Describe sound as a longitudinal wave, explain pitch and loudness, and use distance = speed times time for echoes

What this dot point is asking

SEAB wants you to describe sound as a longitudinal wave that needs a material to travel through, to link the pitch of a sound to its frequency and the loudness to its amplitude, and to use speed $=$ distance $\div$ time to solve echo problems. The big idea is that sound is a vibration passed from particle to particle, so it cannot cross empty space.

The answer

Sound as a longitudinal wave

Sound is made when something vibrates, such as a speaker cone or a guitar string. The vibration pushes the nearby particles back and forth, and they pass the disturbance on. This makes sound a longitudinal wave: the particles vibrate along the same direction the wave travels, creating regions where particles are squashed together and regions where they are spread out.

Sound needs a medium

Because sound travels by making particles vibrate, it needs a material (a solid, liquid or gas) to travel through. It cannot travel through a vacuum, because there are no particles to vibrate. This is why there is no sound in space, and why a ringing bell in a jar goes silent as the air is pumped out.

Sound travels fastest in solids (particles close together), slower in liquids, and slowest in gases such as air (about $340\ \text{m s}^{-1}$).

Pitch and frequency

The pitch of a sound is how high or low it sounds. Pitch depends on frequency:

• A higher frequency gives a higher pitch.
• A lower frequency gives a lower pitch.

A whistle has a high frequency and high pitch; a drum has a low frequency and low pitch.

Loudness and amplitude

The loudness of a sound depends on the amplitude of the wave:

• A larger amplitude gives a louder sound.
• A smaller amplitude gives a quieter sound.

Hitting a drum harder gives the wave a larger amplitude, so it sounds louder, but its pitch stays the same.

Echoes

An echo is sound that has reflected off a hard surface and come back to the listener. Because the sound travels to the surface and back, the total distance is twice the distance to the surface. Using speed $=$ distance $\div$ time, you can find a distance from the echo time, remembering to halve the total for a round trip.

Examples in context

Example 1. Sonar and ultrasound. Ships find the depth of the sea by sending a sound pulse to the seabed and timing its echo. The same idea, using very high-frequency sound (ultrasound), is used to scan an unborn baby and to check for cracks inside metal, because the echoes reveal hidden boundaries.

Example 2. Thunder and lightning. You see a lightning flash before you hear the thunder, even though they happen together, because light travels far faster than sound. Counting the seconds between the flash and the thunder, then using the speed of sound, tells you roughly how far away the storm is.

Try this

• Cue. Explain why an astronaut on the Moon cannot hear a hammer striking unless touching the surface. [2 marks] The Moon has no air, so there is no medium for sound to travel through; sound can only pass through the solid surface by direct contact.

• Cue. State what happens to a note if you increase the frequency of the wave. [1 mark] The pitch of the note gets higher.

• Cue. An echo returns from a wall $0.60\ \text{s}$ after a shout, with sound speed $340\ \text{m s}^{-1}$. Find the distance to the wall. [3 marks] Total distance $= 340 \times 0.60 = 204\ \text{m}$; distance to the wall $= \dfrac{204}{2} = 102\ \text{m}$.