Audio (Basics): Part 2

Hi again.

At the very end of Audio (Basics), Part 1 we introduced the concept of Frequency when discussing waves. We also mentioned some basic information about the speakers in the room, which create the waves that we eventually hear. This article will build from there. You can skip all of this and you can still talk to a technician, but it is really simple. It just looks long because there are a lot of examples.

It was said that we can think of sound like the waves made by a falling pebble on the surface of a pond. This isn’t exactly true, but that is the trouble with analogies – they are similar by not exact. At least you can see a wave in water. A sound wave, not so much. So, we’ll proceed with this analogy as far as we can and explain the difference later. Because we have to learn about sound, and sound is made of waves that are created by the speakers.

So, the experiment is dropping a marble in a pond from the same height each time. If we look closely at the expanding waves, we will notice that the first wave is always taller. As the wave moves away from the source point it gets shorter and shorter. But, while we can see it, the distance from the peak of one wave to the peak of the next stays the same.

What we are seeing is that the power is getting distributed around the water in the circle, so the height goes down. But while it was happening, the number of waves going past the place you were looking at was constant. If we could look while also measuring time, we would notice that the number of waves that go past in the first 5 seconds is the same number of waves that go past in the next 5 seconds.

The distance between the peaks of the waves is called the Wavelength. The number of waves every minute or waves per second is called the Frequency. These two are completely related – as the number of one goes higher, the number of the other goes lower. The higher the Frequency – that is, the number of waves going past per second or minute – the shorter the peak-to-peak Wavelength. And, the opposite; the lower the frequency, the wave length is longer.

An easier example of these opposites is waves at the beach. If we see them crashing to the shore at 15 a minute, we can probably look into the distance and see several waves coming in. (High frequency, short wavelength.) But if you see the surfer who has to stand on her board to see the next wave – that is, the peaks are very distant, that means they have a long wavelength, and sure enough, there is a low frequency – you will see that there are only a few waves per minute crashing on the shore.

You can almost see this with a piano or guitar or harp string. When the low note is hit or picked, the string travels back and forth so slowly that you can practically see it (although, no matter how fast I can count, I can’t keep up.) But the actual sound wave that it is generating is very long. For example, the low note on the piano moves back and forth 27.5 times every second – we say 27.5 cycles per second. The wavelength – and you’ll just have to believe the science people on this – is over 10 meters long…over 33 feet!

And, here is the important part – you can pound on that note hard or soft, but the frequency of the strings and the sound will be the same…and the wavelength will be the same! And the same is true of a high frequency note, which might have a wavelength of only 6 inches (.15 meter), and a frequency of 2,500 cycles per second.

So, let’d end Part 2 here. Just one more silly thing.

Mr. and Mr. Hertz raised a very clever son who figured out that the theories of a very clever guy from Scotland named Maxwell were probable. The theories were about electricity and magnetism in a time when they were both considered spooky actions at a distance. It was a classic example of what Issac Asimov meant when he said “Any sufficiently advanced technology is indistinguishable from magic.” Anyway, his work was all about understanding waves and you will hear (or read) “cycles per second” called Hertz (abbreviated ‘Hz’), or kilohertz (kHz is the formal abbreviation, but the slang abbreviation is just ‘k’ – so you’ll hear, “The explosion had no sound above 1k”, meaning, there were no high frequencies above 1,000 Hz (kilo- means ‘thousand’)

Next we will tie these all together, add a little power and figure out what these terms have to do with your auditorium.

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