BASIC ACOUSTICS

SUWANDI FACULTY OF SCIENCE*TEKNIK FISIKA

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Akustik dasarKuantifikasi suaraTekanan suara, level tekanan (skala dB)Intensitas Suara, Daya SuaraKombinasi sumber suaraFrekuensi SuaraSumber suara sederhanaDirektivitas

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Kuantifikasi SuaraMenunjukkan jumlah pasti yang menggambarkan suara dan tingkat suara.Izin yang tepat, analisis ilmiah dari suara yang mengganggu (cara obyektif untuk perbandingan)Membantu perkiraan Kerusakan perkiraan pendengaran.Alat diagnostik yang powerfull untuk program pengurangan kebisingan: Bandara, Pabrik, Rumah, studio Recording, Jalan, dll*TEKNIK FISIKA

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Daya / Intensitas / Tekanan Intensitas dan tekanan diukur menggunakan alat ukurDaya dihitung Daya adalah ukuran dasar dari energi akustik yang dihasilkan dan tidak bergantung lingkungan *

Daya / Intensitas / TekananDaya Suara: for noise rating of machinesunique descriptor of noisiness of sourceTekanan suara: evaluasi bahaya dan gangguan dari sumber kebisinganIntensitas Suara:Jumlah daya per satuan luas*

Pengukuran intensitas suara juga dapat memperkirakan tingkat sumber gangguan*TEKNIK FISIKA

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Sound Intensity Laju rata-rata aliran energi per satuan luas*

Sound Intensity Time averaged rate of energy flow per unit areaTEKNIK FISIKA

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Hubungan daya dan intensitas*

Steady background noise is not a problem*TEKNIK FISIKA

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RANKING*TEKNIK FISIKA

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Sound FieldsISO 3745*TEKNIK FISIKA

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Kuantifikasi SuaraRoot Mean Square Value (RMS) of Sound PressureMean energy associated with sound waves is its fundamental featureenergy is proportional to square of amplitude

Variabel-varibel Akustik: Tekanan dan Kecepatan PartikelTEKNIK FISIKA

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Range of RMS pressure fluctuations that a human ear can detect extends from 0.00002 N/m2 (Pascal)(threshold of hearing)to20 N/m2(Pascal)(sensation of pain) 1,000,000 times largerpeak pressure of loudest sound is 3500 times smaller than atm. pressure*

Very large range of sound intensity which the ear can accommodate,

from the loudest (1 watt/m2)

to the quietest (10-12 watts/m2),

energy received from a 50 watt bulb *TEKNIK FISIKA

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LevelsA unit of a logarithmic scale of power or intensity called the power level or intensity level.

The decibel is defined as one tenth of a bel

One bel represents a difference in level between two intensities (one of the two is ten times greater than the other)

Thus, the intensity level is the comparison of one intensity to another and may be expressed:Intensity level = 10 log10 (I1 /Iref) (dB)

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Why log ratio?Logarithmic scale compresses the high amplitudes and expands the low ones

The other reason: Equal relative modifications of the strength of a physical stimulus lead to equal absolute changes in the salience of the sensory events (Weber-Fechner Law) and can be approximated by a logarithmic characteristics (Ear responds logarithmically to stimulus)*

Acoustic parameters are expressed as logarithmic ratio of the measured value to a reference valueThe Bel (B) is a unit of measurement invented by Bell Labs and named after Alexander Graham Bell. The Bel was too large, so the deciBel(dB), equal to 0.1 B, became more commonly used as a unit for measuring sound intensity Power Ratio of 2 = dB of 3Power Ratio of 10 = dB of 10Power Ratio of 100 = dB of 20dB SCALE*

Sound Pressure LevelIn acoustics, the reference pressure Pref=2e-5 N/m2 or 20Pa (RMS) loudest sound pressure that a normal person can barely perceive at 1000HzIn linear vibroacoustics, time averaged power values are proportional to the squared rms-amplitudes of the field variables (e.g., pressure, particle velocity)

Thus to calculate logarithmic levels from the field variables, it is these squared rms-amplitudes that must be used. *

Corresponding to audio range of Sound Pressure2e-5 N/m2- 0 dB20 N/m2- 120 dBNormal SPL encountered are between 35 dB to 90 dB

For underwater acoustics different reference pressure is usedPref = 0.1 N/m2It is customary to specify SPL as 52dB re 20PaSound Pressure Level*

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Typical average decibel levels (dBA) of some common sounds. *

Threshold of hearing0 dBMotorcycle (30 feet)88 dBRustling leaves20 dBFoodblender (3 feet)90 dBQuiet whisper (3 feet)30 dBSubway (inside)94 dBQuiet home40 dBDiesel truck (30 feet)100 dBQuiet street50 dBPower mower (3 feet)107 dBNormal conversation60 dBPneumatic riveter (3 feet)115 dBInside car70 dBChainsaw (3 feet)117 dBLoud singing (3 feet)75 dBAmplified Rock and Roll (6 feet)120 dBAutomobile (25 feet)80 dBJet plane (100 feet)130 dB

