Sound Level Exposure Indicator

Abstract

An extremely simple electrical indicator is provided for sensing noise levels and durations, and for providing an indication of the noise level of an environment, insofar as the possibility of ear damage to personnel within the environment is concerned. The indicator includes a microphone for converting the environmental noise into electrical signals, a diode for rectifying the electrical signal from the microphone, and an integrator for integrating the rectified signals on a time base. A threshold device is interposed between the diode and the integrator so that only rectified signals above a predetermined "safe" threshold are actually introduced to the integrator.

Description

BACKGROUND OF THE INVENTION

It has long been appreciated that irreversible destruction of the sensory end organs of a person's inner ear can be caused by sounds of great intensity, or by sounds above a known minimum threshold, of lesser intensity but of relatively long duration. That is, certain high level sounds can be injurious even if sustained over relatively short intervals, and levels of sound below those tolerable for short intervals can be permanently damaging to hearing, if long sustained and if above the aforesaid minimum threshold. Sounds below the minimum threshold can be tolerated indefinitely without ear damage. This "noise trauma" to the ears of personnel working in a noisy environment is now considered a relatively common industrial and environmental health problem.

Equipment for the study of sound frequency and intensity levels is described, for example, in the "American Standard Specification for Sound Level Meters," approved Jan. 1961, by the American Standards Association.

Since the problem is relatively well understood, safe limits for the exposure of personnel working in such noisy environments have been set in many industrial and military establishments. It is known, for example, that sounds must be above a certain minimum threshold of the order of 80 decibels, before they can be harmful; also certain frequencies of sound are more likely to cause ear damage than others.

It has been found that the damage produced to the ear is not only a function of the intensity noise, but as suggested above, is also dependent on the duration of the high intensity noise, and also its frequency within the audio range. Therefore, in order to protect workers adequately, not only must the maximum permissible noise intensity be established, but also the duration and frequencies of noises of lesser intensities, but above the minimum threshold.

It is a relatively simple matter, of course, to establish maximum exposure periods of personnel to environments of known sound levels, and in which the sounds remain at a substantially constant level and pitch over relatively long periods of time. However, in many instances, the noise levels in the environment change rapidly and, prior to the advent of the indicator of the present invention, no simple means was available accurately to determine the safe exposure time limits of personnel in the latter type of environment.

A number of instruments for this purpose have been proposed in the past. These instruments of the prior art have been made frequency selective as well as intensity responsive in an effort to provide an accurate indication of harmful noise levels in an environment; and some include the factor of time. However, all the prior art instruments of which the present inventor is presently aware, fail in one respect or another accurately to indicate the level of harmful noises in an environment.

It is, accordingly, an object of the present invention to provide a simple instrument which may be exposed to the sound levels to be experienced by the individual carrying the instrument, for example, and which indicates when the maximum exposure to the particular sounds in the particular environment has been reached. To this end, the sound exposure instrument of the invention is constructed to respond to the individual noises in the environment above the "safe" threshold, and to their durations and frequencies, in reaching its result. In this manner the instrument is capable of recognizing the dominant damage producing noises in the environment.

Another object of the invention is to provide a simple, completely self-contained electrical instrument, which is small and light in weight so that it is capable of being worn or carried without interference by the individual in the performance of his normal duties, and which will indicate directly to the wearer the amount of damaging sounds to which the individual is exposed.

It has been established, as mentioned above, that any sound becomes a potential threat to the ear, only when it exceeds a certain minimum threshold. This threshold, for example, is of the order of 80 decibels. Any sounds below that level can be tolerated indefinitely by most individuals without any harm to their ears. Therefore, an important aspect of the instrument of the invention is that it is made to be responsive only to sounds above the minimum threshold, and which are potentially dangerous to the ear. Therefore, it does not provide false indications in response to low amplitude long duration noises which in actuality are not harmful.

SUMMARY OF THE INVENTION

A microphone, or other electromechanical transducer is used in the instrument of the invention to convert the sound signals in the environment to which the instrument is exposed into corresponding electrical signals. A threshold device is included in the circuit, so that only the sounds above the predetermined "safe" threshold are effective in the instrument. An appropriate integrator is coupled to the aforesaid transducer, through the threshold device, so that all signals above the predetermined threshold may be integrated on a time base. By this means, an indication is provided, when the total exposure of all the sounds in the environment exceeds the safe level, and when the total exposure is potentially dangerous to the ears.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows in schematic form the electrical details of a simple instrument constituting one embodiment of the sound exposure meter of the present invention;

FIG. 1A is a sectional view of a time integrator and indicator cell which may be included in the system of FIG. 1; and

FIG. 2 is perspective view of an instrument which may be constructed to incorporate the teachings of the invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

The system shown in FIG. 1 includes, for example, an electromechanical transducer 10. This transducer may be in the form of a dynamic microphone which responds to the noise levels in the environment in which the instrument is carried, so as to produce corresponding electrical signals. The frequency response of the microphone 10 may be mechanically adjusted, in accordance with known techniques, to correspond to the relative damage potential of the various signal frequencies throughout the audio spectrum.

