Chapter 5: Sound techniques

In the audiovisual industry, sound is always the poor relation, it is always the one we talk about at the end, it is always the one we almost forget. And yet, it is no less essential than the image. Let’s explore its nature and techniques.

What is sound? And how is it that we hear?

In our ear, a complex system transforms a vibration in the air into a sensation of sound in our brain, via specific electrical impulses. When the string of a guitar vibrates, this vibration is transmitted to the air in which it is located, which in turn vibrates. It is the same principle when a stone is thrown into water. A wave forms in concentric circles around the impact.

This vibration of the air is what we call sound waves. It has an intensity (the sound is louder or softer, this is also called the volume of the sound), and a pitch (the sound is more or less low or high). The low pitches come from a slow vibration of the air (the thickest guitar strings), and the high pitches come from a very fast vibration of the air (the thinnest guitar strings). This pitch is called, in physical terms, the frequency (measured in Hertz - the number of vibrations per second).

A real sound is composed of the very complex addition of all the vibrations that occur at the same time. For example, in the case of a guitar, the string vibrates in several places (the sound is not pure), which are added together; this is called the harmonics, which is what makes the specific “color” of each instrument. Moreover, there is not only the string that vibrates, there is also the body of the guitar that resonates, that vibrates too, in a particular way to its manufacture, and then there is the noise of the cars that pass in the street that are added. In the same way, when we speak, our vocal apparatus forms a set of frequencies, which are mixed, which vary very quickly in height and in volume, to form the various phonemes and intonations.

With electronics, we can create pure frequencies by making a membrane vibrate in a regular way. For example, the tone of the telephone is a frequency of 440 Hertz (the frequency of A). You can use it to tune your guitar.

In the ear, therefore, like our whole body, our eardrum picks up this vibration of the air in a very fine way by being itself put in vibration, then the inner ear transforms this vibration into very complex bio-electrical information which will be interpreted by our brain, to make us hear. The ear is what one calls a mechano-bioelectric transducer. It transforms a wave, mechanical, into bio-electrical information that the brain is able to interpret.

The microphone, the loudspeaker and the tape recorder

Capture and transmit the sound:

• If we tighten a very fine and flexible membrane, it will, in the presence of a sound, make like the tympanum of our ear, vibrate, i.e. make a movement of before and behind proportional to all the variations of acoustic pressure due to the sound. It is the ancestor of the microphone.
• And on the other hand, if we take a membrane and make it vibrate, for example a rather flexible metal plate held in the hand, we will produce a sound, which will be the exact reflection of the vibrations of the object. It is the ancestor of the loudspeaker.

We can see that a microphone and a loudspeaker are the same thing, except that for the microphone its role is to pick up the vibrations that surround it, whereas for the loudspeaker its role is to transmit its own vibration to the air that surrounds it. Moreover, if you take a speaker and plug it into the microphone socket of your computer, for example, it will act as a microphone, and you will be able to record yourself with it (you will just have to speak very loudly to make its membrane vibrate, which is not very fine). And if you plug a microphone into the headphone jack of your computer, you will hear the sound through the microphone, which will act like a speaker. There is no risk for a speaker to work as a microphone, but there is a risk for a microphone to be used as a speaker...

We understand the principle, it is very simple, and children often do it with two empty plastic yoghurt pots, between which a string, very long, is stretched: one speaks into a yoghurt, whose bottom vibrates, this vibration is transmitted by the stretched string to the other yoghurt pot, whose bottom will vibrate proportionally and make the sound heard. The bottom of the yoghurt pot acts as both a microphone and a speaker.

How to record sound?

The principle is also very simple. On the membrane of the microphone, a small metal point is fixed, which will rise and fall according to the vibrations of the membrane.

Place a roll of soft wax in contact with this point, which rotates slowly. In the wax roll, a groove will be dug by the point, more or less deep according to the movements of the point, with more or less wide undulations according to the bass and treble.

Now let’s harden the wax. Let’s increase the surface of the membrane linked to the metal tip, to have a greater resonance. Let’s put the tip in contact with the roller, and let’s rotate the roller. The hollows and bumps of the groove in the roller will vibrate the tip, which will vibrate the membrane, which in turn will vibrate the surrounding air. And then we hear a sound that is the exact reproduction of what was recorded.

This is a primitive tape recorder.

The wax reel (invented by Thomas Edison in 1877) has been perfected into the LP record, which works on the same principle.

Analog sound

The variable depth of the groove is a very faithful reproduction (“analog”) of the variation of sound pressure in the air. This is why this method of recording sound is called “analog”. This is theoretically the best quality that can be obtained, because the slightest inflection of the sound pressure is reproduced in the groove.

On the other hand, the friction of the tip damages (except with very high quality equipment) the surface of the record, and leads to the characteristic crackling of LPs, for those who remember it!

The electric microphone

A small iron bar is attached to the tip, which makes its alternating movement. We surround the small iron bar with a coil of electric wire (without contact). The movement of the bar in the coil generates a variable electric current in the wire coil. This is the phenomenon of electromagnetic induction. The intensity and frequency of this current are proportional to the movement of the iron bar, thus to the variation of the sound wave.

The electric current produced by the microphone is like the variable depth of the groove, an “analog” image of the real sound.

A microphone is an electro-acoustic transducer: it transforms acoustic energy into electrical energy.

If, on the other hand, a variable electric current is sent through the wire coil, the iron bar will start moving, making the membrane vibrate, which will then emit a sound.

So here again, an electric speaker is the same as an electric microphone, used in the other direction.

Magnetic recording

To record sound, the wax roll or the LP record is not the most practical thing. The magnetic tape was invented (1888, but really perfected in 1930). What is magnetic tape? A tape, on which, to simplify, a kind of very fine iron filings is glued. This tape is passed in front of an electromagnet, which magnetizes the metal to a greater or lesser extent, depending on the electrical intensity it receives. Thus, if it receives a variable current, the magnetization recorded on the strip will vary, in a similar way, always, proportionally to the original acoustic variation. The variable depth of the record groove is replaced by a variable magnetization.

When the magnetized tape is passed in front of the same electromagnet, by induction, a current is produced in the electromagnet which varies according to the magnetization on the tape, and is transmitted to the coil, which sets the metal rod in motion, which animates the membrane, sets the air in vibration and makes us hear the sound.

But, as time goes by, the magnetization of the tape slowly decreases, and the tape, rolled up on itself, transmits the magnetization a little. In short, analog sound preserved on tapes is theoretically the most faithful to real sound, but it is also quite fragile.

We will move on to digital sound in the next chapter.