NOISE

Sound and Noise

Sound is what we hear.

Noise is unwanted sound .

The difference between sound and noise depends upon the listener and the circumstances.

Ex. . Rock music can be pleasurable sound to one person and an annoying noise to another

In either case, it can be hazardous to a person's hearing if the sound is loud and if he or she is exposed long and often enough. • Noise does not necessarily have any particular physical characteristics that distinguish it from wanted sound. No instrument can distinguish between a sound and a noise –only human reaction can.

PRODUCE: Sound is produced by vibrating objects and reaches the listener's ears as waves in the air,water or other media.

HOW

•When an object vibrates

•It causes slight changes in air pressure

• These air pressure changes travel as waves through the air and produce sound

Ex.

• Imagine striking a drum surface with a stick

•The drum surface vibrates back and forth

•As it moves forward, it pushes the air in contact with the surface

•This creates a positive (higher) pressure by compressing the air

•When the surface moves in the opposite direction, it creates a negative (lower) pressure by decompressing the air

•Thus, as the drum surface vibrates, it creates alternating regions of higher and lower air pressure

•These pressure variations travel through the air as sound waves

Other Examples such as Consider plate suspended in midair.When struck, the vibrates rapidly back and forth just like drum .

CHARACTERISTICS OF SOUND WAVES:

As a sound energy is transmitted through a medium in waves, it exhibits certain properties.

1) Firstly, the longitudinal waves travel at a velocity or speed. The speed of sound differs depending upon medium temperature and pressure in which it is traveling. At 00 C and one atmosphere of air pressure, the speed of sound is accepted as 331.3 m.s-1 As the temperature (T) increases, the speed of sound also increases at approximately 0.60 m.s-1 for each 10 C.

Example1). What would be the speed of sound(c) at 200 C and 1 atmosphere?

C = (331 + 0.6T) m.s-1

= (331 + 0.6(20) )

= (331 + 12)

= 343m.s-1

2) What would be the speed of sound © at 480 C and 1 atmosphere in paint industry at Ankleshwar?

Pressure: Small fluctuations in air pressure that are caused through the vibration of air particles are known as acoustic pressure.

• For simple sounds, the acoustic pressure is described as a sinusoidal (sine) curve that considers:

1. The angular nature of the wave

2. The period or time duration of the wave(t)

3. Amplitude or maximum value of the curve(A).

Figure of Pressure verses Time (sec)

Equation : p = A.sin 2πft

• Since the pressure of the wave changes direction (compression and rarefaction), the maximum amplitude (A) of the curve is both positive and negative, when compared against its overall amplitude would be zero since the positive and negative amplitudes of the wave would cancel each other out.

•Therefore, to determine the equivalent amplitude of the pressure wave(or the effective pressure {Peff }),the root mean square (rms) value is taken.

This is expressed in following equation:

Peff= Prms ~ 0.707Pamplitude

The term rms is determined by squaring the mean values of points along the pressure wave, and then taking the square root of this value.

• The time taken to complete the cycle is known as the period(t) and is measured in seconds. Period and frequency are inversely related shown in following equation. t= 1/f Where: t is period (sec),f is frequency (Hz)

• The other characteristic of a sound wave is its wavelength(λ ), measured in meters (m). A wavelength is the distance between two successive crests, or two identical points on the wave.

SOUND WAVES:Sound waves are a particular form of a general class of waves known as elastic waves.

• Sound waves can only occur in media that have the properties of mass (inertia) and elasticity.

• Because air possesses both inertia and elasticity,a sound wave can be propagated (spread) in air.

• One sound wave may have three times the frequency and one third the amplitude of another sound wave.

Show the curves of frequency verses intensity

• Curve of normal conversation

• Curve of higher pitch than a sound

• Curve of louder (high intensity) than the normal intensity .

Hearing mechanism :The hearing mechanism of the ear senses the sound waves and converts them into information which it relays to the brain. The brain interprets the information as sound. Even very loud sounds produce pressure fluctuations which are extremely small (1 in 10,000) compared to ambient air pressure (i.e., atmospheric pressure). The hearing mechanism in the ear is sensitive enough to detect even small pressure waves. It is also very delicate: this is why loud sound may damage hearing.

SOME PROPERTIES OF NOISE THAT CAN BE MEASURED :

The properties of noise which are important in the workplace are:

•frequency

•sound pressure

•sound power

•time distribution

FREQUENCY:

•The number of pressure variations per second is called the frequency of the sound,which was formerly measured in cycles per second but is expressed in hertz(Hz),which represents cycle per second.

