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Sound Wave Properties

Sound Wave Properties. What we have learned:. Sound is a longitudinal wave. (A mechanical wave caused by a vibration) Sound (mechanical wave) requires a medium . Wavelength and frequency are inversely related. Molecules interact , producing sound

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Sound Wave Properties

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  1. Sound Wave Properties

  2. What we have learned: • Sound is a longitudinal wave. (A mechanical wave caused by a vibration) • Sound (mechanical wave) requires a medium. • Wavelength and frequency are inversely related. • Molecules interact, producing sound • Examples: Vocal chords, guitar or piano strings, tuning fork, etc.

  3. Longitudinal Wave • Referred to as a PRESSURE WAVE • A sound wave has high pressure and low pressure regions moving through a medium • The high pressure regions are called compressions, (molecules are compressed) • The low pressure regions are called rarefactions, (molecules are spread out)

  4. Sound and Frequency • The frequency of a sound wave (or any wave) is the number of complete vibrations per second. • The frequency of sound determines its pitch • The higher the frequency, the higher the pitch • The lower the frequency, the lower the pitch

  5. Wavelength • Wavelength is the distance between two high pressures or two low pressures • Wavelength and frequency are inversely related • A short wavelength (high frequency) results in a high pitch • http://phet.colorado.edu/en/simulation/sound

  6. Frequency and the human ear • Humans can hear a range of frequencies from 20 Hz to 20,000 Hz • The older you get, the hearing range shrinks • Sound waves with frequencies below 20 Hz are called infrasonic • Sound waves with frequencies above 20,000 Hz are called ultrasonic

  7. Hearing Range Frequencies • http://www.movingsoundtech.com/ • http://www.noiseaddicts.com/2009/03/can-you-hear-this-hearing-test/

  8. Amplitude • The human ear is sensitive to difference in pressure waves • The AMPLITUDE of a sound wave determines its loudness or softness • This means the more energy in a sound wave, the louder the sound • Sound intensityis a measure of how much energy passes a given point in a time period • Intensity is measured in decibels

  9. DECIBEL • Every increase of 10 dB has a 10x greater amplitude • Most people perceive an increase of 10 dB to be about twice as loud as the original sound

  10. Reducing Sound Intensity • Cotton earplugs reduce sound intensity by approximately 10 dB. • Special earplugs reduce intensity by 25 to 45 dB. • Sound proof materials weaken the pressure fluctuations either by absorbing or reflecting the sound waves. • When the sound waves are absorbed by soft materials, the energy is converted into thermal energy.

  11. Sound Behaviors: Reflection • Reflection of sound results in an echo • http://www.youtube.com/watch?v=sAYt-lf4AWk • Sound waves leave a source, travel a distance, and bounce back to the origin • Animals, like bats, use echoes to locate prey • Other uses include determining distances between objects, echocardiograms • The distance the sound travels to get back to the origin is 2x the distance between the sound source and boundary

  12. Sound Behavior: Refraction • Refraction occurs when sound moves from one medium to another • The wave bends, and the speed changes • Even when sound moves from warmer areas to cooler areas, refraction occurs

  13. Sound Behavior: Diffraction • Diffraction occurs when sound waves pass through an opening or through a barrier • Low pitched sound waves travel farther than high pitched sound waves • Animals use diffraction for communication • http://video.nationalgeographic.com/video/animals/mammals-animals/elephants/elephant_african_vocalization/

  14. Velocity • Velocity of sound depends on the medium it travels through and the phase of the medium • Sound travels faster in liquids than in air (4 times faster in water than air) • Sound travels faster in solids than in liquids (11 times faster in iron than in air) • Sound does not travel through a vacuum (there is no air so sound has no medium)

  15. Velocity and Temperature • In air at room temperature, sound travels at 343 m/s (at 20°C). This is about 766 mph. • As temperature increases, the velocity of sound increases v= velocity of sound in air T=temperature of air in °C v=331 + (0.6)T

  16. Wave Equation • The basic wave equation is also applied to sound: • V= velocity, measured in m/s • λ= wavelength, measured in meters • f= frequency, measured in hertz

  17. Example Problems: • Sound waves travel at approximately 340 m/s. What is the wavelength of a sound wave with a frequency of 20 Hz? • What is the speed of sound traveling in air at 28º C? • If the above sound wave has a frequency of 261.6 Hz, what is the wavelength of the wave?

  18. Resonance

  19. Natural Frequency • Nearly all objects when hit or disturbed will vibrate. • Each object vibrates at a particular frequency or set of frequencies. • This frequency is called the natural frequency. • If the amplitude is large enough and if the natural frequency is within the range of 20-20000 Hz, then the object will produce an audible sound.

