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Sound Physics

Sound Physics. Outline. What is sound? Graphic representation of sound Classifying sounds The Acoustic Filter Resonance The decibel. What is sound?. It may be defined as the propagation of a pressure wave in space and time. propagates through a medium.

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Sound Physics

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  1. Sound Physics SPPA 4030 Speech Science

  2. Outline • What is sound? • Graphic representation of sound • Classifying sounds • The Acoustic Filter • Resonance • The decibel SPPA 4030 Speech Science

  3. What is sound? • It may be defined as the propagation of a pressure wave in space and time. • propagates through a medium SPPA 4030 Speech Science

  4. Medium is composed of molecules Molecules have “wiggle room” Molecules exhibit random motion Molecules can exert pressure A B Sound-conducting media SPPA 4030 Speech Science

  5. Mass (inertia) Elasticity Friction Spring Mass Model SPPA 4030 Speech Science

  6. Model of air molecule vibration (Time 1) Air molecules sitting side by side Rest positions SPPA 4030 Speech Science

  7. Model of air molecule vibration (Time 2) SPPA 4030 Speech Science

  8. Model of air molecule vibration (Time 3) SPPA 4030 Speech Science

  9. Model of air molecule vibration (Time 4) SPPA 4030 Speech Science

  10. Model of air molecule vibration (Time 5) SPPA 4030 Speech Science

  11. a b c d Time 1 2 3 4 5 Distance Model of air molecule vibration SPPA 4030 Speech Science

  12. Time 1 2 3 4 5 Distance Wave action of molecular motion

  13. Amplitude waveform Position Time SPPA 4030 Speech Science

  14. Amplitude waveform Question: How long will this last? Amplitude Time SPPA 4030 Speech Science

  15. Model of air molecule vibration Time 1 2 3 4 5 Pressure measuring device Questions: Where is a region of compression? Where is a region of rarefaction? SPPA 4030 Speech Science

  16. For example… Pressure Time SPPA 4030 Speech Science

  17. Pressure vs. time (pressure waveform) Amplitude Phase (deg) Phase: when a period begins Frequency (F): rate that waveform repeats itself (1/T) Pressure Period (T) Time SPPA 4030 Speech Science

  18. Phase SPPA 4030 Speech Science

  19. Initiating a sound waves that differ only in phase A force is applied to molecule at frequency fand time t same force applied at frequency f at time t+a where a < the period of vibration SPPA 4030 Speech Science

  20. Features of a pressure waveform • Amplitude • Measured in pressure units • peak amplitude • peak-to-peak amplitude • Instantaneous amplitude • Period and Frequency • Period measured in time (basic quantity) • Frequency is a rate measure (per unit time) expressed as Hertz (s-1) • May be expressed as octaves, semitones, etc • Phase • Measured in degrees (relative to period length) • 0-360 degrees SPPA 4030 Speech Science

  21. Frequency representation: The octave • Octave shift: doubling or halving of frequency • Non-linear SPPA 4030 Speech Science

  22. Spatial variation in pressure wave wavelength () is the distance covering adjacent high and low pressure regions SPPA 4030 Speech Science

  23. For example… Wavelength () Pressure Distance SPPA 4030 Speech Science

  24. Relation between frequency and wavelength =c/F where : wavelength F: is the frequency c: is sound speed in medium (33,600 cm/sec) SPPA 4030 Speech Science

  25. Additional Concepts • Propagation of waves • Transmission • Absorption • Reflection • Reverberation SPPA 4030 Speech Science

  26. Time domain Called a waveform Amplitude plotted as a function of time Frequency domain Called a spectrum Amplitude spectrum amplitude vs. frequency Phase spectrum phase vs. frequency May be measured using a variety of “window” sizes Graphic representation of sound SPPA 4030 Speech Science

  27. Same sound, different graphs Time domain Frequency domain SPPA 4030 Speech Science From Hillenbrand

  28. Classification of sounds • Number of frequency components • Simple • Complex • Relationship of frequency components • Periodic • Aperiodic • Duration • Continuous • Transient SPPA 4030 Speech Science

