Fundamentals of Noise Measurements

NIDHIN MANOHAR

• Sound - Defined as any pressure variation in a medium (Air, Water or other mediums) that the human ear can detect

• Sounds unpleasant and unwanted are called “Noise• “Audible dynamic range of human ear

20 x 10-6 Pa to 200 Pa

20 x 10-6 Pa Threshold of Hearing of 0 dB

200 Pa - Threshold of Pain - 140 dB

160 dB - Threshold of permanent deafness• Sounds of different frequency, at a constant sound pressure level

do not evoke equal loudness sensations• Loudness level expressed as Phon

0 Phon is 0 dB SPL at 1000 Kz

40 Phon is 40 dB SPL at 1000 Hz

The Ear’s Most Important Function : To Receive Speech

Important Ranges of Human Bearing :

Range of Hearing 20 Hz – 20 kHz

All sounds preceived by normal subjects

Range of Speech 100 Hz – 10 kHz

All sound produced in human speech

Range of Articulation 200 Hz – 6 Hz

Range needed to hear every syllable of English speech

Range of Intelligibility 500 Hz – 2500 Hz

Range needed to understand English speech (even though every syllable may not be properly heard)

Minimum Audible Field (MAF)

Level at which each tonal signal is barely received by people with normal hearing

a) Highest Sensitivity 500 – 2500 Hz

b) Lowest Sensitivity 20 – 100 Hz, 15,000 – 20,000 Hz

Why measure sound? • Provide definite quantities which describe and rate

sound• Permit precise,scientific analysis of annoying sound• Engineering aspects of noise control• Ascertain the probable damage to ears• Improvement in building design• Diagnostic tool

Parameters1. Sound Pressure2. Sound Power3. Sound Intensity

Sound pressure level (SPL)

P = RMS Sound PressurePo = Reference sound pressure 20 x 10-6 Pa

(Threshold Hearing)

dBPo

PLOG20

Equivalent sound pressure level

T = MeasurementP(t) = Sound PressureLeq has the same energy content as the varying sound level

SPL Depends on - Distance- Orientation of Receiver relative to machine- Environment of measurement room

( )dt

oP

tP

TLeq ∫=

2

21log10

Why Use A Logarithmic Measure Of Sound ?1. Enables coverage of entire hearing range on single meter.2. Corresponds roughly to ear’s behavior: 1 unit Loud.10 units Twice as loud.32 units Three times as loud.100 units Four times as loud.1000 units Eight times as loud. 3. Corresponda to ear’s perception of change: 1 db change…….. Hardly noticeable.3 db “ ……... Noticeable, not significant.5 db “ ……... Significant10 db “ ……... Double loudness0.5 db “ ……... Noticeable only in “A-B” tests.

Sound power level (SWL)

W = Acoustic power of sourceW0 = Reference sound power = 10-12

• SWL is a function of source only and is independent of the acoustic environment

• Widely used for rating and comparing equipment and also in noise control.

dBPo

PLOG20

Sound Intensity level

SIL =

Io = Reference Intensity = 10-12 W/M2

• Average rate of flow of energy per unit time through a unit area

• Uses special intensity probes – Two microphones for measurement

• Used for identification of noise sources

• Provides information on direction of acoustical energy flow

0

110

ILOG

Intensity

1. Intensity is a vector quantity as it has both magnitude and direction.

2. Intensity is a measure of the concentration of acoustic power across a unit area.

3. Intensity is dependent on the source’s properties and distance from the source.

4. Intensity is difficult to measure directly, but can be determined indirectly from sound pressure measurements.

Equipment A – Weighted

Sound Power

Levels

Co Comprehensive (3.5 – 17 m3 min) 85 - 120

Pneumatic hand tools 105 –123

Axial flow fans (0.05 m3 min-50 m3min); 10 mm H2O 61-88

Axial flow fans (0.05 m3 min-50 m3min); 300 mm H2O 88 - 120

Centrifugal fans (0.05 m3 min-50 m3 min); 10 mm H2O 45 - 77

Centrifugal fans (0.05 m3 min-50 m3 min); 300mm H2O 75 - 108

Propeller fans (0.05 m3 min-50 m3 min); 10 mm H2O 62 - 94

Propeller fans (0.05 m3 min-50 m3 min); 300 mm H2O 94 - 125

Centrifugal pumps (>1600 rpm ) 105 - 132

Screw Pumps ( >1600 rpm ) 110 - 137

Equipment A – Weighted

Sound Power

Levels

Reciprocating pumps(>1600 rpm) 115 - 138

Pile driving equipment (upto to 6 ton drop hammer 103 - 131

Electric saws 96 - 126

Generators ( 1.25 – 250 kVA) 99 - 119

Industrial vibrating screens 100 - 107

Cooling towers 95 - 120

Room air-conditioners (up to 2 hp) 55 - 85

Tractors and trucks 110 - 130

Sound power level data are useful

• To calculate the approximate SPL at a given distance from a

machine operating in a specified environment

• To compare the noise radiated by machines of same type and size

• To compare the noise radiated by machines of different types and sizes

• To determine whether a machine complies with a specified upper limit of sound emission

• To plan in order to determine the amount of transmission loss or noise control required under certain circumstances

• To assist in developing quiet machinery and equipment.

R = Room constant

Q = Directivity factor depends on the shape and the

complexity of source

dBRr

QLwLP ]

4

4[10log10

2++=

π

αα

−=

1

S

21

221 ...........1

SS

SS

+++= αα

Free space, spherical radiation Q = 1

Centre of flat surface, hemispherical radiation Q = 2

Centre of edge formed by junction of two

adjacent flat surfaces Q = 4

Corner formed by junction of three

adjacent surfaces Q = 8

For a typical air conditioned room at a distance

of 1M

LP = Lw – 8 dB

Meter response control

FAST – Meter responds quickly to step changes in continous sound level, approximately 0.2 second required

SLOW –Meter responds slowly to step changes in continuous sound level. Approximately 1.0 second required

IMPULSE – Meter responds to maximum RMS value of repetitive impulsive sounds.

PEAK -Meter responds to maximum peak value of impulsive sound, even of single impulses