3 THE ORIGIN OF SOUNDAll sounds are produced by the vibrations of objectsA.k.a a disturbanceWaves are created by some form of a disturbance or vibration
4 THE ORIGIN OF SOUNDAs the particles are moved from their rest position, they exert a force on the adjacent particlesThen transfer their kinetic energyThus sound energy travels outward from the source.
5 Mechanical waveMechanical waves require a medium in order to transport their energyExamples:Slinky waves, water waves, stadium waves and sound wavesSound waves are incapable of traveling through a vacuum
6 Wave propagationcreating a pressure disturbance consisting of an alternating pattern of compressions and rarefactions
7 Wave propagation For example: a vibrating guitar string forces surrounding air molecules to be compressed and expandedThe amount of energy transferred to the medium is equal to the amplitude squared of the wave
8 Pressure Wave? In terms of pressure what is a sound wave composed of? Illustration of the pressure-time fluctuationsDo not conclude that sound is a transverse wave which has crests and troughs
10 Longitudinal Wave parts RAREFACTIONa disturbance in air (or matter) in which the pressure is loweredThe transfer of a low pressure air frontSimulated guitar string
11 Longitudinal Wave parts COMPRESSIONA pulse of compressed aira disturbance in air (or matter) in which the pressure is increasedThe transfer of a high pressure air front
12 Bobby’s hummer is a mess. There are papers piled up on the seats Bobby’s hummer is a mess! There are papers piled up on the seats. Whitney, decides to straighten up.Whitney quickly and forcefully opens the door. Predict what will happen to the papersThe air in the car will be rarefied and the papers will go flying.Bobby sees this, gets very upset. He forcefully closes the door. Now, predict what will happen to the papersThe air in the Hummer will be compressed, and the papers will go flying again!
13 Why are sound waves longitudinal waves? How is wavelength measured?
14 MEDIA THAT TRANSMIT SOUND Sound can travel through:AirSolidsLiquidsSound cannot travel through…A vacuumThere is nothing to compress & expand
15 The two main factors affecting the speed of sound… The mediumThe temperatureMinor factorsHumidityAir pressure
16 Density & the Speed of sound The speed of sound is not always the sameIt is easier for sound waves to go through solids than through liquids because…The closer the molecules are to each other the tighter their bonds,the less time it takes for them to pass the sound to each other
17 Density & the Speed of sound MaterialSpeed of SoundRubber60 m/sAir at 40oC355 m/sAir at 20oC343 m/sLead1210 m/sGold3240 m/sGlass4540 m/sCopper4600 m/sAluminum6320 m/sDensity affects the speed of sound in a materialDensity describes the mass of a substance per volume
18 Temperature and the speed of sound Heat, like sound, is a form of kinetic energyhigher temperatures have more energy, therefore they vibrate fastermolecules vibrating faster means, sound waves can travel more quicklyIf no temperature is stated we will use 340 m/s
19 If no temperature is stated we will use 340 m/s Speed of SoundThe speed of sound in dry air is given approximately byV sound in air = ( * Tc ) m/s@ 0o C speed of sound is approx. 331 m/s or 742 mi/hr@ 20oC speed of sound is approx. 343 m/s or 769 mi/hrIf no temperature is stated we will use 340 m/s
20 THE SPEED OF SOUNDThe speed of sound is slower than the speed of lightSpeed of Light = 3.00 x 108 m/s
21 How far away is the storm? If you see a lightning flash and five seconds later you hear the thunder how far away is the storm?Assume speed of sound to be 340 m/sec or .21 miles/secIf v = d / t then d = v * td = 340 m/sec * 5.0 secd = 1700 meters awayd = 0.21 miles/sec * 5.0 secd = 1.1 miles awayExtra practice: Suppose the delay is 8 seconds.
22 Frequency vs. Speed Frequency refers to the… number of vibrations that an individual particle makes in a specific period of timeFrequency refers to how often a wave passes through a certain pointSpeed refers to how fast a wave passes through the point.
