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© 2010 Pearson Education, Inc. Conceptual Physics 11 th Edition Chapter 20: SOUND.

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Presentation on theme: "© 2010 Pearson Education, Inc. Conceptual Physics 11 th Edition Chapter 20: SOUND."— Presentation transcript:

1 © 2010 Pearson Education, Inc. Conceptual Physics 11 th Edition Chapter 20: SOUND

2 © 2010 Pearson Education, Inc. This lecture will help you understand: Nature of Sound Origin of Sound Sound in Air Media That Transmit Sound Speed of Sound in Air Reflection of Sound Refraction of Sound Energy in Sound Waves Forced Vibrations Natural Frequency Resonance Interference Beats

3 © 2010 Pearson Education, Inc. Nature of Sound Sound is a form of energy that exists whether or not it is heard.

4 © 2010 Pearson Education, Inc. Origin of Sound Most sounds are waves produced by the vibrations of matter. For example: In a piano, a violin, and a guitar, the sound is produced by the vibrating strings; in a saxophone, by a vibrating reed; in a flute, by a fluttering column of air at the mouthpiece; in your voice due to the vibration of your vocal chords.

5 © 2010 Pearson Education, Inc. Origin of Sound The original vibration stimulates the vibration of something larger or more massive, such as –the sounding board of a stringed instrument, –the air column within a reed or wind instrument, or –the air in the throat and mouth of a singer. This vibrating material then sends a disturbance through the surrounding medium, usually air, in the form of longitudinal sound waves.

6 © 2010 Pearson Education, Inc. Origin of Sound Under ordinary conditions, the frequencies of the vibrating source and sound waves are the same. The subjective impression about the frequency of sound is called pitch. The ear of a young person can normally hear pitches corresponding to the range of frequencies between about 20 and 20,000 Hertz. As we grow older, the limits of this human hearing range shrink, especially at the high-frequency end.

7 © 2010 Pearson Education, Inc. Origin of Sound Sound waves with frequencies below 20 hertz are infrasonic (frequency too low for human hearing). Sound waves with frequencies above 20,000 hertz are called ultrasonic (frequency too high for human hearing). We cannot hear infrasonic and ultrasonic sound.

8 © 2010 Pearson Education, Inc. Sound in Air Sound waves are vibrations made of compressions and rarefactions. are longitudinal waves. require a medium. travel through solids, liquids, and gases.

9 © 2010 Pearson Education, Inc. Sound in Air Wavelength of sound Distance between successive compressions or rarefactions

10 © 2010 Pearson Education, Inc. Sound in Air How sound is heard from a radio loudspeaker Radio loudspeaker is a paper cone that vibrates. Air molecules next to the loudspeaker set into vibration. Produces compressions and rarefactions traveling in air. Sound waves reach your ears, setting your eardrums into vibration. Sound is heard.

11 © 2010 Pearson Education, Inc. Media That Transmit Sound Any elastic substance — solid, liquid, gas, or plasma — can transmit sound. In elastic liquids and solids, the atoms are relatively close together, respond quickly to one another’s motions, and transmit energy with little loss. Sound travels about 4 times faster in water than in air and about 15 times faster in steel than in air.

12 © 2010 Pearson Education, Inc. Speed of Sound in Air Speed of sound Depends on wind conditions, temperature, humidity –Speed in dry air at 0  C is about 330 m/s. –In water vapor slightly faster. –In warm air faster than cold air. Each degree rise in temperature above 0  C, speed of sound in air increases by 0.6 m/s Speed in water about 4 times speed in air. Speed in steel about 15 times its speed in air.

13 © 2010 Pearson Education, Inc. You watch a person chopping wood and note that after the last chop you hear it 1 second later. How far away is the chopper? A.330 m B.More than 330 m C.Less than 330 m D.There’s no way to tell. Speed of Sound in Air CHECK YOUR NEIGHBOR

14 © 2010 Pearson Education, Inc. You hear thunder 2 seconds after you see a lightning flash. How far away is the lightning? A.340 m/s B.660 m/s C.More than 660 m/s D.There’s no way to tell. Speed of Sound in Air CHECK YOUR NEIGHBOR

15 © 2010 Pearson Education, Inc. Reflection of Sound Reflection Process in which sound encountering a surface is returned Often called an echo Multiple reflections—called reverberations

16 © 2010 Pearson Education, Inc. Reverberations are best heard when you sing in a room with A.carpeted walls. B.hard-surfaced walls. C.open windows. D.None of the above. Reflection of Sound CHECK YOUR NEIGHBOR

17 © 2010 Pearson Education, Inc. Reflection of Sound Acoustics Study of sound Example: A concert hall aims for a balance between reverberation and absorption. Some have reflectors to direct sound (which also reflect light— so what you see is what you hear).

