Guide to Audio Measurements (in late 1950s)--Article Index

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  1. Introduction (this page--see below)


  2. Measurement techniques---Absolute and comparative criteria. Relative output level. Instrument limitations. Standards. Method of establishing equipment accuracy. Voltage comparisons. Waveform effects. Invalidation of reading due to waveform change. Loading. Spurious effects. Invalidation of readings. Faults in making audio measurements. Avoiding ground loops and spurious effects.


  3. Test equipment---Audio oscillators. Heterodyne or beat-frequency audio oscillator. Zero beat. R-C feedback oscillators. Characteristics of various types of audio generators. Vacuum-tube voltmeters. Scales. Audio wattmeter. Calibrated attenuators. Oscilloscopes. Square-wave generators. Harmonic-distortion meters. Intermodulation test meters. Wave analyzers. Bridge circuits. Flutter and wow meters.


  4. Basic measurements---Frequency. Instrument calibration. Voltage and current. Phase. Resistance and impedance. Commonly used ac bridges. Drysdale, Hay and Maxwell bridges. Wheatstone bridge. Direct-reading capacitance checker. Direct-reading impedance meter. Impedance-admittance bridge. Acoustic velocity and pressure. Groove velocity. Disc compliance and viscosity. Tape magnetization. Tube and transistor dynamic characteristics.


  5. Basic amplifiers---Frequency response. Audio-oscillator comparator techniques. Gain. Power output characteristic. Power response. Harmonic distortion. Intermodulation distortion. Intermodulation tests. Hum and noise. Measuring input impedance characteristics. Output impedance. Impedance reflection. Effect of supply variation.


  6. Output transformers---Specifying transformer performance. Efficiency factors. Insertion loss of an output transformer. Measurement of core loss. Frequency response. Low-frequency performance. High-frequency performance. Measuring magnetizing current and waveform. Leakage inductance. Checking winding capacitance. Notch in transformer response.


  7. Preamplifiers---Frequency response. Method for checking equalization characteristics. Automatic plotting of response' curves. Interaction between bass and treble tone controls. Variable-slope controls. Loudness contours. In correct loudness compensation. Gain and sensitivity. Distortion. Noise. Dynamic range. Cross-talk termination. Microphony. Supply changes.


  8. Pickups and arms---Arm resonance. Frequency response. Pickup performance. Frequency discrimination. Stylus compliance. Measuring the response of a pickup. Shadowgraph techniques. Sensitivity. Impedance. Compliance and dynamic mass. Checking arm mechanical impedance characteristic. Stylus force. Distortion. A-N IM test. Utilization features. Counterbalanced arms.


  9. Turntables and changers---Speed constancy. Cyclic changes. Flutter and wow. Test with stroboscope for speed fluctuation. Frequency test discs. Flutter and wow meter. Relative sensitivity of human hearing. Frequency response of weighting network. Rumble. Setup for testing for rumble. Vibration pickup. Hum radiation. Measuring motor field radiation. Measuring drag produced by a tone arm.


  10. Tape recorders---Speed constancy. Varying tape tension. Flutter and wow. Method of preparing a standard tape for a speed constancy test. Tape handling. Setup for measuring start and stop times electronically. Incorrect alignment. Response of a good head. Frequency response. Distortion characteristic for a typical tape. High-frequency bias. Maximum density levels of different tapes. Dynamic range.


  11. Microphones---Acoustic properties. Using the Rayleigh disc. Deducing microphone response. Reciprocal test method. Using a calibrated microphone. Anechoic chamber for testing. Taking microphone response automatically. Adding a warble tone to heterodyne-type audio oscillator. Proximity of microphones. Symmetrical placement. Directivity. Transient response. Phase response. Impedance. Distortion and noise. Vibration isolation. Effect of housing.

Introduction

THESE days everything seems to be an offshoot from something else: radio started as an offshoot from electricity; electronics and television were offshoots from radio; and somewhere in the picture audio got its start as a separate branch.

But with time the pattern changes: now electronics is seen as the more basic field, and audio pertains not only to frequencies that hive been or will be sound-although that is its most obvious application-but it is characterized particularly by the handling of frequencies over an exceptionally wide band, for whatever purpose.

With the development of a new field, at the beginning, who ever is concerned jumps into the breach and does his best. Many musicians got their feet wet in electronics because they were not satisfied with the efforts of electrical hams at recording and transmission. But as the audio "art" grew, the necessity for proper instrumentation became evident. Here again methods were devised to meet the need by those who had gotten involved in this rapidly developing field.

Now, after several decades of audio as an entity in its own right, it seems there is no single documentation of audio measurements largely because everyone "in the business" has been too busy doing it to write about it! As a result, much time is spent in passing out, individually, information on this subject. To save this waste of time, the author has organized as much as possible of this information into the present guide, to provide a handy reference on the subject.

The guide is arranged so it can easily be used as a text book. But the sequence has also been chosen as a logical one for reference or hand book purposes. Its coverage ranges from types of measurements used professionally and for standardization purposes, to simplified ones that can use a minimum of equipment. In this way it will serve engineers at all levels and at the same time technicians and hobbyists.

In collating material for presentation in a guide such as this, it is inevitable that one finds it necessary to use a guillotine to meet space requirements. The textual treatment has been kept concise, as far as is consistent with maintaining lucidity, and references are given at the end of each section where further information may be obtained, if necessary. But this does not mean the guide is a precise, or simplification, of material already published: a very large proportion of it is completely original.

Even this usage of the guillotine was not sufficient. Subject matter had to be cut. In the original plan, loudspeakers and crossovers were to have been included. But for successful coverage, they need a guide for themselves. In a sense too, they are not strictly audio, but get rather more involved in acoustics than do other items in the guide; and there is already more in the literature covering this area than there is for some of the things we have covered.

Acknowledgement should be made here to the many people who have helped in the preparation of this guide. Credit is given under individual illustrations where these have been supplied. Many who have written the author over the years, and people with whom he has worked, have helped him to get the practical slant-knowing just where you'll need help-that runs through the guide.

[This guide is based on 1958 book by NORMAN H. CROWHURST]

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Updated: Saturday, 2019-06-29 21:42 PST