Physics

# Sound and Vibration

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Bang a drum, blow a whistle, or pluck a string: however you make a sound, it spreads out like ripples on a pond. However, there are some differences. Waves in water have an up-and-down motion as they move away from their source. Sound waves are more complex, and of course, invisible, which makes them harder to understand.

Sound travels through air as a wave of pressure that pushes air molecules together, and after it’s passed they spread out again. If that’s hard to visualize, think of a coiled, spring-like Slinky and how a ripple can move from end to end: the coils bunch together and then spread out as the wave passes. There are animations of this process on the Fear of Physics website, and also at The Physics Classroom.

A single burst of sound, like the thump of a drum, moves through the air as a single compressed wave of energy. Other sounds, like a vibrating guitar string, keep putting energy into the air, creating a whole series of pressure changes that are detected by our ears.

Since the transmission depends on molecules acting on their neighbors, it follows that the speed of sound depends on the medium it’s passing through. In simple terms, the closer the molecules are packed together the less distance each has to move to affect its neighbor. In other words, the denser the medium the faster the pressure wave travels.

If you’ve ever watched the drummer in a marching band you may have noticed that the sound of the drum reaches your ears while the stick is raised in the air. This happens because sound moves relatively slowly through air – actually at about 340 meters per second or around 760 miles per hour. Sound moves much faster in water  – around 1,500 meters per second or over 3,000 miles per hour – while it can’t pass through a vacuum at all. So it’s true that in space no one can hear you scream!

Temperature also has an effect on how quickly sound moves, as noted on the Johns Hopkins University web page, “The Propagation of Sound”. Temperature affects the density of a liquid or gas: when it gets warmer the molecules spread out and density reduces. As a result, sound waves move slower through a hot medium than one that is cold.

Sound has two characteristics that we’re all aware of: it can be loud or soft, and it can vary in pitch from very low or deep to very high.

Loudness, also termed intensity, is measured in decibels. The important thing to know about decibels is that they rise on a logarithmic scale, meaning that volume doubles with every increase of ten decibels. To put that in proportion, a sound of 80 decibels – about the level of a food blender - is actually twice as loud as one of 70 decibels, such as a vacuum cleaner.

Pitch is measured in Hertz, or wavelengths per second. The human ear can detect sounds ranging from around 20 Hertz all the way up to 20,000 Hertz, although sensitivity to higher frequencies reduces as people age. Some animals can hear much higher frequencies. According to work at Louisiana State University, dogs can detect frequencies as high as 45,000 Hertz while the hearing of bats goes as high as 110,000 Hertz, far above human hearing.

Bats of course are famous for the ability to navigate by sound. They send out extremely high frequency ‘chirps’ and listen for the echoes. This also is the principle behind the ultrasound scanners used in medicine. These are highly complex machines but the principle is the same as that used by bats. Pulses of high frequency sound are sent out, and reflect back an echo every time they encounter a change in density. So for example, the sound wave will pass through soft tissue but will reflect back from bone.

In an ultrasound system a highly sensitive receiver listens for the returning echoes. By measuring the time it take the sound to travel out and return it’s possible to calculate a distance to a change in density. From this signal it’s possible to construct the highly detailed images of unborn babies that we’ve all become familiar with.

Sound is one of those phenomena most of us take for granted, never wondering how it reaches our ear. Delve into the science of sound, or acoustics, to give it it’s rightful name, and it becomes apparent that sound influences our lives in many ways, from music to healthcare.

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