Sound Wave

Sound Wave

• Sound wave is a mechanical wave that requires a medium for its propagation. Therefore sound wave cannot propagate in vacuum.
• The medium for propagation can be solid, liquid or gas.
• Sound waves propagate fastest in solid and slowest in gas.

Sound Wave as a Longitudinal Wave

Sound wave is a series of compression and rarefaction of layers of air molecules repeatedly through space.

The forward and backward vibration of the air molecules in the direction of motion of a sound wave shows that sound is a longitudinal wave.

Types of Sound Wave

• Human ear is capable of hearing sound with frequency in the range of 20Hz – 20,000Hz, and the sound wave with frequency in this range is called an audio/Sonic wave.
• Sound wave with frequency lower than 20Hz is called an Infrasonic Wave.
• Sound wave with frequency higher than 20,000Hz is called an Ultrasonic wave.

Speed of Sound Wave

• Speed of sound wave in solids is greater than in liquids, which is greater than in gases.
• Speed of sound in air is not affected by pressure, but is affected by the temperature.
• As temperature increases, speed of sound in air (and other gases) is also increases.
• Sound usually travels more slowly with greater altitude, due to reduced temperature.Speed of sound can be calculated by the equation

Pitch and Loudness

Loudness is influenced by the amplitude of the sound wave.The greater the amplitude of sound wave, the higher the loudness of the wave.

The pitch of a sound is high or low of the sound. The pitch of sound is determined by its frequency. The higher the frequency, the higher the pitch.

Reflection of Sound Wave

The clicking sound of the stop watch can be heared clearly when the angle of incidence, i is equal to the angle of reflection, r.

Sound waves obey the law of reflection. That is, the angle of incidence is equal to the angle of reflection.

Refraction of Sound Wave 1

The amplitude of the sound wave increases when a balloon filled with carbon dioxide is placed between the speaker and the microphone.

Explanation:

• Carbon dioxide is denser than air.
• The sound wave is refracted approaching the normal when the wave propagates from the air into the balloon, and away from normal when moving out from the balloon.
• As a result, the balloon acts as a biconvex lens and converge the sound wave at a point.

Refraction of Sound Wave 2

The amplitude of the sound wave decreases when a balloon filled with helium gas is placed between the speaker and the microphone.

• Helium gas is less dense than air.
• The sound wave is refracted away from the normal when the wave propagates from the air into the balloon, and closer to the normal when moving out from the balloon.
• As a result, the wave is diverged to a wider area and causes the amplitude of the sound wave decreases.

Refraction - Natural Phenomenon

The sound of a distance train is clearer at night.

Explanation:
At night, the air closer to the ground is cooler than the air further from the ground.

Sound wave travel slower in cool air. As a result, the sound wave is refracted in the path of curve towards the ground instead of spreading to a wider area (as at daytime).

Diffraction of Sound Wave

The street cleaner can hear the sound of the radio even though it is behind an obstacle.

Explanation:
The sound of the radio spreads around the corner of the wall due to diffraction of sound wave.

Interference of Sound Wave

Alternating loud and soft sounds are detected as the microphone is moved from left to right.

Explanation:
The two loud speakers are sources of two coherent sound waves as they are connected to the same audio signal generator.
The alternating loud and soft sounds are caused by interference of the sound wave.

Formula of Interference

Wavelength of the sound wave is given by the equation