

SONAR stands for SOund Navigation And Ranging.

One of the reasons for using ultrasound for sonar is that humans using the water for swimming or diving don’t hear it and so are not disturbed.

Bats and dolphins use a similar method, called echolocation, to detect their surroundings and to find food.

Example

A sonar system on a boat sends an ultrasound pulse towards the seabed.The pulse is reflected, and it is detected 0.16 s later by the system.

Calculate the depth of water if the speed of sound in water is 1 500 m/s.

Answer

distance = speed × time

speed = 1 500 m/s

time for ultrasound to travel to seabed and back again = 0.16 s

We are calculating the depth of the seabed, so we only need the time for the ultrasound to travel to the seabed (be careful to remember this part of the calculation)

time for the ultrasound just to travel to the seabed = 0.16 s ÷ 2 = 0.08s

distance to seabed = 1 500 × 0.08 = 120 m

The depth of water is 120 m.

We are subjected to sounds pretty much all the time. Many sounds are nice, and we enjoy them. But what is nice to you might not be so nice to somebody else who is also hearing the sound.

Noise is unwanted sound

Noise doesn’t have to be loud sound – it just has to be unwanted. Chattering quietly in a library is noise, as is loud music played from a smart speaker on the beach, if it disturbs other users.

When the sound level of noise rises to higher levels, we refer to this as noise pollution.

Noise can be irritating and annoying, but it can also cause tiredness, loss of concentration or even damage hearing

Sound levels and damage to hearing

Sound levels are measured on a scale known as the decibel scale (dB). The higher the number the greater is the sound level.

Sound level can be measured with a device called a decibel meter. It samples and measures sound, giving a readout. Decibel meters (also called sound-level meters) can even be accessed on a smartphone through apps. Measuring the sound level with a smartphone may help protect your hearing.If the sound level is too high, hearing can be damaged.

• Sound levels above 70 dB over a long period of time may start to damage your hearing
• Hearing 90 decibel sound levels for a long time can cause permanent hearing loss
• Loud noise above 120 dB can cause immediate harm to your ears.
• Hearing a brief sound level of 140 decibels will cause pain and can cause permanent damage to hearing

Eighty-five decibels is considered the highest safe sound level for up to a maximum of eight hours. The time for safe listening decreases as sound levels increase.For example, a sound as high as 100 dB can be safely listened to for only 15 minutes each day.

• users of earphones usually choose to set the volume between 75 to 105 dB
• at nightclubs and discos the average sound levels can range from 104 to 112 dB
• sound levels at pop concerts may be even higher

Even a short duration of exposure to high-decibel levels such as these can be harmful. Doing it regularly will almost certainly lead to hearing loss over time.

The good news is that noise-induced hearing loss can be prevented by following safe listening.

Make listening safe by:

• keeping the volume down – have the volume so you can hear your music comfortably, but no higher
• use noise-cancelling earphones or headphones – do not just turn the volume up to cover up outside noise
• limit the use of earphones to no more than an hour at a time
• move away from loudspeakers at noisy venues and try to take a break from the loud sound every 15 minutes
• always wear ear protection when using noisy machinery

You can easily damage your ears without knowing it.

Summary: sound

• sound waves are longitudinal
• sound waves are produced by a vibrating object
• sound can travel through solids, liquids and gases
• sound cannot travel through a vacuum – it needs a medium to travel through
• the speed of sound in air is approximately 340 m/s
• a high frequency sound wave has a high pitch
• large amplitude sound waves are loud
• reflected sound is called an echo
• the range of human hearing is 20 Hz to 20 000 Hz
• sound waves with a frequency above 20 000 Hz are called ultrasound
• eighty-five decibels is considered the highest safe sound level