1


NBSIR 75-962

An Outdoor Noise Monitoring

System with Automatic

Calibration and Remote Digital

Display

D. M. Corley

Applied Acoustics Section

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

October 1975

Final Report

Prepared for Office of Noise Abatement and Control

U. S. Environmental Protection Agency

Washington, D. C. 20460

D. S. Blomquist

J. S. Forrer

NBSIR 75-962

AN OUTDOOR NOISE MONITORING

SYSTEM WITH AUTOMATIC

CALIBRATION AND REMOTE DIGITAL

DISPLAY

D. S. Blomquist

J. S. Forrer

D. M. Corley

Applied Acoustics Section

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

October 1975

Final Report

Prepared for Office of Noise Abatement and Control

U. S. Environmental Protection Agency

Washington, D. C. 20460

.A -i S1B of

U.S. DEPARTMENT OF COMMERCE, Rogers C.B. Morton, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson. Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director 

TABLE OF CONTENTS

Page

1. Introduction 1

2. Description of System 1

3. Conclusion 6

Acknowledgements 6

i

An Outdoor Noise Monitoring System With Automatic

Calibration and Remote Digital Display

D. S. Blomquist

J. S. Forrer

D. M. Corley

An Outdoor Noise Monitoring System is described. This system uses a microprocessor for automatic calibration. The design concepts of the

system are applicable to other remote noise monitoring systems.

Key Words: Acoustics (sound); environmental acoustics; instrumentation;

noise monitoring; outdoor noise.

1 . INTRODUCTION

The National Bureau of Standards has developed an instrumentation system

which provides the capability for measuring noise at a remote location and

displaying the noise level on a large digital display. The system is automatically calibrated at approximately one hour intervals utilizing an electrostatic actuator. On the basis of this calibration signal, system gains are automatically adjusted to compensate for any ac and conversion gain errors. Overall dynamic range of the system is TO dB. The system can measure noise

levels in the range from 30 to 160 dB(A).

This system was developed for the Office of Noise Abatement and Control,

U. S. Environmental Protection Agency. The outdoor microphone unit is mounted

on the roof of the EPA offices in Arlington, Virginia. The digital display

is located in the reception room of the Deputy Assistant Administrator for

Noise Control Programs. A second digital display and remote interface are located in the building's analytical center. The interface enables a desk-top

calculator to derive statistical descriptors from the collected data.

2. DESCRIPTION OF SYSTEM

The system consists of two major components: a commercially available

outdoor microphone unit and NBS designed control and calibration instrumentation and remote readout. A simplified block diagram of the complete system

is shown in Figure 1. A dashed line surrounds the NBS-designed portion of

the system.

In order to provide reliable, all weather operation, the outdoor microphone unit (see Figure 2) utilizes a one-half inch air-condenser microphone

with a thin quartz coating on the diaphragm to reduce the effects of corrosion.

To reduce the effects of humidity, the microphone is back vented into approximately 75 grams of silica gel. The microphone is protected by a rain cover
r

OUTDOOR

MICROPHONE

UNIT

RMS DETECTOR

LOG CONVERTER

I

GAIN

CONTROLLED

AMPLIFIER

ANALOG TO

DIGITAL

CONVERTER

CLOCK

AND

TIMER

MICROPROCESSOR

Th - DIGIT

LED DISPLAY

SERIAL

TRANSMITTER

2ft - DIGIT

DISPLAY

RECEIVER

REMOTE

TERMINAL

INTERFACE

J

Figure 1. Simplified "block diagram of monitoring system.

2
3
that has a built-in electrostatic actuator. The microphone and rain cover are enclosed in a windscreen. The electronics for the outdoor microphone system

are housed in a waterproof case and include a preamplifier, an A-weighting

network, an amplifier with up to 50 dB of gain in manually-selected 10 dB

steps and a calibration oscillator. When the calibration signal is applied to

the electrostatic actuator, the microphone experiences the equivalent of a 90 dB sound pressure level. The oscillator design is such that calibration

can be initiated by remote control.

The output signal from the outdoor microphone unit is fed through approximately 30 metres of cable to a remote readout to a true root-mean-square (rms)

detector/log converter. The detector has an averaging time which is equivalent

to the fast dynamic characteristics of ANSI Sl.U-1971

and an output that is directly proportional to the logarithm of

the rms value of the amplitude of the input signal. The detector output signal

is connected, through a system to correct for ac and conversion gain errors,

to a three-digit, binary- coded-decimal (BCD) analog-to-digital converter (ADC).

