Chapter 26 Sound Web Page

Origin of Sound All sounds are produced by the vibrations of material objects For example in a piano, violin, or guitar a sound wave is produced by vibrating strings

Sound is made when something vibrates. Origin of Sound Sound is made when something vibrates. The vibration disturbs the air around it. This makes changes in air pressure. These changes in air pressure move through the air as sound waves.

Sound Waves Alternating areas of high & low pressure in the air ALL sound is carried through matter as sound waves Sound waves move out in ALL directions from a vibrating object

Nature of Waves Waves (Def.) – A wave is a disturbance that transfers energy. Medium – Substance or region through which a wave is transmitted. Speed of Waves – Depends on the properties of the medium.

Properties of Waves  or T pressure horizontal axis could be: space: representing snapshot in time time: representing sequence at a par- ticular point in space Wavelength () is measured from crest-to-crest or trough-to-trough, or upswing to upswing, etc. For traveling waves (sound, light, water), there is a speed (c) Frequency (f) refers to how many cycles pass by per second measured in Hertz, or Hz: cycles per second associated with this is period: T = 1/f These three are closely related: f = c

Longitudinal vs. Transverse Waves Sound is a longitudinal wave, meaning that the motion of particles is along the direction of propagation Transverse waves—water waves, light—have things moving perpendicular to the direction of propagation

Why is Sound Longitudinal? Waves in air can’t really be transverse, because the atoms/molecules are not bound to each other can’t pull a (momentarily) neighboring molecule sideways only if a “rubber band” connected the molecules would this work fancy way of saying this: gases can’t support shear loads Air molecules can really only bump into one another Imagine people in a crowded train station with hands in pockets pushing into crowd would send a wave of compression into the crowd in the direction of push (longitudinal) jerking people back and forth (sideways, over several meters) would not propagate into the crowd but if everyone held hands (bonds), this transverse motion would propagate into crowd

Questions A wave in which the particles of the medium move at right angles to the direction of the wave is a __________. Transverse wave High pitch is produced by sounds that have__________. High frequency

Speed of Sound Sound speed in air is related to the frantic motions of molecules as they jostle and collide since air has a lot of empty space, the communication that a wave is coming through has to be carried by the motion of particles for air, this motion is about 500 m/s, but only about 350 m/s directed in any particular direction Solids have faster sound speeds because atoms are hooked up by “springs” (bonds) don’t have to rely on atoms to traverse gap spring compression can (and does) travel faster than actual atom motion

Examples of Sound Speeds Medium sound speed (m/s) air (20C) 343 water 1497 gold 3240 brick 3650 wood 3800–4600 glass 5100 steel 5790 aluminum 6420

Components of Sound Pitch (how high or low) Loudness (volume) Timbre (tone color)

Vibration Back and forth movement of molecules of matter For example,

Does high density material tend to cause sound to go faster or slower? Is low temperature cause sound to go faster or slower? Is highly elastic material cause sound to go faster or slower?

The Physics of Sound

Pitch … is the "highness" or "lowness" of a tone. Pitch corresponds to frequency.

Pitch The vibration patterns of some sounds are repetitive. Vibration patterns are also called waveforms. Each repetition of a waveform is called a cycle. We can hear frequencies between 20 hertz or cycles (vibrations) per second (low pitches)to 20 kilohertz, i.e. 20,000 Hz (high pitches).

Pitch Continued Infrasonic frequencies < 20 Hz Ultrasonic Human hearing range frequencies between 20 Hz and 20,000 Hz

When the frequency of a sound doubles we say that the pitch goes up an octave. We can hear a range of pitches of about ten octaves. Many animals can make sounds and hear frequencies that are beyond what we can hear.

Could you hear a pitch of 19 Hz?

Sound in Air ...a longitudinal wave in air caused by a vibrating object. Demo: slinky

The Ear Sound is carried to our ears through vibrating air molecules. Our ears take in sound waves & turn them into signals that go to our brains. Sound waves move through 3 parts of the ear; outer ear, middle ear, & inner ear. Middle Ear

The Ear The outer ear gathers sound waves, passing them through the ear canal to a tough membrane called the eardrum The vibrating eardrum passes the sound to three tiny bones in the middle ear – the hammer, anvil, and stirrup – which amplify the sound wave

The Ear The stirrup vibrates and transfers the sound to a membrane in the oval window, then on to the inner ear’s cochlea, a spiral-shaped structure that contains hair cells As the hair cells in the cochlea vibrate, nerve impulses are sent through the auditory nerve to the brain

The Ear The sound waves cause pressure changes against our ear drum sending nerve impulses to our brain.

