CHAPTER 4Types of SoundsPeriodic/Aperiodic SoundsSimple/Complex SoundsA Complex SoundLooking at a WaveformWaveform and SpectraHarmonic SeriesTransientsEnvelope--The Outline of the WaveformOne Interesting EnvelopeAM Tone: Waveform & SpectrumSpectrum of an AM tone:Square WaveSlide 15Frequency Modulation (FM)Not Everything is so RegularGaussian Noise WaveformAmp. Spectra: White & Pink NoiseLevels of a Band of NoiseOverall Level Equals Spectrum Level Plus Bandwidth LevelExample of Deriving LsCombining Sound SourcesWorking out the example:Working it out (cont’d)How About a SHORT CUT?CHAPTER 4COMPLEX STIMULITypes of Sounds•So far we’ve talked a lot about sine waves=periodic=energy at one frequency•But, not all sounds are like thatPeriodic/Aperiodic Sounds•Periodic -- Repeating regular pattern with a constant period•Aperiodic-- no consistent pattern repeated.Simple/Complex Sounds•Simple -- Having energy at only one frequency• have a sinusoidal waveform•Complex -- Having energy at more than one frequency• may be periodic or aperiodicA Complex SoundLooking at a Waveform•You may not be able to tell much about frequencies present in the sound•Another way of displaying sound energy is more valuable:AMPLITUDE SPECTRUM--display of amplitude (y-axis) as a function of frequency (x-axis)Waveform and SpectraHarmonic Series•When energy is present at multiples of some frequency•Lowest frequency = FUNDAMENTAL FREQ•Multiples of fundamental = HARMONICSTransients•Brief acoustic signals or “clicks”Envelope--The Outline of the WaveformOne Interesting Envelope•Amplitude Modulated (AM) Tone•Tone whose energy is varied is called CARRIER •You can also talk about the FREQUENCY OF MODULATION--How many times a second does amplitude cycle up and down and back again.AM Tone: Waveform & SpectrumSpectrum of an AM tone:•Has Energy at 3 frequencies:1. at the frequency of the CARRIER2. at Carrier freq PLUS Modulation freq.3. at Carrier freq MINUS Modulation freq.Square Wave•Similar to transient spectrum•Fundamental frequency at reciprocal of period •Can also be a modulator•All harmonics are at a 90 degrees phase angleSquare WaveFrequency Modulation (FM)•Waveform shape changes like a spring•Looking at TIME domainNot Everything is so Regular•Aperiodic sounds vary randomly•= NOISE•Waveforms may look wild•EXAMPLE:•White Gaussian Noise = equal energy at all frequenciesGaussian Noise WaveformAmp. Spectra: White & Pink NoiseLevels of a Band of Noise•Overall Level = SPL (Total Power) •Spectrum Level = Ls level at one frequency•Bandwidth Level = Lbw freq width (in dB) Lbw = 10 log (bandwidth (in Hz)/ 1 Hz)•SPL = Ls + LbwOverall Level Equals Spectrum Level Plus Bandwidth LevelLbwLsSPLExample of Deriving Ls•Given SPL = 80 dB•and Bandwidth = 1000 Hz•Lbw = 10 log (1000Hz / 1Hz) = 30 dB•SPL = Ls + Lbw•80 dB = Ls + 30 dB•50 dB = LsCombining Sound Sources•Adding additional (identical) sources produces summing of intensities•e.g., adding a second speaker playing the same signal•If one produced 60 dB IL, what would two produce?Working out the example:•one produces 60 dB IL•60 = 10 log (Im/10-16 W/cm2)•6 = log (Im/10-16 W/cm2)•106 = Im/ 10-16 W/cm2•10 6 + (-16) = Im•10 -10 = Im•2 x 10 -10 = Intensity of two sources•New IL = 10 log (2 x 10 -10 /10-16 W/cm2)Working it out (cont’d)•New IL = 10 log (2 x 10 -10 - (-16) )• = 10 log (2 x 10 6)• = 10 (6.3010)• = 63 dB ILHow About a SHORT CUT?•New IL = IL of OLD # + 10 log (new #/old #)• = 60 + 10 log (2/1)• = 60 + 3• = 63 dB