11.2 Frequency of Sound
Sound frequencies may be divided into the following categories, depending on whether or not they are within
the range of human hearing:
- Infrasonic.(f<20Hz) These sounds are at frequencies too low to be audible to humans.
- Audible.( 20 Hzf20;000Hz) This is the range of human hearing.
- Ultrasonic.(f > 20;000Hz) These sounds are at frequencies too high to be audible by humans.
These are only approximate ranges. In particular, there is a strong correlation between the highest audible
frequency and the person’s age; as we get older, we become less able to hear very high-frequency sounds.
Infrasonic sounds are inaudible to humans, but can be heard (and produced) by some animals like whales
and elephants. Some natural phenomena like earthquakes also produce infrasonic sounds.
Ultrasonic sounds are also inaudible to humans, but can be heard by some other animals, like dogs, bats,
and dolphins. The familiar dog whistle produces a high-pitched sound that is inaudible to humans, but can be
heard by dogs. Ultrasound has several practical uses: it is used in some cleansing processes, and for medical
imaging.
Table 11-2 shows the hearing ranges (frequencies) audible to different animals.
Table 11-2. Hearing ranges for various animals. [6]Species Hearing range (Hz)
Turtle 20 – 1,000
Goldfish 100 – 2,000
Frog 100 – 3,000
Pigeon 200 – 10,000
Sparrow 250 – 12,000
Human 20 – 20,000
Chimpanzee 100 – 20,000
Rabbit 300 – 45,000
Dog 50 – 46,000
Cat 30 – 50,000
Guinea pig 150 – 50,000
Rat 1,000 – 60,000
Mouse 1,000 – 100,000
Bat 3,000 – 120,000
Dolphin (Tursiops) 1,000 – 130,000