The Ear

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Noise Measurement Analysis Phons- SPL at 1000Hz Double value loudness= double Value sone

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L10 is the noise exceed 10% of the time

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1D Sound Wave Assumptions: Mean Pressure and Density are constant in time and space. There is a zero mean flow in the pipe or duct. The pressure fluctuations are very (very) small. Thus, small velocity and density fluctuations. The processes are isentropic and the perfect gas laws apply.

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Impedance tube

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Silencer reactive-reflect, resistive-dissipate

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P: internal perimeter; S: CSA of air flow Limitations: accuracy about , Freq range: 250-2000Hz, , Circular ducts D>0.15m, Rectangular w/h<1m, 0.5<w/h=2.0. use splitter and thicker and better material

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Spherical Waves of Sound

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To find power

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( ) Pulsating Sphere Assumptions: Velocity at surface=particle velocity of air in contact with surface

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Sound Propagation

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Sound in Rooms

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When power stops, W=0

po2 is the initial mean square when t=0

Reverberation is the time taken for SPL to fall 60 dB from its initially steady value.

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Eyring-Norris; assume c=343m/s

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(Shape,F): (cube,3.5), (cylinder H=D, 4.2), (sphere,6), (hemisphere,4.5), (normal rooms,4)

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Optimum TR by Stephens and Bate ( )

K is 4 for speech, 5 for orchestras, 6 for choirs

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Q is 1 for centre of room, 2 for centre of ‘wall’, 4 for intersection of ‘wall’, 8 for corner

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Increasing R, Little effect on distant receiver (direct field) but large effect on reverberant field SIL-arithmetic average at 1kHz, 2kHz, 4kHz Reverberation chamber In steady state, power entering through specimen= power dissipated in receiving room

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Transmission of Sound through Walls Assume sound directed perpendicular to wall

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at resonance wm = k/w, at mass region wm dominate

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m is mass per unit area, h is thickness At angle approach

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To draw graph, firstly, calculate Field Transmission loss using mass law at 20Hz. Follow 6dB/octave to plateau height. Use frequency ratio x read frequency. Follow 10dB/octave.eg 6db/octave=6/lg 2 dB/dec

Acceptable Noise Level ( ) ( )

Type1-residential, Type 2- business, type3-heavy industry

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If not, noise limit=zoning limit Noise Source Fans

P is in cm of H2O

Flow Noise in Ducts

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Air Compressor

Electric Motor Noise

Spurs Gear Noise at 1m

Jet Noise; peaks at strouhal number =0.2

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Truck-Diesel Engine Noise ( )

B is bore of engine cylinder in cm ( ) ( )

Trucks-tyre noise from 15m ( )

V is speed (km/h), N is number of asles, L is load per type (kg) B tyre thread Pattern (36,neutral) (30,cross lug) Traffic Noise

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Q is flow rate(vehicles/hr), d is distance from centre line (m), v is mean speed (m/s), p is % of heavy vehicles

Basic noise level at 10m from freeway ( ) ( )

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( ) Noise Control Nominal reduction of 6 dB per doubling of distance for point and 3dB for line source. 65<Noise Pollution Level<75

Traffic Noise Index ( )

Day-Night Level L

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Noise and Number Index F=(0.3,oct)(0.15,1/3oct)

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N is no of aircraft, NNI of 20 lil annoyance, 40 moderate annoyance, 50 much annoyance Noise Exposure Forcast<20-30 NN no of night flight

t is time PNL within 10db of the peak, F=3 Extra Notes Six types of silencer

1) Absorptive Silencer -most common -contains absorptive material to absord reflected waves

2) Lined Bend -Force to change direction and lose energy, with absorptive material present.

