1. INTRODUCTION TO SOUND Longitudinal Waves

2. S OUND W AVES  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. S OUND W AVES  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. S OUND S PECTRUM  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. S OUND S PECTRUM  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. S OUND S PECTRUM 1. 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. 2. Audible sounds are in the range of 20 Hz to 20 kHz 3. Ultrasonic sounds have frequencies greater than 20 kHz

8. S PEED OF S OUND  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.

9. S PEED OF S OUND  There are 3 major factors that effect the speed of sound through a medium: 1. The distance between particles in the medium 2. The stiffness of the particles  Stiffness is a measure of how much force is required to create a compression in a given substance 3. Temperature

10. S PEED OF S OUND

11. 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.

12. S PEED OF S OUND

13.  What is the wavelength of a sound of frequency 225 Hz that is produced in air at a temperature of 20.0°C?

14. I NTENSITY  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.

15. I NTENSITY  The intensity of a sound is the energy per unit area that passes a point each second.  It has units of (J/m 2 )/s or J/s·m 2,  which is the same as W/m 2.  Our ears can respond to sounds as faint as one-trillionth of a watt per square metre

16. D ECIBEL S CALE

17.  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.

18. D ECIBEL S CALE

19.  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.

20. D ECIBEL S CALE

21.  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/m 2 to 5 x10 -7 W/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.

22. I NTENSITY  L OUDNESS  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.