GB2551724A

GB2551724A - Sound exposure meter

Info

Publication number: GB2551724A
Application number: GB1611153.6A
Authority: GB
Inventors: Bicknell Nigel; Gostkiewicz Przemyslaw
Original assignee: Pambry Electronics Ltd
Current assignee: Pambry Electronics Ltd
Priority date: 2016-06-27
Filing date: 2016-06-27
Publication date: 2018-01-03
Anticipated expiration: 2036-06-27
Also published as: WO2018002601A1; GB201611153D0; GB2551724B

Abstract

A portable sound dosimeter comprises a microphone 10 for sensing sound; an accelerometer 12 for detecting vibrations caused by mechanical impacts with the dosimeter; and a signal processor 23 that, in determining sound exposure, selectively disregards sounds from the microphone based on accelerometer signals. Thereby, mechanical induced noise, which is not relevant to sound dose calculations, may be ignored, removing a source of error. Mechanical induced noise might be caused by the dosimeter being handled, dropped, or vibrated. Selectively disregarding sound data may involve comparing accelerometer signals to a predetermined threshold; or correlating microphone and accelerometer signals in the frequency or time domain. The accelerometer may comprise a high-pass filter 16 to ignore vibrations below a threshold frequency. The sound exposure meter can be worn or carried by a user.

Description

(71) Applicant(s):

Pambry Electronics Limited

Pambry House,Units 7 & 8 Ventura Centre,

Ventura Place, Upton Industrial Estate, Upton, Poole, Dorset, BH16 5SW, United Kingdom (72) Inventor(s):

Nigel Bicknell Przemyslaw Gostkiewicz (74) Agent and/or Address for Service:

Barker Brettell LLP

Medina Chambers, Town Quay, SOUTHAMPTON, Hampshire, SO14 2AQ, United Kingdom (51) INT CL:

G01H 3/14 (2006.01) G01H 1/16 (2006.01) G01H 3/04 (2006.01) (56) Documents Cited:

EP 2608198 A1 EP 1879180 A1

US 5930372 A JP S5855721 (58) Field of Search:

INT CLA61F, G01H Other: EPODOC, WPI (54) Title of the Invention: Sound exposure meter

Abstract Title: Removing mechanically-induced noise from sound dose calculations (57) A portable sound dosimeter comprises a microphone 10 for sensing sound; an accelerometer 12 for detecting vibrations caused by mechanical impacts with the dosimeter; and a signal processor 23 that, in determining sound exposure, selectively disregards sounds from the microphone based on accelerometer signals. Thereby, mechanical induced noise, which is not relevant to sound dose calculations, may be ignored, removing a source of error. Mechanical induced noise might be caused by the dosimeter being handled, dropped, or vibrated. Selectively disregarding sound data may involve comparing accelerometer signals to a predetermined threshold; or correlating microphone and accelerometer signals in the frequency or time domain. The accelerometer may comprise a highpass filter 16 to ignore vibrations below a threshold frequency. The sound exposure meter can be worn or carried by a user.

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SOUND EXPOSURE METER

Technical Field

The present invention relates generally to sound exposure meters.

Background

The concept of a sound or noise dose measuring equipment has been around for many years and are all referred to as Noise Dosimeters.

According to the International Standard IEC:61252 1.1, a sound exposure meter is intended to measure sound exposure as the time integral of the square of the instantaneous A-frequency weighted sound pressure. Noise dosimeters are designed to indicate noise dose as a percentage of a legal limit, for health and safety reasons say in a noisy industrial environment, the settings of and value being determined on a country-by-country basis.

A standard (and high cost) laboratory style noise dosimeter comprises two principal component parts, a microphone and a display controller. This enables the microphone to be capable of being specially protected from rough handling and weather conditions and thus ensuring an accurate reading of sound pressure and noise dose.

However, when the requirement is to provide an integrated and miniature unit see Figure 2, suitable to be used as a body worn unit, this protection becomes far more difficult. A particular aspect of relevance is the unfortunate ability of the microphone that collects noise signals for sound and noise dose processing is also sensitive to mechanical induced noise. Mechanical induced noise is not relevant to the dose calculations and will introduce an error to the dose results. It comes from the case when being handled, dropped or vibrated, typically when the dosimeter is being used in a ruggedized environment, transforms mechanical vibrations into an undesired electrical signal noise.

