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Chapter 2

Chapter 2. Impulsive Sounds, Alone and in Sequence. WAVE DESCRIPTION. Crest. l. Wavelength. A. Amplitude. Trough. Picture of a Transverse Wave. Transverse and Longitudinal Waves. Wave Motion Click on the phrase above and then select either transverse or longitudinal waves.

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Chapter 2

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  1. Chapter 2 Impulsive Sounds, Alone and in Sequence

  2. WAVE DESCRIPTION Crest l Wavelength A Amplitude Trough Picture of a Transverse Wave

  3. Transverse and Longitudinal Waves Wave Motion Click on the phrase above and then select either transverse or longitudinal waves • Sound is a longitudinal wave disturbance • Wavelength is the distance between compression.

  4. Sound is a Longitudinal Wave The tuning fork moves air molecules back and forth in the direction the sound wave travels

  5. Wavelength (l) • Distance between adjacent crests in a transverse wave • Distance between compressions in a longitudinal wave • Distance a wave travels during one vibration • Units - meters or feet

  6. Period (T) • Time required to make one vibration. • Time required to generate one wave. • Time required for the wave to travel one wavelength.

  7. Frequency (f) The number of vibrations per unit of time made by the vibrating source. Units - cycles per second 1/s Hertz (Hz)

  8. Examples of Frequency • What is the frequency of the second hand of a clock? Frequency = 1cycle/60 sec Period = 60 sec • What is the frequency of US Presidential elections? Frequency = 1 election/4 yrs Period = 4 yrs

  9. or In symbolic form

  10. NATURAL FREQUENCY • Demo - Drop Different Sounding Objects • Rap on Table • Finger Snap • Tuning Fork • Objects have natural frequencies at which they vibrate. • The natural frequency depends on elasticity and shape.

  11. A Selection of Natural Frequencies

  12. RESONANCE • Resonance occurs when successive impulsesare applied to a vibrating object at the object’s natural frequency. • Result - increased amplitude • Examples: Swinging Marching on a bridge Tacoma Narrows Bridge

  13. FORCED VIBRATIONS • Demo - Tuning Fork Touching a Table • Sound is intensified because of the larger surface area that can vibrate the air. • The surface is forced to vibrate at the frequency of the tuning fork. (It is not a resonance phenomenon.) • Examples: Musical sounding boards

  14. Equally Tempered Scale In most acoustics texts the octaves are numbered from the left side of the piano keyboard. The note labeled C4 is middle C.

  15. Octaves • If the note has the same pitch name, then the frequency is related by a power of 2. • C4 = 256 Hz A4 = 440 Hz • C5 = 512 Hz A5 = 880 Hz

  16. Repetition Rates • Consider a drummer playing with his left hand (o) and his right hand (x). He might play -------o-------o-------o-------o-------o-------o-------o-------o • Next he plays a flam -------o-x-----o-x-----o-x-----o-x-----o-x-----o-x-----o-x----- Or, -------o---x----o---x----o---x----o---x----o---x----o---x----o

  17. Repetition Rates • We would have no trouble identifying this as the same frequency as the original • But put the beat of the other hand at the exact midpoint and the listener hears twice the repetition rate.

  18. Shaper 1 Audio Ampl. Frequency Generator Sum Delay D Shaper 2 Electronic Experiment

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