13.2Chapter 13: Sound Waves

Pitch and frequency — Alhussan Physics - Stage 9 Revision Notes & Topical Questions

5 min read 8 parts Lesson 2 of 5 2 quizzes

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Lesson summary

Physics 13.2 Pitch and frequency — Hertz (Hz), Wavelength and Audible Range — Alhussan International Academy — Physics (Stages 7–9) notes: Describe what affects the pitch of a sound, the audible range of humans and animals, and waveforms showing different frequencies.Frequency = …

High and low
Loud and pitch
Infrasound
Ultrasound
Different ranges
Medical scans

Part 1 of 6Frequency and pitch

Frequency

Frequency the number of complete vibrations (waves) per second.

Unit

Frequency is measured in hertz (Hz), where 1 Hz is one vibration per second.

Kilohertz

High frequencies are measured in kilohertz, where 1 kHz = 1000 Hz.

Pitch

Pitch how high or low a sound is, and it depends on the frequency.

High and low

A high-pitched sound has a high frequency and a short wavelength.
A low-pitched sound has a low frequency and a long wavelength.

Strings

The pitch of a string depends on how many times it vibrates each second.

Part 2 of 6Frequency and amplitude are different

Amplitude

It determines how loud the sound is, not its pitch.

Loud and pitch

Loud sounds may be high-pitched like a scream or low-pitched like an earthquake.

Change	Effect on oscilloscope trace	Effect on sound
Higher frequency	More waves in the same time, squashed together	Higher pitch
Greater amplitude	Taller waves	Louder

Part 3 of 6Frequency from a trace

Period

Period the time for one complete wave, read from the horizontal scale.

Equation

Frequency = 1 ÷ period.

Doubling

A trace with twice as many waves across the screen shows twice the frequency, which is one octave higher.

Strings

A shorter, tighter, thinner string vibrates faster and gives a higher pitch.

Part 4 of 6The audible range

Audible range

Audible range the range of frequencies a person can hear, from about 20 Hz to 20 000 Hz for humans.

Age

Young people can hear a bigger range than older people.
As people get older they find it harder to hear very high frequencies.
By the age of 30 a person may hear only up to about 14 000 Hz.

Infrasound

Sound with a frequency below 20 Hz, used by some animals such as elephants.

Ultrasound

Sound with a frequency above 20 000 Hz, which humans cannot hear or make.

Part 5 of 6Animals and ultrasound

Animal	Approximate audible range (Hz)
Elephant	16 to 12 000
Cow	23 to 35 000
Dog	67 to 45 000
Dolphin	100 to 100 000
Mouse	1000 to 91 000

Different ranges

Animals have very different audible ranges from humans.

Ultrasound users

Some animals make and detect ultrasound, such as bats and dolphins, whereas grasshoppers make sounds they cannot hear.

Part 6 of 6Uses of ultrasound

Medical scans

Ultrasound is used to scan unborn babies.

Cleaning and testing

It cleans delicate objects and detects cracks in metal.

Echoes

Bats use ultrasound echoes to find prey in the dark and ships use sonar to measure sea depth.

Key terms

Term	Meaning
Frequency	The number of vibrations per second, in hertz.
Pitch	How high or low a sound is.
Hertz (Hz)	The unit of frequency, one vibration per second.
Audible range	The range of frequencies that can be heard.
Ultrasound	Sound with a frequency above 20 000 Hz.
Infrasound	Sound with a frequency below 20 Hz.