Although a part of Electrics, Sound is actually almost a separate department

Sound itself is the movement of a medium like air as acoustic energy passes through it. Sound travels as pressure waves which cause air molecules to vibrate. This in turn is detected by the eardrums. The number of vibrations per second are referred to as Hertz. The human ear is capable of detecting from about 25 Hz to 25,000 Hz. The higher the frequency, the higher the perceived pitch. The low end is called Bass and the high end Treble.

Sound is measured in Bells, which are further divided into 10 deciBels, or 10 dB. A Bell is actually a ratio of one level to another. It takes an increase of about 3 dB to make a noticeable change in volume, and an increase of 10 dB to make sound TWICE as loud as the previous level. But, since the ear has a logarithmic response, decibels are also logarithmic. An increase of 10 dB represents 10 times the actual power, and 20 dB, twice as loud as 10 dB, would be 100 times the power of 0 dB! A doubling of a small level sound takes a relatively small increase in power, but doubling a loud sound takes a HUGE increase in power.

For reference, 0 dB in Sound Pressure Level (or SPL) has been set as the softest sound most young people with undamaged ears can hear. 100 dB is about the loudness of loud classical music, and in actual power is about 10,000,000,000 times as powerful as 0 dB. Rock music runs about 110 dB and the threshold of pain about 120 dB (if you aren't already deaf)!

Sound energy is actually quite small, and hard to manipulate. It must be converted to another form of energy to be controlled easily: electricity. A device which changes one form of energy to another is called a Transducer. A Microphone is the basic transducer that changes sound to electricity. Other sound transducers include the magnetic pickups of a tape deck or electric guitar, and the laser and photocell of a CD player, and also a speaker, which changes electricity back into sound. Because sound is alternating pressure, the resulting electricity is always Alternating Current, or AC.


There are several types of microphones in common use. Dynamic mics and Ribbon mics make their own electricity, but Condenser mics only regulate power, and so require an external power source, which is called Phantom Power.

Wireless mics may be either Dynamic or Condenser, but because a condenser element can be made smaller more easily than that of a dynamic mic, most wireless mics are condensers. A wireless mic is actually a mic and a radio transmitter, usually FM. The radio battery also provides the phantom power for the mic.

Floor mics are used to pick up sound from a distance. Two are based on the pressure zone principle. Sound travels along the floor and into a thin gap, the pressure zone. There it is picked up by a condenser mic element. The PZM or Pressure Zone Microphone, is omnidirectional. It picks up sound from all sides equally. The PCC, or Phase Coherent Condenser, is similar, but unidirectional. It picks up sound on its front side only, making it more useful for stage work.

While some mics can be fairly rugged, they are in general rather delicate. They should be protected from impact shocks and from dirt, as even a little of either can damage a pickup element.

It is possible but not advisable to parallel microphones into an input; that is, don't two-fer mics. It can cause intermodulation distortion and signal cancellation as parts of the two signals interact.


The amount of electricity produced by a transducer or sound source is quite small. A typical microphone produces only about .001 volts. It must be amplified to a higher level to be controlled, and this is accomplished with a Preamp. A preamp raises the output of a source to about 1 volt, or Line Level.


Preamps are often built into a master control console called a Mixer. A mixer combines several sources into a few signals, which are sent on to the next stage. The gain or volume of each source can be individually controlled, and the combined signal can also be adjusted.

Sound is adjusted at line level. From the preamp for each source the signal typically goes to a simple equalizer, then though a potentiometer or "pot", which adjusts the gain, and then to a submaster or master.


At line level, the signal can be manipulated by a number of devices.

Equalizers adjusts various frequencies, boosting some, attenuating others. They range from basic bass/treble controls to multi-band graphics equalizers, which adjust discrete narrow bands of the sound spectrum.

Compressors and Limiters are used to keep the signal within specified limits.

Echo and Reverb machines add these effects to the sound.

All these line level devices can be added to the chain anywhere between the preamps and the amplifiers.


Once the signal has been adjusted it must be boosted high enough to run the speakers. This takes a significant amount of power, perhaps several hundred watts at 10+ volts. This is done by the power amps, each powering on or several speakers.


Speakers do the reverse of mics; they turn AC electricity back into sound.

No single speaker can handle all frequencies equally well, so the signal is generally split up by a cross-over network, which sends different frequencies to the speakers best able to reproduce them. Cross-overs may be built into a speaker array, but it is better if it is placed before the amps and the split signal is then directed to separate amps for each frequency range. This is referred to as a Bi-amp system, and gives better control of the sound balance.

Speakers are named according to the frequencies they handle best.

Tweeters are high frequency speakers, and are commonly small cone speaks, small horns, or dome radiators.
Mid-range speakers are usually medium size cones or horns.
Woofers are low frequency speakers, usually large cones or rarely very large horns.
Sub-woofers are very low frequencies, sometimes too low to hear, but which you can definitely feel! They are usually very large cones.

since using multiple speakers is the norm, attention must be paid to keeping all speakers in phase with each other. That is, the positive lead of each device and speaker must be attached to the positive of the next device in the chain, and the negatives to the negatives. Otherwise some speaker cones will be moving forward while others are moving back, and they will cancel each other out.

The entire sound path, then, would be:
Source to Preamp to Mixer to Amp to Speaker.
Sound system
EQ and effects can be inserted anywhere between preamp and amp.
Along the way signal level will have increased from -80 dB to over 100 dB!

When powering a system, move from small to large. Turn on external AC, then sources, then mixer, then line-level devices, then amps. Power down in reverse order: amps, then Line level devices, then mixer, then sources, then AC. Doing so will help prevent accidental damage to equipment.


3 conductor: since mics put out such a small signal, they require shielded cable to prevent interference and hum. Professional mics use 3 pin XLR connectors with 3 conductor wire, the shield being the third conductor. Pin 1 is ALWAYS the shield, Pin 2 is usually ( + ) and Pin 3 is usually ( - ). The shields are all connected together but should be connected to earth ground ONLY ONCE to prevent ground loops. Usually about 24 ga.

Headset systems also use 3 conductor mic cable between belt packs, but use 4 conductor cable for the headset to belt pack.

Speakers: as high power devices, speakers are not susceptible to interference, and so use 2 conductor wire. However, because of the high power levels, speaker wires need to be heavy, often 12 ga.

The Sound dept. is also usually responsible for any CCTV, or Closed Circuit Television. CCTV is commonly used to allow the stage manager and crew to see the stage from the front, and the cast to see the orchestra conductor while backstage. CCTV is usually connected.together w/ 75 ohm coax cable.

When running a live show, it is best if the sound operator hears the same thing as the audience, and so the best location for the mixing console is in the middle of the house. However, this is prime seating, and while it is common in a concert setting, it rarely happens in theatre.

Every space is unique, emphasizing some frequencies and absorbing others. Therefore one of the first tasks a show sound operator does after the system is running is to"EQ the House". This is the process of adjusting all frequencies so they sound equally loud when played at the same setting.

Copyright © 2002 Mick Alderson