• The time required for each cycle is known as the period of the wave and is simply the reciprocal of the frequency.

•Frequency is perceived as pitch.

•Most everyday sounds contain a mixture

of frequencies generated by a variety of sources.A sound’s frequency composition is referred to as its spectrum.The frequency spectrum can be a determining factor in the level of annoyance caused by noise;high frequency noise generally is more annoying than low frequency noise.

• FrequenLow pitched or bass sounds have low frequencies. High-pitched or treble sounds have high frequencies. A healthy, young person can hear sounds with frequencies from roughly 20 to 20,000 Hz. The sound of human speech is mainly in the range 300 to 3,000 Hz.

• Also,narrow frequency bands or pure tones( single frequencies) can be somewhat more harmful to hearing than broadband noise.

WAVELENGTH:Wavelength is the distance measured between two analogous points on two successive parts of a wave. In other words, wavelength is the distance that a sound wave travels in one cycle.

• The Greek letter lambda( ) is used to express wavelength,and it is measured in feet or meters.

Wavelength is an important property of sound. For example, sound waves that have a wavelength that is much larger than the size of an obstacle are little affected by the presence of that obstacle; the sound waves will bend around it. This bending of the sound around obstacles is called diffraction.

• If the wavelength of the sound is small in comparison with the size of the obstacle (such a wavelength is generated by high frequency sounds), the sound will be reflected or scattered in many directions, and the obstacle will cast a shadow.

Actually, some sound is diffracted into the shadow, and there is significant reflection of the sound. As a consequence of diffraction, a wall is of little use as a shield against low-frequency sound ( long wavelength), but it can be an effective barrier against high frequency sound ( short wavelength) .

Velocity: The velocity with which the analogous pressure points on successive parts of the wave pass a given point is called the speed of sound. The speed of sound is always equal to the product of the wavelength and the frequency

C=fƛ

In the formula,c= speed of sound(feet or meters per second);

F = Frequency of sound

ƛ = wavelength (feet or meters)

 

Sound pressure :Sound pressure is the amount of air pressure fluctuation(pressure changes above and below atmospheric pressure) a noise source creates.

• Most common sounds consist of a rapid,irregular series of positive pressure disturbances(compression) and negative pressure disturbances (rarefactions) measured against the equilibrium pressure value. If we were to measure the mean value of a sound pressure disturbance, we would find it would be zero,because there

are as many positive compressions as negative rarefactions. Thus, the mean value of sound pressure is not a useful measurement. We must look for a measurement that permits the effects of rarefactions to be added to (rather than subtracted from) the effects of compressions.

•If the drum in above example is hit very lightly, the surface moves only a very short distance and produces weak pressure fluctuations and a faint sound. If the drum is hit harder, its surface moves farther from its rest position. As a result, the pressure increase is greater. To the listener, the sound is louder.

•Sound pressure also depends on the environment in which the source is located and the listener's distance from the source.

•The sound produced by the drum is louder two meters from the drum if it is in a small bathroom, than if it is struck in the middle of a football field.

•Sound pressure is usually expressed in units called pascals (Pa). A healthy, young person can hear sound pressures as low as 0.00002 Pa. A normal conversation produces a sound pressure of 0.02 Pa.

•A gasoline-powered lawn mower produces about 1 Pa. The sound is painfully loud at levels around 20 Pa. Thus the common sounds we hear have sound pressure over a wide range (0.00002 Pa - 20 Pa).

•It is difficult to work with such a broad range of sound pressures. To overcome this difficulty we use decibel (dB, or tenth (deci) of a Bel)). The decibel or dB scale is more convenient because it compresses the scale of numbers into a manageable range. The decibel is named after Alexander Graham Bell, the Canadian pioneer of the telephone who took great personal interest in the problems of deaf people.

Sound pressure level :Sound pressure converted to the decibel scale is called sound pressure level (Lp). Following figure compares sound pressures in pascals and sound pressure levels in decibels (dB). . Figure compares sound pressures in pascals and sound pressure levels in decibels (dB). The zero of the decibel scale (0 dB) is the sound pressure of 0.00002 Pa. This means that 0.00002 Pa is the reference sound pressure to which all other sound pressures are compared on the dB scale. This is the reason the decibels of sound are often indicated as dB re 0.00002 Pa.