18 © 2010 Pearson Education, Inc. Refraction of Sound Refraction Bending of waves—caused by changes in speed affected by –wind variations. –temperature variations.

19 © 2010 Pearson Education, Inc. When air near the ground on a warm day is warmed more than the air above, sound tends to bend A.upward. B.downward. C.at right angles to the ground. D.None of the above. Refraction of Sound CHECK YOUR NEIGHBOR

20 © 2010 Pearson Education, Inc. In the evening, when air directly above a pond is cooler than air above, sound across a pond tends to bend A.upward. B.downward. C.at right angles to the ground. D.None of the above. Refraction of Sound CHECK YOUR NEIGHBOR

21 © 2010 Pearson Education, Inc. Reflection and Refraction of Sound Multiple reflection and refractions of ultrasonic waves Device sends high-frequency sounds into the body and reflects the waves more strongly from the exterior of the organs, producing an image of the organs. Used instead of X-rays by physicians to see the interior of the body.

22 © 2010 Pearson Education, Inc. Reflection and Refraction of Sound Dolphins emit ultrasonic waves to enable them to locate objects in their environment.

23 © 2010 Pearson Education, Inc. Forced Vibrations Forced vibration Setting up of vibrations in an object by a vibrating force Example: factory floor vibration caused by running of heavy machinery

24 © 2010 Pearson Education, Inc. Natural Frequency Natural frequency Own unique frequency (or set of frequencies) Dependent on –elasticity –shape of object

25 © 2010 Pearson Education, Inc. Resonance A phenomenon in which the frequency of forced vibrations on an object matches the object’s natural frequency Examples: Swinging in rhythm with the natural frequency of a swing Tuning a radio station to the “carrier frequency” of the radio station Troops marching in rhythm with the natural frequency of a bridge (a no-no!)

26 © 2010 Pearson Education, Inc. Resonance Dramatic example of wind-generated resonance

27 © 2010 Pearson Education, Inc. Interference

28 © 2010 Pearson Education, Inc. Interference Application of sound interference Destructive sound interference in noisy devices such as jackhammers that are equipped with microphones to produce mirror-image wave patterns fed to operator’s earphone, canceling the jackhammer’s sound

29 © 2010 Pearson Education, Inc. Application of sound interference (continued) Sound interference in stereo speakers out of phase sending a monoaural signal (one speaker sending compressions of sound and other sending rarefactions) As speakers are brought closer to each other, sound is diminished. Interference

30 © 2010 Pearson Education, Inc. Beats Periodic variations in the loudness of sound due to interference Occur with any kind of wave Provide a comparison of frequencies

31 © 2010 Pearson Education, Inc. Beats Applications –Piano tuning by listening to the disappearance of beats from a tuning fork and a piano key –Tuning instruments in an orchestra by listening for beats between instruments and piano tone

32 © 2010 Pearson Education, Inc. This lecture will help you understand: Noise and Music Musical Sounds Pitch Sound Intensity and Loudness Quality Musical Instruments Fourier Analysis Digital Versatile Discs (DVDs)

33 © 2010 Pearson Education, Inc. Noise and Music Noise corresponds to an irregular vibration of the eardrum produced by some irregular vibration in our surroundings, a jumble of wavelengths and amplitudes. –White noise is a mixture of a variety of frequencies of sound.

34 © 2010 Pearson Education, Inc. Noise and Music Music is the art of sound and has a different character. Musical sounds have periodic tones–or musical notes. The line that separates music and noise can be thin and subjective.

35 © 2010 Pearson Education, Inc. Musical Sounds Musical tone Three characteristics: –Pitch determined by frequency of sound waves as received by the ear determined by fundamental frequency, lowest frequency –Intensity determines the perceived loudness of sound

36 © 2010 Pearson Education, Inc. Musical Sounds Musical tone Three characteristics (continued): –Quality determined by prominence of the harmonics determined by presence and relative intensity of the various partials

37 © 2010 Pearson Education, Inc. Pitch Music is organized on many different levels. Most noticeable are musical notes. Each note has its own pitch. We can describe pitch by frequency. –Rapid vibrations of the sound source (high frequency) produce sound of a high pitch. –Slow vibrations (low frequency) produce a low pitch.

38 © 2010 Pearson Education, Inc. Pitch Musicians give different pitches different letter names: A, B, C, D, E, F, G. –Notes A through G are all notes within one octave. –Multiply the frequency on any note by 2, and you have the same note at a higher pitch in the next octave. –A piano keyboard covers a little more than seven octaves.

