Why the frequency range is limited

1. Every solid part that converts sound into vibration, such as an eardrum or a microphone diaphragm, has mass and stiffness.
2. Together, the mass and stiffness give the part a natural frequency at which it vibrates most easily.
3. When sound drives the part at or near its natural frequency, it vibrates with a large amplitude, an effect called resonance.
4. The further the frequency of the sound is from the natural frequency, the smaller the amplitude of the vibration becomes.
5. At very high frequencies, the part's mass gives it too much inertia to reverse direction quickly enough, so it barely moves.
6. At very low frequencies, the slow pressure changes are too weak compared with the part's stiffness to make it vibrate usefully.
7. Outside a certain range of frequencies, the vibrations are so small that no useful signal is produced.
8. This is why every microphone and loudspeaker has a stated frequency range, and why small earphones struggle to reproduce deep bass notes.
9. The same limit applies in reverse, because a solid can only produce sound waves over the range of frequencies at which it can vibrate.

The range of human hearing

1. The human ear can only turn sound into nerve signals over a limited range of frequencies, called the audible range.
2. For a young person with healthy hearing, this range is about $20Hz$ to $20000Hz$, or $20kHz$.
3. Sound below $20Hz$ is called infrasound, and sound above $20000Hz$ is called ultrasound.
4. Infrasound and ultrasound are still sound waves that carry energy, but the parts of the ear cannot respond to them strongly enough to produce signals.
5. Within the audible range, the frequency of a sound sets its pitch, so a higher frequency is heard as a higher note.
6. The ear is most sensitive to frequencies of a few thousand hertz, which helps you pick out the sounds of speech.
7. Other animals have ears built differently, so dogs can hear up to about $45000Hz$, bats hear well above $100000Hz$, and elephants communicate using infrasound.

How hearing changes with age

1. Inside the cochlea, rows of hair cells each respond best to a particular frequency.
2. Hair cells near the entrance of the cochlea respond to high frequencies, while those deep inside the coil respond to low frequencies.
3. Hair cells are gradually damaged over a lifetime and, unlike many cells in the body, they are not replaced.
4. The cells that detect high frequencies are usually lost first, so the upper limit of hearing falls as people get older.
5. By middle age many adults cannot hear sounds above about $15000Hz$, and the limit keeps dropping with age.
6. The eardrum and ossicles can also become stiffer with age, so they pass on vibrations less well.
7. Repeated exposure to loud sound, such as from concerts, power tools or earphones turned up high, damages hair cells faster and brings hearing loss on earlier.
8. Some UK shops have used devices that give out a tone of about $17000Hz$ to stop groups gathering outside, because most teenagers can hear it while most older adults cannot.