Decibels and Sound Pressure Level:

•Even though the weakest sound pressure perceived as sound is a small quantity,the range of sound pressure perceived as sound is extremely large.

•The weakest sound that can be heard by a person with very good hearing in an extremely quiet location is known as the threshold of hearing.

• At a reference tone of 1000 Hz, the threshold of hearing is equal to a sound pressure of 20 microPa.(0.0002 microbar).

• The threshold of pain, or the greatest sound pressure that can be perceived without pain, is approximately 10 million times greater. It is therefore move convenient to use a relative scale of sound pressure rather than an absolute scale.

•Equation to calculate sound pressure level

Lp = 20 log p/po Where, Lp = the sound pressure level,p= rms sound pressure, po= a reference sound pressure and log= logarithms to the base 10.

•So, the sound pressure level of the quietest noise the healthy young person can hear is calculated in this way:

Lp = 20 log (0.00002/ 0.00002) = 20 log (1) = 20 X 0 = 0 dB

• The sound pressure level or Lp in a very quiet room, where the sound pressure is 0.002 Pa, is calculated:

•Lp = 20 log (0.002/ 0.00002) = 20 log (100) = 20 X 2 = 40 dB

•The sound pressure level of a typical gasoline-powered lawn mower, which has a sound pressure of 1 Pa, is calculated

•Lp = 20 log (1/0.00002) = 20 log (50 000) = 20 X 4.7 = 94 dB

sound power:•The sound power is the sound energy transferred per second from the noise source to the air.

•A noise source, such as a compressor or drum, has a given, constant sound power that does not change if the source is placed in a different environment.

• Power is expressed in units called watts (W). An average whisper generates a sound power of 0.0000001 watts (0.1 W), a truck horn 0.1 W, and a turbo jet engine 100,000 W.

Like sound pressure, sound power (in W) is usually expressed as sound power levels in dB. Following figure provides examples of sound power level calculations.

Following figure relates sound power in watts to sound power level in decibels. Note that while the sound power goes from one trillionth of a watt to one hundred thousand watts, the equivalent sound power levels range from 0 to 170 dB.

The manufacturer can often provide the sound power of equipment. A number of international standards are available for labeling machines and equipment with their noise emission levels. From the sound power of a compressor, one can calculate the expected sound pressure and sound pressure level at a certain location and distance. This information can be helpful in determining possible noise exposures and how they compare to the noise guidelines.

Sound Power Level Calculations

Sound power levels or Lw are determined by the following formula:

Lw = 10 log (Sound Power Level / Reference Power Level )

The reference power is one trillionth of a watt (0.000000000001 W). Therefore

Lw = 10 log (Sound Power Level / 0.000000000001)

Thus, the sound power level associated with an average whisper, which has a sound power of 0.0000001 W, is calculated

Lw = 10 log (0.0000001/ 0.000000000001) = 50 dB

KINDS OF NOISE :Noise can be

• continuous

• variable

• intermittent or impulsive

Continuous noise is noise which remains constant and stable over a given time period. The noise of boilers in a power house is relatively constant and can therefore be classified as continuous.

• Most manufacturing noise is variable or intermittent. Different operations or different noise sources cause the sound changes over time. Noise is intermittent if there is a mix of relatively quiet periods and noisy.

• Impulse or impact noise is a very short burst of loud noise which lasts for less than one second. Gun fire or the noise produced by punch presses are examples of such noise.

A-weighted decibels :

The sensitivity of the human ear to sound depends on the frequency or pitch of the sound. People hear some frequencies better than others. If a person hears two sounds of the same sound pressure but different frequencies, one sound may appear louder than the other. This occurs because people hear high frequency noise much better than low frequency noise.

Noise measurement readings can be adjusted to correspond to this peculiarity of human hearing. An A-weighting filter which is built into the instrument de-emphasizes low frequencies or pitches. Decibels measured using this filter are A-weighted and are called dB(A). Legislation on workplace noise normally gives exposure limits in dB(A).

A-weighting serves two important purposes:

1. gives a single number measure of noise level by integrating sound levels at all frequencies

2. gives a scale for noise level as experienced or perceived by the human ear

Some of the Examples:

Typical Noise Levels

Noise Source dB(A)

pneumatic chipper at 1 metre 115

hand-held circular saw at 1 metre 115

textile room 103

newspaper press 95

power lawn mower at 1 metre 92

Diesel truck 50 km per hour at 20 metres 85

passenger car 60 km per hour at 20 metres 65

conversation at 1 metre 55

quiet room 40

The three variables that affect the risk of exposure to noise are:

• Frequency composition:

1. A healthy, young ear is able to detect sound waves between 20 Hz and 20 kHz.

2. Exposure at some of the higher frequencies can produce permanent hearing loss.

3. Occupational noise consists of many frequencies.This is known as broadband sound.

4)Where sound has only one frequency, it is referred to as a pure tone. These are found less frequently in industrial settings.

