2 Sound WavesSound waves are areas of alternating high and low molecular densities.LongitudinalCaused by vibrationsCompression-areas of high molecular densityRarefactions-areas of low molecular density
3 Speed of SoundThe speed of sound depends on the medium in which it propagates.Solids Liquids GasesHot Cold(Fast to Slow)
4 Human Hearing Audible Sounds Humans can hear frequencies (known as pitch) between 20 Hz and 20,000 HzFrequencies higher than we can hear…UltrasonicFrequencies lower than we can hear…infrasonic
5 Sound Characteristics Pitch-results from differences in frequencyThe higher the frequency, the higher the pitchDon’t directly observe wavelengthInfrasonic and UltrasonicLoudness-results from amplitudeLogarithmic relationship between intensity and perceived loudness.Increasing intensity by 10 times only results in a doubling of perceived loudness.
6 Intensity Rate at which energy flows Inverse square relationship Units of W/m2We will assume the sound spreads in all directions equally so the area we are dealing with is the surface area of a sphere (4πr2).
7 Intensity and Human Hearing Intensity ranges:Threshold of Hearing to Threshold of Pain1.0 x W/m2 to 1 W/m2This is a large range:Logarithmic scale compresses this rangeUse Decibel levels
8 The Decibel Scale Hearing damage starts at 85dB For each 10dB step, the intensity increases by 10 times and the perceived loudness increases by 2 times.
9 ExamplesCalculate the intensity of the sound waves from an electric guitar’s amplifier at a distance of 5.0 m when its power output is equal to each of the following values:0.25 W0.50 W2.0 WIf the intensity of a person’s voice is 4.6 x 10-7 W/m2 at a distance of 2.0 m, how much sound power does that person generate?The power output of a tuba is 0.35 W. At what distance is the sound intensity of the tuba 1.2 x 10-3 W/m2?
10 The Doppler EffectThe shift in perceived frequency of a wave due to relative motion between the source and the observer.Observable in both sound (pitch) and light (red and blue shifts)As the sound approaches - higher fAs sound leaves - lower fOccurs as either listener or source is moving
11 Forced Vibrations and Natural Frequency Forced Vibration – a vibration that occurs when a periodic force causes an object to vibrate at a particular frequencySingingNatural Frequency – the frequency an object will vibrate at when given a one time forceDepends on shape and material of object
12 ResonanceThe amplified wave that occurs when a forced vibration on an object matches the natural frequency of the object.Makes the wave progressively larger.Swing set example
13 MusicDifferent notes have different mathematical relationships between their frequencies.Specific frequency combinations are considered pleasant (harmony) and others unpleasant (dissonance).2:1 = Octave (C to the next C)5:4 = Major Third (C to E)4:3 = Perfect Forth (C to F)3:2 = Perfect Fifth (C to G)
14 InstrumentsThe sound produced by the vibration of a piece of the instrument (string, reed, lips) is amplified and shaped through resonance by the rest of the instrument.Standing waves are produced within the instrument at certain frequencies depending on either the properties of the string or the shape and size of the instrument.
15 Strings Physically, wavelength is restricted to certain values. To change those values, the length of the string needs to be changed (different keys on a piano, different finger placements on a guitar)The longest wavelength on the string is the sound you hear as the note being played and is called the fundamental or first harmonic.
16 WavelengthsThe higher frequency vibrations are played simultaneously and are called overtones. Therefore, the next longest wavelength will be the second harmonic or first overtone.
17 FrequenciesSince all of the waves are on the same medium and travel at the same speed, there will be a pattern to the frequency as well as the wavelength.
18 Closed Pipe Node on one end, antinode on the other. Diagrams help with determining wavelength pattern.
19 Wavelengths and Frequencies Where n=1, 3, 5…Closed pipes have only odd harmonics
20 Open PipesThe same as the closed pipe, however there is an anti-node at each end for molecular movement and a node at each end for pressure variance.
22 BeatsBeats are a result of two waves with close, but not identical frequencies.A pattern of constructive and destructive interference forms creating a warbling sound.Useful for tuning instruments.Beat frequency is equal to the difference between the two component frequencies.
23 TimbreThe unique combination of intensities of fundamental and overtone frequencies that makes instruments sound different when playing the same note.TrumpetFluteCello
24 ExamplesWhat is the fundamental frequency of a 0.2 m long organ pipe that is closed at one end, when the speed of sound in the pipe is 352 m/s?A flute is an open pipe. The length is the flute is approximately 66.0 cm. What are the first three harmonics of a flute when all keys are closed, making the length of air equal to the length of the flute? Use 340 m/s for the speed of sound.What is the fundamental frequency of a guitar string when the speed of waves on the string is 115 m/s and the effective string lengths are as follows:70.0 cm50.0 cm40.0 cmA violin string that is 50.0 cm in length has a fundamental frequency of 440 Hz. What is the speed of the waves on this string?