Salam (May God Bless You). Sound waves are mechanical longitudinal waves which can travel only through a medium. Sound is more easily described as a disturbance in a medium, more commonly air. It has the same properties as any other wave, it can refract and it can reflect. A reflected sound is called an echoe. Its reflection and refraction is already discussed in my previous article Waves.

#### Sound Waves

Sound waves occurs when an object vibrates. The vibration of the object causes the air particles to vibrate as well causing the air particles to form rarefactions and compressions. A compression is an area in which particles are squashes together causing an area of high pressure. A rarefaction is an area in which particles are further apart from each other. This creates an area of low pressure. The wavelength of the wave is found by measuring the length from the middle of a compression to the other compression or from the middle of a rarefaction to the other rarefaction.

#### Pitch and Loudness

The pitch of a sound wave depends upon the frequency of a wave or more accurately the wavelength of the wave. The shorther the wavelength the higher the pitch and the longer the wavelength the smaller the pitch.

The loudness of a wave depends upon the amplitude of the sound wave. The longer the amplitude, the more loud the noise is and if the sound wave has a smaller amplitude, the sound will be much more quiet.

#### Harmonics

Different instruments tend to five different sound when playing the same note. Even if the frequency and the amplitude are same the sound will always be different. This is because these instruments produce a whole complex waveform. Each waveform contains a set of waves called harmonics or sometimes also known as an overtone (plural for harmonic). Each harmonic has different frequency, the first harmonic tends to have the lowest frequency and the frequency increases with the next harmonic.

#### Audible sounds

Humans can only hear sound waves which have the frequency of 20 Hz – 20,000 Hz. Human can not hear any frequency beyond or below that. This slowly decreases with age and also due to continouos exposure to loud noises.

#### Ultrasound and its uses

Sound waves with the frequency above 20,000 Hz is called ultrasound. Ultrasound has a lot of uses. One of its most common uses includes prenatal dioagnosis. A handheld device is used to emit ultrasound waves, these waves are then reflected back when it crosses the boundries between the different tissues of the mother and the fetus. The waves are then recepted by the device and then sent to the computer to form an image.

Other uses include, it is used to clean delicate objects. Once emitted, the dirt particles vibrate at the same rate as the air particles in the wave. This causes it to break down and fall. Other than that it is also used to find faults in building structures and other things. It uses the same basic principle, the waves are reflected when the waves touches the faults to the device to form images of the faults.