  29. Simple periodic sound • Simple: one frequency component • Periodic: repeating pattern • Completely characterized by • amplitude • period (frequency) • phase • Other names: sinusoid, simple harmonic motion, pure tone SPPA 4030 Speech Science

  30. Simple periodic sound: Graphic appearance SPPA 4030 Speech Science From Hillenbrand

  31. Complex periodic sounds • Complex: > one frequency component • Periodic: repeating pattern • Continuous • Frequencies components have a special relation • Lowest frequency: fundamental frequency • Symbol: fo • Frequency component with longest period • Higher frequency components: harmonics • integer (whole number) multiples of the fo SPPA 4030 Speech Science

  32. Complex periodic sounds: Graphic appearance • Time domain: • repeating pattern of pressure change • within the cycle, things look complex • Frequency domain: • spectral peaks at evenly spaced frequency intervals • “picket fence” appearance • Auditory impression: sounds ‘musical’ SPPA 4030 Speech Science

  33. Complex periodic sounds: Graphic appearance SPPA 4030 Speech Science From Hillenbrand

  34. (Complex) Aperiodic sounds • Complex: > one frequency component • Aperiodic: Does not repeat itself • Frequency components are not systematically related • May be • Continuous • Transient SPPA 4030 Speech Science

  35. Aperiodic sounds: Graphic appearance • Time domain: • no repeating pattern of pressure change • Frequency domain: • the spectrum is dense • No “picket fence” • Auditory impression: sounds ‘noisy’ SPPA 4030 Speech Science

  36. Aperiodic sounds: Graphic appearance SPPA 4030 Speech Science From Hillenbrand

  37. Analysis of complex waves • Waves can be summed • Complex waves are the sum of simple waves • Fourier: French Mathematician: • Any complex waveform may be formed by summing sinusoids of various frequency, amplitude and phase • Fourier Analysis • Provides a unique (only one) solution for a given sound signal • Is reflected in the amplitude and phase spectrum of the signal • Reveals the building blocks of complex waves, which are sinusoids SPPA 4030 Speech Science

  38. The “envelope” of a sound wave • Amplitude envelope: • imaginary smooth line that follows the peak of the amplitude of a sound pressure waveform • Spectrum envelope: • Imaginary smooth line drawn on top of the amplitude spectrum SPPA 4030 Speech Science

  39. Amplitude envelope SPPA 4030 Speech Science From Hillenbrand

  40. Spectrum envelope SPPA 4030 Speech Science From Hillenbrand

  41. Amplitude Spectrum: Window Size • “instantaneous” vs. average spectrum SPPA 4030 Speech Science

  42. “Instantaneous” Amplitude Spectra SPPA 4030 Speech Science

  43. (Long Term) Average Amplitude Spectrum SPPA 4030 Speech Science

  44. SPPA 4030 Speech Science

  45. What is a filter? SPPA 4030 Speech Science

  46. “Acoustic” Filter • holds back (attenuates) certain sounds and lets other sounds through - selective. SPPA 4030 Speech Science

  47. Why might we be interested in filters? • human vocal tract acts like a frequency selective acoustic filter • helps us understand how speech is produced and perceived. SPPA 4030 Speech Science

  48. Frequency Response Curve (FRC) Center frequency + 3 dB passband Gain lower cutoff frequency upper cutoff frequency - low high Frequency

  49. Operation of a filter on a signal NOTE: Amplitude spectrum describes a sound Frequency response curve describes a filter SPPA 4030 Speech Science

  50. Kinds of frequency selective filters Low-pass filters • Lets low frequencies “pass through” and attenuates high frequencies High-pass filters • Lets high frequencies “pass through” and attenuates low frequencies Band-pass filters • Lets a particular frequency range “pass through” and attenuates other frequencies SPPA 4030 Speech Science

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