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Introduction to Sound

Introduction to Sound. Longitudinal Waves. Sound Waves. A sound wave is a travelling disturbance of compressions — regions in which air pressure rises followed by rarefactions regions where air pressure drops compared to quiet or still air. Sound Waves.

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Introduction to Sound

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  1. Introduction to Sound Longitudinal Waves

  2. Sound Waves • A sound wave is a travelling disturbance of compressions — • regions in which air pressure rises • followed by rarefactions • regions where air pressure drops compared to quiet or still air.

  3. Sound Waves • Sound is an example of a longitudinal wave • the wave moves in a direction parallel to the direction in which the medium moves. • The wavelength of a sound wave is the distance between successive compressions or rarefactions. • The amplitude of the sound wave is measured by how much the medium moves from its equilibrium state..

  4. Sound Spectrum • Sounds come to us in an entire spectrum of frequencies. • Conversation with your friends is in the range of 80 Hz to about 300 Hz. • A opera singer can reach frequencies as high as 1100 Hz • Musical instruments extend this range from as low as 20 Hz to as high as 5000 Hz • Bird songs can reach higher than 15 000 Hz. I

  5. Sound Spectrum • In general, your ears are able to respond to sounds in the range of 20 Hz to 20 000 Hz, • But they are most sensitive to sounds in the range of about 2000 Hz to 5000 Hz.

  6. Sound Spectrum • Infrasonic sounds have frequencies of less than 20 Hz. • Rather than being able to hear sounds in this range, you may “feel” them as a rumble that passes through your body. • Audible sounds are in the range of 20 Hz to 20 kHz • Ultrasonic sounds have frequencies greater than 20 kHz

  7. Speed of Sound • The speed of sound through a medium depends on the physical characteristics of the medium. • If you compare sound waves in a solid medium (such as steel) with those in a gaseous medium (air), you will find that they have very different speeds. • The speed of sound in steel is almost 5800 m/s, whereas in air it is only about 350 m/s.

  8. Speed of Sound • There are 3 major factors that effect the speed of sound through a medium: • The distance between particles in the medium • The stiffness of the particles • Stiffness is a measure of how much force is required to create a compression in a given substance • Temperature

  9. Speed of Sound

  10. Speed of Sound If an object is 6.56 m from the camera and sound travels at 344 m/s, determine the length of time it takes the emitted sound pulse to return to the camera.

  11. Speed of Sound

  12. Speed of Sound • What is the wavelength of a sound of frequency 225 Hz that is produced in air at a temperature of 20.0°C?

  13. Intensity • Our ears are marvellous organs. • They can respond to the faintest of whispers or the roar of a jet engine. • The ability to distinguish variations in loudness is an important environmental cue that humans and other animals use to navigate in their surroundings.

  14. Intensity • The intensity of a sound is the energy per unit area that passes a point each second. • It has units of (J/m2)/s or J/s·m2, • which is the same as W/m2. • Our ears can respond to sounds as faint as one-trillionth of a watt per square metre

  15. Decibel Scale

  16. Decibel Scale • The Greek letter beta, , is commonly used to represent sound intensity expressed in dB. • The faintest sound that humans can hear represents the start of the decibel scale and is given a value of 0 dB. • The decibel scale is a logarithmic scale that corresponds to how our ears perceive loudness.

  17. Decibel Scale

  18. Decibel Scale • In general, the smallest difference in loudness that can be detected by the human ear is 1 dB. • When using the decibel scale, every 3-dB increase in SIL is a doubling in intensity. • A 10-dB increase increases the intensity by 10 times.

  19. Decibel Scale

  20. Decibel Scale • 1. By what factor would the sound intensity increase if the sound intensity level in an office increased from 68 dB to 77 dB? • 2. A sound changes from an intensity of 5 x10-6 W/m2 to 5 x10-7W/m2. Has the sound intensity level increased or decreased? • 3. By how much has the sound intensity level changed in question 2? Express your answer in decibels.

  21. Intensity  Loudness • The terms “loudness” and “intensity” do not have the same meaning. • Loudness is a measure of the ear’s response to sound. Two sounds can have equal intensity, • but you may hear one sound as louder than another because your ears can detect it better.

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