Basic Audio (Part One)

Sound is generated by a particular mechanical process. Some physical object moves at a certain speed and then the air around it moves. The simplest example would be a hammer hitting a tuning fork vibrating at a certain frequency and the sound it produces is transmitted through the air until it reaches your ear. The ear converts the movement of air into electrical energy and sends it to your brain. Then your brain interprets that information (see Figure 1).

There is also another way to explain this complicated process as a conversion from analog to digital. The hammer hitting the fingerboard is comparable to a digital signal that is then converted into an analog signal: the sound wave. The wave is transmitted through the air. Your ear receives this analog signal and converts it into an electrical signal. This would be called an analog-to-digital conversion. (see Figure 2).

The microphone can be considered as a conversion apparatus. It takes the sound wave and converts it into electrical energy (see Figure 3). There are three basic steps to explain this process:

A: The sound wave moves the plastic diaphragm. B: The voice coil that is attached to the diaphragm and inside the magnetic structure (magnet/magnetic compartment) also moves. C: The process produces a small amount of electrical energy. This type of microphone is called dynamic or moving coil. The sound wave must move the mass (diaphragm and coil assembly) but has some difficulties. It usually gives a small peak, between 2-5 KHz, which is sometimes called the presence effect.

Extremely high or low frequencies can give the microphone some problems. This type of microphone is usually very durable and does not require any special strokes.

The amount of electrical energy you can draw out partially depends on the amount of acoustic energy. The harder the sound, the more electrical energy the microphone will produce. Of course, some factors limit this situation.

The sound we hear is usually measured in dB SPL, a measure that tells us how hard the sound we hear is. This ranges from 25dB SPL (e.g. a very quiet studio for music recording) to 140dB SPL (a siren). A full symphony orchestra has a dynamic range of 20 to 100 dB. The human ear has a dynamic range of 120 dB, which is one trillion to one, while analog recording is at 60 dB or 1 million to one, and digital recording is 90 dB, that is, one trillion to one. As we can see, the human ear is a very fine instrument and should be treated with respect. Hence, the microphone that converts the movement of air into an electrical signal within this large dynamic range, has a difficult task to fulfill.

The microphone must be close to the generating object. The voltages produced by the microphone fall to the square inverse of the distance (Figure 4). This simply means that the closer the microphone is to the object, the more voltage it will produce. In other words, more audio level. This also has its disadvantages. When the microphone is too close to the sound source, some effects are generated–proximity–. Low frequencies have a greater effect than high frequencies.

Remember that the microphone is an acoustic device. Temperature and humidity are important factors in the sound reproduction process. This is why recording in a studio can be very uniform, while outdoor recording – remotely – will be different from time to time. These factors are very important to keep in mind because something that worked well once may not be right another time.

There are five different ways to generate voltage from an acoustic source. The first and most common corresponds to the dynamic type that has a small diaphragm with a coil of magnet wire glued together, in a permanent magnetic field (see Figure 5).

The electret capacitor type is, after the dynamic one, the most popular in the current market. It consists of a permanent capacitor loaded with a FET amplifier in its structure (Figure 6). The capacitor microphone is a descendant of the electret type, but requires a power source to charge its plates (Figure 7). An R:F capacitor microphone: is another way to generate quality electrical voltage. There were many other types of microphones but they were already discontinued from the market.

I will describe them in order according to their use. The following descriptions of collection patterns have some elements in common. The figures shown are at 1 KHz for purposes of simplification. Depending on the various frequencies there are different collection patterns and each microphone will have its own polar pattern. The manufacturer usually provides such information with the microphone. The idea is to know what is the right pattern to use in a particular application.

The most commonly used type is omnidirectional (Figure 8). It receives the acoustic waves in a pattern that is the same around, in circular mode. The drawing shows that the collection in the back suffers a loss. Next is the cardioid, which has better collection in front of the microphone than behind (Figure 9). This microphone is very popular for live sound for many good reasons. One of the biggest is because it can reduce acoustic feedback.

Of course, unidirectional is not the only answer to the acoustic feedback problem. Hypercardioids and supercardioids have some features of the cardioid, but even more intense, for example, a narrower collection range.

It is worth repeating that there is no microphone that solves all acoustic problems. After a little practice you will acquire a knowledge of what can work in different situations.

The last type is the gun/interference used for collection over long distances (Figure 11), which is used especially for ENG applications, where the object is difficult to microphone and a wireless microphone cannot be used for any technical reason. They are also used in film and TV production to record dialogues. They are very directional, so the longer they are – shotgun type – they can be very effective.

In my next article I will discuss other types of microphones and their uses. This basic audio series is designed for those with knowledge in production areas, but not specifically in audio. My goal is to inform my readers.

If there are issues you want to know, please write to me at the magazine's editorial offices, Avenida Eldorado No. 90-10, Bogotá, Colombia, or fax me at (57-1)410-49-16.

* Author's note: Franklin Miller is the founder of Sescom, Inc., a company that has manufactured more than 350 audio accessories for broadcast and recording. Mr. Miller has been dedicated to the audio industry for 28 years and has written several articles, including the chapter on audio transformers in the manual for sound engineers, The New Audio Encyclopedia, as well as several collaborations for Radio World, dB sound Engineering, Electronis Now and Teleconferencing Magazine. In addition, he is the creator of the practical manual Audio Demystified.