- Introduction (this page--see below)
- 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
- 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.
- 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.
- 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.
- 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.
- 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.
- Phono pickups (cartridges) and tonearms---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
- 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.
- 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.
- 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.
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
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
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
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
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]