The principle of operation of the sound level meter

MINISTRY OF EDUCATION AND SCIENCE

RUSSIAN FEDERATION

_______________

Federal State Educational Institution of Higher Professional Education

PENZA STATE UNIVERSITY

NOISE ASSESSMENT AND METHODS FOR ITS REDUCTION

Guidelines for laboratory work No. 7

Course “Life safety”

PENZA 2013*

__________________________________________________

* Reissue of MU from 2001

UDC 658.382.3

About 93

The paper considers the measurement method, the principles of noise regulation at workplaces, outlines some methods for reducing its level, and provides individual elements of acoustic calculations.

Methodical instructions are addressed to senior students of all specialties and forms of education.

Il. 3, tab. 4, bibliography. 3 names

Compiled by: Cand. tech. Sciences D. P. Gruzin ,
cand. tech. Sciences A. I. Chernikov

Scientific editor V. I. Simakin , head of the department “Ecology and life safety”, Ph.D. tech. Sciences, Associate Professor

Reviewer: V. S. Arbuzov, professor, academician of MANEB

Guidelines

to laboratory work No. 7

on the course “Life safety”

Noise assessment and methods of its reduction

The purpose of the work is to obtain practical skills in measuring and normalizing noise. Acquaintance with the elements of acoustic calculations.

I. THEORETICAL PART

Brief information about noise

From a physical point of view, noise is a combination of sounds of different intensity and frequency, propagating in waves in a continuous elastic medium.

From a physiological point of view, noise is a set of sounds that are unfavorable for a person and interfere with his normal work and rest.

Sound waves arise as a result of various oscillatory processes in an elastic medium.

A person hears sounds in the frequency range from 16 to 20,000 Hz. Oscillations with a frequency below 16 Hz are called infrasonic, and those with a frequency above 20 kHz are called ultrasonic, and at a certain intensity, all of them can affect a person.

Sound waves lead to a change in the pressure of the medium and are characterized by sound pressure, which is defined as the difference between the total pressure in the presence of sound and the average pressure in the medium in its absence.

For hygienic noise assessment and acoustic calculations, the main characteristic of noise is the sound pressure level, the value of which is expressed by the dependence:

dB(1)

where P is the sound pressure at a point in space, Pa; P 0 – threshold sound pressure equal to 2·10 -5 Pa. It is the lower threshold of human hearing at a frequency of 1000 Hz.

Acting on the central nervous system, noise has an adverse effect on the activity of the entire human body, causing headache, dizziness, weakening of attention, which can cause an accident.

The area of human perception of the sound pressure level is in the range from 0 to 130 dB, where 0 dB is the hearing threshold; 130 dB – pain threshold.

At sound levels above 100 dB, partial or complete hearing loss may occur.

Noise levels of 150 dB will destroy the hearing aid and can be fatal.

According to temporal characteristics, noise should be divided into:

– constant, the sound level of which for an 8-hour working day changes over time by no more than 5 dBA;

– intermittent, the sound level of which changes over time by more than 5 dBA over an 8-hour working day.

Intermittent noise should be divided into:

– oscillating in time, the sound level of which continuously changes in time;

– intermittent, the sound level of which changes in steps (by 5 dBA or more), and the duration of the intervals during which the level remains constant is 1 s or more;

– pulse, consisting of one or more sound signals, each less than 1 s long and exceeding the background noise level by more than 7 dBA.

Noise rating

Normalized noise parameters at workplaces are determined by sanitary standards SN 2.2.4/2.1.8.562-96. This regulatory document establishes the maximum permissible sound pressure levels (in dB) in octave frequency bands, sound levels and equivalent sound levels (in dBA) depending on the nature of work in various rooms, as well as the maximum sound levels (in dBA) for non-permanent noise.

Noise parameters are measured according to the procedure in accordance with GOST 12.1.050-86. For constant noise, sound pressure levels (in dB) are measured with a sound level meter equipped with a set of octave filters on the “slow” time characteristic, and sound levels (in dBA) are measured with a sound level meter in frequency reduction mode “A” on the “slow” time characteristic.

Equivalent and maximum (except impulse) sound levels (in dBA) for non-constant noise are calculated based on sound level meter measurements in frequency reduction mode “A” on the “slow” time characteristic, and the maximum value for impulse noise on the time characteristic “I”.

