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Sound

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  • Sound is a longitudinal wave, creating compression and rarefaction areas. Vibrating sources cause air molecules to vibrate, not move. Compression is an area of higher than normal air pressure and rarefaction is an area of lower than normal air pressure. Often easier to represent as a graph of pressure vs distance, which will look like a sine wave. (see p354 bottom). Wavelength, frequency, amplitude, and the mathematic relationships between them all hold. Pitch is often substituted for frequency in music references.
  • Speed of sound is highly affected by the temperature and the density of the medium. In general, the speed of sound is greater the more dense the medium is, although there are exceptions (lead < water). Speed of sound in air = (332 + 0.6T) m/s (at normal atmospheric pressure) T is temperature in celsius. Otto von Guericke 1654, did experiments with sound finding that the intensity of sound decreases as air is removed. Also found that sound travels through water. SOUND NEEDS A MATERIAL MEDIUM FOR ITS TRANSMISSION. IT WILL NOT TRAVEL IN A VACUUM.
  • Intensity of sound or loudness is difficult to measure because the amount of energy involved is small in comparison to other forms of energy and because the potential range of sound intensity is great. Intensity and intensity level on the decibel scale both decrease as the distance from the source increases. This is due to the energy spreading itself more and more thinly over increased area.
  • Measured in decibels, where 0 dB is the threshold of human hearing; 160 dB will destroy the eardrum instantly. Intensity measures Power per unit area; intensity levels (dB) is a nonlinear relationship measuring the difference between two intensities.
  • Humans respond to frequencies between 20 and 20,000 Hz (male speaking voice 120 Hz; female, 250 Hz). Infrasonic denotes frequencies of less than 20 Hz; Ultrasonic, greater than 20,000 Hz.
  • A concave reflector will focus sound the same as a mirror focuses light at a single point. The reflecctor collects the sound waves and converges them into a small area. Echoes are produced when sound is reflected by a hard surface such as a wall. Can only be heard by the human ear when the time interval b/w the echo and the original sound is greater than 0.1 s and the distance b/w the person and the reflecting surface is greater than 17 m. Other applications: parabolic microphone to pick up distant sounds; echo-sounder to measure depth of sea; sonar devices to locate submarines, monitor fetuses, etc; radar, similar to sonar, but uses radio waves instead of sound.
  • Acoustics in buildings. Reverberation occurs when the reflecting surface in less than 17 m away, the echo follows so closely behind the original sound that the original sound appears to be prolonged. Reverberation time is defined as the time required for sound to die away and become inaudible.
  • Transcript

    • 1. Sound
    • 2. All sounds are caused by vibrations
      • Vibration – back and forth movement of matter.
      • Sound waves are generated by any vibrating object
      • The vibrations create molecular motions and pressure oscillations in the air
      • The oscillations create a periodic disturbance of the surrounding air
      • The effect of these waves is heard as sound
    • 3.
      • The human voice comes from vibrations of the vocal cords
      • Air from the lungs is what causes the vocal cords to vibrate
      • The frequency of vibration is controlled by the singer’s muscular tension placed on the cords
      • The human voice uses the throat and mouth cavity as a resonator
      The Human Voice
    • 4.
      • BRASS
        • The lips of the performer vibrate resulting in sound
      • REED
        • Have a thin wooden strip that vibrates as a result of air blown across it
      • WIND
        • The air blown across the opening in the pipe (mouthpiece) sets the column of air in the instrument into vibration
      • STRING
        • A wire or string is set into vibration. The wire is attached to a sounding board that vibrates with the string
      Source of Sound in Instruments
    • 5. Sound waves are longitudinal  
    • 6. Sound waves travel faster as medium it travels through becomes warmer.
      • Speed of sound in various mediums Air 340 m/s Water 1500 m/s Iron 5100 m/s
    • 7. Loudness Vs. Intensity
      • Loudness – perception of the rate at which sound energy enters your ear.  
      • Intensity – Measurement of amplitude.  
      • Intensity and loudness decreases with distance  
    • 8. Intensity
      • Decibel (dB) Unit of intensity level – compares to lowest possible limit of human hearing
      • Typical intensity levels Breathing 10 dB Whisper 30 dB Loud stereo 90 dB Jet 150 dB  
    • 9. Sound waves that enter your ear cause the eardrum to vibrate.
      • Intensity levels greater than 90 dB can cause hearing loss. 
    • 10. Frequency and Pitch  
      • Pitch – the perception of frequency Humans can detect a wide range of frequencies and are sensitive to a large range of amplitudes.
      • Humans (hearing) 20-20 000 Hz
      • Humans (making sounds) 85-1100 Hz Bat 1000 – 120 000 Hz Dog whistle 20 000 – 24 000 Hz   Waves with frequencies greater than 20,000 Hz are called Ultrasonic.   Waves with frequencies less than 20,000 Hz are called Subsonic .
    • 11. Reflection of Sound Waves
      • Echo – reflected sound wave
      • Follows the Law of Reflection, where the angle of incidence is equal to the angle of reflection.
      • Applications:
      • Sonar – sound navigation and ranging
      • Ultrasound – viewing the fetus in the womb.
    • 12. Diffraction
      • – bending of sound waves around barriers (you can hear people in the hallway)  
    • 13.
      • It is divided into three parts: outer ear (pinna), middle ear, inner ear.
          • The first chamber of the ear contains tiny hairs and cells that produce ear wax
          • Sound waves pass by the pinna and beat against the ear drum, causing it to vibrate
          • The vibration of the ear drum causes three bones in the inner ear (hammer, anvil, stirrup) to vibrate carrying motion to the inner ear, which is filled with fluid.
          • The cochlea converts the vibration of the bones to nerve impulses that the brain receives via the auditory nerve
          • The inner ear also controls balance
      The Human Ear
    • 14. Diagram of the Ear
    • 15.
      • Even though two instruments may be playing the same note, their sounds can differ because each sound contains a number of frequencies with different intensities
      • Timbre : the quality of a steady musical sound that is the result of a mixture of harmonics present at different intensities
      Quality of Sound
    • 16. Music  
      • Shorter strings produce sounds with higher pitch. (higher frequency) Shorter vibrating columns produce higher pitch. (higher frequency) A vibrating surface is responsible for loudness.    
    • 17.
      • The variation from soft to loud and back to soft is called a beat
      • The number of beats per second corresponds to the difference between frequencies
      • The ability to detect beats depends upon an individual’s hearing and musical training. The average human ear can distinguish beats up to a frequency of ten beats per second
      BEAT
    • 18. Beats are formed by the interference if two waves of slightly different frequencies traveling in the same direction. In this case one beat occurs at T-2 where constructive interference is greatest. BEAT (continued)
    • 19.
      • The equation for beat frequency is:
      •  F=  f 2 -f 1 
      • EXAMPLE PROBLEM
      • A 420 Hz tuning fork and a 620 Hz tuning fork are struck at the same time. What beat frequency will be produced?
      • Solution:
      • 620-420 = 200, Therefore the beat frequency produced is 200 Hz.
      BEAT (continued)
    • 20.
      • Consists of a large number of frequencies with no relationship to each other
      • White noise occurs when all frequencies are present in equal amplitudes.
      • The human voice uses the throat and mouth cavity as a resonator
        • The quality of the tone depends on the shape of the resonator
      NOISE
    • 21. Acoustics
      • – study of how materials influence the production and movement of sound  
    • 22. the end