Sound PowerIntensity : Average Rate of energy transfer per unit areaSound Power Level:Reference Power Wref =10-12 WattdBPeak Power output: Female Voice 0.002W, Male Voice 0.004W, A Soft whisper 10-9W,An average shout 0.001W Large Orchestra 10-70W,Large Jet at Takeoff 100,000W 15,000,000 speakers speaking simultaneously generate 1HP*

Sound Intensity

For air, 0c 415Ns/m3 so that For plane progressive waves;Hold true also for spherical waves far away from sourceReference Intensity Iref =10-12 Watt/m2*

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Effect of multiple sound sourcesLp1Lp2*TEKNIK FISIKA

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If intensity levels of each of the N sources is same,Thus for 2 identical sources, total Intensity Level is 10Log2 i.e., 3dB greater than the level of the single sourceFor 2 sources of different intensities: L1 and L2COMBINATIONS OF SOURCESL1=60dB, L2=65.5dB LT=66.5dBL1=80dB, L2=82dB LT=84dB

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Correlated and uncorrelated sources*

Which source to first take care?*

FREQUENCY & FREQUENCY BANDSFrequency of sound ---- as important as its levelSensitivity of earSound insulation of a wallAttenuation of silencerall vary with freq.

20000HzInfrasonicAudio RangeUltrasonic*

Musical InstrumentFor multiple frequency composition sound, frequency spectrum is obtained through Fourier analysis Pure toneFrequency Composition of Sound*TEKNIK FISIKA

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Complex Noise PatternNo discrete tones, infinite frequenciesBetter to group them in frequency bands total strength in each band gives measure of soundOctave Bands commonly used (Octave: Halving / doubling)produced by exhaust of Jet Engine, water at base of Niagara Falls, hiss of air/steam jets, etc*TEKNIK FISIKA

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Octave Filters*TEKNIK FISIKA

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Octave and 1/3rd Octave band filters mostly to analyse relatively smooth varying spectra

If tones are present,1/10th Octave or Narrow-band filter be used*TEKNIK FISIKA

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Radiation from SourceRadiates sound waves equally in all directions (spherical radiation)W: is acoustic power output of the source; power must be distributed equally over spherical surface areaConstant termDepends on distance from sourceWhen distance doubles (r=2r0) ; 20log 2 + 20log r0 means 6dB difference in the Sound Intensity/pressure Level

Inverse Square Law

Point Source (Monopole)*TEKNIK FISIKA

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If the point source is placed on ground, it radiates over a hemisphere, the intensity is then doubled and VsFor source not on groundPressure level gets doubled at the same point*TEKNIK FISIKA

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Line Source(Long trains, steady stream of traffic, long straight run of pipeline)If the source is located on ground,and has acoustic power output of W per unit lengthradiating over half the cylinderIntensity at radius r, When distance doubles; 10log 2 + 10log r means 3dB difference in the Sound Intensity Level *TEKNIK FISIKA

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In free field condition,Any source with its characteristic dimension small compared to the wavelength of the sound generated is considered a point sourceAlternatively a source is considered point source if the receiver is at large distance away from the sourceSome small sources do not radiate sound equally in all directionsDirectivity of the source must be taken into account to calculate power from the sound pressure VALIDITY OF POINT SOURCE*TEKNIK FISIKA

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Directivity of Sound Source*TEKNIK FISIKA

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Sound sources whose dimensions are small compared to the wavelength of the sound they are radiating are generally omni-directional; otherwise when dimensions are large in comparison, they are directionalDIRECTIVITY OF SOUND SOURCE*TEKNIK FISIKA

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Directivity Factor & Directivity IndexDirectivity FactorDirectivity IndexRigid boundaries force an omni-directional source to radiate sound in preferential direction*TEKNIK FISIKA

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Radiated Sound Power of the source can be affected by a rigid, reflecting planesStrength and vibrational velocity of the source does not change but the hard reflecting plane produces double the pressure and four-fold increase in sound intensity compared to monopole (point spherical source) in free spaceIf source is sufficiently above the ground this effect is reduced

EFFECT OF HARD REFLECTING GROUND*TEKNIK FISIKA

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Measurements made in semi-reverberant and free field conditions are in error of 2dB*TEKNIK FISIKA

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If hemisphere surface is used then the above equation changes toSound Power Estimation from Pressure level measurements*TEKNIK FISIKA

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Measurement of Power in Reverberant RoomWhich is called room constant team used to describe acoustic characteristic of a room Alternatively, L = Lp + 10 log V 10 log T60 - 14*TEKNIK FISIKA

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Semi-reverberant field technique When sound field is neither free nor completely diffuse.

Use calibrated sound source with known power spectrum.

Then use L = Lr - Lpr + Lp *TEKNIK FISIKA

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Semi-reverberant field technique To take care of nearby reflecting surfaces and background noise,

Measure at number of locations on measuring surfaceLpd = Lp 10log10(d/r)2 Then use L Lpd + 10log10 (2d2)Lpd is equivalent sound pressure level at the reference radius d, and Lp is mean sound pressure level measured over surface of area S, and radius r= (S/2)

Background noise < 10dBr*TEKNIK FISIKA

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What we learntSound Pressure, Intensity and PowerdB levelsMultiple Sound SourcesTypes of Sound Sources Directivity*TEKNIK FISIKA

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Thanks !!*TEKNIK FISIKA

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