Much thought and care have been exercised in determining the final location of the panel controls. While simplicity and ease of operation were dominant factors in determining control locations, electrical performance was never sacrificed. As illustrated in Figure 1, the important and most used controls have been placed at the right. The

indicating meter and attenuator control have been located side by side and given equal prominence. The mask was added to the attenuator control to reduce the possibility of errors in reading the sound level, which is the sum of meter and attenuator readings. The microphone is shown in operating position. When not in use, it is folded down so that it rests in the small well at the right. The on-off switch is operated by the microphone swivel post so that the instrument is automatically turned on when the microphone is raised to its operating position.


The welded aluminum case designed to house this new meter is light in weight, strong, durable, and attractive. Four large rubber feet mounted on its base serve as a first stage in shock and vibration isolation for the high-gain amplifier carried within. The end frames of the case are molded from high-impact-strength bakelite and serve to protect the panel and controls when the instrument is in use. They make possible a simple U-shaped cover design, which can be attached quickly and securely, and they prevent marring and scarring surfaces when the instrument is set down in other than its operating position.

Interior Construction

Figure 3 is a view of the instrument removed from its cabinet. The amplifier cover has been removed and the amplifier shelf has been raised to show accessibility to all parts of the amplifier. The seven sub-miniature tubes are just visible along the top of the amplifier shelf. Full constructional details of the amplifier shelf are shown in Figure 4. The amplifier case is supported at three points by soft rubber bushings, which make a

Figure 3. View of Type 1551-A removed from its case. Amplifier cover removed, and amplifier raised to show accessibility.

second stage of shock and vibration isolation. In addition, the input tubes for the pre-amplifier and main amplifier rest between pieces of light cellular rubber when the amplifier cover is in place. The attenuator switch and weighting switch are enclosed in the two cylindrical shield cans at the left of Figure 3.


The battery complement for the new instrument is visible at the top of the photograph. It consists of two D-size flashlight cells for the filament supply and one portable radio B battery for the plate supply. Batteries used in this instrument are popular sizes manufactured by a number of battery companies and are readily available at almost any radio store or supply house. One set of A batteries will give 6 to 7 days' operation at

Figure 4. Detail view of panel side

Figure 4. Detail view of panel side

8 hours a day or 30 to 35 days' operation at 2 hours a day. The plate battery will ' give 18 to 20 days' operation at 8 hours a day or 90 to 100 days' operation at 2 hours a day. Tests to date indicate that over most of the useful life of the batteries, 8-hour stability of the instrument is within 0.5 db and 2-hour stability is within 0.2 db.

A-C Power Supply

For applications where continuous use of the Type 1551-A Sound-Level Meter is contemplated, a small a-c power supply has been designed. This power unit, the Type 1262-A Power Supply, is so constructed that it will fasten directly to the end plate of the sound-level meter case as is indicated in Figure 5. No regulation is provided in this supply, because the stability of the amplifiers in the sound-level meter is such that variations in line voltage over the range of 105 to s 125 volts cause meter reading changes of the order of only 1 db, so that normal line voltage changes have little effect on the meter reading.


The diaphragm-type Rochelle-salt crystal microphone supplied with the Type 1551-A Sound-Level Meter is a good low-cost microphone and serves as a satisfactory pickup for most noises encountered in the home, office, or factory. It has high sensitivity, flat response to sounds of random incidence, from very low frequencies to frequencies well above 1 kilocycle, and good response up to 8

nd bottom side of Amplifier Shelf.

nd bottom side of Amplifier Shelf.

kilocycles. For sounds arriving at 90° incidence, it is essentially non-directional (in the horizontal plane) for frequencies up to 6 kilocycles. Figure 6 is a group of generalized curves showing the response of this microphone to sounds incident at 0 degrees, 90 degrees, and from random directions in the vertical plane. The upper curve in Figure 7 shows the overall response to sounds of random incidence obtained for a typical microphone used with the Type 1551-A Sound-Level Meter. The lower curves show the response characteristics of the electrical circuits in the sound-level meter.

Temperature Effects

The open-circuit voltage of the microphone changes by about 0.02 decibel for each degree Fahrenheit change in temperature as is shown by the dotted curve of Figure 8. This relatively small change in output is accompanied by a rather large change in the capacitance of the microphone.4 As long as the microphone is connected directly to the input of the Type 1551-A Sound-Level Meter, this large capacitance change is of little consequence. The response of the Type 1551-A as a function of temperature changes at the microphone for this condition is shown by the upper solid curve in Figure 8. The low input capacitance

4E. E. Gross. "A Dynamic Microphone for the Sound-Level Meter," General Radio Experimenter, April. 1951.

