Waves and sound · System
Hearing and auditory range
The words infrasound and ultrasound sound like two different kinds of sound. They are not. They are two statements about the ears of one particular animal, and that animal is us.
Start here
The whistle nobody can hear.
A dog whistle is blown hard in a quiet room. Every person there agrees it made no sound at all. The dog is already on its feet and looking at the door.
What is actually coming out of the whistle?
Put a microphone in front of it and it registers a strong, steady tone at about 30 000 Hz. Nothing is missing from the wave — the air is being squeezed and released just as vigorously as by any audible note. What is missing is a pair of ears able to respond that fast. Turning it up would not help.
Every ear has a band of frequencies it can respond to, and that band is called its auditory range. Below the bottom of the range the vibrations are too slow to set the ear working; above the top they are too fast. A healthy young human ear runs from about 20 Hz up to about 20 000 Hz. Sound below the bottom of the human range is called infrasound; sound above the top of it is called ultrasound.
Those names are about us, not about the sound. Ultrasound is ordinary sound in every physical respect: it is made the same way, it travels at the same speed through the same material, and it reflects and is absorbed by the same rules. The only thing that makes it ultra is that our particular ears stop at 20 000 Hz. A bat hearing a 60 000 Hz call is not hearing something exotic. It is just hearing.
Ranges are best drawn on a scale where every step multiplies rather than adds, because the numbers involved span five powers of ten. On the bench below, each mark along the axis is ten times the one before it.
At the bench · a tone generator and one listener at a time
Sound one tone. Ask one pair of ears.
Change a control to begin
A single steady tone at a frequency you choose, and one listener in the room. The axis multiplies by ten at every mark, because the numbers run from a few hertz to a few hundred thousand.
Commit first. A dog whistle sounds at about 30 000 Hz. A person hears nothing. Why?
Who is listening
251 Hz
The tone
—
This listener hears from
—
at the bottom of the range
up to
—
at the top of the range
Can this listener hear it?
—
The figure
All seven on one multiplying scale
Read the shaded strip as the human range. Everything to the left of it is infrasound and everything to the right is ultrasound, and both names are about us. On a scale that multiplied by ten at every mark, a bar reaching one mark further right is a range going ten times higher — which is why a straight ruler scale would have squashed every one of these bars into the same left-hand centimetre.
Key fact
An auditory range is the band of frequencies an ear can respond to, and a healthy young human ear covers about 20 Hz to 20 000 Hz. Sound below that band is infrasound and sound above it is ultrasound; both names describe our ears rather than the sound, which behaves in exactly the same way at every frequency. Many animals hear well outside our band, and the top of the human range falls with age.
Think again
“A dog whistle makes no sound.”
It makes a great deal of sound, and a microphone put in front of it registers a loud steady tone at about 30 000 Hz. Nothing is missing from the wave: the air is being squeezed and released just as vigorously as it would be by an audible note, and if you stood close enough with the right instrument you could measure it. What is missing is a pair of ears able to respond that fast. The word inaudible is a statement about the listener.
“Losing the top of your hearing range just makes everything a bit quieter.”
It takes pieces out rather than turning a dial down. High frequencies are what make consonants distinct — the difference between s and f, or t and k, sits mostly above 4000 Hz — so the first thing people notice when the top of their range drops is not that speech is quiet but that it is mumbled, especially in a noisy room where the low frequencies are still arriving perfectly well. That is also why simply shouting at someone with hearing loss often does not help: the missing information was never in the loud part.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Read the ranges
A tone of 30 000 Hz is sounded in a room holding a young person, a dog and an elephant. Who can hear it?
Rung 2 · The one that catches people
A dog whistle is blown and no person in the room hears anything. Which statement is right?
Rung 3 · Explain
A bat hunts small insects in complete darkness by sending out calls and listening for what comes back. Explain why a range reaching up to about 110 000 Hz suits that job better than a range like ours would.
Rung 4 · Take it somewhere new
Some shops have fitted a device that sounds a continuous tone at about 17 000 Hz outside the door. Explain who is likely to hear it and who is not, and give one reason people have argued about whether such devices should be allowed.
Key note
An auditory range is the band of frequencies an ear can respond to. A healthy young human ear runs from about 20 Hz to about 20 000 Hz; below that is infrasound and above it is ultrasound, and both names describe our ears rather than the sound itself. Other animals have their bands in different places: a bat reaches to about 110 000 Hz and an elephant down to about 16 Hz. The top of the human range falls with age and with exposure to loud sound, and that loss is permanent.
Going further
Where an animal's range sits is usually about the job it has to do. Echolocation needs high frequencies, because a short wavelength is what reflects off something small: a bat calling at 100 000 Hz uses a wavelength of a few millimetres and can pick a moth out of the air, while a call at 1000 Hz would tell it only where the walls are. Long-distance communication needs the opposite. Low frequencies are absorbed far less by air and by ground cover, so elephant rumbles near 16 Hz carry for kilometres, and the loudest thing in the ocean is a blue whale calling below 20 Hz.
Hearing loss from noise is a physical injury, not a tiredness. The hair cells in the inner ear that respond to the highest frequencies are the ones nearest the entrance, they take the most punishment, and they do not grow back in humans. That is why the damage shows up at the top of the range first, why it accumulates over years, and why it is one of the few injuries that is completely preventable and completely permanent. Earplugs at a concert and a volume limit on headphones are cheap; the hair cells are not replaceable at any price.
Before this lesson
Connects to
At GCSE this becomes
- The human audible range and how the ear converts pressure changes to electrical impulses, ultrasound and infrasound applications, and noise-induced hearing loss.
Where to next
Ask Mr Badmus AI
Wondering which animals could hear a particular frequency?
Hearing damage from loud sound builds up quietly and does not repair itself, so ringing ears after a night out, or finding you keep turning the volume up, are worth mentioning to someone rather than ignoring. You can talk to a doctor, a school nurse or any adult you trust. Childline is free, confidential and open at any hour, on 0800 1111, and you do not have to give your name.
Every range on this page is an approximate figure for a typical healthy adult of that species, and individuals vary widely; published values differ between studies, partly because they disagree about how quiet a sound has to be before an animal counts as hearing it. The 20 Hz and 20 000 Hz limits of the human range are round conventions rather than measurements, and very few adults reach 20 000 Hz. The human-at-50 figure is a typical outcome, not a schedule: age-related loss varies enormously between people and is made much worse by a lifetime of loud sound. A range says only whether an ear can respond to a frequency at all; it says nothing about how well.
Lesson content © MrBadmusAI.