SOUND UNIT NOTES

The Nature of Sound Sound is a disturbance that travels through a medium as a longitudinal wave

Review of Waves Waves move and carry energy Waves travel in a medium (gas,liquid,solid, eg.air,water,rope) Waves are created when a source of energy (wind,drum stick) causes a medium to vibrate Waves spread out as they move Waves have different sizes and shapes Waves pass through each other Waves continue in the medium until they reach a barrier Waves do not disappear at barriers (waves diffract around corners) Waves move and carry energy Waves travel in a medium (gas,liquid,solid, eg.air,water,rope) Waves are created when a source of energy (wind,drum stick) causes a medium to vibrate Waves spread out as they move Waves have different sizes and shapes Waves pass through each other Waves continue in the medium until they reach a barrier Waves do not disappear at barriers (waves diffract around corners)

Vibrations & Waves

Which of the following waves has a bigger amplitude? Which required more energy to produce?

Sound Sound is produced when objects vibrate and cause molecules of air or some other material to also vibrate. (This is why there is no sound in outer space!) Volume – the height of each peak in the sound wave (aka amplitude or loudness) Frequency – (sometimes referred to as pitch) the distance between the peaks. The greater the distance, the lower the sound. Sound is produced when objects vibrate and cause molecules of air or some other material to also vibrate. (This is why there is no sound in outer space!) Volume – the height of each peak in the sound wave (aka amplitude or loudness) Frequency – (sometimes referred to as pitch) the distance between the peaks. The greater the distance, the lower the sound. Image: scanned from Multimedia Concepts – James Schuman

Frequency The number of complete waves that pass a given point in a certain amount of time (shorter the wavelength the higher the frequency) Sound - Hz (Hertz) = 1 wavelength / second ; 2 Hz = 2 wavelengths / second We hear as pitch High frequency = high pitch (ultrasound = above range of human hearing) 20,000 Hz Low frequency = low pitch (infrasound = below range of human hearing) 20 Hz Amplitude: related to the amount of energy carried by the wave; intensity Sound intensity is measured using the decibel scale (dB) = volume (Each 10 dB increase in sound level represents a tenfold increase in intensity) The number of complete waves that pass a given point in a certain amount of time (shorter the wavelength the higher the frequency) Sound - Hz (Hertz) = 1 wavelength / second ; 2 Hz = 2 wavelengths / second We hear as pitch High frequency = high pitch (ultrasound = above range of human hearing) 20,000 Hz Low frequency = low pitch (infrasound = below range of human hearing) 20 Hz Amplitude: related to the amount of energy carried by the wave; intensity Sound intensity is measured using the decibel scale (dB) = volume (Each 10 dB increase in sound level represents a tenfold increase in intensity)

The smallest change in sound levels detectable by the ear is around one decibel. A change of around 3 decibels is noticeable, and a change of around 6 decibels is obvious. An increase of about 8 to 10 decibels is required before a noise appears significantly louder (twice as loud as before).

Vibrations & Waves Water Waves - move stick up and down in a pond Sound Waves - move air back and forth Click here to go to another web site with information on wavesClick here to go to another web site with information on waves

Wave Motion Consider our stick moving up and down in the water Create circular waves that move out from the disturbance Water moves up and down, the wave moves outward away from the disturbance Waves carry energy of the moving stick A bug in the water will move up and down

Wave Speed Wavelength is distance between peaks Frequency is number of peaks per second going past a point in the water Frequency depends on wavelength and how fast the wave moves!!!

Wave Speed Cont. Speed = Wavelength x Frequency Frequency = Speed  Wavelength Wavelength = Speed  Frequency A sound wave has a wavelength of 4m and a frequency of 2 Hz (2/sec). What is it’s speed? Speed = Wavelength x Frequency Frequency = Speed  Wavelength Wavelength = Speed  Frequency A sound wave has a wavelength of 4m and a frequency of 2 Hz (2/sec). What is it’s speed?

