Noise Pollution Introduction Wave A wave is a periodic disturbance that carries energy from one point to another and require a source and medium of propagation. It may lose energy with distance or as they propagate through the denser medium. Wave can be of two typesTransverse wave- e.g. electromagnetic waves such as light Longitudinal wave- e.g. soundwaves Transverse Wave Transverse wave is a wave in which the motion of medium is at right angle to the direction of the wave The waves you can make on a jump rope, on a guitar string, electromagnetic waves like visible light and X-rays are all transverse waves Transverse Wave Longitudinal Wave The displacement of the medium is parallel to the propagation of wave Particles move left and right which makes the other particles start to oscillate Wavelength – the distance between the same points on successive waves (meters). Period – the amount of time it takes a wave to complete one oscillation or cycle, i.e., wavelength expressed in units of time (seconds). Frequency – number of oscillations per second (s-1 or Hertz). Amplitude - the maximum displacement of a wave media from its undisturbed position. Properties of wave • Diffraction- bending of waves as they pass through narrow openings or around sharp corners. • Refraction- the change of speed and direction that occurs when a wave goes from one medium to another. • Reflection- a form of scattering that may be described with a simple geometric relationship, i.e. angle of incidence equals angle of reflection. • Scattering - when waves interact with matter. Sound • Sound (or noise) is the result of pressure variations, or oscillations, in an elastic medium (e.g., air, water, solids), generated by a vibrating surface, or turbulent fluid flow. • Sound is a vibration that is audible to ear • It propagate as longitudinal wave in air, water and solids and also as transverse waves in solids • When the waves are generated from a source, it creates vibrations in the surrounding medium. • As the sources continue to vibrate the medium, the vibration propagate away from the source, thus form the sound wave. • Sound waves can travel through variety of substances. • Sound generated from a point source obey Inverse Square Law which states that intensity of sound is inversely proportional to the square of the distance from the sources. Sound Field- the technical name given to the dispersion of sound energy within given boundaries. • Free field- The free field is a region in space where sound may propagate free from any form of obstruction. • Near field-The near field of a source is the region close to a source where the sound pressure and acoustic particle velocity are not in phase. • Far field- The far field of a source begins where the near field ends and extends to infinity. • Direct field- The direct field of a sound source is defined as that part of the sound field which has not suffered any reflection from any room surfaces or obstacles • Reverberant field- The reverberant field of a source is defined as that part of the sound field radiated by a source which has experienced at least one reflection from a boundary of the room or enclosure containing the source. Sound waves are characterized by • Sound Intensity • Sound Pressure • Speed of Sound Sound Intensity Sound power is the rate at which energy is radiated (energy per unit time). Sound intensity is defined as the sound power per unit area. The basic units are watts/m2 or watts/cm2 Sound intensity gives a measure of direction as there will be energy flow in some directions but not in others. Therefore sound intensity is a vector quantity as it has both magnitude and direction. Sound Pressure Sound Pressure: is the difference between the pressure produced by a sound wave and the barometric (ambient) pressure at the same point in space. It is a measure of force per unit area and the unit is pascal (Pa) (Newton/meter square) Speed of Sound The speed of sound is the distance travelled per unit time by a sound wave propagating through an elastic medium. It is actually the speed of transmission of a small disturbance through a medium. The SI unit of the speed of sound is the metre per second (m/s). In dry air at 20°C, the speed of sound is 343.2 metres per second (1,126 ft/s). • The speed of sound varies from substance to substance. • Sound travels faster in liquids and non-porous solids than it does in air. • It travels about 4.3 times as fast in water (1,484 m/s), and nearly 15 times as fast in iron (5,120 m/s), as in air at 20°C. Sound Perception The physical reception of sound in any hearing organism is limited to a range of frequencies. Humans normally hear sound frequencies between approximately 20 Hz and 20,000 Hz. The upper limit decreases with age. Unwanted sound which disturbs the human being and cause an adverse effects on mental and psychological well being is called noise. Sound • People generally hear sounds between the “threshold of hearing” and the “threshold of pain” • In terms of pressure, this is 20 μPa – 100 Pa. • To cope with the problem of measuring noise in terms of pressure unit, a scale based on the logarithm of the ratio of the measured quantities is used. • Measurements on this scale are called Levels. • Unit for these measurement scales is the bel, named after Alexander Graham Bell • L’= log Q/Q0 • Where, L’= Level, bels; Q = measured quantity; Q0= reference quantity; log= Logarithm in base 10 • A bel turns out to be a large unit, thus for the convenience it is divided into 10 subunits called decibels (dB). • Levels in decibels are computed as follows: L= 10 log Q/Q0 It does not have any physical unit. The decibel scale generally ranges from approximately 0 to 130. • Sound Power Level Lw = 10 log W/10-12 For noise measurement, reference power level has been established as 10-12 W. Sound Intensity Level Li = 10 log I/10-12 Because the threshold of hearing is 20 μPa Sound pressure for known sounds Basic Formulas to solve • For antilog calculation 𝑦 = 𝑎𝑛𝑡𝑖𝑙𝑜𝑔 𝑥 = 10𝑥 • For logarithmic calculation 𝐿𝑜𝑔 10𝑥 = 𝑥 Questions • If the sound pressure is 0.02 Pa, what is sound pressure level? • Noise level is 80 dB, then calculate the corresponding sound pressure level. • If the sound power from a point source is 0.001 watt. What is the sound level in dB?