3) Plenum Chamber -combination of 2&4; changes in direction causes energy loss, with absorptive material use for machine enclosure

4) Expansion chamber reduce sound by reflection of sound wave back to source to cancel out each other

5) Reactive resonator

6) Diffuser Emit gases at high pressure/velocity

7) Side branch resonator (1/4wave reson.) Employee enclosure Most cost effective and prominent. In the event acccess is need, increase the pressure of the room by means of increasing temperature, pumping air by exhaust. Enclosure should enclose the source as much as possible and have adequate stiffness and damping and uniform mass and away from resonance with no cracks and be lined on the inside with sound absorbing materials. Use composite silencer. Types: 1) localised (silencer/sheet metal &wire mesh cover). 2) Partial (sound absorbing baffles, 1 side open with another side close)increase room pressure 3) Complete (control room, floor plate isolated from machinery)-close fitting, provide structural damping, keep resonance away from frequency that required insertion loss Factors: 1) Transmission loss of wall 2) Surface area of wall 3) Temperature and absorption coefficient of source and receiver 4) location, size, machine, accessibility, visibility Health Hazard due to Loud Noise Exposure Permitted level is ( ) and

( ). Over exposure may cause temporary hearing impairment and Tinnitus and Noise Induced Hearing Loss. Under exposure may cause stress and raise blood pressure and heart rate. Designing the source In designing, isolation of noisy component (ie flywheel), preferential assembly (weld>bolts>rivet). In machinery, regular maintenance and lubrication is expected. Replacement of bearings, conveyer belt, using improved gears (helical>spurs) are common solutions. In operation, avoiding metal-metal contact by using dampers, avoid unbalanced force, ensure steady flow, control speed,reduce rate of force also means avoid impulse force for less annoyance due to less fundamental resonant frequency, enclose machine. Generally, a smaller source(<lambda), decreasing amplitude and increasing distance will help reduce noise. Using panels vibrating out of phase and lightly damped (constrained layer damping) can reduce noise. Designing Transmission Path (air borne & structure borne) Common solutions include distancing, noise barrier, absorbing material on noise barrier, usage of sound-absorbing baffles and acoustic silencer. Dense and heavy wall make a good barrier but poor sound absorber. Notes on Personal Hearing Protector Must be worn at all times as removal of 15min would be not effective. Long usage will contribute ear infection. Generally causes discomfot and inconvenience and poses harm when

warning sound not heard. Administrative control such as sign posting and notification to employee, job rotation and audiometric testing and issue protected areas must be used to comprehend usage. Designing Door A solid dense with sweep seal and gasketed perimeter is expected. Double panel window can be used. Designing Wall Wall must be extend to true ceiling and sealed at ends with gypsum. Wall is built with staggered studs and fibrous insulation in cavity air space and double layer gypsum. Use wall isolator if duct is connect to walls.

Designing Roof: gaps in corrugated roof filled Designing Common (ceiling +floor) Floated floor with plaster ceiling .

Designing Room

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to reduce RT by making space: reducing

suspended ceiling/wall/door. Usage of heavy curtains, sound diffusive panel and sound absorption panel to direct noise and eliminate noise for all frequency. Use individual branch feed ventilation for each room. Increasing the room absorption will only affect reverberant field (close) and not direct field (far). Partial enclosure is more effect in the near field. Partial with top enclosure can help direct field. Trigonometry Identity

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Ambient noise is the measurement of all noise including the offending noise. Background noise is the L90 without the presense of the offending noise. Leq is equivalent noise without the presence of offending noise. Specific noise is the logarithmic subtraction os ambient and residual. Rating level is adjustment made to the specific noise. Tonal noise: containing a prominent frequency and characterised by a definite pitch. White noise is the sound sound containing all audible frequencies at equal intensity. Pink noise sound containing all audible frequencies at equal intensity. A-weigthing scale that designed to simulate the response of the human ear. It is less effective for noise dominated by low-frequency content or for very loud sources of noise. B-weighthing is used by the motor industry for many years. The B-weighting was more critical of lower frequencies than the A-weighting network. C-weigthing is commonly used for high level measurements and peak SPL. A used for general purpose but C correlates better with hum response to high noise.D-weighting developed for measuring high level aircraft noise especially non-bypass military engine. L10 measure of the peaks in the noise and how ‘annoying’ the noise is, L90 background noise, L50 measure the average or mean noise level. Leq, continuous equivalent noise level: level of a steady noise which would contain the same acoustic energy, relevant when considering hearing damage from exposure to noise vary time. SEL = Leq + 10 log T (seconds). Effective Noise Level: level depending on Duration: offending noise only exist for short period time, Tonal component: impulsive noise having a high peak of short duration or a sequence of such peaks, Intermittency: intermittent noise the level suddenly drops to that of the background noise several times during the assessment period and Reflection: measurement point above reflecting surface.