Sound is mechanical vibrations that travel through the air or any medium and can be heard when they reach a persons or animals ear. Sound is very important for communication. Normally a healthy person can detect sounds between 20 Hz to 20 kHz. So it is impossible to hear frequencies below the range or above the range. The sound which frequency is below 20 Hz is referred to infrasonic. On the other hand sound which frequency is above 20 kHz is referred to ultrasonic. We can’t hear infrasonic and ultrasonic because of the special characteristics of our ear.
We can’t hear ultrasonic sound because the eardrum which is established in the last part of the external ear prevents the sound to reach into the inner hair cell. Normally when a sound is produced it vibrates the air molecule. The pinna of the external ear collects this energy and transmits it to the auditory channel. Then the frequency of the sound vibrates the eardrum. Eardrum vibrates with the frequency of the sound and ossicles of the ear also vibrate. ossicles are made up of three tiny bones. The three tiny bones of the middle ear act as levers and amplify the vibrations of the sound wave. They carry the sound waves to the inner ear. The inner ear consists of a cochlea, the semicircular canals, and the auditory nerve. The cochlea contains two types of hair cells. They are called inner hair cell and outer hair cell. Inner hair cells transform the wave into nerve impulses. This process is called transduction.
So we can understand that inner hair cell is mainly responsible for humane hearing. If the frequency or energy of the sound are unable to reach into the inner hair cell, we won’t be able to hear the sound. The eardrum does not allow ultrasonic sound to reach into the cochlea because our eardrum is heavy and thick. On the other hand ultrasonic sound has a higher frequency. So our heavy eardrum does not vibrate properly. The ossicles also vibrates with the frequency. Our ossicles should be light to hear higher frequency. Many animals such as bats can detect frequencies beyond 100 kHz, possibly up to 200 kHz. Because the eardrum of bats is thin and it can vibrate properly in case of ultrasonic sound. Some ultra sonic sounds are occurred in space. Sound cannot pass without medium. So we can’t hear these sounds.
We can’t hear infrasonic. The range of infrasound is between 0-20 hertz. Outer hair cells cancel the infrasonic sound. Infrasound can travel for long distances without losing strength. There are about 12000 outer hair cell and 3500 inner hair cell. These outer hair cells are considered as a miniature which helps to amplify the sound. Another function of these cells is to counteract very low frequency, infrasonic vibrations. So, these cells cancel the infrasound. That’s why we can’t hear infrasonic.