39 © 2010 Pearson Education, Inc. Pitch Different musical notes are obtained by changing the frequency of the vibrating sound source. This is usually done by altering the size, the tightness, or the mass of the vibrating object.

40 © 2010 Pearson Education, Inc. Pitch High-pitched sounds used in music are most often less than 4000 Hz, but the average human ear can hear sounds with frequencies up to 18,000 Hz. –Some people and most dogs can hear tones of higher pitch than this. –The upper limit of hearing in people gets lower as they grow older. –A high-pitched sound is often inaudible to an older person and yet may be clearly heard by a younger one.

41 © 2010 Pearson Education, Inc. Sound Intensity and Loudness The intensity of sound depends on the amplitude of pressure variations within the sound wave. The human ear responds to intensities covering the enormous range from 10 – 12 W/m 2 (the threshold of hearing) to more than 1 W/m 2 (the threshold of pain).

42 © 2010 Pearson Education, Inc. Sound Intensity and Loudness Because the range is so great, intensities are scaled by factors of 10, with the barely audible 10 –12 W/m 2 as a reference intensity called 0 bel (a unit named after Alexander Bell). A sound 10 times more intense has an intensity of 1 bel (W/m 2 ) or 10 decibels (dB)

43 © 2010 Pearson Education, Inc. Sound Intensity and Loudness Sound intensity is a purely objective and physical attribute of a sound wave, and it can be measured by various acoustical instruments. Loudness is a physiological sensation. –The ear senses some frequencies much better than others. –A 3500-Hz sound at 80 decibels sounds about twice as loud to most people as a 125-Hz sound at 80 decibels. –Humans are more sensitive to the 3500-Hz range of frequencies.

44 © 2010 Pearson Education, Inc. Quality We have no trouble distinguishing between the tone from a piano and a tone of the same pitch from a clarinet. Each of these tones has a characteristic sound that differs in quality, the “color” of a tone —timbre. Timbre describes all of the aspects of a musical sound other than pitch, loudness, or length of tone.

45 © 2010 Pearson Education, Inc. Quality Most musical sounds are composed of a superposition of many tones differing in frequency. The various tones are called partial tones, or simply partials. The lowest frequency, called the fundamental frequency, determines the pitch of the note. A partial tone whose frequency is a whole-number multiple of the fundamental frequency is called a harmonic. A composite vibration of the fundamental mode and the third harmonic is shown in the figure.

46 © 2010 Pearson Education, Inc. Quality The quality of a tone is determined by the presence and relative intensity of the various partials. The sound produced by a certain tone from the piano and a clarinet of the same pitch have different qualities that the ear can recognize because their partials are different. A pair of tones of the same pitch with different qualities have either different partials or a difference in the relative intensity of the partials.

47 © 2010 Pearson Education, Inc. Musical Instruments Vibrating strings –Vibration of stringed instruments is transferred to a sounding board and then to the air. Vibrating air columns –Brass instruments. –Woodwinds—stream of air produced by musician sets a reed vibrating. –Fifes, flutes, piccolos—musician blows air against the edge of a hole to produce a fluttering stream.

48 © 2010 Pearson Education, Inc. Musical Instruments Percussion –Striking a 2-dimensional membrane. –Tone produced depends on geometry, elasticity, and tension in the vibrating surface. –Pitch produced by changes in tension.

49 © 2010 Pearson Education, Inc. Musical Instruments Electronic musical instrument differs from conventional musical instruments uses electrons to generate the signals that make up musical sounds modifies sound from an acoustic instrument demands the composer and player demonstrate an expertise beyond the knowledge of musicology

50 © 2010 Pearson Education, Inc. Fourier Analysis The sound of an oboe displayed on the screen of an oscilloscope looks like this. The sound of an clarinet displayed on the screen of an oscilloscope looks like this. The two together look like this.

51 © 2010 Pearson Education, Inc. Digital Versatile Discs (DVDs) The output of phonograph records was signals like those shown below. This type of continuous waveform is called an analog signal. The analog signal can be changed to a digital signal by measuring the numerical value of its amplitude during each split second.

52 © 2010 Pearson Education, Inc. Digital Versatile Discs (DVDs) Microscopic pits about one-thirtieth the diameter of a strand of human hair are imbedded in the CD or DVD –The short pits corresponding to 0. –The long pits corresponding to 1. When the beam falls on a short pit, it is reflected directly into the player’s optical system and registers a 0. When the beam is incident upon a passing longer pit, the optical sensor registers a 1. Hence the beam reads the 1 and 0 digits of the binary code.


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