4. Ultrasound's and its Application

Characteristics of Sound Wave

• The characteristics of sound waves are : wavelength, frequency, amplitude, time period and

velocity.

→ When a wave travel in air the density and pressure of air changes from their mean position.

→ Compression is shown by crest while rarefaction is shown by trough.

→ Compression is the region of maximum density or pressure.

→ Rarefaction is the region of minimum density or pressure.

Wavelength

→ In sound waves the combined length of a compression and an adjacent rarefaction is called

wavelength.

→ The distance between the centres of two consecutive compressions or two consecutive

rarefactions is also called its wavelength.

→ It is denoted by the Greek letter lamda (λ). Its SI unit is metre.

Frequency

→ No. of complete waves produced in one second or number of vibrations per second is called

frequency.

→ Number of compressions or rarefactions passed in one second is also frequency.

→ Frequency of wave is same as the frequency of the vibrating body which produces the wave

• The SI unit of frequency is hertz (Hz). The symbol of frequency is v (nu).

• 1 Hertz: One Hz is equal to 1 vibration per second.

• Bigger unit of frequency is kilohertz kHz = 1000 Hz.

Time Period

→ Time taken to complete one vibration is called time period.

→ Time required to pass two consecutive compressions or rarefactions through a point is called

time period.

• SI unit of time period is second (s). Time period is denoted by T.

• The frequency of a wave is the reciprocal of the time period.

• v = 1/T

Amplitude

→ The maximum displacement of the particle of the medium from their original undisturbed

position is called amplitude of the wave.

• Amplitude is denoted by A and its SI unit is metre (m).

→ Sound have characteristics like pitch and loudness and timbre.

• Pitch: The pitch of sound depends on the frequency of sound (vibration).

→ It is directly proportional to its frequency. Greater the frequency, higher is the pitch and lesser the

frequency, lower is the pitch.

→ A woman’s voice is shrill having a high pitch while a man’s voice is flat having low pitch.

• Loudness: The loudness depends on the amplitude of the sound wave.

→ Loudness is the measure of the sound energy reaching the ear per sec.

→ Greater the amplitude of sound wave, greater is the energy, louder the sound; short is the

amplitude, less is the energy, soft is the sound.

→ Loudness is measured in decibel ‘dB’.

• Quality or Timbre: The timbre of a sound depends on the shape of sound wave produced by

the characteristic of musical sound.

→ It helps us to distinguish between two sounds of same pitch & loudness.

• Sound of single (same) frequency is called tone while a mixture of different frequencies is called

note.

• The distance travelled by a wave in one second is called velocity of the wave.

• Its SI unit is metre per second (m/s ).

Velocity = Distance travelled/Time taken

⇒ v = λ/T

(λ is the wavelength of the waves travelled in one time time period T)

v = λv (1/T = v)

So, Velocity = Wavelength × Frequency

This is the wave equation.

Example: What is the frequency of sound wave whose time period is 0.05 second ?