Sound

When we think of sound, we think of our ears and what we hear. But what is sound? To start, sound is a wave. Specifically, it is a mechanical, longitudinal wave. From a physicist's point of view, in order for a sound to exist, first there has to be a source of sound. Second, the energy is transferred from the source via longitudinal sound waves. Finally, there has to be a detector. In our case, it is our ear that hears the sound. If a tree falls in the woods and no one heard it, was there a sound? Philosophically, maybe. Physically, probably not.

Sound must have a medium to travel in - it cannot travel in a vacuum. Sound travels through different media at different speeds. We are all familiar with the difference between the speed of sound and the speed of light - just think about seeing a lightning strike and then waiting to hear the thunder. This is because the speed of sound in air is much slower than the speed of light. For objects that are nearby, we probably don't notice any lag. For objects at a distance, it is very noticeable. Some objects may travel faster than the speed of sound - we refer to these as supersonic. The speed of sound in air is 331 m/s at STP. At other temperatures, it is given by:

V of sound in air = (331 + .6T) m/sec T=temp in oC

For example, as temperature goes up to 20oC, the speed of sound increases to 343 m/s. The speed of sound can vary greatly for different materials. In helium, for example, sound travels at 1005 m/s. In fresh water, it travels at 1440 m/s, and in aluminum, it travels at about 5100 m/s.

The average human ear can detect sounds in the frequency range of 20 Hz - to 20,000Hz. We call this the audible range. The actual frequency we refer to as pitch. Sounds below 20 Hz are referred to as infrasonic. We feel these rather than hear them. Above 20 kHz, sound waves are referred to as ultrasonic. High frequency, or ultrasonic waves can be used in medical imaging and distance measurement. Many animals hear sounds in these frequencies, some as high as 50 kHz.

Sound waves are longitudinal waves. They consist of a compression of the medium molecules, followed by an expansion or rarefaction of the molecules. Think of a loudspeaker or a drum head vibrating back and forth. This, in turn, causes the air molecules to vibrate back and forth and produce a longitudinal wave. Tuning forks, strings, and pipes all cause air to start vibrating with longitudinal waves. Compressions are the crests, or high pressure regions of the wave, and the rarefactions are the troughs, or low pressure regions of the wave. Sound waves travel in all directions and obey inverse square laws for intensity vs. distance. We'll talk about this in a later lesson. All other characteristics of a wave (speed, frequency, wavelength relations) remain the same.

For more on sound, try http://www.physicsclassroom.com/Class/sound/.

For Practice Problems, Try: Giancoli Multiple Choice PracticeQuestions (Questions 1-12)