By HARDIN STRATMAN
Simple technique using audio generator, scope and pair of speakers can be employed to properly phase the loudspeakers in public-address system.
Author is shown here in front of the church whose speakers he has just phased. Behind the signal generator at the left is a tall pole with the transmitting speaker and the transmitter directly below it. The pole at the right, behind the scope mounts the receiving speaker and receiver above hand.
PUBLIC-ADDRESS systems are used in large auditoriums, halls, or churches to insure that the performer's words carry to every part of the room. Besides the normal routine of providing enough speakers, adequate amplifier power, and acoustical treatment, there is one additional thing that can be done to improve listening-at no additional cost. This involves phasing all speakers to each other as well as to the performer. The end result is maximum sound power at minimum amplifier power, improved clarity, less garbling, and a sound that has a one-point source rather than seeming to come from all directions.
Many home hi-fi or stereo entertainment systems have a phase switch labeled "normal" and "reverse." This switch merely reverses the connection to one speaker so that mono sound seems to originate from a single source. Bass frequencies are usually reinforced. This same principle may be applied to auditoriums to give large audiences improved audibility. By using the test equipment to be described, one person can accomplish the desired result scientifically without having to rely upon the personal preferences of several people with varied viewpoints (and hearing). Relative Speaker Phasing Two kinds of speaker phasing are possible. One is absolute, the other relative. Absolute systems are used in elegant auditorium installations where money is no object.
This involves taking into account the difference in propagation time between sound in air and electrical impulses in wires and associated electronic gear. Some sort of mechanism is used at each speaker to delay the electrical impulses applied to that particular speaker so that the sounds agree absolutely in time, amplitude, and phase with sounds corning direct from the performer or the other speakers.
A delay system of this sort may take the form of a tape recorder with the recording and playback heads arranged on a single loop of rotating tape so that sound from the performer or other speakers coincides exactly with sounds emitted from the speaker in question.
Since no simple and low-cost delay system is presently available, we will consider only a relative phasing system which involves proper placement of speakers and their correct connection. Results, using the relative system, can be quite satisfying to one who has struggled with a public-address system in attempting to gain maximum clarity at low cost.
As a rough rule-of-thumb, sound travels at a speed of 1100 feet per second at normal room temperature. If a performer sings at one end of a hall 100-feet long, it takes 90 milliseconds for the sound of his voice to reach the other end. If an amplifier and associated speaker are placed mid-way down the hall to assist the performer, sound from the speaker will reach a listener at the rear of the auditorium before sound from the performer unless something is done to delay sound from the speaker.
It stands to reason that maximum clarity is realized if rarefactions and condensations coming from the speaker system coincide with the rarefactions and condensations from the performer's lips. In addition, vibrations from all of the speakers should coincide with each other as far as possible as the sound reaches the listener's ears.
If a performer sings a note of 200 Hz, there will be 18 repeats of condensations and rarefactions down a 100-foot long hall before the sound first reaches a listener at the other end. Also, the distance from condensation to condensation or rarefaction to rarefaction is 5.5 feet. At this frequency a speaker need only be moved one-half wavelength or 2.75 feet to place it at a point of maximum or minimum reinforcement of the sound wave. Approximately the same results can be obtained by merely reversing the leads to the speaker. It is probably just as well to go ahead and place all speakers where most convenient and phase them in a relative manner later.
When one considers all the possible audio frequencies, all possible angles that the sound waves may make With various walls, and the many other variables, it is obvious that the relative-phasing method is not 100% foolproof. However, a speaker system will work best for a large band of frequencies when connected properly. This applies particularly to the lower audio frequencies from male voices.
Test equipment to provide a good solution to the problem involves a transmitter and a receiver. The transmitter duplicates the performance and consists of an audio signal generator, audio amplifier, and a loudspeaker. The speaker should be mounted on a pole or tripod at about the height of a performer's lips. The receiver duplicates a listener's ears and consists of a microphone or loudspeaker, an audio amplifier, and an oscilloscope or meter to indicate levels. An oscilloscope is preferable because it is more sensitive and more easily observed than a meter.
The microphone or receiving loudspeaker should be mounted on a pole or tripod at about the level of a listener's ears.
The phasing method used by the author and recommended for any installation is as follows:
1. Disconnect all but one "key" speaker and terminate all the disconnected wires with resistors equal to the impedance of each speaker. This will allow the public-address system amplifier to be turned on without damaging any internal components. The "key" speaker should be the one closest to the microphones.
2. Place the transmitter in front of one of the microphones, turn the transmitter on, and adjust the output to an audible level that may be heard at least 20 to 30 feet away. Set the frequency to 200 Hz.
3. Place the receiver in a listening area 20 to 30 feet away and turn the receiver on. Adjust gain of amplifier or scope for a visible sine-wave pattern.
4. Turn public-address system on and observe amplitude change on the scope. Reverse connections to the "key" speaker. Move receiver around in an arc of 5 to 10 feet. Leave connections to "key" speaker in the position which provides the greatest amplitude, as observed on the scope.
5. Move transmitter to any other microphone location but leave receiver in the same position. Reverse connections to the "key" speaker and observe amplitude change. If maximum amplitude does not agree with the phase of first microphone, reverse leads to the second microphone if possible. Do the same with any other microphones that are used.
6. Place transmitter in front of the most often used microphone with generator still set at 200 Hz and the "key" speaker properly phased.
7. Move receiver toward the end of the hall so as to be in front of other speakers being used. Remove terminations from wires one by one and connect each speaker in a way that will increase the scope amplitude a maximum amount. Move the receiving loudspeaker about somewhat to be certain of the best system loudspeaker connections.
8. Continue on down the hall and reconnect all speakers one by one for the best general amplitude increase as observed on the receiving scope.
After phasing has been completed, listen for results with a live performer under actual audience conditions. Expect that normal listening level will occur with gain controls of the public-address system down somewhat from where they were before executing the phasing operation. Better clarity and improved listening should result.
Fig. 1. Circuit diagram of transmitter hooked up to the generator. Unit
was constructed by author and used an 8-in loudspeaker. Fig. 2. Circuit
of the receiver whose output feeds the scope.
Also see: Chip Capacitors for IC's