The Soundsurvey Meter

A Simple, Pocket-Size Instrument for Noise-Level Measurements

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• MANY SOUND M E A S U R EM E N TS do not require the accuracy and versatility of a standard sound-level meter/ and many others are economically feasible only with a low-cost meter. For these applications, the Type 1555-A Sound-Survey Meter, shown in Figure 1, has been developed. It is similar in operating characteristics to the standard sound-level meter and is comparable in accuracy, stability, and frequency response to the commercially available sound-level meters of only one year ago. At the same time it is smaller, lighter in weight, easier to use, and much lower in cost than standard instruments.

The Sound-Survey Meter has a wide range of applications in nearly all fields of sound measurement. For example, it can be used for determining the noise level from machinery, for preliminary surveys of environmental

IE. E. Gross, "Tyfe 1551-A Sound-Level Meter," General Radio Experimenter, XXVI, 10, March, 1952.

Figure 1. View of the Sound-Survey Meter, held in hand, with thumb in position to operate level control.

IE. E. Gross, "Tyfe 1551-A Sound-Level Meter," General Radio Experimenter, XXVI, 10, March, 1952.

Figure 1. View of the Sound-Survey Meter, held in hand, with thumb in position to operate level control.

1555a General Radio

noise levels, for simple acoustic measurements, and as a teaching and laboratory aid in education.

Description

The photograph of Figure 1 shows that the instrument has been designed particularly for ease of use. It is shaped to fit the hand but can also be set on a table or mounted on a tripod. The indicating meter is large for a hand-size instrument, so that it can be easily read. The controls and the meter are all on the face of the instrument. Controls are simple, a function switch at the left and a continuous level control at the right, both arranged for easy, finger-tip operation. Total weight with batteries is only 1 pound, 14 ounces.

Although the instrument is small enough to be carried in the coat or trousers pocket, many users will find it convenient to have the carrying case shown in Figure 2, which is available as an accessory.

The Sound-Survey Meter is shown partially disassembled in Figure 3. The

Figure 2. Convenient carrying case is made of brown, blister-proof, top-grain cowhide and has a shoulder strap. Space is provided for two spare flashlight cells and one spare plate battery.

Figure 2. Convenient carrying case is made of brown, blister-proof, top-grain cowhide and has a shoulder strap. Space is provided for two spare flashlight cells and one spare plate battery.

entire unit is mounted in a simple, two-piece, aluminum case. The microphone cartridge is visible at the top, fastened to the case. The amplifier chassis is in the middle, showing the four sub-miniature tubes, and this chassis is readily removed from the case for ease in servicing. The batteries are one size-C flashlight cell and one 30-volt hearing-aid B-battery.

Circuit

As shown by the simplified schematic of Figure 4, the instrument consists of a microphone, a calibrated potentiometer, a four-stage amplifier with weighting networks, and an indicating meter. A voltage proportional to the current in the meter circuit is returned to the grid of the second stage as negative feedback, which maintains the gain of the amplifier reasonably independent of normal changes in battery voltage and aging of tubes. This stabilization makes it practical in this simplified instrument to dispense with the usual front-panel gain adjustments. An internal adjustment is provided, however, which can be used if tube replacements make it necessary.

Components

While low price was an important objective in the design of this instrument, high-quality components have been used throughout. For example, the capacitors are hermetically-sealed units; low-noise, low-microphonic tubes are used in the first two stages; the meter is rugged, accurate, and comparatively large; the switch used is a high-quality miniature one; and the potentiometer is of a type well known for stability and long life.

Level Control

The calibrated potentiometer is a continuous level control, which is an inno vation in commercial noise meters, it permits one, when measuring noise, to adjust the level control so that the fluctuating reading of the meter balances about the zero-decibel mark on the meter. Then the level is given directly by the setting of the attenuating potentiometer, which covers the most often used range of from 50 to 100 decibels/ An additional 30-decibel attenuation is also provided, and this with the —10 to +6 decibel range of the meter makes the total sound-pressure-level range of the instrument from 40 to 136 decibels.2

The continuous level control also permits the full 16-decibel dynamic range of the meter to be utilized. For example, some noises have a fairly steady background level with occasional bursts to higher levels. The level control can then be set so that the background level is at — 10 decibels on the meter, and bursts of noise up to 1G decibels higher can be observed directly on the meter. This freedom of adjustment is not possible with the usual 10-decibel step control.