Speakers: Inverse Eardrums Speakers vibrate and push on the air pushing out creates compression pulling back creates rarefaction Speaker must execute complex motion according to desired waveform

What type of wave would moving a rope attached to a wall simulate? Question What type of wave would moving a rope attached to a wall simulate?

Sound in Air Continued Sound requires a medium. solid, liquid or gas Demo: Bell in a evacuated Bell Jar Sound waves have compression and rarefaction regions.

Noise Measuring Equipment Sound Level Meters (SLM) Continuous on-mobile sources Noise Dosimeters Mobile/variable noise sources

Questions Would sound travel faster on a train track or in a pool?

Compression Where molecules are being pressed together as the sound waves move through matter For example, a wave travels through the springs just like sound waves travel through the air the places where the springs are close together are like compressions in the air.

Rarefaction The change in direction of a wave as it crossed the boundary between two media in which the wave travels at different speeds

Questions Refraction occurs because waves__________. Move at different speeds in different mediums In rarefaction, the molecules of the medium are _____. Spaced apart

The Physics of Sound

The Physics of Sound

Are the air molecules more spread out in compression or rarefraction?

Media that Transmits Sound Most sounds you hear are transmitted through air. Solids and liquids are generally good conductor of sound— much better than air Sounds cannot travel through a vacuum

Speed of Sound in Air Speed of sound = 340 meters/second or 760 miles/hour Sound travels faster in hot, humid climates Speed of sound depends on elasticity

Speed of Sound in Air Speed of sound = (331.5 + 0.6T) m/s So at higher temperatures the speed of sound is faster because of the faster moving molecules. At 0 degrees the speed in air is 331.5 m/s. This speed increases with the temperature at about 0.6 m/s per degree Celsius. Speed of sound = (331.5 + 0.6T) m/s T = temperature in Celsius

Speed of Sound and Refraction Sound travels faster in warm air. Why?

Late one summer night ( T = 20 degrees Celsius), lightning is seen from an approaching storm and five seconds later, thunder sound is heard. How far away is the storm? Speed of sound = ((331.5) + 0.6(20))m/s Speed of sound = 343.5 m/s V = d/t 343.5 m/s = d / 5s D = 1717.5 m

Wavelength & Frequency Wavelength is the distance between one part of a wave and the same part of the next wave Frequency is the number of waves moving past a point in one second

Can you predict how far a storm is if you here thunder 8 seconds away? 340m/s x 8 s = 2720 m

Sound Intensity and Loudness Intensity of Sound refers to the amplitude of the pressure variations in the sound wave

Loudness The physiological sensation directly related to the sound intensity Measured in bels (10 bels = 1 decibels)

As the sound spreads out from its source, the concentration of power becomes less. As the distance from the source increases the amount of power is spread over a greater area. The amount of power per square meter is called the intensity of the sound.

Loudness To create vibrations energy is used. The greater amount of energy used the louder the sound. The strength of the changes in air pressure made by the vibrating object determines loudness.

Loudness A sound of 10 decibels is or 101 or 10 times as intense as 0 decibels. 20 decibels is 102 or 100 times the intensity 0 decibels.

Source of Sound Loudness (db) Threshold of Hearing 0 Conversation 60 Ear Damage Begins 85 Amplified Music 110 Jet Airplane at 30 meters 140

Common Sound Intensities

Hearing Protection Devices and Their Noise Reduction Ratings EAR Foam Plugs (NRR = 29 dB) EAR CARBOFLEX (NRR= 20 dB) Moldex PURAFIT Foam Plugs (NRR=30 dB)

The human perception of intensity is known as _____. Loudness Sound waves are _______________ waves. Compressional The speed of sound depends on? Temperature Density Elasticity Reflection of a wave occurs when the wave______. Strikes a boundary and bounces back The pitch of a sound depends on its__________. Frequency

Humans do not perceive sound intensity linearly. For us to perceive a sound as twice as loud its intensity must be ten times greater. The perceived intensity level of sound is measured in a logarithmic scale using a unit called the decibel (dB) 2

From the perspective of the logarithmic scale the threshold of pain is 1,000,000,000,000 times as great as the threshold of hearing.

What is the loudest setting should set their ipod at to maintain a healthy ear? 85 decibels

Timber is the specific property of sound that enables us to determine the difference between a piano and a harp.

The predominant pitch is called the fundamental frequency. An extremely broad variety of tone colors exist because most sounds that we perceive as pitch actually contain many frequencies. The predominant pitch is called the fundamental frequency.

Although we would perceive a string vibrating as a whole, it actually vibrates in a pattern that at first appears to be erratic producing many different overtone pitches. What results are particular tone colors or timbres of instruments and voices.