• Amplitude:

1. The amplitude of sound can be measured as an intensity level (IL); although more commonly as the sound pressure level (SPL).

2. For an ideal point source in a free field (where the source is in open air or where reflection is limited) the intensity of sound radiated is given in following equation.

I = W/4π r2

Where: ‘I’ is intensity of radiated sound

‘W’ is power (Watts)

‘r’ is the distance from the source (m)

3. Example of this type of noise may be a small loud speaker operating at low frequency.

• Continuousness:

The continuousness of sound relates to whether the sound is produced intermittently or constantly.

BASIC RULES OF WORKING WITH DECIBEL (DB) UNITS

• The decibel [dB, and also dB(A)] is a logarithmic scale.

• For mathematical calculations using dB units, we must use logarithmic mathematics . However, in our day-to-day work we do not need such calculations.

•The use of dB unit makes it easy to deal with the workplace noise level data provided we use a set of simple rules as summarized in following Table.

Table

Decibel (dB) basics

Change in dB Change in sound energy

3 dB increase Sound energy doubled

3 dB decrease Sound energy halved

10 dB increase Sound energy increased

by factor of 10

10 dB decrease Sound energy decreased

by factor of 10

20 dB increase Sound energy increased by factor of 100

20 dB decrease Sound energy decreased

by factor of 100

SOUND PRESSURE WEIGHING

• Simple to build an electronic circuit whose sensitivity varied with frequency in the same way as the human ear.

• Resulted in three different internationally standardized characteristics termed weighting networks “A”,”B”,”C”.

• The A –network was designed to approximate the equal loudness curves at low sound pressure levels.

•The “B” –network was designed for medium sound pressure levels

•The “C”-network was designed for high levels.

•The very low frequencies are discriminated(Distinguish),attenuated or filtered out quite severely the A-network,moderately filtered out by the B-network and hardly attenuated at all by the C-network.

• The A-weighted sound level measurement has become popular in the assessment of the overall noise hazard since this level is thought to provide a

rating industrial broadband noises that indicates the injurious effects such noise has on the human ear.

• As a result of its simplicity in rating the hazard to hearing, the A-weighted sound level has been adopted as the measurement of assessing noise exposure by the ACGIH.

• The A-weighted sound levels have also been shown to provide reasonably good assessments of speech interference and community disturbance conditions and have been adopted for these purposes.

NOISE LEVEL ADDITION METHOD:

•Sound pressure levels in decibels (dB) or A-weighted decibels [dB(A)] are based on a logarithmic scale .

• They cannot be added or subtracted in the usual arithmetical way. If one machine emits a sound level of 90 dB, and a second identical machine is placed beside the first, the combined sound level is 93 dB, not 180 dB.

•Following table shows a simple way to add noise levels.

Addition of Decibels

Numerical difference Amount to be added

between to the higher of the

two noise levels [dB(A)] two noise levels [dB or dB(A)]

0 3.0

0.1 - 0.9 2.5

1.0 - 2.4 2.0

2.4 - 4.0 1.5

4.1 - 6.0 1.0

6.1 – 10 0.5

10 0.0

Step 1: Determine the difference between the two levels and find the corresponding row in the left hand column. Step 2: Find the number [dB or dB(A)] corresponding to this difference in the right hand column of the table. Step 3: Add this number to the higher of the two decibel levels.

For instance, using the example of two machines each emitting a noise level of 90 dB:

•Step 1: The numerical difference between the two levels is 0 dB (90-90= 0), using the first row.

•Step 2: The number corresponding to this difference of 0, taken from the right hand column, is 3.

•Step 3: Add 3 to the highest level, in this case 90. Therefore, the resulting noise level is 93 dB.

When the difference between two noise levels is 10 dB(A) or more, the amount to be added to the higher noise level is zero. In such cases, no adjustment factor is needed because adding in the contribution of the lower in the total noise level makes no perceptible difference in what people can hear or measure. For example if your workplace noise level is 95 dB(A) and you add another machine that produces 80 dB(A) noise, the workplace noise level will still be 95dB(A).