2 Objectives: After completing this module, you should be able to: Compute intensity and intensity levels of sounds and correlate with the distance to a source.Apply the Doppler effect to predict apparent changes in frequency due to relative velocities of a source and a listener.
3 Acoustics DefinedAcoustics is the branch of science that deals with the physiological aspects of sound. For example, in a theater or room, an engineer is concerned with how clearly sounds can be heard or transmitted.
4 Audible Sound WavesSometimes it is useful to narrow the classification of sound to those that are audible (those that can be heard). The following definitions are used:Audible sound: Frequencies from 20 to 20,000 Hz.Infrasonic: Frequencies below the audible range.Ultrasonic: Frequencies above the audible range.
5 Comparison of Sensory Effects With Physical Measurements Sensory effects Physical propertyLoudnessPitchQualityIntensityFrequencyWaveformPhysical properties are measurable and repeatable.
6 Sound Intensity (Loudness) Sound intensity is the power transferred by a sound wave per unit area normal to the direction of wave propagation.Units: W/m2
7 Isotropic Source of Sound An isotropic source propagates sound in ever-increasing spherical waves as shown. The Intensity I is given by:lIntensity I decreases with the square of the distance r from the isotropic sound source.
8 Comparison of Sound Intensities The inverse square relationship means a sound that is twice as far away is one-fourth as intense, and one that is three times as far away is one-ninth as intense.I1I2r1r2Constant Power P
9 Example 1: A horn blows with constant power Example 1: A horn blows with constant power. A child 8 m away hears a sound of intensity W/m2. What is the intensity heard by his mother 20 m away? What is the power of the source?Given: I1 = 0.60 W/m2; r1 = 8 m, r2 = 20 mI2 = W/m2
10 The same result is found from: Example 1: (Cont.) What is the power of the source? Assume isotropic propagation.Given: I1 = 0.60 W/m2; r1 = 8 m I2 = W/m2 ; r2 = 20 mP = 7.54 WThe same result is found from:
11 Range of IntensitiesThe hearing threshold is the standard minimum of intensity for audible sound. Its value I0 is:Hearing threshold: I0 = 1 x W/m2The pain threshold is the maximum intensity Ip that the average ear can record without feeling or pain.Pain threshold: Ip = 1 W/m2
12 Intensity Level (Decibels) Due to the wide range of sound intensities (from 1 x W/m2 to 1 W/m2) a logarithmic scale is defined as the intensity level in decibels:Intensity leveldecibels (dB)where b is the intensity level of a sound whose intensity is I and I0 = 1 x W/m2.
13 Example 2: Find the intensity level of a sound whose intensity is 1 x 10-7 W/m2. Intensity level: b = 50 dB
14 Intensity Levels of Common Sounds. Leaves or whisper20 dB65 dBNormal conversationSubway100 dBJet enginesdBHearing threshold: 0 dB Pain threshold: 120 dB
15 Comparison of Two Sounds Often two sounds are compared by intensity levels. But remember, intensity levels are logarithmic. A sound that is 100 times as intense as another is only 20 dB larger!20 dB, 1 x W/m2Source AIB = 100 IA40 dB, 1 x 10-8 W/m2Source B
16 Difference in Intensity Levels Consider two sounds of intensity levels b1 and b2
17 Example 3: How much more intense is a 60 dB sound than a 30 dB sound? Recall definition:I2 = 1000 I1
18 Interference and Beats f f’+=Beat frequency = f’ - f
19 The Doppler EffectThe Doppler effect refers to the apparent change in frequency of a sound when there is relative motion of the source and listener.Sound source moving with vsApparent f0 is affected by motion.Left person hears lower f due to longer l.Right person hears a higher f due to shorter l
20 General Formula for Doppler Effect Definition of terms:f0 = observed frequencyfs = frequency of sourceV = velocity of soundv0 = velocity of observervs = velocity of sourceSpeeds are reckoned as positive for approach and negative for recession
21 The truck is approaching; the boy is fleeing. Thus: Example 4: A boy on a bicycle moves north at 10 m/s. Following the boy is a truck traveling north at 30 m/s. The truck’s horn blows at a frequency of 500 Hz. What is the apparent frequency heard by the boy? Assume sound travels at 340 m/s.30 m/s10 m/sV = 340 m/sfs = 500 HzThe truck is approaching; the boy is fleeing. Thus:vs = +30 m/sv0 = -10 m/s
23 Summary of AcousticsAcoustics is the branch of science that deals with the physiological aspects of sound. For example, in a theater or room, an engineer is concerned with how clearly sounds can be heard or transmitted.Audible sound: Frequencies from 20 to 20,000 Hz.Infrasonic: Frequencies below the audible range.Ultrasonic: Frequencies above the audible range.
24 Sensory effects Physical property Summary (Continued)Sensory effects Physical propertyLoudnessPitchQualityIntensityFrequencyWaveformMeasurable physical properties that determine the sensory effects of individual sounds
25 Summary (Cont.)Sound intensity is the power transferred by a sound wave per unit area normal to the direction of wave propagation.Units: W/m2
26 Summary (Cont.)The inverse square relationship means a sound that is twice as far away is one-fourth as intense, and one that is three times as far away is one-ninth as intense.
27 Summary of Formulas: v = fl Hearing threshold: I0 = 1 x W/m2 Pain threshold: Ip = 1 W/m2v = flBeat freq. = f’ - f