Sound and noise

Listen to this. ... and to this. Which one is louder? Now listen to this. Is it even louder, still?

What about this? These are four tones. Let's listen to them again. Number two and three are loud. They have higher amplitudes than the first and the last one.

Number three and four are bass tones. They have a lower frequency and a larger wavelength than the first two. Tones are regular patterns of compressions and rarefactions that show up as smooth curves when measured with an oscilloscope, like here. Now, listen to these sounds. They all play the same tone - a C - at the same intensity.

But even though both the tone and the volume are the same, each instrument sounds distinctly different. That's because instruments don't produce just a single tone. Little variations, over-tones, noise, and the way the tone changes slightly while being played, all make up the tone quality, or timbre of the tone. Each instrument or voice has its own timbre (tambre). What about these sounds?

Those aren't tones at all. This is noise. Or, well... Noise isn't really a physical concept... Noise is unwanted sound.

But what's noise to one person may be music to another. However, it's not only a matter of personal taste -- noise matters. If you live or work in a place where there is a lot of noise... ... then, in the long run, noise could affect your health. Noise can cause your stress levels to rise, which has a negative effect on your sleep.

It raises blood pressure, and weakens the immune system. A weird thing is that the noise doesn't even have to annoy you. It can do all these things, even though you may have become so used to the noise that you don't even think about it. Noise can also damage your hearing of course. Well, that can happen even if you do enjoy the sound you're listening to.

Playing music too loud in your headphones can lead to a nasty thing called tinnitus. This is when you experience wheezing or humming in your ears, when in reality there is perfect silence. Unwanted sounds that come from farther away make less of a noise than sounds from a nearby source. If you double the distance, the intensity of the sound drops to just a quarter. The sound intensity drops as the inverse square of the distance.

If you can't increase the distance from the noise source, you could insulate it. If you place some material between yourself and the sound source, you will hear less of the sound. If the material has the right properties, it will absorb the energy of the sound without passing it on. If that isn't possible either, then there is one more, quite fascinating way to block out the noise. Here, the noise that Philip is making encroaches upon Kim.

But Kim is wearing headphones. These special headphones have a tiny microphone on them that captures the noise, and immediately plays it back, but inversed. Where there is a compression in the noise, the speakers cause a rarefaction. And when a rarefaction happens in the noise, the speakers immediately create a compression. This way, the noise is cancelled.

These are active noise-cancelling headphones. Aaaah, peace and quiet, at last!