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Wednesday, December 15, 2010

SOUND sensation

Those cicadas are still chirping outside the window, and the computer is whirring. These sensations are clearly conveyed to me by sound. But what is sound? The mechanical nature of sound Like light, sound is also a form of physical energy, but this type of energy is mechanical. Sources of sound cause the air molecules next to them to vibrate with certain frequencies; these vibrations are transmitted to neighbouring molecules and cause waves of vibration to spread outwards from the source, just like waves spread on a calm pond if you throw a pebble into it. In this way, sound can travel around corners, unlike light. So sound conveys a very different form of information than light. Since it is not constrained to travel in straight lines, it can tell us about things that are out of sight – but at a price. The price is that sound cannot tell us about spatial location with as much precision as light can; this is a consequence of its physical properties, and nothing to do with our ears.

As sound travels through the air, the air pressure at a given point will change according to the frequency of the sound. We are sensitive to a range of frequencies [frequency the rate at which a periodic signal repeats, often measured in cycles per second or Hertz (Hz); the higher the frequency, the higher the perceived pitch] from about 30 Hz (Hertz in full, which means cycles per second) to about 12 kHz (or kiloHertz, meaning thousands of cycles per second). Figure 7.5 shows the patterns of waves reaching us from objects vibrating at a given frequency.
Using sound to locate objects
As we have already seen, sound also travels much more slowly than light, with a speed of about 300 metres per second. Even though this is still pretty fast, it is slow enough for our brains to process time-of-arrival information. It takes sound just under one millisecond to travel from one side of the head to the other. This information can be encoded by neurons (Moore 2003; see also chapter 3), giving information about what direction the sound is coming from. Sound also gets reflected or absorbed by surfaces. Think about echoes in a cave. These are extreme examples of a process that happens less spectacularly, but more usefully, in everyday life. Subtle echoes give us clues about the location of large objects, even in the absence of vision. Blind people tend to develop this skill to a higher level, using sticks to tap the ground and listen for new echoes. Bats use echolocation to fly at night.

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