LETTERS TO THE EDITOR

4.21, 4.4, 4.7, 4.11; 0.1 Received 7 April 1966

Errata'

Remote Masking in the Absence of Intra-aural Muscles

I-J. Acoust. Soc. Am. 39, 103-108 (1966)-] RO•3ERT C. BILGER

.Bioacoustics Laboratory, Eye and Ear Hospital, and School of Medicine, University of Pittsburgh, Pennsylvania

Figures 3 and 4, together with their captions, appeared out of sequence on pp. 106 and 107.

On p. 107, Col. 1, line 21: instead of "neural losses of hearing (&, ¸)...," read "neural losses of hearing (&, ©)...."

5.19; 9.9 Received 3 January 1966

Use of Thermosensitive Paper in Sound-Spectrum Analysis ARNOLD ABRAMOVITZ

Department of Psychology, University of Cape Town, Rondebosch, Cape Province, South Africa

The use of thermosensitive paper is mooted in the design of an all-purpose sound-spectrum analyzer that would instantaneously and continuously provide both display and permanent graphic record. Some applications and extensions of the technique are suggested.

As A PSYCHOLOGIST CONCERNED WITH AUDITORY PERCEPTION, I have keenly felt the need for a sound-spectrum analyzer that would at one and the same time provide a visual display, as well as a permanent graphic record, in terms of frequency, amplitude, and time, of any complex acoustic signal.

I should like to draw attention to the possibilities and advan- tages of thermosensitive paper in designing just such an instrument.

The basic technique that I have in mind is a very simple one developed in the Bell Telephone Laboratories by Dudley and Gruenz • for a "visible-speech translator." They fed the outputs of a bank of fixed filters to a vertical column of grain-of-wheat incandescent lamps that excited strips of phosphorescence on a moving belt of phosphor-coated plastic.

My proposal is simply that the lamps be replaced by miniature heating elements, and the endless belt of phosphor-coated plastic by an unrolling web of thermosensitive paper.

Thermofax© paper has some useful properties in this connection. First, no processing is necessary and printout is immediate. Second, since it varies with the thermal energy applied, printout intensity will be a function of the channeled signal amplitude. Third, printout occurs when a heat source is applied to the reverse side of the paper, so that placing the vertical column of heating elements behind the paper will result in an unobstructed viewing area. Since integrated-circuit techniques will allow the number of filter channels to be quite large, I see no reason why a very compact, all-purpose analyzer-recorder cannot be designed.

Apart from research and testing applications, such an instru-

ment should prove invaluable (especially if adaptable as a pitch indicator) to those concerned with training and therapy in speech and hearing. I also believe that, with the designing of a corre- sponding photocell readout unit, and microminiaturization of the printout and readout elements, there exists potentialities for a new approach to speech and music synthesis, as well as to data transmission and information storage.

1H. Dudley and O. O. Gruenz, Jr., "Visible Speech Translators with External Phosphors," J. Acoust. Soc. Am. 18, 62-73 (1946).

Received 17 February 1966 8.1; 0.5

Why Strike Out the Mighty kc? W. Dixon WAlm

Hearing Research Laboratory, Department of Otolaryngology, University of Minnesota, Minneapolis, Minnesota

Mandatory use of "kc/sec" in place of "kc" is protested.

THE FORCES OF CONFORMITY-FOR-CONFORMITY•S-SAKE HAVE DEALT

us another blow. Nor content with forcing on everyone the in- congruity of a lower case letter followed by a capital letter, for example "dB" (but not deciBel, strangely), they have now decided that using "kc" as an abbreviation for "kilocycles per second" is no longer to be tolerated. Unlike the change from db to dB, how- ever, which has some arguments in its favor, the switch from "kc" to "kc/sec" is, in my opinion, arbitrary, unnecessary, and, forsooth, not even an improvement in consistency.

Arbitrary. Even if it be granted that a single abbreviation should be mandatory (though I do not), we have to decide among kc, kcps, kc/s, and kc/sec. That is, one could choose a 2-, 4-, or 6- character abbreviation. It seems silly to select the longest possible alternative unless the interests of clarity and lack of ambiguity are thereby served. This is clearly not the case here. Both kcps and kc/s are just as explicit as kc/sec.

Unnecessary. But even kc is actually free of ambiguity, be- cause it is always read as "kilocycles per second :" the "per second" has come to be understood. When I returned my last (expurgated) galleys to the editorial office at The American Institute of Physics, I challenged the anonymous censor to show me a single instance in which the use of kc has led to ambiguity or error of interpreta- tion. My note was ignored, which is tacit admission that none could be found. Indeed, it is difficult to think of an instance in which one would like to use "kilocycles" without the "per second." If one had a 1-msec pulse of a 1 000 000-cps wave, perhaps this could be described as "a 1-kc burst of a 1-Mc/sec wave"; however, it seems to me that most of us would in such a case spell it out this way: "a 1000-cycle burst .... "(It is not, of course, a 1000- cps burst.)

Inconsistent. The crowning absurdity of the rule is that we are supposed to follow completely different conventions for cycles per second and kilocycles per second. If "cps" is to be retained, then obviously "kcps" is the logical choice, from the viewpoint of consistency.

978 volume 39 number 5 part 1 1966

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LETTERS TO THE EDITOR

Perhaps, however, this apparently capricious situation was carefully planned by the internationalists. Their scheme may be to get us so confused and irritated that we will all "compromise" by throwing out cycles completely and going to "kHz" (which, as everybody knows, stands for a thousand rental cars).

