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- Describe the production of sound by vibrating sources.
Sound travels in the form of waves which are generated by vibrating particles.
- Describe transmission of sound in air in terms of compressions and rarefactions.
When a surface vibrates, it generates a sound wave.
Let me explain it in more detail – as the surface pushes outwards, the air in front of it is pushed closer to more air. This creates an area of compression. As the air collides with more air, that air then goes on to collide with even more air, causing the area of compression to move – a wave of compression is emitted.
When the surface pulls back, it creates a small vacuum, and air rushes to fill up that space. This causes a region of lower air density. We call it a rarefaction. More adjacent air rushes to fill the empty spaces, causing the rarefaction to move out from the surface.
In this way, the vibrating surface emits alternating waves of compression and rarefaction, which together make up a sound wave.
Overal, during the emission of a sound wave, air particles tend to vibrate back and forth around a fixed point, parallel to the direction that the wave travels in. This means that a sound wave is a longitudinal wave.
- State the approximate human range of audible frequencies.
Frequencies are measured in Hertz. Hertz is a measure of the number of waves that pass a single point per second, so 1 Hertz (Hz) = 1 wave per second. As in, a total of one entire wave passes through a single point in one second.
The average human’s audible range is from 20 hertz to 20,000 hertz. Sounds below 20 hertz and above 20,000 hertz cannot be heard by humans. This range may differ slightly from person to person and tends to get smaller as you get older.
- Demonstrate understanding that a medium is needed to transmit sound waves.
Sound waves are produced from vibrating particles, so when there aren’t any particles (ie. there is no medium), sound cannot be generated, eg. in outer space. So a medium is always needed for sound waves to be transmitted.
- Describe and interpret an experiment to determine the speed of sound in air.
You can measure the speed of sound in air by dividing the distance of sound travelled by the time it took to travel that distance. (speed = distance/ time)
One method involves measuring the time taken for you to hear an echo from a sharp clap. You stand a long distance from a wall, clap, and listen for the echo. The distance travelled is twice the distance from you to the wall (because the sound has to travel to the wall and back).
One way to reduce timing errors in this method is to clap in time to the echoes. This means that the time between each clap is the journey time for the sound. You then measure the time for 11 claps, which is the time for 10 journeys by the sound. This time can then be used to calculate an average time for the sound to travel to the wall and back.
(Adapted from BBC Bitesize)
- State the typical values of the speed of sound in air, liquids and solids.
The speed of sound in air: 330m/s
The speed of sound in liquids: 1500m/s
The speed of sound in solids: 6000 m/s
Sound is fastest in solid since particles are closer together for waves to travel through them and slowest in the air because the particles are farther away from each other.
- Relate the loudness and pitch of sound waves to amplitude and frequency.
Amplitude: this is the maximum displacement of the particle from its equilibrium position. Assuming the frequency remains constant, the higher the amplitude, the more energy the wave has. As a result, the wave exerts more pressure on its surroundings, making it louder. Therefore, the higher its amplitude, the louder the wave. The loudness of waves are measured in decibels (dB).
Frequency: frequency is measured as the number of waves that pass a fixed point in space per second. The unit of frequency measurement is Hertz (Hz for short). the higher the frequency, the faster the pressure fluctuation. The human ear detects these fluctuations as pitch. Therefore, the higher the frequency, the higher the pitch.
- Describe how the reflection of sound may produce an echo.
When a sound wave is reflected, its frequency and wavelength are still the same, only the direction is different. So to a person, it is heard as another sound.
Note: I understand that it can be quite difficult understanding or visualising some of this – especially the explanation to point 2 (the transmission of sound) – so I found a video on YouTube that actually explains most of this pretty clearly. The only downside is that the narrator’s voice is really annoying, but it’s worth watching it if you need more help understanding 🙂 Here’s the link: https://www.youtube.com/watch?v=GkNJvZINSEY
Notes submitted by Lintha and edited by Sarah.
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