Chapter 8 of 8

All 8 chapters
  1. 1Reflection, readunderstood
  2. 2Refraction, readunderstood
  3. 3Lenses and the eye, readunderstood
  4. 4The colours of light, readunderstood
  5. 5Why things have colour, readunderstood
  6. 6Sound as a wave, readunderstood
  7. 7The speed of sound, readunderstood
  8. 8Pitch, loudness and hearing, readunderstood

S3.9.8 Chapter 8

Pitch, loudness and hearing

What makes one sound high and another loud, and how do loud sounds hurt my ears?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Link pitch to frequency in hertz and loudness to vibration size
  • Explain why animals hear different ranges of sound
  • Explain how loud sounds, measured in decibels, damage hearing

Ana plucks a short guitar string. It makes a high note. She plucks a long string. It makes a low note. She plucks the long string hard. The note is the same, but louder. What changed?

How high or low a note is, is its pitch. Pitch depends on frequency. Frequency is the number of vibrations in each second. We measure it in hertz, written Hz. One hertz is one vibration each second.

A string that vibrates 200 times each second has a frequency of 200 Hz. A string that vibrates 400 times each second has a frequency of 400 Hz. Its note is higher.

How loud a sound is depends on the size of the vibration. That size is the amplitude. A bigger vibration pushes the air harder and carries more energy. Plucking harder makes a bigger vibration.

Draw it first

Draw a low note, a high note and a loud note

  1. Three pictures of a string and the wave it makes. In the first, the wave has few waves, labelled few waves: low pitch. In the second, the wave has many waves, labelled many waves: high pitch. In the third, the wave is tall, labelled tall: loud, middle pitch. Labels: few waves: low pitch, many waves: high pitch, tall: loud, middle pitch.few waves: low pitch

    1 Draw a wave with only a few humps in the space. It is a low pitch, a small frequency.

  2. Three pictures of a string and the wave it makes. In the first, the wave has few waves, labelled few waves: low pitch. In the second, the wave has many waves, labelled many waves: high pitch. In the third, the wave is tall, labelled tall: loud, middle pitch. Labels: few waves: low pitch, many waves: high pitch, tall: loud, middle pitch.many waves: high pitch

    2 Draw a wave with many humps in the same space. It is a high pitch, a large frequency.

  3. Three pictures of a string and the wave it makes. In the first, the wave has few waves, labelled few waves: low pitch. In the second, the wave has many waves, labelled many waves: high pitch. In the third, the wave is tall, labelled tall: loud, middle pitch. Labels: few waves: low pitch, many waves: high pitch, tall: loud, middle pitch.tall: loud, middle pitch

    3 Draw a tall wave. The vibration is big, so the sound is loud. Pitch is about how many humps, and loudness is about how tall.

Two waves one above the other. The top wave is tall, labelled loud. The bottom wave is short, labelled quiet. Both have the same number of waves. Labels: loud, quiet.loudquiet
The same pitch, but a bigger vibration makes a louder sound.
A bar chart called highest sound heard (thousand Hz). The bar for person reaches 20. The bar for dog reaches 45. The bar for bat reaches 100. The scale goes from 0 to 100 in steps of 20. Labels: highest sound heard (thousand Hz), person, dog, bat, 0, 20, 40, 60, 80, 100.020406080100persondogbathighest sound heard (thousand Hz)
About how high each animal can hear, in thousands of hertz. Some bats hear 100 000 Hz or more.

People hear sounds from about 20 Hz to about 20 000 Hz. Dogs hear higher, up to about 45 000 Hz. Bats hear much higher still. Elephants hear very low sounds, below 20 Hz, that people cannot.

Bats make very high squeaks and listen for the echoes to find insects in the dark. Elephants call with very low sounds that carry far. The herd hears them from a long way off.

Each kind of animal has ears and a brain fitted to its own life. A sound that is too high or too low for one animal can be clear to another. A dog whistle sounds silent to us. A dog hears it well.

A table with two columns, vibrations and seconds. The row says 600 vibrations in 3 seconds. Labels: vibrations, seconds, 600, 3.vibrationsseconds6003
A string vibrates 600 times in 3 seconds. What is its frequency?

Worked example

A string vibrates 600 times in 3 seconds. What is its frequency in hertz?

  1. Frequency = number of vibrations ÷ seconds.
  2. 600 ÷ 3 = 200.
  3. The frequency is 200 Hz.

Answer: 200 Hz

Loudness is measured in decibels, written dB. Each 10 dB more is a sound with about ten times the energy. So 20 dB more is 10 × 10 = 100 times the energy.

See it three ways

As a picture

A table with two columns, sound and dB. whisper, 30. talking, 60. traffic, 80. loud music, 100. Labels: sound, dB, whisper, 30, talking, 60, traffic, 80, loud music, 100.sounddBwhisper30talking60traffic80loud music100

In words

A whisper is quiet and a loud band is very loud. The numbers climb fast, because each 10 dB is ten times the energy.