The output of the ADC is connected to a 2 1/2 digit light emitting diode (LED)

display and to a four bit microprocessor. As will be discussed subsequently,

the microprocessor is utilized to control the system calibration and to make

the necessary gain changes in order to achieve a calibrated output from the ADC.

The clock commands the ADC to digitize the input signal ten times per second

and signals the microprocessor to calibrate the system every 5^- minutes. The

system also has an analog output available which allows one to drive a variety

of output devices; e.g., a tape recorder or a loudspeaker. This output is

turned "off" when the electrostatic actuator is turned "on".

Details of the gain error correction system used for calibration are shown in Figure 3. The microprocessor has been included in this diagram for

purposes of explanation. When the microprocessor receives the calibrate

command pulse from the clock, it samples the output of the ADC. If the value

sampled is less than 80 dB (10 dB less than the 90 dB calibration level), the

microprocessor turns on the electrostatic actuator and initiates calibration.

If the noise level is greater than 80 dB, the microprocessor continues sampling

the ADC until the ambient noise is less than 80 dB. Once the electrostatic

actuator is on, the microprocessor samples the 90 dB calibration signal. If

the calibration signal is between 85 and 95 dB, the microprocessor will change

offset signal in steps corresponding to 0.5 dB until the output of the ADC is

90 dB. If the calibration signal is not within the correction range (85-95 dB)

,

the microprocessor will turn on the decimal points for the two least significant digits of the local display, indicating a malfunction. During the normal

calibration sequence the decimal point of the most significant digit is on.

To change the offset of the gain error correction system, the microprocessor shifts out serial information into a storage register. The storage

register is parallel-connected to a digital-to-analog converter (DAC). The

DAC produces a voltage proportional to the digital number. The DAC output is

connected to a summing amplifier which sums the output of the DAC with the

output of the log converter. Since the output of the log converter is the

logarithm of the input signal, the addition of a dc offset effectively

corrects for gain and conversion errors in the system.
5
To ensure calibration was satisfactorily achieved, i.e., no high level

noises during calibration, the microprocessor continues to sample the output

of the ADC after performing the calibration. If the value sampled immediatelyafter calibration is greater than 80 dB the calibration routine is repeated.

It should be noted that the entire calibration routine can be completed in a few seconds.

When the system is initially powered, a thirty second warm-up time is

required for the system to stabilize. The calibration routine should then be

initiated manually by pushing the "start" switch on the display unit. Each

time the calibration routine is initiated (either manually or automatically)

a constant is shifted into the storage register to provide nominal calibration

for the system.

To allow remote digital data processing of the data, a remote readout

(see Figure h) is included in the system. The output of the ADC is connected

to a universal asynchronous receiver/transmitter (UART). The UART converts

the parallel data from the ADC to a serial word. A start bit and odd parity

bit are also generated. The most significant digit (one bit) is transmitted as a separate digit. The data are transferred through approximately 20 metres of

cable to the remote readout which contains a second UART for converting the

data stream to a parallel word for a digital data processor.

3. CONCLUSION

This design has demonstrated the feasibility of a microprocessor con- trolled outdoor noise monitoring system with remote digital display. This

design concept is directly applicable to other noise monitoring systems

especially where automatic calibration is required.

ACKNOWLEDGEMENTS

The authors wish to thank James Heinen and Mitchell Tarica for help in

assembly of the circuits and other hardware. This work was supported by

the Office of Noise Abatement and Control, U. S. Environmental Protection

Agency

.

USCOMM NBS-DC 6

NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLIC ATION OR REPORT NO. NBSIR 75-962

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

An Outdoor Noise Monitoring System With Automatic

Calibration and Remote Digital Display

5. Publication Date October 1975

6. Performing Organization ("ode

7. AUTHOR(S)

D. S. Blomquist, J. S. Forrer, and D. M. Corley

8. Performing Organ. Report No. NBSIR 75-962

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2000U51

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Office of Noise Abatement and Control

U. S. Environmental Protection Agency

Washington, D. C. 20H60

13. Type of Report & Period Covered

FINAL

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) An Outdoor Noise Monitoring System is described. This system uses a microprocessor for automatic calibration. The design concepts of the

system are applicable to other remote noise monitoring^systems

.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Acoustics (sound); environmental acoustics; instrumentation; noise monitoring;

outdoor noise.

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