Meter Characteristics

The negative feedback from the meter circuit to the second stage provides a high-impedance source for the rectifier-type meter. The resultant meter current is very closely proportional to the average value of the rectified signal, over the full calibrated range of the meter, with little dependence on temperature and individual rectifier characteristics. The scale distribution on the meter is correspondingly appreciably better than that obtained when a low-impedance source is used for driving the rectifier.

The metering system fails to meet the requirements of the two-signal test for r-m-s reading by only ^ decibel. An

3American Standard for Sound-Level Meters, Z24.3

(1944), American Standards Association.

investigation made during the development of this instrument showed, however, that this discrepancy is not important for a simple Sound-Survey Meter. It was checked experimentally that for almost all sounds the difference in reading that could be ascribed to the rectifier characteristic compared to that in standard sound-level meters was less than one decibel.

The meter meets the ballistic characteristics specified for sound-level meters, which includes a limit of one decibel on the overshoot. The speed of response is only slightly faster than a VU meter,4 so that those familiar with the behavior of that instrument will find this one very similar.

Frequency Response

Typical over-all frequency response curves of the instrument are shown in Figure 5. These curves show the relative meter reading as a function of frequency for constant free-field sound pressure produced by a plane-wave source. The response curves include the diffraction effects of the instrument, but not those of the observer. Results are shown for two different angles of incidence and for the three different weighting networks. Those who are familiar with the usual microphone characteristics will realize that this over-all response is remarkably good.

4A. S. A., "Volume Measurements of Electrical Speech and Program Waves," C16.5 — 1942.

Figure 3. View of Sound-Survey Meter partially disassembled to show construction.

The weighting characteristics are intended to approximate the relative response of the ear to pure tones at three different levels: the A network corresponding approximately to a 40-decibel level, the B network to a 70-decibel level, and the C network to a 100-decibel level. In this Sound-Survey Meter the minor differences at high frequencies have been ignored, and the weighting networks affect only the low-frequency response.

Maintaining uniform response at high frequencies for the different networks makes the instrument more suitable for preliminary surveys for determining possible hearing damage. It also makes it possible to estimate better the nature of the frequency spectrum being measured. For example, if a marked reduction in reading occurs when switching from the C to the B and from the B to the A networks, then most of the energy is concentrated at low frequencies. The extent of the reduction sometimes permits one to estimate the approximate frequency at which the energy is concentrated. In contrast, the usual weighting networks modify the high-frequency response as well, and this modification makes the estimate of the spectrum more uncertain.

Comparative Measurements

One important question that should be considered is: How does the noise-level reading as measured by the Sound-Survey Meter compare with that measured on a sound-level meter? There are many factors that enter into the answer to this question. The most important are the frequency spectrum of the noise and the frequency response characteristic of the instruments being compared. When these two factors are known, a fairly good estimate of possible differences can be made. This effect is important, not only when the Sound-Survey Meter is compared with a sound-level meter; it also is important when comparing two different types of sound-level meters that use different microphones.

For example, when the Type 759-P25 Dynamic Microphone is used with the Type 1551-A Sound-Level Meter to measure a 30-cycle signal, the reading will be about 7 decibels lower than when the Rochelle-salt microphone furnished with the instrument is used. For this frequency and the C network, the Type 1555-A Sound-Survey Meter reading would tend to be near that obtained when the Type 759-P25 Dynamic Microphone is used on the Type 1551-A Sound-Level Meter. Differences of similar magnitude can occur at the very high frequencies, while the differences will be appreciably less in the range from 100 to 2,000 cycles. That such differences are normal can be verified by checking the tolerances allowed in the ASA Specification on sound-level meters.5

Because of the limitations imposed by small size and low cost, more variation can be expected in the low-frequency response for the different networks in this simplified instrument than occur in the Type 1551-A Sound-Level Meter.

sSee footnote 3.

