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CHAPTER 4

CHAPTER 4. COMPLEX STIMULI. Types of Sounds. So far we’ve talked a lot about sine waves periodic energy at one frequency But, not all sounds are like that. Periodic/Aperiodic Sounds. Periodic -- Repeating regular pattern with a constant period Aperiodic -- no consistent pattern repeated.

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CHAPTER 4

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  1. CHAPTER 4 COMPLEX STIMULI

  2. Types of Sounds • So far we’ve talked a lot about sine waves • periodic • energy at one frequency • But, not all sounds are like that

  3. Periodic/Aperiodic Sounds • Periodic -- Repeating regular pattern with a constant period • Aperiodic-- no consistent pattern repeated.

  4. 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 aperiodic

  5. A Complex Sound

  6. Looking 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)

  7. Waveform and Spectra

  8. Harmonic Series • When energy is present at multiples of some frequency • Lowest frequency = FUNDAMENTAL FREQ • Multiples of fundamental = HARMONICS

  9. Transients • Brief acoustic signals or “clicks”

  10. Envelope--The Outline of the Waveform

  11. One 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.

  12. AM Tone: Waveform & Spectrum

  13. Spectrum of an AM tone: • Has Energy at 3 frequencies: 1. at the frequency of the CARRIER 2. at Carrier freq PLUS Modulation freq. 3. at Carrier freq MINUS Modulation freq.

  14. 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 angle

  15. Square Wave

  16. Frequency Modulation (FM) • Waveform shape changes like a spring • Looking at TIME domain

  17. Not Everything is so Regular • Aperiodic sounds vary randomly • = NOISE • Waveforms may look wild • EXAMPLE: • White Gaussian Noise = equal energy at all frequencies

  18. Gaussian Noise Waveform

  19. Amp. Spectra: White & Pink Noise

  20. Levels 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 + Lbw

  21. Overall Level Equals Spectrum Level Plus Bandwidth Level SPL Ls Lbw

  22. Example 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 = Ls

  23. Combining 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?

  24. 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)

  25. 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 IL

  26. How About a SHORT CUT? • New IL = IL of OLD # + 10 log (new #/old #) • = 60 + 10 log (2/1) • = 60 + 3 • = 63 dB IL

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