Intro to Sound

1. Intro to Sound

2. Sound Waves • Sound waves are produced by a vibrating object. • Sound waves are longitudinal mechanical waves. • Often demonstrated with a tuning fork – as tines vibrate, they disturb the surrounding air molecules

3. Sound as a Longitudinal Wave • Individual particles of medium move parallel to the direction of the wave • As particles push on adjacent particles, some regions are created where particles are pressed together (high pressure) and in some regions they are spread apart (low pressure).

4. Sound waves • Compressions – the areas of high pressure • Rarefactions – the areas of low pressure • Since there is a repeating pattern of compressions and rarefactions, sound waves are also called “pressure waves” • A wavelength is the distance from compression to compression (or rarefaction to rarefaction)

5. Transverse vs. Longitudinal • Waves traveling through a solid medium can be either transverse or longitudinal • Waves traveling through a liquid or gas are ALWAYS longitudinal

6. Transmission of pulse in different mediums • Wave speed differs in different types of media • Waves are faster in less dense media • At the boundary, waves will either reflect back or transmit through the other medium

7. Sound Behaviors: Reflection • Reflection of sound results in an echo • 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

8. Reflection • Law of Reflection: The angle of incidence is equal to the angle of reflection

9. 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

10. 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/

11. Interference • Interference of sound waves causes beats • Beats occur due to constructive and destructive interference between sound waves of very similar frequencies • The beat frequency will be the difference in the 2 sound frequencies: • Ex: 2 sound waves with frequencies of 256 Hz and 254 Hz will have a beat frequency of 2 Hz • Humans can hear beats with frequencies of 7 Hz and below

12. Pitch and Frequency • Frequency – how often the particles of the medium vibrate when a wave passes through the medium (Hz) • Frequency and Period are inversely related: • High frequency = short wavelength period • Low frequency = long wavelength period • The sensation of a frequency is referred to as the pitch • High pitch = high frequency • Low pitch = low frequency

13. Pitch and Frequency • Certain sound waves when played/heard simultaneously will produce a pleasing sound and are called “consonant” • Such sound waves form the basis of intervals in music • Any 2 sounds whose frequencies make a 2:1 ratio are separated by an octave (one sound has twice the frequency of the other) • Ex: sounds with frequencies of 512 Hz and 256 Hz

14. Intensity • The human ear is sensitive to differences 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 intensity is a measure of how much energy passes a given point in a time period • Intensity is measured in decibels (db)

15. Intensity • As sound waves travel through a medium, the intensity decreases with increasing distance from the source • Intensity varies inversely with the square of the distance • Ex: if distance doubles, the intensity goes down by 4x

16. The Decibel Scale • Based on powers of 10 • 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

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

19. Velocity • Velocity (speed) 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 are no particles, so sound has no medium) • Vsolid > vliquid>vgas

20. 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

21. Wave Equation

22. 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?

23. What is the Doppler Effect? • http://molebash.com/doppler/home.htm

24. Doppler Effect • The Doppler effect is a change in the apparent frequency due to the motion of the source or the receiver • Example: As an ambulance with sirens approaches, the pitch seems high. As the ambulance moves away the pitch lowers.

25. Doppler Effect • Sound waves move out in all directions • As the wave travels outward, the front of the wave bunches up, producing a shorter wavelength • We hear a higher frequency

26. The back of the wave spreads out, producing a longer wavelength • We hear a lower frequency • http://www.sounddogs.com/searchresults.asp?Keyword=Doppler

27. Doppler Effect Example • http://www.animations.physics.unsw.edu.au/jw/doppler.htm#example

28. Observer A hears a low pitch (lower frequency) • Observer B hears the correct pitch (no change in frequency) • Observer C hears a high pitch (high frequency)

29. When the source goes faster, the wave fronts in the front of the source start to bunch up closer and closer together, until...

30. The object actually starts to go faster than the speed of sound. A sonic boom is then created.

31. Uses of the Doppler Effect • Police use Doppler to measure your speed with radar • A frequency is sent out with a radar gun • The sound wave hits your car and bounces back to the police car • Speed can be determined based on the frequency changes received • Radar can be used to determine the speed of baseballs • Astronomers can determine the distance to other galaxies • Bats use Doppler to locate prey

32. Acoustics • Acoustics- field of study related to sound • Acoustic designers try to maximize the quality of sound reaching the audience • Control the size, shape, and material used • They try and control the reflection

33. 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.

34. 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

35. Sound starts at the Pinna

36. Then goes through the auditory canal

37. 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 eardrum is a very durable and tightly stretched membrane that vibrates as the incoming pressure waves reach • The stirrup is connected to the inner ear

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

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

40. 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

41. The stapes will then vibrate the Cochlea

42. 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. 

43. Standing Waves • Standing waves are produced only at specific frequencies, called “harmonics” • Different frequencies create different standing wave patterns:

44. 1st harmonic pattern • 2 nodes and 1 antinode • Simplest harmonic pattern • Low frequencies 2nd harmonic pattern • 3 nodes and 2 antinodes • 2 times the frequency as 1st 3rd harmonic pattern • 4 nodes and 3 antinodes

45. Resonance

46. 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.

47. 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

48. 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)

49. 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.

50. 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.