Basic Noise

Change in Sound LevelResponse by human ears3 dBBarely perceptible5 dBClearly perceptible10 dBDramatic, twice as loud20 dBStriking, fourfold change

2 dB + 2 dB = ? Answer

Example A: 2 dB + 2 dBL(total)= 10 log (102/10 + 102/10) = 10 log (1.585 + 1.585)= 5 dB

Example B: 74dB + 76 dBL(total)= 10 log (1074/10 + 1076/10) = 10 log (107.4 + 107.6)= 78 dB

Difference of the two levelsIncrement to be added to higher level0 13 dB2 42 dB5 91 dB10 or more0 dB

Converting dB Linear to dB(A)

Octave Band Frequency (Hz)631252505001K2k4k8kSound Press in dB(L)7982858587827568A-weightedCorrection factor-26-16-9-30+1+1-1Sound Press in dB(A)5366768287837667

Noise Spectrum and Overall Noise LevelNoise Spectrum is the noise level which taken at each frequency e.g.

Overall Noise Level is the summation of the noise spectrum to become a single value e.g 92 dB

Octave Band Frequency (Hz)631252505001K2k4k8kSound Press 5366768487868267

Converting Noise Spectrum to Overall Noise Level909192 dB(A)Overall Noise Level

Octave Band Frequency (Hz)631252505001K2k4k8kLA5366768487868267


Summation of Overall Noise Level94 dB(A)97 dB(A)99 dB(A)98 dB(A)88 dB(A)102 dB(A)103 dB(A)104 dB(A)104 dB(A)


Summation of Overall Noise Level94 dB(A)97 dB(A)79 dB(A)98 dB(A)88 dB(A)101 dB(A)102 dB(A)102 dB(A)Compare with previously which was 104 dB(A)..


One factor may influence the accuracy of the measurement is the level of the background compared to the level of the sound being measuredThe following is the procedure of subtraction of the background noise1. Measure the total noise (Lm + La) with the machine running2. Measure the background noise (La) with the machine off3. Find the difference of these two readings and apply the subtracting procedure.

Example:Total noise = 60 dBBackground noise = 53 dBDifference = 7 dBCorrection from the chart = 1 dBNoise of the Machine= 60 1 = 59 dB

Chart3

6.6

4.25

3

2.2

1.6

1.24

1

0.8

dB Difference Between Measurement and Background Noise

dB To Be Subtracted From Measurement

Effects of Background Noise

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6.64.2532.21.61.310.8

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Level Difference Between Measurement and Background Noise

Level to be Subtracted From Measurement

Background Noise Correction Factor

Sheet2

6.64.2532.21.61.2410.8

12345678

Sheet2

0

0

0

0

0

0

0

0

dB Difference Between Measurement and Background Noise

dB To Be Subtracted From Measurement

Effects of Background Noise

Sheet3

6.64.2532.21.61.2410.8

12345678

Sheet3

00

00

00

00

00

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00

For a point source noise will reduce by 6 dB for every doubling distance 20 log rFor a plane source noise will reduce by 3 dB for every doubling distance 10 log r. This is also true within an enclosed space.

The most common interference is provided by a solid boundary. Sound striking a solid boundary may be either transmitted, reflected, or absorbed, as shown in Figure A3.

Sound striking a solid surface can cause the surface to vibrate, just as the ear drum vibrates when it is met by a sound wave. This vibration which is of the same frequency as the sound wave may set up another air-borne sound wave on the other side of the solid.

Sound may also be reflected from a solid surface in much the same way as a ball bounces from a wall. Reflected sound will increase the sound level on the source side of the solid. The most common example of this is a noise source such as a machine located above a hard concrete floor.

Insertion Loss Commonly used for duct silencer. Insertion Loss is obtained by performing a test on a section of ductwork through which acoustic energy is transmitted with and without a silencer. The Insertion Loss is the difference of the SPL measured in the receiving chamber.

Transmission Loss (TL)TL is obtained by putting a test piece between two reverberant chambers. It is the difference of the SPL between these chambers, related to the area of the test sample and the absorption of the receiving chamber.Common Acoustic Terminology- Continue

AttenuationAnother word for noise reductionAbsorption CoefficientThe ability of a material to absorb the sound. Normally given in percentage format

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Basic Noise