Sound · SoundLab

How sound is produced

Strike the tuning fork with the rubber mallet and hold it next to the hanging ping-pong ball: the ball is knocked away again and again. The fork vibrates too fast and too slightly for the eye to follow; the ball magnifies the motion — this is called the conversion method.

When the drum is hit, the paper bits on the drumhead jump up. Grip the fork or press the drumhead: the vibration stops, and so does the sound.

Sound is produced by vibrating objects

An object that is producing sound is called a sound source. Solids, liquids and gases can all vibrate and make sound: we speak with vibrating vocal cords, a flute sounds through its vibrating air column, a stream babbles because the water vibrates.

How sound travels

Put a ringing electric bell inside a glass jar and gradually pump the air out: the gauge reading drops and the ringing grows fainter — you can still see the hammer striking, yet you can hardly hear it. Let the air back in and the ringing gets loud again.

The jar can never be pumped to a perfect vacuum, and a little sound still leaks out through the base. So we reason: with no air at all inside the jar, we would hear nothing —

Sound needs a medium to travel; it cannot travel through a vacuum

Switch to the “Sound waves” view: the air particles in the jar thin out and the waves leaving the jar grow weaker. Astronauts in space can only talk by radio.

Speed of sound

Sound runs along three 1.2 m tracks at the same moment: a steel pipe, a trough of water and a stretch of air. Sensors at the finish feed an oscilloscope, which records when the sound arrives. The picture is slowed down more than a thousand times.

v = s / t

Sound is fastest in steel (about 5200 m/s), about 1500 m/s in water and about 340 m/s in air at 15 °C: in general sound travels faster in solids than in liquids, and faster in liquids than in gases; the warmer the air, the faster the sound.

Strike one end of a very long iron pipe and a person at the other end hears two sounds: first through the iron, then through the air.

Pitch

Press a steel ruler firmly onto the table with one end sticking out over the edge, and pluck it. Change how far it sticks out while plucking with the same force (controlling variables): the shorter the free end, the faster it vibrates and the shriller the sound.

The number of vibrations per second is the frequency, measured in hertz (Hz). The denser the waveform on the oscilloscope, the higher the frequency.

The higher the frequency, the higher the pitch

Human ears hear frequencies from roughly 20 Hz to 20000 Hz. Sound below 20 Hz is infrasound; above 20000 Hz it is ultrasound. Let the ruler stick out a long way and pluck it: you can see it vibrate, yet hear nothing.

Loudness

Hit the drum with different force: the harder the hit, the larger the drumhead's vibration, the higher the paper bits jump and the larger the sound level meter reading.

The size of a vibration is its amplitude. How loud a sound is, its loudness, is usually given in decibels (dB).

The larger the amplitude, the louder the sound

Loudness also depends on how far you are from the source: move the meter away and the reading drops — sound spreads out as it travels. Megaphones and stethoscopes keep the sound together, reducing the spread and increasing the loudness.

Timbre

Play the same note on different sources (for example A4 = 440 Hz): the waveforms on the oscilloscope are equally dense (same frequency, same pitch) but differently shaped.

Sources differ in material and structure, so their sounds differ in timbre (tone colour). A tuning fork gives an almost pure sine wave; a piano or ukulele mixes many overtones in with the fundamental — the small chart on the right shows how strong each harmonic is.

Timbre is how we recognise a person by their voice or tell which instrument is playing.

Pitch · loudness · timbre — the three characteristics of sound

Echoes · Using sound

Sound bounces back off obstacles, producing an echo. We can only tell an echo from the original sound if it arrives at least 0.1 s later, so the obstacle must be at least 17 m away.

The ultrasonic rangefinder sends out a burst of 40 kHz ultrasound and listens for the echo from the board. Measure the round-trip time t:

s = v t / 2

Bats hunt by echolocation, fishing boats find shoals with sonar, doctors look at organs with ultrasound scans — sound can carry information; ultrasonic cleaners clean glasses and shock waves break up kidney stones — sound can carry energy. Soft, porous materials absorb sound, so their echo is very weak.

Noise: harm and control

In physics, noise is sound from an object vibrating irregularly; from an environmental point of view, any sound that disturbs people's rest, study or work is noise.

Sound levels are measured in decibels (dB): 30–40 dB is a pleasantly quiet place; above 50 dB disturbs rest; above 70 dB interferes with conversation; living with more than 90 dB for a long time damages hearing.

Stop noise being made · block its path · keep it out of the ears

Mufflers on motorbikes and soundproof hoods over machines; noise walls and trees along roads; earmuffs for workers — the three stages of noise control.

In-class testing