Figure 4. Elementary schematic circuit diagram.

FINAL AMPLIFIER

SO-lOOdb ATTENUATOR

Sound Level Calibrator Design Circuit

Figure 4. Elementary schematic circuit diagram.

FINAL AMPLIFIER

SO-lOOdb ATTENUATOR

RECTIFIER METER -

Oil the whole, however, the differences between readings taken on the Sound-Survey Meter and a commercial sound-level meter are not significantly greater than the differences that can be expected between sound-level meters of different manufacture.

Microphone

The microphone used in the Sound-Survey Meter is a Rochelle-salt-crvstal diaphragm type similar in characteristics to the one supplied as standard with the Type 1551-A Sound-Level Meter. It operates into a high impedance, which limits the variation of sensitivity with temperature to about 0.03 db per degree F. Like all Rochelle-salt devices, it is limited to a maximum safe operating temperature of 46°C. or 115°F.; and the crystal is destroyed if kept above 55°C. or 131°F. Long exposure to extremes of humidity should also be avoided.

Maintenance

Routine maintenance checks are easily made. A battery check position is provided on the function switch. The plate battery is a 30-volt hearing-aid B-battery, which lasts for about 100 hours at two hours per day, while the filament battery operates for 20 hours at two hours per day. The filament battery is a size-C flashlight cell available in many local stores. The marked discrepancy in life of the two batteries serves to make less costly any oversight in failing to turn off the instrument. The inexpensive, readily-obtained filament battery runs down first and saves the plate battery.

When necessary, the over-all calibration can be checked accurately with the Type 1552-A Sound-Level Calibrator6 as shown in Figure 6.

Applications

The Sound-Survey Meter can be used in many measurements that have hitherto been made by the more expensive sound-level meter. For example, many noise surveys, appliance noise tests, and frequency response tests can be made satisfactorily with this new instrument. Some of these tests, however, must still be made with an instrument like the Type 1551-A Sound-Level Meter. When the noise must be analyzed or recorded, when a wide-frequency range system is necessary, or when a product-acceptance test requires the use of a standard sound-level meter, the

6e. e. Gross, "ail Acoustic Calibrator for the Sound-Level Meter," General Radio Experimenter, XXIV, 7, December, 1949.

Figure 5. Typical frequency-response curves.

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Figure 5. Typical frequency-response curves.

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IOO 300

FREQUENCY IN CYCLES

IOOO PER SECOND

3000

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IOO 300

FREQUENCY IN CYCLES

IOOO PER SECOND

3000

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Type 1551-A Sound-Level Meter is recommended. In addition, some noise levels are beyond the range of measurement of the Sound-Survey Meter. For example, quiet electric clocks have a noise level well below 40 db, and the background level in a broadcast studio is usually in the range from 20 to 30 db on the A-weighting network. But these low levels are exceptional, and the usual noise levels to be measured are well within the operating range of the Type 1555-A Sound-Survey Meter.

In addition to these generally accepted applications for a noise meter, the low cost of the Sound-Survey Meter makes it economically practical to use for many applications that were not so feasible before. Some of these will be discussed briefly, and, because of its importance, the application of this meter for preliminary noise surveys with regard to deafness risk will also be considered.

Deafness-Risk Surveys

Much w^ork is being done at present by industrial hygienists, otologists, psychologists, physicists, engineers, and others on the problem of hearing loss from long-period exposure to excessive noise. This work will lead to essential information for judging when ear protection is necessary. Some preliminary conclusions have been reached, but, because the problem is very complicated and adequate data are not available, the present conclusions are tentative and will be modified when a better understanding of the problem develops. Some of the factors that make the problem difficult are (1) the large differences between individuals in their susceptibility to damage by noise; (2) the normal loss in hearing with age; (3) the effects of some diseases on hearing; (4) the much higher level of noise that can be tolerated without permanent damage for short exposures than for repeated longtime exposures; and (5) the higher levels that can be tolerated when the noise is dominated by low-frequency components rather than components in the higher audio-frequency range.

