International projectionist (Jan 1963-June 1965)

power amplifier serving them. The total impedance of a number of individual ""loads'" is greater than the individual impedances when thev are connected in series. less when thev are connected in parallel. For the maximum transfer of power, the two impedances, source and load, must be of equal ohmic value. If the "'load resistance" has a value (ohms) substantially lower than the impedance of the source, the current (amperes) flowing through the circuit will be relative!) great, but the load voltage drop will be small. Electrical power in watts is found by multiplying amps, by volts: and in a case like this flow voltage, high amperage I . the watts of power expended in the load resistance will be small. On the other hand, if the load resistance has an ohmic value substantial!) higher than the impedance of the source, the voltage drop across it will be great, but the amount of current able to force its way through the high resistance will be small. Here we have a case of high voltage and low amperage: and the mathematical product of amps, times volts equals watts will again be small. When the ohmic resistances are equal, however, the watts of power obtained bv multiplying the voltage drop across the load bv the current in the circuit will be at a maximum. Matched impedances accordingly permit the maximum transfer of power. \everthele>s. quite a bit of leeway in the match of ohmic values is allowable. One impedance mav have a resistance I ohms i as low as onehalf that of the other, or as high as twice that of the other, and the power loss through mismatch will amount to only about 10 per cent. This is well below the maximum power variation which can be detected in sound reproduction bv the human ear. Rad mismatches between sound-circuit components have a much more serious effect than the mere loss of audio power, however. The impedance of transformer coils and the voice coils of speakers varies with the frequency of the audio signal. When we say, for example, that a certain speaker has an impedance of 6 ohms, we mean that the voice coil of the speaker has a resistance of 6 ohms to AC at a frequency of 1000 cycles per second. The resistance will be less than 6 ohms at lower frequencies, and higher than 6 ohms at higher frequencies. Now. if a speaker or its matching transformer is improperly matched to the output transformer of the amplifier, the frequency characteristics of the system will be altered. The result is distorted, unnatural sound. It is generallv believed that the human ear is unable to detect harmonic distortions in sound reproduction amounting to less than 5 per cent of the total power output. Distortion is much reduced in certain amplifier circuits bv the use of deliberate impedance mismatching! For example, it has been found that second-harmonic distortion is greatly reduced by making the impedance of a load resistance about twice as great as the plate resistance of the tube supplying it. By a thoughtful choice of impedances and by the use of degenerative feedback, modern theatre amplifiers have an overall distortion factor of less than 2 or 3 per cent. A drive-in theatre absolutely requires amplifiers of the same high quality as those used in indoor theatres. Speakers Weak Link in Chain Drive-in sound seldom equals the high quality of good indoor-theatre sound even when the very best amplifiers are used. As in all sound systems from the International Projectionist May 1963 jwtmr #1 jwwwwv *3 Impedances in series: R= Rx+ Rz + R3+ . . . R\ Rz /?3 Impedances in parallel: FIG. 1 — When impedances (resistors, transformer windings, speaker voice coils, etc.) are connected in series, the total impedance in ohms is the simple sum of the individual impedances. When connected in parallel, however, the ohmic values of individual impedances are each divided into 1, the quotients added, and the sum divided into 1. Thus three 25-ohm resistors in series have a total impedance of 25 + 25 + 25 = 75 ohms; in parallel, 1/(1/25 + 1/25 + 1/25) = 1/0.12 = 8.33 ohms. mightiest public-address system to the smallest home "hi-fi." the speaker, itself, is the weakest link in the chain of sound components. Even when the amplifiers introduce no perceptible distortion in the reproduction, the sound may have a slightly unnatural quality. This is due entirely to distortions arising in the speaker. Because a loudspeaker is a mechanical device as well as an electrical one, it is extremely difficult to design one which perfectly translates the fluctuating soundsignal currents into air-pressure waves. It is therefore unfair to expect an "unbaffled" drivein speaker having a 4-, 5-, or 6-inch cone, and no separate "woofer," or low-frequency unit, to equal the sound-reproducing quality of a large and carefully engineered indoor-theatre speaker assembly. But in spite of these unavoidable physical limitations, we have a right to expect acceptably good sound in the drive-in theatre. In-car speakers should be specially designed and constructed for drive-in use. Cheaply priced speakers "adapted" from units originally designed for small radio sets or phonographs are unsatisfactory in almost every respect. Drive-in speakers should be completely waterproof in order to resist the effects of the rain and damp weather to which they will be exposed. They must also be reasonably heat-resistant — the summer sun is sometimes hot enough to fry an egg! And last but not least is ruggedness. In-car speaker units must hold up under the rough usage that the rank and file of the public accords to property not its own. Spare Speakers and Amplifiers Prospective purchasers of in-car speakers should make certain that the units they intend to buy have constant electrical impedance regardless of the setting