SOUND

This week we will be studying the speed of sound and ways that sound can be characterized.

The sound that we detect with our ears is due to the vibrations of the air. Sound waves in air create regions of high pressure and low pressure. In general these variations in pressure are very complicated and produce the myriad of different sounds that we hear.

One particular sound that we all can identify is the musical note. It is a special sound because it is clearly identifiable but also because it plays a special role in the way we understand the physics of sound. A "pure" note has a pressure vs time and a pressure vs location dependence that vary according to the sine function. A"pure" note consists of many of these oscillations. This type of sound therefore has a long duration and is spread out. A graph of a sine wave is shown below. (The word note is used here to identify those oscillations that are only sinusoidal. A tuning fork can produce an almost perfectly sinusoidal oscillation. Real instruments combine higher harmonic waves with the fundamental sound wave to produce a note. Saxophones and violins therefore sound different when playing the same note. In this discussion we will use the term note only to refer to pure sine waves.)

The way we describe a general sound that may have a very complicated pattern of pressure variations, is to assume that it can be produced by playing an appropriate combination of notes. This is a profound and surprising result. Any sound can be through of as a sum of sine waves of varying amplitude. Finding the amount of a given note within a general sound is referred to as performing a Fourier transform.

In the first part of the experiment we would like to measure the speed of sound by timing a sound pulse as it travels a known distance. A tuning fork produces a note and this type of sound would not be appropriate for such a measurement because the sound is spread out. A finger snap or click is a short duration. It is interesting that the Fourier transform of a click or snap will show that a large range of sine waves are required. This type of sound bears little resemblance to a note but still can be described as a sum of many simultaneously played notes.

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This weeks quiz. Read the lab in the manual and be sure you glance at any materials that are available for this week's lab on the web.