23 PITCH & FREQUENCY Pitch – impression of frequency Some might say the two are synonymousHigh-pitched vs. Low-Pitched
24 NATURAL FREQUENCYAll objects have a natural frequency or set of frequencies at which they vibrateA natural frequency is one at which minimum energy is required to force vibrationsNatural frequency depends on factors such as the density, elasticity and shape of an object
25 FORCED VIBRATIONForced vibration is when one object is connected to a larger one,forcing its sound vibrations on the larger surfaceThe resulting sound is relatively louder
26 RESONANCEResonance – when the frequency of a forced vibration of an object matches the object’s natural frequency, there is a dramatic increase in amplitude.To resonate, objects need a force to pull them back to their starting positionFor ex: pumping a swing in rhythm with its natural frequency produces larger amplitudes.
27 Energy Transport and the Amplitude of a Wave A wave transports energy along a medium without transporting matterThe amount of energy carried by a wave is related to the amplitude of the wave
28 IntensityThe amount of energy which is transported past a given area of the medium per unit of timeThe greater the amplitude,the greater the rate at which energy is transported, and therefore the more intense the sound wave
29 LOUDNESS Loudness is a physiological sensation sensed in the brain It is subjective, but relates to sound intensityIntensity of sound is measured with the decibel (dB)0 dB is the threshold of hearing for a normal earAn increase of 10 dB means that it increase by a factor of 1010 dB = 10 x 1 dB20 dB = 10 x 10 dB30 dB = 10 x 10 x 10 dB
30 Decibel quiz 1. The decibel level that causes pain 1. (130)dB 2. The decibel level that can break bones in the ear2. (150)dB3. The decibel level that is 1000 times more powerful than 10 dB3. (30)dB
32 Human Hearing Thresholds Infrasonicfrequencies below 20 hertzUltrasonicfrequencies above 20,000 hertzWe cannot hear these sounds
33 Decibel HistoryA bel (symbol B) is a unit of measure of ratiosDeci (symbol d) is the metric prefix meaning tenInvented by engineers of the Bell Telephone Laboratory, it was originally called the transmission unit or TURenamed in in honor of the laboratory's founder and telecommunications pioneer Alexander Graham Bell
34 Reflection of SoundSound reflects or bounces from all surfaces-walls, ceiling, floor, furniture, and peopleAn echo is reflected soundSound and light follow the same rules about reflections
35 Reflection of Sound Acoustics the study of the reflective properties upon surfacesReverberationsPersistence of a sound, as in a echo , due to multiple reflections
36 Reflection of SoundReflection of sound in a room makes it sound lively and fullIn a concert hall, highly reflective surfaces are often placed behind and above the stage to direct sound out to an audience
37 Reflection of SoundWhen sound reflects off a special curved surface called a parabolait will bounce out in a straight line no matter where it originally hitsMany stages are designed as parabolas so the sound will go directly into the audience, instead of bouncing around on stage
38 Example: Parabolic Stage Location: HOLLYWOOD BOWL
39 Reflection of SoundIf the parabola is closed off by another curved surface, it is called an ellipseSound will travel from one focus to the other, no matter where it strikes the wallA whispering gallery is designed as an ellipse
40 Example: Ellipse Design In here if you and a friend stand in opposite corners, as if being punished, you can carry on a whispered conversation that others can’t hearLocated in Grand Central Station
41 Refraction or (bending) of Sound Sound waves are refracted or bent when parts of a wave front travel at different speedsthis happens in uneven winds or…when sound is traveling through air of uneven temperatures
42 Refraction of SoundOn a warm day the waves tend to bend away from the warm ground…making it appear that the sound does not carry wellOn a cold day the speed of sound is reduced on the ground…causing the waves to bend towards the earth, carrying the sound longer distances
43 Pop QuizHow does sound reach you when are in a different room than the source?Refraction and Reflection!
46 INTERFERENCE Waves can be made to interfere with each other. When the crests of one wave overlap those of another wave, there is a constructive interference and increase in amplitude.When the crests of one wave overlap the overlap the troughs of another, there is a destructive interference and a decrease in amplitude.
47 INTERFERENCE AND SOUND Interference effects loudnessIf one is equally distant from two speakers simultaneously triggering identical sound waves, the sound is louder because the waves add.Destructive interference is usually not a problem, and there is enough reflection to fill in canceled spots.“Dead spots” are present in poorly designed theaters and gyms.
49 BEATS Periodic variation in the loudness of sound is called a beat. When two tones of slightly different frequency are sounded together, a fluctuation in the loudness of the combined sounds is heard.Beats can occur with any kind of wave.