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Acoustics Applications

Inhabitants in big cities are immensely open to effects of environmental noise. The increase of environmental noise in urban areas is remarkably impressive. Recently, environmental noise has emerged as the most displeasing and harmful environmental factor.

At the treshold of 21 th century, Europen Union Commission Report has stated that twenty percent of the European population (approximately 80 millions ) are located in the area which is described as the black area where the environmental noise level exceeds 65dB

Approximately 170 millions live in the grey area where the environmental noise level exceeds 55 dB.

More than 65 % of the european population have been effected by excessive noise.

For an average city,the source of ourdoor noise is transportation systems ( 75% motor vehicles, 10% railway noise, 5% aircraft noise and 10% stem from industrial and residential constructions. These values may vary depending on the area and the cities to a certain extend.

Noise dissipating in air can generally be controlled with insulation and damping techniques. Noise path changes enable less annoying level of noise.

Following methods can be applied for a solution of noise problem.

a. Control of source

b. Control of noise path

c. Protective precautions from receiver

It might be costly to reduce the noise at source compared to the cost of total product therefore noise control at source is limited.

The cost of noise barrier is comparatively lower . Noise barriers with reasonable cost are preferred for environmental corcern as well.

Noise is the impact given by ear to pressure changes of oscillating objects in elastic environment. Sound transmission occurs with vibration of elements and transfering to adjacent particules.

Unwanted and discomforting sound is called noise. Sound is an objective notion. It is measurable and unchangeable. Noise is subjective. It depends on the individual to define the sound as noise but it is apparent that most of sound type can be recognized as noise.

Noise impacts on human health can be classifed as follows:

	Hearing defects due to noise
	Prevention of speaking
	Sleeping disorders due to noise
	Physical and cardiovascular impacts due to noise
	Effects on mental health
	Effects on work performance
	Effects on general behaviour and discomfort in residential areas
	Effects of combined noise sources
	Noise sensitive communities

Noise can also be categorized depending on frequency spectrum and sound level changes

a) a) Frequency spectrum is divided to broad and narrow band

b) b) Sound level changes in time can be watched as determined and and determined noise.

Periods Under the Continuous and Different Level of Noise according to Occupational Health and Safety Regulation

Maximum noise level (dBA)	Accepted period ( hour/day)
90	8
92	6
95	4
97	3
100	2
102	1,5
105	1
110	0,5
115	0,25

30-65 dBA	1.Degree of noise · Discomfort · Disturbance · Oppression · Anxiety · Disorder of concentration · Sleeping disorder
65-90 dBA	2.Degree of noise · Pulse changes · Accelaretion of respiration · Depression in brain
90-120 dBA	3.Degree of noise · Headache
120-140 dBA	4.Degree of noise · Defect in inner ear
140> dBA	5.Degree of noise · Tear of eardrum

SOME DESCRIPTIONS RELATED TO NOISE

Frequency ( f ) : frequency is the speed in a second and measured by Hertz. Audible frequency ranges 16-16000Hz to human ear. Most sensitive frequency is 3000 Hz. Talking are realized in between 200-10000 Hz. Frequencies from 1000 Hz to 2500 Hz are sufficient for clear understanding.

Octave Band: octave band is the frequency intervals to be examined in sound analysis. Octave bands and 1/n octave bands ( n=2,3,10,12 ) are used for standartization of sound and noise analysis. 1/3 and 1/12 octave bands analysis are pratically available. Octave interval are divided to 3 or 12 respectively for these analysis.

Frequencies from 63 Hz to 250 are low frequencies

Frequencies from 500 Hz to 1000 are middle frequencies

Frequencies from 2000 Hz to 8000 are high frequencies

Wavelength is the distance between two waves and measured by meter.

C=f*λ

C: speed of wave

f: frequency

λ: wavelength

Speed of sound (c): Transmission of sound are indicated in several materials below

Material	Transmission velocity m/h
Air ( 0°c )	331
Air ( 21°c )	344
Mushroom	500
Lead	1200-2400
Water	1450
Wood	3600
Concrete	3200-3600
Marble	3800
Glass	4000-5600
Brass	4700
Copper	3600-4760
Iron	5100-6000
Cast Iron	3500-5600
Steel	5800-6000
Aluminum	5100-6400

As you can see from the table above, sound diffusion speed in solids are much higher than the sound diffusion speed in the air.

The speed of sound may change slightly depending on temparature, pressure and humidity.

