How Is Sound Amplified?

The first step in modern sound amplification is to convert sound vibration, which is a form of acoustical or mechanical radiant energy, into electrical energy. This function is performed by the microphone. Sound waves, which are actually variations in air pressure, are converted by the microphone into a corresponding electrical voltage that varies in frequency and strength according to the “pressure” of the sound vibrations. Microphones are necessarily delicate instruments and should be given special care.

Since the output of the microphone is a very minute electrical voltage or signal, that signal must be strengthened or amplified many thousands of times in order to “drive” a loudspeaker. For this purpose, an audio amplifier is used. Many amplifiers have provision for receiving the signals from several microphones or other sources, combining them, and then amplifying the combined program to the power required for the audience to hear easily. Occasionally a separate preamplifier is used to increase the low-level signals from the microphones, mix them together and then distribute the combined program to any point, near or far, where it can be further amplified as needed. Large sound systems may utilize many amplifiers, each working to supply the program to a specific area, where the audience may consist of a few persons or many thousands.

Finally, the amplified electrical signal is fed into one or more loudspeakers. The loudspeaker acts as a sort of microphone in reverse. A cone or a diaphragm is set to vibrating by the amplified electrical current. Electrical energy is thereby converted into mechanical energy, setting up vibrations in the adjacent air once again with sound waves that are audible to our hearing.

Selection of Components

Since the objectives in amplifying speech are (1) to allow everyone to hear well and (2) to preserve the naturalness of each speaker’s voice, careful selection of the right components for your sound system is vital. The adage “You get what you pay for” applies here. So-called “bargain parts” do not usually contribute to your objectives with the dependability and quality that you need. It is good to consult an experienced sound technician when purchasing a public-address system. This will aid you to get a system of good quality and of sufficient power, but not too powerful or expensive for your needs.

The first link of your sound system is the microphone. Each kind of mike is designed for a specific application. There is no best one for all situations. For the amplification of speech, the dynamic microphone usually will fill your needs. It has good fidelity, is rugged, relatively trouble-free, and moderately priced.

Next, decide what kind of microphone pickup pattern would best suit your needs. Should you choose an omnidirectional unit, which picks up sound from all directions, or a unidirectional one? Figure-eight pickup patterns are also available.

A unidirectional microphone is also called a cardioid mike because it responds to sounds in a somewhat heart-shaped pattern around the front of the unit. Since the back of the cardioid microphone is relatively dead, its use generally aids in reducing acoustical feedback, the squealing noise that occurs when sound from the loudspeakers is picked up by the mike. However, we sacrifice some advantages with the selection of the cardioid unit. If the speaker is slightly “off mike” he may not be picked up well. Also, the cardioid unit is not usually as rugged as other types of microphones.

There are certain definite advantages in using an omnidirectional microphone. Where speakers may not be talking directly into a mike, such as when it is passed among a group of people, the omni pattern is most practical. This microphone is usually a less expensive unit and is more rugged. Hence, in many applications it will do an excellent job. However, such microphones usually are more likely to pick up extraneous noises and to contribute to acoustical feedback, especially where two or more microphones are in use at the same time.

Some microphones tend to produce an undesirable popping sound when words having a hard “p” or “t” are spoken directly into them. A suitable windscreen to prevent the “breath blast” of the speaker from entering the microphone will usually rectify this.

Consideration must also be given to microphone impedance. A microphone is said to be either of high or of low impedance, which is a description of its electrical characteristics and does not necessarily relate to its quality or sensitivity. High-impedance microphones are frequently selected from the standpoint of economy. However, there are certain inherent problems that reflect on the apparent quality of the system. For example, if the microphone cable does not exceed 50 feet (15 meters) in length, a high-impedance system is fine. But if the cable exceeds 50 feet, a low-impedance system would be recommended, in order to minimize high-frequency losses created by the capacitance of the cable. The low-impedance system also helps to eliminate the pickup of hum and possible interference from nearby radio transmitters, such as CB, police radio, and so forth. Low-impedance microphones should be connected through low-impedance microphone cables, and their use may require adjustments in the amplifier, such as adding input transformers to receive the low-impedance signal and match it to the amplifier. All professional sound installations utilize balanced low-impedance microphone circuits.

Sound Suggestions

A microphone should be adjusted to a distance of about six inches (15 centimeters) from a speaker’s mouth, with consideration given to the raising and lowering of his head. It must be close enough to provide adequate volume without any trace of acoustical feedback, yet not so close as to make the speaker uncomfortable or to cause the sound to fade with normal head movement. A microphone adjusted too close to the speaker is also subject to annoying “popping” due to the explosive characteristic of the speaker’s breath when certain words are spoken.

Proper sound level and good quality are extremely important. The amplifier volume and tone controls should be adjusted so that each speaker’s voice is reproduced in crisp, intelligible, natural tones, permitting all to hear comfortably and without straining. If volume is insufficient, or the quality is poor, some words will be missed, forcing the listener to strain in order to understand. After a time, mental fatigue may take over, for the natural tendency is to stop listening rather than to continue straining to comprehend the message. On the other hand, sound that is excessively loud becomes oppressive and distracting, resulting in “listener fatigue.” Here again the listener “turns off” his attention and is deprived of the message.

If a microphone is not going to be used for several minutes, its volume should be turned down so as to avoid picking up extraneous speech or sounds. Logically, a full-time sound-system operator is helpful. If this operator is alert and informed in advance as to the sound needs of each speaker, all will benefit from the program.

Should “roving mikes,” that is, movable mikes, be used for audience participation? That depends on the size of your audience, the acoustical qualities of your auditorium, and the ability of all to hear. If each one commenting is encouraged to speak with good volume, this, of course, is preferable to losing time through carrying the roving mikes to commentators in the audience.