In symbols

each 10 dB more = about 10 times the energy.

Worked example

Loud music is 100 dB. Traffic is 80 dB. How many times more sound energy does the music have?

  1. 100 − 80 = 20 dB more.
  2. Count the steps of 10 dB. 20 ÷ 10 = 2.
  3. Each step is 10 times the energy. 10 × 10 = 100.

Answer: 100 times

Deep inside your ear are thousands of tiny hair cells. They shake when sound arrives and send signals to the brain. A very loud sound shakes them too hard. They can be damaged, and they do not grow back.

Sounds above about 85 dB, heard for a long time, damage hearing. A very loud sudden sound, such as a firework close by, can harm at once. Damage builds up slowly, and you may not notice it for years.

To protect your hearing, turn the sound down and move away from the source. Cover your ears. Take breaks from loud places.

Try it

Make high and low notes with a ruler

You need: a plastic ruler, or a flat strip of wood or card, a table with a firm edge.

Be safe: Flick gently, and keep your face clear of the free end. Keep the sound quiet.

  1. Hold one end of the ruler flat against the table with your hand. Let the other end stick out over the edge.
  2. Flick the free end gently down, and let go. Listen to the note.
  3. Slide the ruler so less of it sticks out. Flick it gently again.
  4. Now flick the long end, first softly and then a little harder. Listen for loud and quiet, and for high and low.

What you will see: The short end buzzes quickly. It makes a higher note. The long end moves slowly. It makes a lower note. A harder flick makes a bigger movement and a louder sound, but the note stays the same.

If you cannot try it, this is what happens.
Two strips, drawn as strings, each with its wave, one above the other. The top wave has many waves, labelled short end: high note. The bottom wave has few waves, labelled long end: low note. Labels: short end: high note, long end: low note.short end: high notelong end: low note

A wrong idea many people have

A high sound is a loud sound.

Why it feels right: In daily talk, "high" can sound like "up" or "strong". A shriek is high and loud.

The test that breaks it: Whisper in a high squeaky voice. It is high and quiet. Then hum low and strong. It is low and loud.

What is true: Pitch is how fast the vibration is. Loudness is how big it is. They are two separate things.

Key words

pitch
how high or low a note is. It depends on frequency.
frequency
the number of vibrations in one second.
hertz
the unit of frequency. One hertz is one vibration each second. We write Hz.
amplitude
the size of a vibration. A bigger one gives a louder sound.
decibels
the unit of loudness. We write dB.
hair cells
tiny cells in the inner ear that sense sound. They do not grow back.

Back to your guess

The answer is A: A high sound shakes faster. A loud sound shakes bigger.

Pitch depends on how fast the vibration is. Loudness depends on how big it is. A sound can be high and quiet, or low and loud.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Look at the two waves. Which sound is louder, and which has the higher pitch?

    Two waves one above the other. Wave A is tall and has a few waves. Wave B is short and has many waves. Labels: A, B.AB
    Tap your answer.

    1. Not this one. Pitch is about how many waves, not how tall. B has more waves, so B has the higher pitch. Go to S3.9.8. Try another.

    2. Yes, that is right. A taller wave is a bigger vibration, so it is louder. More waves in the same space is a higher frequency, so it is a higher pitch.

    3. Not this one. Loudness is about how tall the wave is. A is taller, so A is louder. Go to S3.9.8. Try another.

    See the answer

    A is louder. B has the higher pitch.

    A taller wave is a bigger vibration, so it is louder. More waves in the same space is a higher frequency, so it is a higher pitch.

    • If you chose A: Pitch is about how many waves, not how tall. B has more waves, so B has the higher pitch. Go to S3.9.8.
    • If you chose C: Loudness is about how tall the wave is. A is taller, so A is louder. Go to S3.9.8.

  2. A string vibrates 800 times in 4 seconds. What is its frequency?

    Tap your answer.

    1. Not this one. You multiplied. Frequency is the number of vibrations in one second, so divide by the seconds. Go to S3.9.8. Try another.

    2. Not this one. That is the vibrations in all of the seconds. Divide by the seconds to find the number in one second. Go to S3.9.8. Try another.

    3. Not this one. You took the seconds away. Divide the vibrations by the seconds instead. Go to S3.9.8. Try another.

    4. Yes, that is right. Frequency = vibrations ÷ seconds = 800 ÷ 4 = 200 Hz.

    See the answer

    200 Hz

    Frequency = vibrations ÷ seconds = 800 ÷ 4 = 200 Hz.

    • If you chose A: You multiplied. Frequency is the number of vibrations in one second, so divide by the seconds. Go to S3.9.8.
    • If you chose B: That is the vibrations in all of the seconds. Divide by the seconds to find the number in one second. Go to S3.9.8.
    • If you chose C: You took the seconds away. Divide the vibrations by the seconds instead. Go to S3.9.8.

