Sound. Sound is produced by vibrating objects. A sound wave consists of mechanical vibrations in air and other substances.

Transcription

1 Sound GCSE Physics

2 Sound Sound is produced by vibrating objects. A sound wave consists of mechanical vibrations in air and other substances. Sound is a longitudinal wave in which the wave energy travels in the same direction as the particles within the wave vibrate. Most other types of wave are transverse. For examaple water and light waves. In these cases the particles making up the wave vibrate at ninety degrees to the direction of energy transfer. LONGITUDINAL WAVE wave direction vibrations TRANSVERSE WAVE vibrations wave direction

3 Through air a sound wave consists of a series of compressions and rarefactions. A compression is a region of slightly higher pressure where the air molecules are closer together than usual. A rarefaction is the opposite. loudspeaker no sound wave sound wave compression wavelength rarefaction

4 Speed of sound Substance Speed in m/s Air at 0ºC Air at 20ºC Water at 20ºC Steel at 20ºC Sound does not travel through a vacuum. In space no one can hear you scream! tagline of the film Alien (1979)

5 Question A thunderclap is heard 12 seconds after a lightning flash. Calculate the distance to the lightning flash. Take the speed of sound = 340 m/s speed = distance / time becomes: distance = speed x time = 340 m/s x 12 seconds = m Distance = 4.08 km

6 Bell jar experiment This experiment shows that sound needs a material medium for transmission. As the air pressure inside the bell jar is reduced the loudness of the sound heard outside decreases. The bell can be still seen to be working normally.

7 Range of hearing Humans can hear sounds in the range 20 Hz to Hz. Age and damage reduces the upper limit. For example an old person or someone exposed to perlonged high sound volume may no be able to hear above Hz. Some animals can hear much higher frequencies: dogs to Hz (depends on the breed) bats and dolphins Hz

8 Reflection of sound An echo is a reflected sound wave. Question: A missguided child shouts Chelsea! at a nearby cliff and hears their echo 1.4 s later. How far away is the cliff? Take the speed of sound = 340 m/s. distance, D The sound travels to and from the cliff, a total distance of 2D speed = distance / time becomes: distance = speed x time = 340 m/s x 1.4 s = 476 m = 2D! Therefore distance to the cliff = 238 m.

9 Sound refraction C A B cooler air The sound produced by person A may be heard more clearly by person B than by person C. The cooler air over the water refracts the sound waves downwards.

10 Sound waves on oscilloscopes An oscilloscope is a device that can be used to display a sound wave. The screen displays a graph of how the amplitude of the sound wave varies with time.

11 Loudness The loudness of a sound increases with the amplitude of the sound wave. quiet loud

12 Pitch The pitch of a musical note increases with frequency. low pitch high pitch Examples: Concert pitch A = 440 Hz; Top C = 523 Hz Doubling the frequency increases the pitch by one octave. Therefore the A above top C will have frequency 880 Hz.

13 Quality The quality or timbre of a musical note is what makes one musical instrument sound different from another. Despite different instruments producing a note of the same loudness and pitch, the shape of the wave will be different.

14 The diagram opposite shows the appearance of a sound wave on an oscilloscope. Draw a second diagram showing the appearance of a sound wave of lower pitch but greater loudness. Question

15 Question Choose appropriate words to fill in the gaps below: Sound is a longitudinal wave that in air consists of a series of compressions and. rarefactions Sound travels fastest through solids but does not travel at all through a. vacuum A reflected sound wave is called an echo. Sound also undergoes. refraction The loudness of a sound increases with wave, amplitude the pitch with wave. frequency WORD SELECTION: rarefactions refraction vacuum amplitude longitudinal reflected frequency solids

16 Ultrasound Ultrasound is very high frequency sound that is above Hz, too high to be heard by humans. Some animals, for example dogs, bats and dolphins can communicate or navigate using ultrasound. Ultrasound echoes can be used to measure distance (e.g. sonar) and to see inside objects (scans).

17 Sonar SOund Navigation And Ranging.

18 Ultrasound scans Ultrasound waves are partially reflected when they meet a boundary between two different media. The time taken for the reflections to reach a detector is a measure of how far away such a boundary is. Ultrasound reflects off each tissue boundary Developing baby Ultrasound transmitter

19 1. Prenatal scanning

20 Ultrasonic testing is a type of nondestructive testing commonly used to find flaws in materials and to measure the thickness of objects. 2. Quality control At a construction site, a technician tests a pipeline weld for defects using an ultrasonic instrument.

21 The oscilloscope trace opposite was obtained from the scan of a metal block. (a) how many flaws are present according to this display? (b) If the width of the block was 300 mm calculate the distance fro the front of the block to each flaw. Question (a) There are two flaws. (b) The width of the block is represented by 8.3 squares. Therefore each square represents 300mm / 8.3 = 36.1 mm per square The two flaws occur after 3.0 and 4.2 squares Therefore: 1 st flaw is at 3.0 x 36.1 = 108 mm from the front 2 nd flaw is at 4.2 x 36.1 = 152 mm from the front

22 Ultrasound cleaning The very high frequency waves of ultrasound can be used to shake off dirt or grease. Jewellery immersed in water Another way of cleaning teeth

23 Question Choose appropriate words to fill in the gaps below: Ultrasound is very high frequency sound. It is above the upper range of human hearing, more than hertz Ultrasound can be used to measure, distance for example SONAR. Ultrasound can be used to obtain, scans for example of a developing baby within a mother s womb. Ultrasound can also be used to clean objects. WORD SELECTION: baby human scans frequency distance clean hertz

24 Sound Simulations Sound - PhET - This simulation lets you see sound waves. Adjust the frequency, volume, and harmonic content and you can see and hear how the wave changes. Move the listener around and hear what she hears Vend diagram quiz comparing light and sound waves - echalk Ultrasound scanning - Explore Science BBC KS3 Bitesize Revision: KS3 Sound Contents Page What is sound? Loudness Pitch Hearing Test bite on KS3 Sound