7Ivarl D. Kryter, "The Effects of Noise on Man," Monograph Supplement 1, September, 1950, American Speech and Hearing Association.

Leo L. Beranek, "Noise Control in Office and Factory Spaces," Transactions Bulletin IS, 1950, Industrial Hygiene Foundation, pp. 26-33.

Proceedings of the Second Annual National Noise Abatement Symposium, October 5, 1951, Technology Center, Chicago 1G, Illinois.

Proceedings of the Course on the Acoustical Spectrum, February 5-8, 1952, School of Public Health, University of Michigan, Ann Arbor, Michigan.

(Left) Figure 6. Sound-Survey Meter with Sound-Level Calibrator in position for over-all calibration check. (Right) Figure 7. Sound-Survey Meter being used to measure noise level produced by pneumatic rock drills.

(Left) Figure 6. Sound-Survey Meter with Sound-Level Calibrator in position for over-all calibration check. (Right) Figure 7. Sound-Survey Meter being used to measure noise level produced by pneumatic rock drills.

Because of the importance of the problem, however, even the tentative conclusions available now are of value; and the Sound-Survey Meter is most helpful for preliminary surveys to determine if operating personnel need to wear ear defenders or if effort to reduce the noise level is justified. If the levels are sufficiently low, a check by the Sound-Survey Meter could be all that is needed. Otherwise, it can show whether or not detailed investigation using the Type 1551-A Sound-Level Meter and the Type 1550-A Octave-Band Noise Analyzer is necessary.

Sound Reproduction

The audio engineer should find the Sound-Survey Meter very useful for custom audio installations. Typical uses here are the following: adjusting the relative levels of the different speakers

__. in a two or three-way speaker system;

checking the dynamic range; setting the initial reference level for a compensated volume control; checking and adjusting low-frequency response to avoid boom-iness.

Speech Classes

The deaf person is obviously unable to judge the relative loudness of his own speech and that of others. A visible indication of level, such as that provided by the Sound-Survey Meter, can be a useful aid to the instructor of the handicapped in showing the student how to adjust this level. Training in adjusting the level of speaking is also needed when a hearing aid is first used, because this aid upsets the apparent balance of level between the user's voice and the background noise or other voices.

Even a person with normal hearing cannot correctly compare on a subjective basis his own voice level with that of others, because of the inherent difference between listening to himself and listening to others. The instructor in speech and drama classes may find the Sound-Survey Meter useful here for demonstrating to the student on an objective basis how his level compares with other voices, and it might be used as an aid to develop the ability of the student to project his voice to cover a reasonable audience without speech reinforcement.

After experience has been obtained with the instrument, it can be used as a guide at rehearsals. It can help in determining whether or not a given performer needs a close microphone pickup, or it may be useful in demonstrating to the performer and the director that such a pickup is necessary.

Physics Laboratories

While many college physics laboratories have sound-level meters, large numbers of high-school physics laboratories and even some of the smaller colleges have not been able to afford one. Now these can consider this new, low-cost meter. It can replace or supplement

Figure 8. Measuring the level of reproduced sound in a theatre.

Figure 8. Measuring the level of reproduced sound in a theatre.

some of the classical powder or flame experiments. For example, standing waves in rooms, the effects of baffles or obstructions, the attenuation of doors and partitions, the comparative intensity of various noise sources as well as other phenomena can be demonstrated.

When schools have a serious noise problem, this instrument can help in determining how to correct it. Simple sound surveys will indicate quickly which classrooms are too noisy and likely to affect the efficiency of the teachers. Experience has indicated that when the noise level exceeds 45 db on the A-weighting network, the students are likely to have difficulty in understanding the teacher.

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