1. SOUND 5 th Six Weeks

2. Intro to Sound The source of all waves (including sound) are vibrations. In a sound wave, a disturbance causes molecules in a medium to move back and forth, those molecules affect others next to them and so on. The frequency of sound is equal to the frequency of the vibrating object that created the longitudinal wave. In this way, energy is transferred from the source of the disturbance to your eardrum (which in turn vibrates, sending signals to the brain). Sound is a longitudinal (or mechanical) wave that moves through an elastic medium such as the air from a vibrating source. Since Sound by definition requires energy to be passed from particle to particle – sound does not occur in space.

3. Pitch Pitch – how high or low sound frequencies appear to be. Humans can normally hear pitches with frequencies from about 20 to 20,000 Hz Sound waves with frequencies below 20 Hz are called infrasonic. Sound waves with frequencies above 20,000 Hz are called ultrasonic. Humans cannot hear infrasonic or ultrasonic sound waves.

4. For all wave motion, it is not the medium that travels, but a pulse that travels. When the prong of a vibrating tuning fork vibrates near a tube filled with air, a compression enters the tube (in effect the tuning fork is “slapping” the air molecules) As the prong swings away, a rarefaction follows the compression. Therefore, as a source vibrates, a series of compressions and rarefactions is produced.

5. Loudness Loudness – is a way of describing sound intensity and amplitude of sound waves. Loudness or sound intensity can be measured by a device called an oscilloscope The unit for intensity of sound (also called loudness) is the decibel (dB). Higher decibel sounds are generally more painful and can cause damage to the ear if exposure is too long.

6. Beats When two or more tones of slightly different frequencies are sounded together a fluctuation in the loudness of the combined sounds is heard (loud, faint, loud, faint, and so on) The periodic variation (or change) in the loudness of sound are called beats. When different sound waves are in step (that is when the compressions occur at the same time) the sound is a maximum When the different sound waves are out of step (when a compression and rarefaction occur at the same time) a minimum sound is heard.

7. Beats, continued The lower frequency will be subtracted from the higher frequency to get the number of beats produced from the interaction. If a tuning fork vibrates 264 times per second and another nearby vibrates at 262 times per second, then they are in step twice each second. A beat frequency of 2 Hz would be heard.

8. Beats - illustrated

9. Bow Waves Waves are produced by vibrating object. However those vibrating objects are not always stationary, they may be vibrating while traveling through space. Note: as the object moves – the speed of those waves remains the same – though the frequency and wavelength change. When the speed of the source is as great as the speed of the waves it produces, the waves pile up. Overlapping circles form a “V” Airplanes with sufficient power can transcend this pile up or “sound barrier” and then fly in smooth, undisturbed air since no sound waves can occur out in front of it. When an object begins to move, the waves in front of it begin to pile up. The wave pattern that results are known as bow waves. When something goes faster than the waves it is creating, the bow wave pattern formed is a cone extending behind the object. V source > V wave V source = V wave V source < V wave Stationary Source

11. The Doppler Effect The apparent change in frequency due to motion of the source (or receiver) is called the Doppler Effect. Wavelength is less and the waves are more frequent in front of a moving object, and they are less frequent and the wavelength is longer after the object. The Doppler Effect exists for all waves not only sound waves When wavelength decreases frequency increases. As wavelength increases frequency decreases

12. The Doppler Effect…illustrated

13. Doppler Effect and the “Red Shift” As previously stated, the Doppler Effect occurs with all waves, not just sound waves In the case of light waves, the effect can be seen in the Red Shift. Visible light that is a higher frequency is more blue (or violet). Lower frequency light is more reddish. Therefore something moving towards you would give off light that was bluish while passing you would be reddish. Red Shift (and Blue Shift) describe how light waves change as they move towards and away from an observer due to the Doppler Effect. At low speeds the effect is quite small and is harder to detect “Red Shift” “Blue Shift”

14. Shock Waves and a Sonic Boom Within the bow waves, matter in a fluid is being compressed in 3 dimensions as the waves pile-up. The compressed area of a bow wave is referred to as a shock wave. Essentially a 3 – dimensional bow wave is a shock wave. The spheres overlap to form a cone The speed of sound in the air (generally – since it depends on temperature and air pressure) is 343 m/s or approximately 760 miles per hour Going the speed of sound is going “Mach 1” (named for the scientist Ernst Mach) and multiples of the speed of sound are Mach 2, Mach 3, etc. Light will not produce shock waves since by definition nothing can go faster than light. V source > V wave

15. Shock Waves and a Sonic Boom As a shock wave produced by something traveling faster than the speed of sound reaches listeners, that mass of compressed air is perceived as a sonic boom. V source > V wave

16. Blast Waves A blast wave is a compression wave that results from a detonated explosive & travels outward at greater than the speed of sound. True blast waves only result from High Explosives (such as C-4, nitroglycerin, & TNT). Low Explosives include items like gunpowder & gasoline – and are so named since the compression wave they produce is less than the speed of sound. Blast waves experience all the same wave interaction as other waves (reflection, diffraction, refraction, etc.)

17. Pressure Difference of a Blast Wave & Blast Wind One of the greatest sources of destruction from a blast wave is the pressure difference that forms between the high positive pressure of the shock wave front (the compression) and the extreme negative pressure of the area that follows (the rarefaction) This phenomenon is also known as “blast wind”

18. Speed of Sound & Density Previously, it was stated that the “speed of sound” was 343 m/s or approximately 760 miles per hour – and that speed was “Mach 1” That speed is the speed of sound in the air only and under normal conditions. The speed of sound is different in materials of different densities. Since sound is a longitudinal wave that requires particle collisions, the denser a material is, the closer particles are to one another, and thus the greater likelihood of collisions. The greater the density the faster the sound wave will move. Speed in Solid > Speed in Liquid > Speed in Gas There is a direct relationship between density and speed

19. Speed of Sound and Temperature Temperature has to do with the energy of particles that make up a given substance…in fact it is the average kinetic energy of the particles in a substance. Hotter things have particles moving faster (and colliding more often) than cooler substances do. Therefore, since energy travelling as sound requires colliding particles to carry the wave forward, sound moves faster through hot objects than cold ones which have slower moving particles.