Figure 5. Type 1551-A with Type 1262-A A-c Power Supply.

achieved in the new sound-level meter accounts for the close adherence of the solid curve to the dotted curve. If the input capacitance is increased as, for instance, when a long cable is used, the indicated output of the microphone will vary more widely with temperature as is shown by the middle and lower curves in Figure 8.

Microphone Mounting

For convenience in use, storage, and transport, the microphone is mounted directly on the instrument. When sounds of random incidence are being measured, this mounting for the microphone is satisfactory, and the instrument is adjusted to conform to the ASA specifications for such sounds. For sounds of other than random incidence, this mounting can be

Figure 6. (Below) Free field frequency response of microphone for sounds incident at 0 degrees, 90 degrees, and from random directions in a vertical plane.

Figure 7. (Right) Typical acoustical and electrical calibration curves for the Type 1551-A Sound-Level Meter.

-10 500

Figure 7. (Right) Typical acoustical and electrical calibration curves for the Type 1551-A Sound-Level Meter.

o* I








90®— 1


IOOO 3000



400 IOOO 3000


10,000 20,000

400 IOOO 3000


10,000 20,000

a source of error because the presence of any object, such as the instrument case, in the sound field of the microphone can distort the field and hence affect the meter readings. In addition, the position of the observer relative to the source of sound and the microphone becomes very important. It has been experimentally determined that when the Type 1551-A Sound-Level Meter is held as shown in Figure 9, these effects are negligible for frequencies up to 2 kilocycles for sounds in the horizontal direction (90° incidence, Figure 6) which arrive from the left- or right-hand side of the observer. In contrast, the observer will affect meter readings by as much as 3 db at frequencies as low as 200 cycles if he holds the instrument as in Figure 9 and faces the source of sound.

For sounds of random incidence over the frequency range of 20 cycles to 8 kilocycles and for directive sounds from 20 cycles to 2 kilocycles, this sound-level meter with attached microphone is an excellent hand-held instrument, suitable for use in many common noise measurement problems. When directive sounds are involved and good results above 2 kilocycles are required, one should mount the microphone on a tripod and use a cable to keep the microphone well away from the observer and the sound-level meter. The Type 759-P25 Dynamic

5H. F. Olson and J. Preston, "Unobtrusive Pressure Microphone," Audio Engineering, Vol. 34, pp. 18-20 (July, 1950).

s.J. K. Hilliard. "Miniature Condenser Microphone," Journal of the Society of Motion Picture and Television Engineers, Vol. 54, pp. 1-12 (March, 1950). 7Type 1551-P1 Condenser Microphone System to be described in a forthcoming issue of the Experimenter.

Figure 9. Typical operating position for Type 1551-A Sound-Level Meter.

Microphone Assembly* will give better results for measurements above 2 kilocycles. For even better results at high frequencies, a Western Electric Type 640-AA Condenser Microphone or one of the recent high-fidelity microphones, such as the RCA Type BK4A5 Pressure Ribbon Microphone or the Altec Type 21-B6 Condenser Microphone should be used. Because good low frequency response is also important in noise measurements, a condenser type of microphone will be offered as accessory for use with the Type 1551-A Sound-Level Meter. The development of a battery-operated preamplifier and power supply7 for this type of microphone will be completed soon.


The sound-level meter is a basic instrument about which a comprehensive sound measuring system can be built. Many types of measurements can be made with it directly, and its usefulness has been extended many-fold by numerous accessories, including a wide range of microphones, vibration pickups, analyzers, and recorders. The sound-level

Figure 8. Variation in response as a function of temperature for the microphone alone and with various lengths of cable between microphone and sound-level meter.




meter is commonly used in industry, in schools, and in laboratories to obtain objective measurements of a wide range of noise levels. The list of its applications for the measurement of noise or unwanted sound is growing rapidly. Many measurements are made in the process of reducing the noise in consumer products or in improving worker comfort, safety, and efficiency. With the advent of high-intensity noise makers, such as jet engines, test facilities must be carefully engineered and designed, not only for protection and comfort of test personnel, but to prevent undesirable noise conditions extending to surrounding communities.

Every effort has been made to make the Type 1551-A Sound-Level Meter an outstandingly useful instrument. The broad frequency-response characteristic, stability, wide dynamic range, and low noise and distortion level, resulting from careful design of the amplifier, attenuators, and output system, make it an excellent foundation on which to build a comprehensive sound-measuring system. Special microphones can be used to full advantage. The output is adequate to operate many pieces of auxiliary equipment, such as frequency analyzers, graphic level recorders, magnetic tape recorders, or cathode-ray oscillographs. In addition, this new sound-level meter is compact and light in weight, so that it is much easier to carry about than the old Type 759-B. Its over-all size is 470 cubic inches, compared to 1200 cubic inches, and its weight is 11 pounds, compared to the 22^ pounds of the Type 759-B.

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