Measuring wave velocity

Question 1

How do you calculate the speed of sound in air using an echo?

Answer 1

1) measure the distance from the surface of the source to the reflecting surface (the wall)
2) measure the time interval, with a stopwatch, between the original sound being produced and the echo being heard
3) use speed = distance/time to calculate the speed of sound in air

repeating the experiment multiple times over a range of distances allows you to obtain accurate and precise results

Question 2

oscilloscope

Answer 2

used to display sound waves or electrical signals on a screen

Question 3

oscilloscope traces

Answer 3

The wave pattern seen on the screen of an oscilloscope.

Question 4

How do you calculate the speed of sound in air using two microphones and an oscilloscope?

Answer 4

1) set up the microphones one in front of the other at different distances in a straight line from a loudspeaker
2) set the frequency of the sound from the loudspeaker to a known, audible value
3) display the two waveforms on the oscilloscope, measure the distance between the microphones
4) move the microphones apart so that the waveforms move apart by one wavelength

5) calculate the speed of sound using the equation:
wavespeed = frequency x wavelength

Question 5

sound waves and particle density

Answer 5

sound is a mechanical longitudinal wave, so is passed on by collisions between particles

the speed the wave moves at depends on the density of particles

Question 6

speed of sound in different materials

Answer 6

fastest in solids
(6000m/s in steel)

medium in liquids
(1500m/s in water)

slowest in gas
(330m/s in air)

Question 7

What happens when sound moves from one medium to another?

Answer 7

change in speed creates a change in wavelength

frequency is unaffected

wavelength is proportional to the wave velocity

Question 8

How do you calculate the speed of ripples on the surface of water using a ripple tank and a strobe

Answer 8

1) set up a ripple tank 5 cm of water and a paddle to create waves vibrating at a known frequency
2) make sure the paddle just touchs the water and set up a light source above to project the waves onto a screen below the tank
3) switch on the tank and motor and adjust until low frequency waves can clearly be seen
4) a strobe light that flashes on and off can be used to ‘freeze’ the waves and make them appear in the same position so wavelength can be measured (or take a picture of the card/screen)
5) measure the length of a number of waves then divide by the number of waves to record wavelength
6) count the number of waves passing a point in ten seconds then divide by ten to record frequency
7) calculate the wave speed using v = frequency x wavelength

Question 9

Hazard of calculating the speed on ripples on water surfaces

Answer 9

electrical components near water

could shock or damage equipment

can be controlled by securing electrical components before adding water,
taking care not to splash

Question 10

How do you calculate the speed of waves in a solid using a metal rod?

Answer 10

1) using the first finger and thumb on one hand, hold a metal rod at its middle point
2) hit one end of the rod with a hammer
3) hold a phone with an app which measures frequency near the end of the rod
4) record the peak frequency from the phone app
5) measure the length of the metal rod and record the length
6) calculate the speed of the sound waves in the rod using the frequency and wavelength recorded

Question 11

Describe one hazard of measuring the speed of waves in a solid

Answer 11

dropping rod or hammer

could cause injury from falling objects

can be avoided by holding equipment carefully

Question 12

a) A hand clap is made 480m from a wall and the echo is heard 3 seconds later.

Calculate the speed of sound

b) two microphones and an oscilloscope are used to determine the speed of sound.
The distance between the two microphones is 35 cm and it represents one wavelength. The frequency of the sound is 1 kHz.

Calculate the speed of sound in air.

Answer 12

a) speed = x/t = 960m/3s = 320m/s

b) v = frequency x wavelength = 1000Hz x 0.35m = 350m/s

Question 13

The frequency of a ripple tank paddle is 4 Hz and the wavelength of the ripples is 8cm.
Calculate the speed of the water waves in m/s.

Answer 13

v = f x wavelength = 4Hz x 0.08m = 0.32m/s

Question 14

Explain how you could obtain a value for the speed of sound in air with a small percentage error.

Answer 14

speed of sound in air is found by measuring a distance for the wave to travel and a time over which the distance is covered

having a large distance and a large time for these measurements using equipment with a high degree of accuracy will lead to a low percentage error

repeating the experiment multiple times and finding an average between the values will also give a more accurate value

Question 15

When determining the speed of waves in fluids using a ripple tank:

Describe the main errors in determining the speed of water waves in this investigation [3]

Answer 15

error in wavelength measured for the wave when distance being measured

error in frequency value of the wave from a generating source

error when taking time values with a stopwatch in order to find for the wave speed

Question 16

When determining the speed of waves in fluids using a ripple tank:

Explain how the investigation can be improved [4]

Answer 16

• more wavelengths gives a greater distance to measure
• with a measuring device of a constant accuracy
• so wavelength values will be more accurate since percentage error in value obtained will be less

-a metre ruler being used to measure a wavelength of 10cm with a 1cm scale gives a 10% error, whereas measuring 10 wavelengths would mean a 1% error

(measure more wavelengths)