Sound

Characteristics of sound waves

• It travels in the form of a longitudinal wave consisting of alternate compressions and rarefactions.
• It cannot travel through vacuum- it needs a medium to travel.
• Speed of sound increases with the density of a medium.
• Speed of sound increases with increase in temperature.
• It can travel through solids, liquids, and gases.
• The approximate rage of audible frequencies for a healthy human ear is 20 Hz- 20000 Hz
• Infra-sound: frequency less than 20 Hz
• Ultra-sound: frequency more than 20000 Hz
• Loudness of a sound wave ∝ amplitude of sound waves
• Pitch of sound waves is the number of cycles of sound waves as heard by a person / second. It is measured in hertz (Hz)
• The frequency of sound waves is the number of cycles emitted by a source per unit time
• Pitch ∝ frequency

Compressions and Rarefactions

• Compressions are the points in a medium at which particles are crowded. Hence, pressure and density would be maximum.
• Rarefactions are the points where particles are widely separated. Hence pressure and density is the least.

Speed of sound

3.4.2 Reflection of Sound: Echo

• Reflection of sound from an obstacle is called an echo.
• For a discernable echo, the time difference between two sound waves reaching the ear should be greater than 0.5 seconds
• The distance between the obstacle and the observer should be:

• where is the distance between the observer and the obstacle,  is the to-and-fro time, and  is the speed of sound in the air.

• This distance seems large, but if the sound can reflect multiple times before reaching our ear, then this echo can take place in smaller rooms.