We have devised an improved portable sound exposure meter.

Summary

According to a first aspect of the invention there is provided an external housing, a microphone for sensing prevailing sound, an accelerometer for measuring vibration of the dosimeter caused by mechanical impact with the housing, a signal processor for processing signals from the microphone and from the accelerometer, and the signal processor configured to be capable of selectively disregarding at least some of the signal from the microphone for determining sound exposure based on the signals from the accelerometer.

The signal processor may be arranged to determine if the signals from the accelerometer are above a threshold value or magnitude.

The signal processor may be arranged to disregard at least one of the signal from the microphone for the time period during which, or contemporaneously with which, the signal from the accelerometer meets certain predetermined criteria are such as being above a threshold value/magnitude.

The functionality of the processor to disregard signals from the microphone may include completely disregarding the signal, or attenuating the signal, or deducting an element of the signal which is detected by, or associated with, the accelerometer.

The signal processor may comprise a data processor which is arranged to receive inputs originating from the microphone and from the accelerometer. The data processor preferably operates in the digital domain.

The dosimeter may comprise a high-pass filter arranged to block frequencies from the accelerometer below a threshold.

The data processor may be arranged to correlate signals from the microphone with signals from the accelerometer. This may be done in frequency and time domains.

The processor may be arranged to subtract the effect of the vibration from the microphone grid.

The accelerometer may comprise a three-axis accelerometer, or a nine-axis accelerometer.

The accelerometer may be rigidly secured to an electronics component mounting substrate. The accelerometer may be arranged to be in the inertial frame of reference of dosimeter.

The dosimeter may be configured, such as by its dimensions and weight, to be worn on or carried (externally) by a user. The dosimeter may be person portable or person wearable. The dosimeter may comprise a clip or attachment to secure the dosimeter to a person's clothes or about the person in some way. The dosimeter may be a body worn unit.

The dosimeter preferably comprises a rigid external housing.

The dosimeter may comprise a housing which is provided with a hole or conduit or pathway, through which sound can pass to the microphone.

The dosimeter may comprise a visual display.

The dosimeter may comprise user input devices or functionality. The user input may comprise one or more buttons.

According to a second aspect of the invention there is provided a method of measuring sound levels, comprising determining when mechanically induced noise is detected by an accelerometer, and causing an audio input detected by a microphone to be disregarded.

The dosimeter may comprise one or more features as described in the description and/or as shown in the drawings, either individually or in combination.

Brief Description of the Drawings

Various embodiments of the invention will now be described, by way of example only, with reference to the following drawings in which:

Figure 1 is a perspective view of the innards of a sound level meter,

Figure 2 is perspective view of the external appearance of the sound level meter of Figure 1,

Figure 3 is a functional block diagram illustrating how detected signals are processed; and

Figures 4A to 4F are amplitude versus time plots of signals in the dosimeter.

Detailed Description

There is now described an improved sound exposure meter 1, also referred to as a noise dosimeter. The dosimeter is arranged to be worn externally on a wearer’s clothing, and comprises a microphone 10 arranged to detect prevailing sound pressure waves. The microphone 10 is located in a housing 35, which also accommodates an accelerometer 12, an LCD visual display 11, a printed circuit board substrate 17, two user input switches 14, and an electrical data socket 15. The meter 1 also comprises, although not referenced in Figure 1, a signal processor, a (rechargeable) power source, a memory, a temperature sensor, a humidity sensor, a clock and a radio module transceiver, for communicating data over an air interface, such as over the industrial, scientific and medical (ISM) band. As will be described in more detail below, the dosimeter is able to more accurately measure sound levels which impinge on the device.

The dosimeter 1 comprises a clip 30 on a rearward surface, which is configured to attach to a wearer’s clothing, such as a shirt or overall. The housing 35 is of rigid construction essentially comprising two parts, a forward part (which accommodates the screen) and a rearward part, each of which may be considered as a shell. As can be seen in Figure 2, the forward shell is provided with an aperture 10a, which is arranged to be aligned with the microphone 10. This allows sound to pass through the opening and reach the microphone to be detected.