  3. A dog hears a whistle that a person cannot hear. Why?

    Tap your answer.

    1. Not this one. Loudness is not the reason. The whistle is too high in pitch for a person, but a dog can hear higher. Go to S3.9.8. Try another.

    2. Not this one. Ear size is not the whole story. Each kind of animal hears its own range, and a dog's range reaches higher than ours. Go to S3.9.8. Try another.

    3. Yes, that is right. People hear about 20 Hz to 20 000 Hz. Dogs hear up to about 45 000 Hz. A whistle in between is silent to us and clear to a dog.

    See the answer

    The whistle is too high for people to hear, but not too high for a dog.

    People hear about 20 Hz to 20 000 Hz. Dogs hear up to about 45 000 Hz. A whistle in between is silent to us and clear to a dog.

    • If you chose A: Loudness is not the reason. The whistle is too high in pitch for a person, but a dog can hear higher. Go to S3.9.8.
    • If you chose B: Ear size is not the whole story. Each kind of animal hears its own range, and a dog's range reaches higher than ours. Go to S3.9.8.

  4. One sound is 70 dB. Another is 90 dB. About how many times more sound energy does the 90 dB sound have?

    Tap your answer.

    1. Not this one. You took the numbers away and stopped. Each 10 dB is a step of 10 times, so count the steps and multiply. Go to S3.9.8. Try another.

    2. Not this one. That is only one step of 10 dB. There are 2 steps from 70 to 90, so 10 × 10. Go to S3.9.8. Try another.

    3. Not this one. You counted the steps and stopped. Each step is 10 times the energy, so multiply 10 by itself. Go to S3.9.8. Try another.

    4. Yes, that is right. 90 − 70 = 20 dB, which is 2 steps of 10 dB. Each step is 10 times the energy, so it is 10 × 10 = 100 times.

    See the answer

    100 times

    90 − 70 = 20 dB, which is 2 steps of 10 dB. Each step is 10 times the energy, so it is 10 × 10 = 100 times.

    • If you chose A: You took the numbers away and stopped. Each 10 dB is a step of 10 times, so count the steps and multiply. Go to S3.9.8.
    • If you chose B: That is only one step of 10 dB. There are 2 steps from 70 to 90, so 10 × 10. Go to S3.9.8.
    • If you chose C: You counted the steps and stopped. Each step is 10 times the energy, so multiply 10 by itself. Go to S3.9.8.

  5. Lucía listens to loud music at 100 dB for hours, day after day. What can happen to her hearing?

    Tap your answer.

    1. Yes, that is right. Loud sound for a long time damages the tiny hair cells. They do not grow back, so the hearing loss stays. She can protect her hearing by turning the music down, moving away, and taking breaks.

    2. Not this one. Ears do not toughen. The hair cells are damaged by loud sound, and they cannot be rebuilt. Go to S2.7.8. Try another.

    3. Not this one. Hair cells do not grow back. Damage from long loud sound lasts, and it adds up over time. Go to S2.7.8. Try another.

    See the answer

    The hair cells in her ears can be damaged, and they do not grow back.

    Loud sound for a long time damages the tiny hair cells. They do not grow back, so the hearing loss stays. She can protect her hearing by turning the music down, moving away, and taking breaks.

    • If you chose B: Ears do not toughen. The hair cells are damaged by loud sound, and they cannot be rebuilt. Go to S2.7.8.
    • If you chose C: Hair cells do not grow back. Damage from long loud sound lasts, and it adds up over time. Go to S2.7.8.

  6. From an earlier chapter

    Review: Kofi counts 12 seconds from a flash to the thunder. About how far away is the storm?

    Tap your answer.

    1. Not this one. You used the seconds as kilometres. Divide the seconds by 3. Go to S3.9.7. Try another.

    2. Yes, that is right. Sound goes about 1 km in 3 s. 12 ÷ 3 = 4 km.

    3. Not this one. You multiplied by 3. Sound goes about 1 km in 3 s, so divide by 3. Go to S3.9.7. Try another.

    See the answer

    4 km

    Sound goes about 1 km in 3 s. 12 ÷ 3 = 4 km.

    • If you chose A: You used the seconds as kilometres. Divide the seconds by 3. Go to S3.9.7.
    • If you chose C: You multiplied by 3. Sound goes about 1 km in 3 s, so divide by 3. Go to S3.9.7.

Keep it

Copy this into your exercise book.

Pitch depends on frequency in hertz, the vibrations each second. Loudness depends on the size of the vibration, in decibels. Each 10 dB more is about 10 times the energy. Loud sound damages hair cells for good.

Draw: A wave with few humps, labelled low. A wave with many humps, labelled high. A tall wave, labelled loud.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (720 KB). It opens on any phone.