Sound Power Level: Sound power is the power of energy transmitted from sound source. The level of this power is sound power level ( Lw ).

When sound power is W ; sound power level Lw is calculated from below equation

W: Sound power level of source

Wo: International reference power level is 10^-12 W

The sound power of a machine is the transmitted part of entire power as only a small percentage of overall.

Sound Pressure Level: Sound pressure changes in a certain point is more important than the sound power of a source since the sound is perceived as the altering pressure of air to ear drum.

Sound pressure level is calculated from following equation

P: Square root value of average of sound pressure squares, effective value or square root of average square

P₀: International reference pressure as 20 micropascal ( 20x10^-6 N/m2 )

Adding in decibels: “n” which sound pressure levels Lpi (i=1,2,…n) can not be found by adding arithmetical sum of sound pressure level of Lpi’s different sound wavelength

Total level of sound pressure can be found by following equation

Mean time total sound power level is calculated the following equation.

Example: The sound pressure levels of two different machines at one point are 80 dB ve 83 dB when they run separetely. What is the sound presure level at the same point if they run together ?

Solution: if you formulate the sound pressure levels in

the result is

Subtracting in decibels: Subtracting in decibels is realized by subtracting sound pressure occured by environment noise( Lpa ) from measured level of sound pressure (Lpt ) when sound pressure level (Lpb) from the source is needed or any other circumstances.

Similar equation has been used for the sound power level.

Averaging in decibels: We might need to find out the average value when more than one different measurement appears at the same point or average sound pressure level is required to be calculated from the measurements at different points

Average sound pressure level is calculated as follows:

Similar equation has been used for the average sound power level.

Example: The measured sound pressure levels are 80 dB , 86 dB, 85 dB ve 88 dB at one point. Can you find our average sound pressure level value?

Reverberation of Sound: when the sound energy is delivered from the edge element to walls of the room, sound waves hitting the surface of the wall are partly reverberated, partly being absorbed and transmitted. A receiver in the volume will be hearing the direct sound from the source and after period of time based on a linear measurement difference between the sound ways, reverberated sound will be heard.

Absorption of Sound: when a sound wave hits an obstacle or a surface of a volume ; part of its enery has been absorbed. Soft and porous materials even human beings absorb great amount of sound waves. When sound goes through the material or change f to another type of energy ( mostly heat energy ) when it hits a surface it is called attentuation. Absorbing materials are being used for reducing noise and echo control and mostly for the control of reverberation time.

Transmission of sound: Transmission is the random dispersion of sound waves from surface and it occurs when surface dimension is equal to wavelength. If sound pressure is the same everywhere in an auditorium , we assume that sound waves move around freely to every direction and the sound area is homogeneous.

Reverberation time: reverberation time is the duration required sound pressure level for 60 dB after the active source has become silent in a volume.

Long reverberation time makes the conversation less understandable and the music out of tune. Short reverberation time diminishes the sound of music and conversation has been suffocated.

Reverberation time depends on the volume of the room and total absorption.

Echo: It is the repeat of the very high and long original sound after hearing of complete sound. Echo does not occur if the back wall is reflective and absorbtive. Other walls rather than the back wall can create echo.

Noise control: noise control is all precautions taken to prevent individuals from noise.

Noise control ;

· Requires reducing noise in the source

· Decreases noise in the transmitting area ( in between the source and receiver )

· Takes precautions where receiver senses the noise.

In principal, noise must be reduced in the source thus protection in whole environment has been ensured. Secondly decreasing the transmitting area must be considered. If these options are not feasible or ineffective ( uneconomic) noise must be reduced at the receiver.

Below find some methods for the noise control at the source , transmitting area and receiver.

A. Methods for the noise control at source

1. Reduce the sound energy from the source

2. Insulate between source and reflecting surface

3. Decrease reflection on the surface

Suggestions for implementation of above mentioned methods.

· Control of noise with planning and maintainance.

· Use of silencers

· Cover of the source with noise reducer and absorbtive material

· Insulation for vibration

· Coatings to vibrating surfaces with vibration reducer materials

· Keeping the noise source in closed unit

· Changing material and design

B. Methods for the noise control at transmitting area

· Seperating the source area with sound insulator material

· Using noise barriers

· Control of noise in transmitting area ( coating and covering the walls and ceilings with sound insulators , use of hanging noise insulators )

C. Methods for the noise control at the receiver

Prevention of individuals who are affected with the noise by taking them insulated areas or make them use ear protectors. These measures do not reduce the noise but protect the individuals.