Reference is made to Figure 3 which shows the functional pathways of the signal processing which is effected by the dosimeter 1. The basic sound noise dosimeter is shown in Path A. During Path A, the sound level is converted into an electrical signal by the internal microphone 1 (or, in an alternative embodiment this could be an external microphone which is connected to the dosimeter by way of an electrical cable or wiring). A switch 19 is pre-set by the user to select which microphone (either internal or external) is to be used. This selection is dependent on how the dosimeter 1 is to be used. An example of the use of an external microphone is a situation where the size of the dosimeter does not physically fit into a particular environment requiring measurement. An external microphone can be dimensionally much smaller than the spatial envelope or 'packaging' of the dosimeter itself. A further example would be where the environmental conditions are outside the dosimeter capability, whereas an external microphone can more readily meet wider climatic and environmental conditions. This preselection process is effected by a user at Input Settings using the buttons 14 provided with the device 1.

An audio signal from the microphone passes to a filter 20 where a preselected filter characteristic has been chosen. For most noise dosimeter uses this is set to Aweighting (such as is required by the international standard IEC 61672 to be fitted to all sound level meters) but in any event is in accordance with the relevant country standards. The filter 20 is configured to implement either a linear, A or C weighting to signals from the microphone. Whilst the A weighting filter may be mandated for a noise dose measurement, the choice of filter is dependent on what measurement is needed. For peak sound measurements or measuring sound with significant low frequency content, linear or C weighting is selected, linear weighting having a wider audio bandwidth than the C weighting filter. Linear weighting is sometimes referred to a Z weighting.

A RMS to DC Convertor 21 provides for the audio signal to be converted to a DC variant which is in turn converted to a linear digital signal of same. This digital signal is then processed by the data processor unit in the digital domain by the unit 23 to provide an equivalent sound noise dose calculation over time.

It will be appreciated that collectively the processing blocks 16, 17, 18, 19, 20, 21, 22 and 23, may be referred to as a signal processor.

The result is presented by the visual display 7 as a percentage of the maximum level allowed over a specified time, usually an eight hour period. A clock function 12 is provided to enable an accurate time to be used.

The introduction of a vibration sensor to the dosimeter provides an additional level of accuracy to the final result. Microphones are naturally sensitive to sound but are also sensitive to vibration. This is sometimes referred as 'microphonic' and is particularly important when a dosimeter measurement is attempted in 'rugged' environment such as firefighting, for example. The mechanically induced noise as measured at the normal microphone Path A in Figure 3 will significantly affect the accuracy of the resultant dose levels and give a false readings.

The vibration sensor is a three-axis accelerometer shown by reference numeral 8 in Figure 3 and is mounted directly onto an electronics mounting substrate 17 as shown in Figure 1, and may be a PCB. The substrate becomes rigidly mounted and connected to the housing when the case parts are screwed together so that the detector can produce an output relevant to any mechanical noise introduced onto the case structure.

The accelerometer 12 provides an output for movements in all three axes (x, y and z). These outputs pass to a High Pass Filter 9 which separates out the low frequency vibration signals and only allows through high frequency components. These are the relevant signals as they also appear as unwanted output signals of the dosimeter microphone.

The filtered mechanical noise signals detected by the accelerometer 12 then pass to a mixer 10 which combines them into a single peak value followed by the threshold circuit 11. This circuit assesses whether the signal is of sufficient level to corrupt the audio in Path A Only a mechanical impact at a level, as measured in the microphone audio path, is important if it exceeds approximately a level greater than -10 db of the maximum audio level set for calculation of the legal noise dose. Levels lower than this are considered as not significantly interfering with the noise dose calculation.

If that is determined to be the case, then an output is provided to the processor 23 which promptly disregards the microphone audio readings provided from Path A. The microphone signal is continued to be disregarded until the mechanical noise has subsided below the pre-set threshold. Put another way, when sound above a predetermined threshold value is detected in Path A, microphone path A is effectively muted for the duration of the vibration signal.

The dosimeter advantageously no longer assesses the unwanted mechanical case noise and vibration and therefore provides a more accurate reading of the actual airborne sound and noise levels.

