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# CSEC Physics: Behaviour of Sound Waves

The speed of sound refers to the distance travelled per unit time (speed) by a sound wave as it propagates through an elastic medium. In dry air at 20Â°C, the speed of sound is 343 m/s (1238 km/h). In water, it is 1481 m/s, and in iron, it is 5120 m/s.

For example, if you have ever seen lightning strike off in the distance, then you will know that the sound of thunder always comes after the lightning bolt has disappeared from your sight. This is because the speed of sound (343 m/s) is far slower than the speed of light (299Â 792Â 458 m/s). Thus, you will see the lightning instantaneously (within a few thousandths of a second) while, depending on the distance, the sound from the sonic shock wave (thunder) will take between a tenth of and several seconds to reach you.

Behaviour of Sound Waves

Reflection

When a sound wave reaches a boundary (like a wall) that separates one space from another, some of the wave is transmitted across the boundary while the other portion is reflected. The amount of sound that is reflected relies mostly on how different the material of the boundary is from the medium the sound wave was travelling in. This is why recording studios are usually built with soft materials like foam, and the world's quietest room (the anechoic chamber at Orfield Laboratories) is built with wedges of fibreglass to absorb all sound waves within the room.

The same is true for acoustic performance halls, which, instead of using hard materials like concrete (which will reflect sound waves) utilize softer materials like acoustic tiles to absorb sound waves. These materials are more similar to air in composition, and thus can absorb sound more readily.

When sound is reflected, there are two possibilities: echoing or reverberation.

Reverberation happens mostly in rooms smaller than 17 meters in height, width and length. This is because the effect of a sound wave on the brain lasts for more than a tiny fraction of a second- that is, the human brain keeps a sound in memory for up to 0.1 seconds. If a reflected sound wave reaches the ear within 0.1 seconds of the initial sound, then it seems to the person that the sound isÂ prolonged. The reception of multiple reflections off walls and ceilings within 0.1 seconds of each other causes reverberations - the prolonging of a sound. Since sound waves travel at about 343 m/s at room temperature (1,115.5 f/s) it will take approximately 0.1 second for a sound to travel the length of a 17-meter room and back, thus causing a reverberation. This is why reverberations are common in rooms with dimensions of approximately 17 meters or less.

You are likely more familiar with echoes, which you might hear when shouting in a new house with no furniture or when honking a car horn in a tunnel or underpass. The sound waves reflect off of the hard surfaces of the walls (some of the wave is absorbed, which is why the echo is always a little weaker than the initial sound).

Refraction

Refraction of sound waves occurs when they move from one medium to another, causing a change in direction. Whenever refraction occurs, there is also a shift in the wavelength and wave speed of the sound wave. For example, when sound passes from air into water, it is refracted, and undergoes these changes. If you have ever dipped underwater in a pool while music is playing, the music will begin to sound muffled and lower pitched. This is because the sound is refracted, because the water is a different medium to the air.

Diffraction

Diffraction is when sound waves change direction when passing through an opening or around an opening in their path. Sound waves diffract (or bend) more sharply with increasing wavelength- that is, sounds with lower frequencies and long wavelengths (lower pitched sounds) tend to diffract more sharply and carry further distances than sounds with higher frequencies and short wavelengths.

This is why you can hear a conversation from around a corner, or even hear someone speaking through an opening in a door.

Diffraction also helps us to explain why bats use ultrasound (sound above 20 kHz) to echolocate their prey. This ultrasound has a very high frequency (sometimes in excess of 100 kHz), meaning it has a very short wavelength, reducing its tendency to diffract. Bats need to detect their minute prey (mere centimetres in length), so they need their sound waves to reflect off of the insects instead of diffracting around them. Their ultrasonic waves are smaller than the dimensions of their prey, so they reflect instead of diffracting, allowing them to echolocate their prey.

Interference

When two sound waves are present from two different sources at the same time, they interact with one another to create a new sound wave.

There are 2 types of interference, constructive and destructive interference.

When the compressions and rarefactions of the two sound waves line up, it is known as constructive interference.

Both waves interact creating a new wave that is the sum of both of the waves interacting. For this to occur, both the troughs and peaks must be lined up, also known as being in phase.

When the compressions and rarefactions are out of phase (not exactly aligned), their interaction creates a wave with a dampened or lower intensity. This is destructive interference. When waves are interfering with each other destructively, the sound is louder in some places and softer in others. As a result, we hear pulses or beats in the sound.

Waves can interfere so destructively with one another that they produce dead spots, or places where no sound at all can be heard. Dead spots occur when the compressions of one wave line up with the rarefactions from another wave and cancel each other.