(a) What are infrasonic waves ? Name two animals which produce infrasonic waves.
(b) What are ultrasonic waves ? Name two animals which can produce ultrasonic waves.
(c) The audible range of frequencies of an average human ear is from 20 Hz to 20 kHz. Calculate the corresponding wavelengths. (Speed of sound in air is 344 ms^–1)

(a) Infrasonic sound waves have frequencies lesser than 20 Hz. It is inaudible as its frequency lies below the human audible range of frequencies (20 Hz–20 kHz). These sounds are produced by objects vibrating very slowly. Whales and elephants can produce these sounds.

(b) Ultrasonic sound waves have frequencies higher than 20 kHz. It is inaudible as its frequency lies above the human audible range of frequencies (20 Hz–20 kHz). These sounds are produced by objects vibrating very rapidly. Bats and dolphins can produce these sounds.

(c) We have to calculate the wavelength range of audible sound.
Given: Speed of sound ν = 344 m/s
Lower frequency of audible range f_L = 20 Hz
Higher frequency of audible range f_H = 20,000 Hz
We know relation of velocity, frequency, and wavelength $\lambda =\frac{\nu }{f}$,
where ν is the velocity, f the frequency, and λ the wavelength.
So, wavelength corresponding to the lower frequency is
$$\begin{gathered} {\lambda }_{\mathrm{L}}=\frac{344}{20} \\ =17.2\mathrm{m} \end{gathered}$$
So, wavelength corresponding to the higher frequency is
$$\begin{gathered} {\lambda }_{\mathrm{H}}=\frac{344}{20000} \\ =0.0172\mathrm{m} \end{gathered}$$
Hence, the range of wavelength in audible region is 17.2 m − 0.0172 m.