ABOVE OUR RANGE
Bats shout in ultrasound
Bat calls are usually too high for humans to hear. The returning echoes tell a bat the size, shape and direction of an insect ahead.
How detectors workBEYOND HUMAN HEARING
The sounds just
out of earshot.
A quiet summer evening is not quiet at all.
Test a sound
20 Hz – 20 kHzthe human window
- Above: ultrasound
- Below: infrasound
- Detectors bring it down
- Different ears, different worlds
People hear from about 20 Hz to 20 kHz. Above that, bats hunt with calls we cannot hear; below it, elephants rumble through the ground. Here is what lies on either side, and how we listen in.
Updated
THE SHORT ANSWER
Our hearing is
just one window.
Other animals hear higher or lower.
Some of their sounds never reach us.
BELOW OUR RANGE
Elephants rumble low
Elephant rumbles are usually under 20 Hz, below what humans can hear, and can travel through the ground as vibrations.
See the numbers
A DIFFERENT EAR
Mice hear higher
Mouse hearing runs from about 1 to about 100 kHz, so a sound that is loud to us can be faint, or silent, to a mouse.
Test a soundA QUICK HEARING TEST
Who can hear
this sound?
Pick a pitch to see whether people and mice can hear it. The bar shows how far it sits from the middle of human hearing.
Ranges from a 2010 study of human and mouse hearing (J Am Assoc Lab Anim Sci), with bat and elephant examples from the Bat Conservation Trust and Physics World. Individual hearing varies with age and noise exposure.
TWO NUMBERS WORTH KNOWING
Higher ears,
longer rumbles.
How far up a mouse can hear,
and how far an elephant's call can travel through the ground.
≈ 100 kHz
Top of the mouse hearing range
Mouse hearing extends into ultrasound, from 1 to about 100 kHz. The human range is between 20 Hz and 20 kHz, so mice hear about five times higher.
MOUSE HEARING · JAALAS 2010
6.4 km
Estimated range through the ground
Bristol and Oxford researchers estimated that a bull elephant's rumble could be detected through the ground up to 6.4 km away, against 3.6 km for a fast walk.
ELEPHANT RUMBLES · PHYSICS WORLD
The same 2010 study found that sound clearly audible to people may be heard less, or not at all, by mice, because of its pitch. High-pitched sound also fades more quickly with distance than low-pitched sound.
HOW BAT DETECTORS WORK
Three ways to
bring a bat down.
The Bat Conservation Trust describes several ways a detector turns ultrasound into something we can hear. Many detectors combine more than one.
Heterodyne: tune and listen
You tune to a frequency, say 50 kHz. A bat calling at 49 kHz is mixed with it and you hear the 1 kHz difference.
Frequency division: divide by ten
Every incoming frequency is divided, normally by ten, so a 50 kHz call becomes 5 kHz, heard in real time.
Time expansion: slow it down
The detector records about a second of sound and replays it usually ten times slower, so 50 kHz drops to 5 kHz.
Full spectrum: record it all
Now the mainstream choice, full spectrum detectors capture ultrasound directly in high definition for later analysis.
WHY IT MATTERS
Loud to us is not
loud to everyone.
Sound levels for people can be given as A-weighted decibels, which roughly match what humans hear by playing down pitches we are less sensitive to. That makes sense for people, but not for animals whose hearing sits higher or lower.
The researchers warn that judging noise only by the human hearing range can lead to wrong conclusions about animals. A jackhammer far away may be loud to us but close to silent for a mouse.
Questions
Questions people ask
What is the range of human hearing?
Roughly 20 Hz to 20 kHz, with hearing most sensitive at about 1 to 4 kHz. Individual hearing varies with age, noise exposure and other factors.
What is ultrasound?
Sound above the range of human hearing. Bat echolocation calls are usually pitched too high for people to hear without a bat detector.
What is infrasound?
Sound below the range of human hearing. Elephant rumbles are usually under 20 Hz and can also travel through the ground as vibrations.
Why might a mouse not hear a distant jackhammer?
In a 2010 study, jackhammer noise stayed loud for people on floors above, but for mice it fell close to the level of the cage's own ventilation fan, partly because high-pitched sound fades faster with distance than low-pitched sound.
Which bat detector should a beginner choose?
Many people start with a heterodyne detector on a bat walk, but the Bat Conservation Trust says beginners increasingly choose full spectrum detectors, many of which also offer heterodyne listening.
Where the facts come from
- Reynolds et al. (2010) Noise in a laboratory animal facility from the human and mouse perspectives, PMC
- Bat Conservation Trust: Flight, food and echolocation
- Bat Conservation Trust: Heterodyne bat detectors
- Bat Conservation Trust: Frequency division bat detectors
- Bat Conservation Trust: Time expansion bat detectors
- Physics World: Elephant rumbles travel through the ground