Production and Transmission of Sound Waves

Vibrating Reed

• As the vibrating reed moves from a to b, it compresses the gas molecules of the air immediately to the right.

• The reed transfers energy to the molecules in the direction in which the compression occurs. At the same time the air molecules to the left expand into the space behind the moving reed and become rarefied. Thus the movement of the reed from a to b produces a compression and rarefaction simultaneously with a net transfer of energy to the right, the direction of the compression.

• As the reed moves in the reverse direction from b to a, it compresses the gas molecules to the left, while those to the right become rarefied.

• The combined effect of this simultaneous compression and rarefaction transfers energy to the left, again in the direction of the compression.

• If we consider just the series of compressions and rarefactions produced to the right, it is apparent that the maximum compression occurs as the reed moves through its equilibrium position o traveling from a to b.

• The maximum rarefaction occurs as the reed moves through its equilibrium position o traveling from b to a.

• Simultaneously, of course, a corresponding series of rarefactions and compressions is produced to the left. The vibration of the reed thus generates longitudinal trains of waves. In these longitudinal waves, vibrating gas molecules move back and forth along the path of the traveling waves, receiving energy from adjacent molecules nearer the source and passing it on to adjacent molecules farther from the source. Sound waves are longitudinal waves.

Sound Transmission

• The requirements for the production of sound waves are:

• 1. A source which initiates a mechanical disturbance.

• 2. An elastic medium through which the disturbance can be transmitted.

Density Matters

• Dense air is a more efficient transmitter of sound than rarefied air.

• At high altitudes, where the density of the air is lower, less energy may be transferred from the source to the air.

• Sound does not travel through a vacuum. It is only transmitted through a material medium.

• Sound travels very effectively through solids and liquids.