ACOUSTICSSound in a MediumIn the Medium:Particle MotionDisplacement of Molecules in the MediumBecause We have Transmission:Sound Travels Out From the SourceWe Can Also Talk About:Wavelength Questions:Question 1:Question 2:EXAMPLE OF SOUND WAVESWhen Talking about Amplitude:Therefore, we are more interested in:Remember :Intuitively, we all know thisThe Physics of the Situation:WHY?Change in IntensityEXAMPLE:Sound Wave PhenomenaSound Encountering an Object:Reflected and Incident Sound MeetGetting around an Object:DiffractionRefractionSound FieldsSound Fields (cont’d)Reverberation:Reverberation TimeEarphonesResonanceStanding WavesStanding Wave IllustrationStanding Waves (cont’d)Pipes produce standing wavesStanding Waves in PipesA closed pipe only produces odd harmonics.ACOUSTICSSound in a MediumSound Wave PhenomenaSound FieldsEarphonesResonance and Standing WavesSound in a MediumVibrating object displaces molecules in mediummolecules move back and forth“bump” into others transmitting vibration thru mediumIn the Medium:We have both OSCILLATION of particlesandTRANSMISSION of energy (or propagation)Particle MotionIn Air, in line with transmission--LONGITUDINALOn Water, perpendicular to transmission--TRANSVERSEDisplacement of Molecules in the Mediumcreates areas of more molecules--increased density--CONDENSATIONand areas of fewer molecules--decreased density--RAREFACTIONBecause We have Transmission:We can talk about how fast sound travels in the medium= SPEED OF SOUND or cDepends on medium, temperature, density, stateIn Air = 344 meters/sec or 1100 feet/secSound Travels Out From the Source•In All Directions•(at the same speed)•So, Until Sound Encounters some object, •the “wavefront” is sphericalWe Can Also Talk About:Distance Traveled during each cycle= WAVELENGTH = c/f•Wavelength = speed of sound / frequencyWavelength Questions:•What is the wavelength in meters of a 1720 Hz sound traveling in air?•What is the wavelength in meters of an 86 Hz sound traveling in air?Question 1:•Freq = 1720 cyc/sec, c = 344 m/sec•wavelength = c/f•=344m/sec /1720 cyc/sec•=0.2 m/cycQuestion 2:•Freq = 86 cyc/sec, c = 344 m/secwavelength = c/f= 344m/sec /86 cyc/sec= 4 m/cycEXAMPLE OF SOUND WAVEShttp://rustam.uwp.edu/GWWM/sound_waves.htmlWhen Talking about Amplitude:•Remember Power is Rate at which Work is done (Work /Time = Power)•But the power in sound doesn’t all travel the same direction•Only some of it reaches you.Therefore, we are more interested in:•How much Sound Power there is in a given area•(e.g., the opening of ear canal, microphone)•New term: INTENSITY = Power/AreaRemember :•Sound Power is spread over the Wavefront•So the farther you are from the sound source:• the larger the area over which power is spread• the smaller the intensityIntuitively, we all know this•The closer you are, the louder the sound•The farther away you are, the softer the soundThe Physics of the Situation:•The relation between distance and intensity is an example of•THE INVERSE SQUARE LAW•Intensity = 1/distance2WHY?•Surface area of sphere = 4 Pi r2•In this case r = distance•The area is proportional to distance squaredChange in Intensity = old d2 / new d2EXAMPLE:Moving from 100 m to 200 m away from sourceDelta I = 100 2/200 2= 1 x 104/4 x 104•= 1/4•=0.25Sound Wave Phenomena•Reflection-bouncing off an object•Absorption-sound trapped (absorbed) by an object•Diffraction-spreading of sound into area beyond an object•Refraction-bending of sound waves in a mediumSound Encountering an Object:•Transmission-setting object into vibration•Reflection-sound bounces back•Absorption-sound becomes trapped in gaps of surface of objectReflected and Incident Sound Meet•Producing INTERFERENCE•Where the two waves meet in phase, the intensity doubles --Constructive Interference•Where they meet out of phase, cancellation --Destructive InterferenceGetting around an Object:depends on size of object and wavelength of soundwhen > object’s diameter, sound passes bywhen < object’s diameter, sound blockedArea of reduced or no sound energy is “sound shadow”Diffraction•Sound passing an object will spread to fill in area beyond it.Refraction •the bending of the sound’s path produced by changes in medium•e.g., temperature changes will bend path of sound propagationSound Fields•FREE FIELD = no objects in medium•ANECHOIC CHAMBER = room with highly absorptive walls; an attempt to create a free field.Sound Fields (cont’d)•SOUND TREATED ROOM = has somewhat absorptive walls, produces some reflections•REVERBERATION ROOM = highly reflective walls set at odd angles; many reflections and complex interactions. Creates a uniform (diffuse) sound field.Reverberation:•Persistence of sound in a sound field after the source is turned off• = time taken for intensity to drop to 1 millionth of initial value•Reverberation  ROOM VOL./ABSORPTION COEF.Reverberation Time•Least for Anechoic Chamber•Most for Reverberation Room•Longer for larger rooms with reflective wallsEarphones•Miniature loudspeakers to introduce sound into the ear.•Supra-aural (sits on the pinna)•Insert (sits within external canal)•Calibrated in “artificial ears” (6cc or 2cc couplers)Resonance•Helmholtz Resonators simulate influence of mass and compliance (stiffness) on resonance. Tube and Cavity.•Mass component--inversely proportional to resonant freq•Compliance component--directly prop. to resonant freq•Resistance -- doesn’t affect resonant freq, but produces broader tuningStanding Waves•Interaction between incident and reflected waves•Produces areas of :•constructive interf. --ANTINODE•destructive interf. --NODEStanding Wave IllustrationStanding Waves (cont’d)•Intensity varies with position•Position of nodes, antinodes depends on frequencyPipes produce standing wavesclosed pipes —antinode at open end and node at closed end open pipes — antinode at each open end closed pipe, length = ¼ l  open pipe, length = ½ lStanding Waves in PipesA closed pipe only produces odd harmonics. Frequency of harmonics = (n c)/4 L,Where n=1, 3, 5, ... c = speed of soundL is the length of the pipe. In music, harmonics are called