Loudness is measured in decibels (dB). Some common sounds on the decibel scale are listed below.

Threshold of Hearing 0dB Normal Breathing 10dB Rustling Leaves 20dB 30dB Library 40dB 50dB Normal Conversation 60dB 70dB Noisy Office with machines 80dB Heavy Traffic 90dB 100dB Construction Noise (up close) 110dB Threshold of Pain 120dB Rock Concert 130dB 140dB Jet Takeoff (from runway) 150dB

Sound waves are loud if their compressions are dense. This is analogous to the wave having high crests.

The higher the frequency, the faster your eardrum vibrates. This is what causes pitch.

The sensitivity range for human hearing depends on the loudness and pitch. Noises along each black line would be heard with the same volume.

Your eardrum acts like a trampoline. When air molecules bounce on it, it vibrates and you hear sound.

Standing Waves

When a wave reflects off of something, it can interefere with its own reflection. The interference is alternately constructive or destructive as the two waves move past each other. This creates a standing wave.

Certain points along the standing wave never move. These points are called nodes. The points that move the most are called antinodes.

Along any length, only waves with certain frequencies can create standing waves. This is because the distance from one node to the next must always be some fraction of the total length (one half, one third, etc.). The different standing waves corresponding to different fractions are called harmonics. The first three harmonics are shown on the right.

Notes

Sounds are waves moving though the air. Notes are just standing waves of a specific frequency. On a piano these notes are organized into an assending scale.

The musical scale used today was first discovered by the Greek mathematician Pythagoras in the 6th century BC. While playing the lyre, he noticed that he could create the different harmonics by placing his finger at fractional points on the string. Where he placed his finger corresponded to the nodes of each harmonic.

Timbre

So if notes are just waves of different frequencies, why do all the different instruments sound different when they play the same notes? Actually an instrument always plays several frequencies at once. The most dominant frequency is that of the note, but many others are played as well. These are determined by the shape of the instrument. This spectrum of frequencies is called the instruments timbre.

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