Training for Non-Technical Cinema Employees

Audio (Sound Basics): Part 3

Hi again. This is a work-in-progress…read and comment where the lesson should go next please.

At the very end of Audio (Sound Basics), Part 2 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 but 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 – the center, where the beginning energy was transferred from the falling rock into the water – the expanding wave 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.

Let’s look at those two sentences a little more closely. The power is getting distributed…the first force, the marble hitting the water, has to go somewhere. The water can’t just eat it, it has to distribute it. If the marble fell in a closed channel, the waves would go down the channel and the waves would stay taller for a longer length of time. But in a circle, the same energy is distributed all around, so the circle of the waves get bigger around, the the height gets smaller.

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, even if they are shorter.

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 3 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 Arther C. Clark 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.




If we hit a bell with a hammer, it goes ‘bong’ or rings with a high pitch, depending on how it was made. If we feel the bell while it rings we can feel it vibrating back and forth just like the guitar string.


In and out, not up and down.

The height of is not the frequency of the wave. The height shows the power of the wave – the force of the energy that is expanding outward from the source point. The frequency is the number of waves that go by a certain point in a particular length of time. The frequency can be the same even if the waves are taller or shortLike the piano strings or a guitar string being struck, we measure sound in wave cycles per second. In this case it is like the water wave, a cycle is measured from the when that top of a wave passes a point until the next top of the wave. If our eyes were able to see the string move as it goes back and forth, it would be a complete cycle from left to right to left again.

When we talk about sound waves, we talk about power as intensity or volume – how loud something is. And that is where we start talking about the son of Mr. and Mrs. Bell.

Basics: Audio (Sound). It is good to have a basic understanding of frequencies and speakers and surround and amplifiers and level and Loudness.

Frequency is a term used when describing both sound and light, so we will need to get a good idea of it. Sound frequencies are very easy to think of when we consider a musical instrument like the piano. From left to right, the notes start with bass and

There are two reasons for this: power and clarity.

Power is simple. The auditorium is large and the sound must get to all of the audience without being too soft for some and without being too loud for others.

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