In the above example the filters are realised using passive filter components (resistors and capacitors) but could easily be realised using equivalent digital components and processing. The functional blocks can therefore, in an alternative embodiment, easily be merged into effectively a signal processor workflow and in this case the process can be arranged to include more sophisticated processing steps. For example, using the audio signal (Path A) and the three vibration signals (Path B) realised directly into digitally sampled inputs, they can be correlated in the frequency and time domains by the processor. This would enable a more accurate view of the effect any vibration signals have on the microphone path and allow the microphone path to continue to be useful even in the presence of noise. In essence the effect of vibration signals is subtracted from the microphone path using this method in almost real time. In order describe the alternative embodiment reference is made to Figures 4A, 4B, 4C, 4D, 4E and 4F. The Figures show amplitude/time plots of various signals and show how the alternative embodiment using real time processing can offer an even more accurate result.

Signal A is a simulation of a simple audio signal as presented to the microphone in Path A, a sine wave in this case. Signal B is a signal from the vibration Path B and would be the result of a simple tapping of the casing of the dosimeter. The resultant Path A audio signal is shown at Signal E where it can be seen that the mechanical noise could significantly corrupt the dosimeter calculations

The first embodiment as described above takes signal B, assesses the same for amplitude and if this is large enough to exceed the threshold (as shown by the broken line in Signal B) then a mute control Signal C is generated. This signal mutes the microphone path A with the resultant Signal D being applied to the noise dose processor function. As can be seen, the noise signal is removed but a significant amount of the microphone signal is also lost in this process as in practice the Mute control Signal C will have to have much longer 'MUTE ON' durations (typically 20 to 100msec) than shown in this diagram with the result of the loss of various periods of the microphone frequency.

The alternative embodiment however works slightly differently, but nevertheless according to the same underlying principle. Both Signals B and E are sampled in real time and for any given point in the time samples the level of the Signal B are measured and subtracted from Signal E by the signal processor. This has the effect of 'cleaning up' Signal E of the offending mechanical noise Signal B with minimal loss of the microphone audio resulting in Signal F. This signal is then passed to the dosimeter calculation processor and a more robust calculation can therefore be expected. The preset threshold circuit function 18 in Figure 3 is not used in this method.

Claims

1. A portable sound dosimeter comprising an external housing, a microphone for sensing prevailing sound, an accelerometer for measuring vibration of the dosimeter caused by mechanical impact with the housing, a signal processor for processing signals from the microphone and from the accelerometer, and the signal processor, in determining sound exposure, configured to be capable of selectively disregarding at least some of the signal from the microphone for based on the signal from the accelerometer.

2. A dosimeter as claimed in claim 1 in which the signal processor is arranged to determine if the signals from the accelerometer are above a threshold value or magnitude.

3. A dosimeter as claimed in claim 2 in which the signal processor is arranged to disregard at least some of the signal from the microphone for the time period during which the signal from the accelerometer is determined to meet predetermined criteria.

4. A dosimeter as claimed in any preceding claim in which the signal processor comprises a data processor which is arranged to receive inputs originating from the microphone and from the accelerometer.

5. A dosimeter as claimed in any preceding claim which comprises a high-pass filter arranged to block frequencies from the accelerometer below a frequency threshold.

6. A dosimeter as claimed in any preceding claim which comprises a multiple axis accelerometer.

7. A dosimeter as claimed in any preceding claim in which the accelerometer is rigidly secured to an electronics component mounting substrate.

8. A dosimeter as claimed in claim 7 in which the accelerometer is arranged to be maintained in the inertial frame of reference of dosimeter.

9. A dosimeter as claimed in any preceding claim which is configured, such as by its dimensions and weight, to be worn on or carried (externally) by a user.

10. A dosimeter as claimed in claim 9 which is person-portable or person-wearable.

11. A dosimeter as claimed in any preceding claim which comprises a rigid external housing.

12. A dosimeter as claimed in any preceding claim in which the housing is provided with a hole, through which sound can pass to the microphone.

13. A dosimeter as claimed in any preceding claim which comprises a visual display.

14. A dosimeter as claimed in any preceding claim in which the signal processor may be configured to correlate signals from the microphone with signals from the accelerometer.

15. A dosimeter as claimed in any preceding claim in which the signal processor arranged to disregard substantially all of the microphone signal or substantially that portion of the microphone signal that relates to signal from the accelerometer.

16. A dosimeter substantially as herein described with reference to the drawings.

Intellectual

Property

Office

Application No: GB1611153.6 Examiner: Mr David Burns