The other frequencies which occur in a mathematical series are called the harmonic or overtone series. When C1 is the fundamental the following pitches represent its first fifteen successive overtones.

Forced Vibrations …the setting up of vibrations in an object by a vibrating force. Examples of Forced Vibration: A tuning fork touching a wood surface Sounding boards for stringed instruments Matching tuning fork boxes

Natural Frequency …the frequency at which an elastic object naturally tends to vibrate. At this frequency, a minimum energy is required to produce a forced vibration. The natural frequency of a body depends on its elasticity and its shape.

What factor besides shape affects natural frequency?

Natural Frequency Examples Dropping Aluminum Rods Ringing Small and Large Bells Xylophone Rubbing a Wine Glass Mass on a Spring

38. What is the speed of a wave if the frequency is 300 Hz and the wavelength is 150m? S = λf S = (150)(300) = 45000 m/s 39. What is the wavelength of a 350 Hz wave traveling at 500m/s? S = λf λ = 1.4 meters 40. How many cycles per second would be characteristic of a 30 m wave traveling at 300 m/s? S = λf 10 cycles/sec (Hz)

All Shapes of Waveforms Different Instruments have different waveforms a: glockenspiel b: soft piano c: loud piano d: trumpet Our ears are sensitive to the detailed shape of waveforms! More waveforms: e: french horn f: clarinet g: violin

Section 3 – Music Music – sounds that are deliberately used in a regular pattern Natural frequency – frequency at which the material vibrates Resonance – the ability of a medium to vibrate by absorbing energy at its own natural frequency

Resonance …is the result of forced vibrations in a body when the applied frequency matches the natural frequency of the body. The resulting vibration has a high amplitude and can destroy the body that is vibrating.

Examples of Resonance breaking a wine glass using sound mass on a spring at resonance a singing rod caused by forced vibration a tuning fork exciting a guitar string In 1940, the Tacoma Narrows Bridge was destroyed by wind-generated resonance.

1. Frequency times wavelength equals_________. 2. What is the number of complete wave cycles per unit time? 3. A ____________ is a region in the medium in which molecules are crowded together. 4. The ability of an object to vibrate by absorbing energy at its natural frequency is called _________.

Sound Interference Overlapping crests of a wave will result in an increased amplitude. Overlapping a crest and a trough results in a decrease in amplitude.

Beats Beats - the periodic variation in loudness of two sounds played together The beat frequency is equal to the difference in the frequency of the two sounds.

Acoustics... ...the study of sound properties. When a sound wave strikes a surface it can be.… (a) reflected. (b) transmitted. (c) absorbed. (d) all of these. 

Reflection of Sound e.g. an echo Reverberation - re-echoed sound, multiple reflections of sound waves from walls Compare reflections from a hard wall with that from a carpet wall. Demo: Whip

Refraction of Sound Refraction - the bending of a wave Sound waves bend toward cooler air. Desert and Lake Example

Radio Broadcasts AM - Amplitude Modulation FM - Frequency Modulation 535 kHz to 1605 kHz FM - Frequency Modulation 88 MHz to 108 MHz Modulation - an impression of the sound wave on a higher frequency radio waves

Modulating Radio Waves © 2000 Microsoft Clip Gallery Modulation - variation of amplitude or frequency when waves are broadcast AM – amplitude modulation Carries audio for T.V. Broadcasts Longer wavelength so can bend around hills FM – frequency modulation Carries video for T.V. Broadcasts

Humans use sonar to locate or map objects An instrument that uses reflected sound waves to find underwater objects For example, Humans use sonar to locate or map objects Animals use sonar or echo location to find their prey; these sounds have such a high pitch or frequency that the human ear cannot hear

Bibliography http://science.pppst.com/sound.html http://www.tzemach.org/music/powerpoint/physics_of_sound.ppt http://www.physics.ucsd.edu/~tmurphy/phys8/lectures/10_sound.ppt http://cmsweb2.loudoun.k12.va.us/539208161203730/lib/539208161203730/Sound.ppt http://lessons.ctaponline.org/~dpower/waves.ppt http://www.greenville.k12.sc.us/eastside/simmons/docs/ps/Module%207.ppt http://www.purdue.edu/physicalfacilities/safety/presentations/NoiseExposure/Noise.ppt http://www.st-and.demon.co.uk/AudioMisc/asymmetry/asym.htm schools.birdvilleschools.net/178720814112933850/lib/.../Chapter_12.ppt kisdwebs.katyisd.org/.../Waves%20and%20sound%20powerpoint.ppt http://hypertextbook.com/physics/waves/sound/