I challenge the editorial staff to put the question to popular vote. I am confident that the consensus of a vast majority of the people who use the concept of frequency would be that "kc" is perfectly acceptable. In the meantime, the only way I can see for us to express disapproval is to use only "cps"--e.g., 4000 cps instead of 4 kc--although my Scottish instincts rebel at using 8 spaces when 4 would transmit the same information.

DRIVING VOLTAGE (RMS) =.•,._e-----o

-o•o• o• • , ••0 • • • I• 0•-, I •••o• o•o •o••• •- I

, , , L/ 4 6 8 I0 I• 14 16

DISTANCE FROM FIXED EDGE (MM)

FIG. 2. Peak-displacement amplitude in k .• vs distance in mm from the fixed edge of a vibrating edge-clamped circular plate. The total diameter of the plate was 32 ram.

12.3 Received 20 January 1966

Laser Interferometric Technique for Measuring Small-Order Vibration Displacements HARRY A. DE•FERRARI AND FRANK A. ANDREWS

The Catholic University of America, Washington, D.C.

A Michelson interferometer using a CW laser-light source has been used to measure the amplitude of small-order vibrational displacements. Measure- ments over the surface of a vibrating plate at points «mm in diameter are reported. Magnitudes of 0.1-5000 X at frequencies of 100 Hz-20 kHz have been measured with higher and lower limits possible. The advantages of the technique are discussed.

THE PRECISE MEASUREMENT O•' ACCELERATION, VELOCITY, AND displacement-amplitude distribution over the face of a small acoustical transducer or other similar vibration surface, is not always possible with existing motion sensors. An accelerometer must be attached to the vibrating surface and, hence, adds mass that can alter the modal shape and the modal frequency of the vibrating surface. A capacitance probe must be placed in close proximity to the test surface, must be mounted in a dielectric medium, and must cover considerable test area if small-order dis- placements are to be measured. Thus, installation difficulties plus lack of resolution across the test surface can be disadvantages when one uses the condenser probe for such measurements.

At Catholic University, interest in the study of acoustical trans- ducers has led to experiments using a laser interferometer as a motion sensing device. The difficulties cited above are eliminated. The device (Fig. 1) has been used to obtain displacement-ampli- tude measurements at a point (--•« mm in diameter) in the range of 0.1-5000 f•, and at frequencies from 100 Hz-20 kHz. Figure 2 shows a series of typical measurements of the displacement ampli-

FIG. 1. Schematic diagram of the laser interferometer used to measure small- order peak-displace- ment amplitudes of a vibrating circular plate.

C[• BSK RCA •_• Vorioble •(7265) I ] [ Filter I I PM. I

I d2',';_ ,I •Attenuotor

,o , • • ••L LASER 6328• I

Perkin Elmer 5200 / • • • • Beamsplitter •

Beom • Local 0scillatorl• / • • Transducer

•ix•d Mirror _• •

• Adjustment • •

tudes of points on the face of an edge-clamped circular plate driven at its first normal mode by a thin barium titanate disk cemented to one side.

In the method reported here, a vibrating surface will phase- modulate Beam 1, the carrier, while Beam 2, the local oscillator, reflected from a fixed mirror, is unmodulated. Beam 1 and Beam 2 are recombined at the beam splitter and then detected by a multiplier phototube. The magnitude of the current passing out of the phototube will then give the displacement amplitude at the point where laser Beam 1 impinged upon the vibrating surface. Phase-modulation of a laser beam using this basic Michelson- interferometer technique was described by Rabinowitz, Jacobs, Targ, and Gould, • who concerned themselves mainly with the phase relation between the local oscillator and carrier beam. Schmidt, Edelman, Smith, and Jones •' described an optical method using a Fizeau interferometer and a mercury light source, which they used to calibrate vibration pickups at small amplitudes. The theory given in this latter paper is basically the same used in the work reported herein.

The phototube responds to the square of the real part of the sum of the complex-field intensities of the local oscillator (Beam 2) and the carrier (Beam 1). The expression for the phototube current (it) is

+••+•,o ir = G( Elø• 2 Xcos0•J0(•) q-2J•.(•)cos2 (Wmt+½) q-''' q-2jn(4-•)cosn(wmt+½)]+•EoE]o Xsin0[2j•(4-•) COS(wmt+½)q-2j•(4-•)

Xcos3(wmtq-qb)q-...q-2Jv(•) cosp(wmtq-q5)] >. (1) In this expression, the symbols are defined as follows: ir is the

current out of the phototube; • is a positive constant less than 1, which indicates the efficiency of the homodyne process; G is a constant of the photodetector; E]o is the amplitude of the electric- field intensity of the local oscillator beam; E• is the amplitude of the electric-field intensity of the carrier beam; 0 is a phase angle between the carrier beam and the local oscillator beam, owing to a difference between the two distance 1• and 1•., where 1•. is the distance from the beam splitter to the adjustable mirror and back and 1 • is the distance from the beam splitter to the mean posi- tion of the vibrating test point and back--thus, 0-2,r/X (1•--1•.); X is the wavelength of the laser light, 6328 f•; Jn and Jp are even- and odd-order Bessel functions--thus, n= 2, 4, 6..., and p= 1, 3, 5-..; • is the peak displacement of the vibrating test point from its mean position; O•m is the vibrating frequency of the test point; ½ is the phase of the displacement of the test point relative to the voltage driving the vibrating surface.

the journal of the Acoustical Society of America 979

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