1.3. Calculation of sound levels at a point
and overall sound level from multiple sources

There may be one or more noise sources in the room. If there is one source of noise in the room, then the sound level at the calculated point is determined by the formula:

(2)

where L ai is the sound level of the noise source, dBA; S i is the area of an imaginary surface surrounding the noise source and passing through the calculated point (for small sources S i = pr i 2 ), m 2 ; r i – distance from the acoustic center of the noise source to the calculated point, m; B – room constant, determined according to Table 2 and Fig. 1 of the Appendix, m 2 .

The determination of the sound level at a given point according to the sound levels from various noise sources is carried out according to the formula:

(3)

where L 1 ,L 2 ,…,L n are sound levels at the calculated point from various sources (determined by formula 2).

When counting, use Table 3 of the Appendix.

Noise reduction methods

The attenuation of noise at the source through the use of low-noise machines and mechanisms is the most radical measure to combat it. However, the implementation of this method is not always possible, especially in operating enterprises, due to the complexity of design changes in machines, so noise reduction in these cases can be achieved by using isolation and absorption methods.

The sound field in a room is created by sound waves that are direct (coming from the noise source) and reflected from the walls of the room. Covering the walls of the room with sound-absorbing lining weakens the sound pressure level by reducing the energy of reflected waves. The effectiveness of sound-absorbing lining (noise reduction) in a room can be determined by the formula:

(4)

where = 10 0.1Li – determined using Table 3 of the Appendix;

L i – sound level at a point from the i -th noise source, dBA;

S i is the area of an imaginary surface surrounding the noise source and passing through the calculated point (for small sources S i = 2pr i 2 ), m 2 ;

n is the number of noise sources;

B, B 1 – permanent premises before and after acoustic treatment (Table 2, Fig. 1 of the Appendix);

L, L region sound levels before and after acoustic treatment, dBA.

The required reduction in sound pressure level is defined as:

L tr u003d LL add . (5)

where L add. – permissible sound level, dBA (Table 1 of the Appendix).

PRACTICAL PART

Installation description

The installation for noise assessment includes (Fig. 1): a tape recorder 1 with additional acoustic systems 2; accurate pulse sound level meter type 00024 – 3; microphone 4.

With the help of a tape recorder and acoustic systems, noise is simulated, which is measured by a sound level meter.

The principle of operation of the sound level meter

Pulse sound level meter 00024 is a device for determining the noise level with any time characteristic. It is intended for noise measurements and evaluation, for research work in engineering, construction and other special areas.

The block diagram of the sound level meter is shown in Fig.2.

The principle of operation of the sound level meter is as follows. The microphone converts air vibrations resulting from the propagation of sound into

electrical vibrations that are amplified and fed to the range switch. The switch position depends on the sound pressure level (0-140 dB). Further, the electrical signal, amplified by the second amplifier, is fed through the reduction switch to the effective value rectifier. The rectifier output is loaded on a pointer indicator device, which indicates the sound pressure level. The indicator scale has a range from -10 to +10 dB. The sound level meter provides connection of external filters for frequency analysis of noise.

2.3. Sound level meter controls 00024
and their purpose (Fig. 3)

1. Switch BEWERTUNG (Cast). Has three positions:

LIN – without reduction;

A – with reduction (measurement of the reduced value of the sound level for all frequencies simultaneously);

EXT – for measurement with external filters.

2. BEREICH switch (Range). Serves for step switching of measuring ranges from 30 to 130 dB (steps – 10 dB).

3 . Button 0/1 (on/off). Pressing the button turns on the device. Switching off is carried out by pressing again.

4 . Button “ “. Power control is carried out by pressing a button. The supply voltage is considered normal if the arrow is inside the field marked “ “.

5. LŐSCHEN button. Briefly pressing this button resets the instrument readings. Measurement is not possible while the LŐSCHEN button is pressed.

6. Button I (Impulse).

7.S/F button (Slow/Fast). With buttons 6 and 7 the following time adjustments can be switched on:

S / F – pressed, I – released – quickly;

S / F – released, I – pressed – impulse;

S/F and I are released – slowly;

S / F and I are pressed – the pulse is held.

Rice. 3. Appearance of sound level meter 00024

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