Problems Cont. S = W x F, 4m x 2/sec. = 8 m/s The speed of a wave on a guitar string is 200 m/s and the frequency is 1,000Hz. What is the wavelength of the wave? W = S  F, 200 m/s  1,000 /s = 0.2m The speed of a wave on a rope is 60 cm/s and its wavelength is 15 cm. What is the frequency? F = S  W, 60 cm/s  15 cm = 4/s or 4 Hz S = W x F, 4m x 2/sec. = 8 m/s The speed of a wave on a guitar string is 200 m/s and the frequency is 1,000Hz. What is the wavelength of the wave? W = S  F, 200 m/s  1,000 /s = 0.2m The speed of a wave on a rope is 60 cm/s and its wavelength is 15 cm. What is the frequency? F = S  W, 60 cm/s  15 cm = 4/s or 4 Hz

Click here to go to web site for answer to previous question: Longitudinal or Transverse? Click here to go to web site for answer to previous question: Longitudinal or Transverse?

Sound Waves

Molecules in the air vibrate about some average position creating the compressions and rarefactions. We call the frequency of sound the pitch.

Sound Waves

Speed of Sound Depends on the elasticity of the material that is vibrating (the material’s abililty to bounce back after being stretched ex. Rubber band vs. clay) Solids are more elastic than liquids or gases because the particles are closer together and move back to their original positions quickly) Depends on the Density of a material. For materials in the same state ie. Liquid,gas, solid, sound travels slower in those that are more dense. Particles in dense materials don’t move as quickly. Ex. Lead = slow, Steel (not as dense) = fast) How about gold? Sound speed in dry air is 330 meters/second at 0 o C (air 25  C 346 m/s, rubber 60 m/s, fresh water 1,498 m/s, steel 5,200 m/s) Depends on Temperature -Faster in warm air, slower in cold air Water 4 times faster, steel 15 times faster

Wave Reflection When a sound wave reflects from a surface we generate an echo Wave reflection from surfaces depends on the characteristics of the surface Smooth hard surfaces reflect best Rough soft surfaces reflect poorly Energy not reflected is absorbed or transmitted through the material

Wave Reflection Think of arrows pointing in the direction of the wave motion We can trace the path of these arrows Angles Equal

Wave Reflection Acoustics of room design is very interesting. Need some reflections to “liven” the room. Too many reflections and the sound gets mushy. Look in a concert hall or auditorium to see the different sound treatments

Wave Refraction If there is a change in the characteristics of a medium, waves are bent This occurs because different parts of the wave front travel at different speeds Think of a marching band moving around a curved track The inside people have to move more slowly than the outside people to keep the lines straight

Wave Refraction

Wave Reflection & Refraction The combination of reflection and refraction enables imaging Ultrasonic medical imaging Naval SONAR for detecting submarines Bats catch mosquitoes

Natural Frequencies Objects have “natural” frequencies based on their size and structure Guitar strings are an example Timpani heads Air columns

Resonance When the forced vibration matches a natural frequency we get a “resonance” condition Think about a swing on a playground You go high when you pump the swing at its natural vibration frequency Sympathetic vibrations in tuning forks Famous Tacoma Narrows bridge collapse

Interference When two or more waves meet they have an effect on each other, this is called interference. There are two types: constructive and destructive Constructive interference is when the waves combine to form a larger amplitude (happens when wave crests align) Destructive interference is when the waves combine to produce a smaller wave or even cancel each other out. When two or more waves meet they have an effect on each other, this is called interference. There are two types: constructive and destructive Constructive interference is when the waves combine to form a larger amplitude (happens when wave crests align) Destructive interference is when the waves combine to produce a smaller wave or even cancel each other out.

Example of positive use of destructive interference

The Doppler Effect Basic doppler effect Click here for animations/diagrams of the doppler effectClick here for animations/diagrams of the doppler effect Click here for another! Click here for yet another Basic doppler effect Click here for animations/diagrams of the doppler effectClick here for animations/diagrams of the doppler effect Click here for another! Click here for yet another