1Chapter 20SoundRemember, sound is a longitudinal wave.22Origin of Sound• Sound is made of vibrations in the air in the form of longitudinal waves• The human ear can hear sounds with frequencies between 20 Hz and 20,000 Hz– Sounds with frequencies lower than 20 Hz are called infrasonic; sounds with frequencies above 20,000 Hz are called ultrasonic– The lower the frequency, the lower the pitch; the greater the amplitude, the louder the soundWe have seen the prefixes ‘infra’ and ‘ultra’ before when we have hear about infrared light and ultraviolet light33Nature of Sound in Air• Sound waves are composed of regions of high and low air pressure•Regions of high pressure are called compressions•Regions of low pressure are called rarefactionsThis is an animation of a vibrating tuning fork. As the right prong moves to the right, it compresses the air in that region and creates a compression. This compression moves outward from the fork. When the prong then moves back to the left, it creates a region of low air pressure called a rarefaction. This rarefaction also moves outward from the fork. These alternating compressions and rarefactions create a longitudinal wave called sound.44Interference of Sound• Sound interferes just like any other wave (as described in last lecture)This is an animation of 2 sound sources that are moving together. The dark lines are lines of compression and the lighter lines are lines of rarefaction. When 2 compression lines or 2 rarefaction lines intersect, there is constructive interference. The a line of compression and a line of rarefaction intersect, there is destructive interference. If everything is just right, these areas of destructive interference can be places where no sound is heard.55Media That Transmit Sound• Sound is most commonly transmitted through air but any elastic substance can transmit sound• Solids can transmit sound better than air since their molecules are closer together– That is why cowboys in the old movies would place their ear on the track; you can hear the train better through the tracks than the air66Speed of Sound• Because molecules are closer together in solids and fluids than in air, they have do not have as far to travel before they hit another molecule• Sound will also travel faster in hot air than cold, because the molecules of air move faster in hotter air• The speed of sound in air at room temperature (~ 20oC), is about 340 m/s77Problem Solving: Speed of Sound• How long does it take thunder to travel 1.6 km (1 mi)?s 7.4~m/s 340m 1600tm/s 340 sm 1600===dTherefore, you should count about 5 seconds in between seeing lightning and hearing thunder to figure out how far away the storm is in miles.88Reflection of Sound•An echo is a reflection of sound• Whenever a sound wave encounters an obstacle, part of the sound if reflected and some of the sound is transmitted or absorbed• More of the energy in sound is reflected if the obstacle is rigid or smooth than if it were soft and irregular• Sound is reflected off of an obstacle at the same angle that it hit it99Refraction of Sound• The bending of sound is called refraction• This bending occurs when different parts of the wave front moves at different velocities• If a layer of cold air is on top of a layer of hot air, sound will be refracted upward• If a layer of hot air is on top of a layer of cold air, sound will be refracted downwardWe will talk about refraction again after the midterm when we discuss optics (this is an important principle behind eye glasses).1010Energy in Sound Waves• Energy in sound waves will dissipate into thermal energy• The higher the frequency the more rapidly the sound’s energy is transformed into thermal energy– That is why foghorns are at very low frequency and by bass sounds travel more easily through floors and wallsRemember, energy is ALWAYS conserves but it often seems to ‘disappear’ into thermal energy.1111Natural Frequency and Resonance• Every object has a frequency that it vibrates best at – this is its natural frequency– As a general rule, the larger the vibrating object, the lower its natural frequency • If a force is applied to the object at the same frequency as its natural frequency, the amplitude of the vibration will increase dramatically – this is called resonanceYou have figured out the resonance of a swing when you adjust how fast you swing your legs to go higher and higher. You go higher because you are causing the swing to be in resonance.1212Sound Shattering a GlassThis is a movie that shows a wine glass starting to resonate because the sound in the air is vibrating at its natural frequency. You see larger and larger waves along the rim until they become so large that the glass shatters.1313Beats• If two objects are making sounds at very nearly the same frequency, they will occasionally interfere to create a constructive beat• For example, a sound that has a frequency of 100 Hz will interfere with another sound at 101 Hz to make 1 beat per secondThe beats kind of sound like a “woop-woop” sound.1414Problem Solving: Beats• You can use beats the measure the frequency of an of another sound. Let’s say you have a tuning for the vibrates a a frequency of 320 Hz. If you hear three beats per second, what is the frequency of the other sound?–It is either 323 Hz or 317 Hz. We said that a beat is caused by a difference in frequency. This difference can be caused by a higher or lower frequency.Remember, beats are caused by the difference in frequency, which can be a higher or lower