  20. Factors Affecting Natural Frequency • Properties of the medium • Modification in the wavelength that is produced (length of string, column of air in instrument, etc.) • Temperature of the air

  21. Timbre • Timbre is the quality of the sound that is produced. • If a single frequency is produced, the tone is pure (example: a flute) • If a set of frequencies is produced, but related mathematically by whole-number ratios, it produces a richer tone (example: a tuba) • If multiple frequencies are produced that are not related mathematically, the sound produced is described as noise (example: a pencil)

  22. Resonance • Resonance occurs when one object vibrates at the same natural frequency of a second object, forcing that second object to vibrate at the same frequency.

  23. Types of Resonance • Resonance is the cause of sound production in musical instruments. • Energy is transferred thereby increasing the amplitude (volume) of the sound. • Resonance takes place in both closed pipe resonators and open pipe resonators. • Resonance is achieved when there is a standing wave produced in the tube.

  24. Boom Whackers • Using the boom whackers, determine what happens to the frequency when the cap is taken off the pipe. • Draw a standing wave fro the pipe when the cap is on and when the cap is off.

  25. Closed pipe resonator • open end of tube is anti-node • closed end of tube is node

  26. Harmonics of Closed Pipe Resonance • The shortest column of air that can have a pressure anti-node at the closed end and a pressure node at the open end is ¼ wavelength long. This is called the fundamental frequency or first harmonic. • As the frequency is increased, additional resonance lengths are found at ½ wavelength intervals. • The frequency that corresponds to ¾ wavelength is called the 3rd harmonic, 5/4 wavelength is called the 5th harmonic, etc.

  27. Open pipe resonator • both ends are open • both ends are anti-node

  28. Harmonics of Open Pipe Resonance • The shortest column of air that can have nodes (or antinodes) at both ends is ½ wavelength long. This is called the fundamental frequency or first harmonic. • As the frequency is increased, additional resonance lengths are found at ½ wavelength intervals. • The frequency that corresponds to a full wavelength is the second harmonic, 3/2 wavelength is the third harmonic, etc.

  29. Problem 1. Tommy and the Test Tubes have a concert this weekend. The lead instrumentalist uses a test tube (closed end air column) with a 17.2 cm air column. The speed of sound in the test tube is 340 m/s. Find the frequency of the first harmonic played by this instrument.

  30. Solution L = λ/4 4 x L = λ 4 x .172 = .688 m v = f λ 340 = f (.688) f = 494 Hz

  31. Problems 2. Matt is playing a toy flute, causing resonating waves in a open-end air column. The speed of sound through the air column is 336 m/s. The length of the air column is 30.0 cm. Calculate the frequency of the first, second, and third harmonics.

  32. Solution • L = λ/2 2 x L = λ 2 x .30 = .60 m v = f λ 336 = f (.60) f = 560 Hz. (first harmonic) 2nd harmonic = 560 + 560 = 1120 Hz. 3rd harmonic = 1120 + 560 = 1680 Hz

  33. Bellwork • Pick up the review guide at the front. • Place yourselves in your lab group from Tuesday • Follow the directions at the lab table where you are seated.

  34. Sound and Hearing • Acoustics is the branch of physics pertaining to sound • The ear converts sound energy to mechanical energy to a nerve impulse that is then transmitted to the brain • Our ears allow us to perceive changes in pitch • Our ears are sensitive to a particular range of frequencies between 1,000 – 4,000 Hz.

  35. Anatomy of the Ear

  36. The Outer Ear • The outer ear consists of the earlobe and the ear canal • Sound enters the outer ear as a pressure wave • The outer ear provides protection to the middle ear and protects the eardrum

  37. Sound starts at the Pinna

  38. Then goes through the auditory canal

  39. The Middle Ear • The middle ear is an air-filled cavity that consists of an eardrum and three tiny, interconnected bones - the hammer, anvil, and stirrup. • The eardrumis a very durable and tightly stretched membrane that vibrates as the incoming pressure waves reach • The stirrup is connected to the inner ear

  40. The sound waves will then vibrate the Tympanic Membrane(eardrum)which is made of a thin layer of skin.

  41. The tympanic membrane will then vibrate three tiny bones: theMalleus (hammer),theIncus (anvil),and theStapes (stirrup)

  42. The Inner Ear • The inner ear consists of a cochlea, the semicircular canals, and the auditory nerve • The cochlea and the semicircular canals are filled with a water-like fluid • The fluid and nerve cells of the semicircular canals provide no role in the task of hearing; they speed up the detection of sound

  43. The stapes will then vibrate the Cochlea

  44. Inside look of the Cochlea • The stapes vibrates the cochlea • The frequency of the vibrations will stimulate particular hairs inside the cochlea • The intensity at which these little hairs are vibrated will determine how loud the sound is. • The auditory nerve will then send this signal to the brain. 

  45. DECIBEL • Every increase of 10 dB has a 10x greater amplitude • Most people perceive an increase of 10 dB to be about twice as loud as the original sound

  46. A mosquito’s buzz is often rated with a decibel rating of 40 dB. Normal conversation is often rated at 60 dB. How many times more intense is normal conversation compared to a mosquito’s buzz? • 2 • 20 • 100 • 200

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