The output from the transducer 10 may be connected directly to a diode rectifier 12, and thence to a Zener diode 14 to provide a passive instrument. However, in some cases it may prove preferable to interpose an amplifier 11 between the transducer 10 and diode 12. The Zener diode 14 is connected to an impedance designated 16 which, in turn, is connected to an appropriate integrator and indicator cell 18. A ground return is provided between the dynamic microphone 10 and the integrator cell 18.

The diode 12 serves to rectify the alternating current output derived from the microphone 10, as it responds to the various noises in the environment in which it is placed. The diode 12, therefore, effectively acts as a detector so that the signals applied to the integrator 18 are unidirectional.

The zener diode 14 is interposed in the circuit as the threshold device, and it serves to isolate all signals below the predetermined minimum threshold from the integrator 18. This threshold may correspond, for example, to noise levels of the order of 80 decibels, and any signals representative of sounds above that level cause the Zener diode to break down, so that the latter signals may be passed to the integrator 18.

The element 16 constitutes an impedance, which, for some applications, may be a simple resistor, as shown. However, it has been found that slight increases in the amplitudes of signals of the higher amplitude levels produce relatively more damage to the ear than similar increases in the amplitudes of signals of lower amplitude levels, and to compensate for this, the impedance 16 may be a nonlinear device, of any appropriate type.

The signals passed to the integrator 18 are integrated thereby with respect to time. The integrator, for example, may be a mercury capillary tube integrator of known construction, and which is readily available. Any other appropriate type of integrator, however, may be used.

The aforesaid mercury-type integrator cell is a known type of chemical device, and it includes a gap G which is observable from the exterior of the instrument. The mercury is transferred across the gap as a function of the charge Q across the integrator, so that the displacement X.sub.1 of the gap from one end of the indicator is a function of the charge. The cell is shown in some detail in FIG. 1A. It includes a glass capillary tube 30 having its bore filled with mercury, except for the narrow gap G. The gap may be filled with an aqueous solution of a mercury salt such as HgC1.sub.2. Leads 32 and 34 are connected to the ends of the mercury column in the tube 30. These leads are embedded in respective blocks 36 and 38 of insulating sealing material. Then, as current flows through the cell, mercury is transferred from one end to the other, and the gap G moves in one direction or the other, depending upon the direction of the current.

Therefore, when the instrument is exposed to sound levels above the predetermined threshold, the resulting signal current through the circuit flow, through the integrator 18, and the gap G is displaced as a function of the magnitude of the current and the time of its flow. Therefore, the inherent damage capabilities of the sound exposure will be indicated as the displacement X of the gap G.

Then, at regular periods, or at any other particular times, the indicator may be automatically reset to zero by appropriate resetting circuitry (not shown) for reuse.

It will be appreciated that the instrument of the invention may be constructed as an extremely small unit which may, for example, be worn on the lapel, or in the pocket of the user, as shown in FIG. 2. In any event, the instrument may be worn so that the indication from the integrator 18 is displayed, in order that the personnel may be apprised when the sound exposure reaches the dangerous level.

As illustrated in FIG. 2, the instrument may be housed in an appropriate housing 20 with the microphone 10 positioned behind a screen 22, and with the gap G of the integrator 18 exposed through the front wall of the housing. The instrument may be worn on the lapel or over a shirt pocket, for example, by means of an appropriate hook 24.

It will be appreciated, of course, that many different embodiments of the invention may be devised, without departing from the spirit of the invention. The scope of the invention is intended to be covered in the following claims.

Claims

What we claim is:

1. An instrument for indicating sound exposure including: an electromechanical transducer for converting incident sound signals into corresponding alternating current electrical signals; first diode means connected to said transducer for rectifying the alternating current signals produced thereby; means including integrating means coupled to said diode means for integrating the rectified electrical signals with respect to time and providing an indication of the output level of said integrating means; and Zener diode means interposed between said first diode means and said integrating means to establish a minimum amplitude threshold for signals to be applied to said integrating means.

2. The instrument defined in claim 1 in which said electromechanical transducer includes a dynamic microphone having a selected frequency response characteristic throughout the audio range.

3. The instrument defined in claim 1 and which includes impedance means connected in circuit between said transducer and said integrating means.

4. The instrument defined in claim 1 in which said integrating means includes a mercury capillary tube.