Standing Waves Flashcards
Standing waves can form in air columns for instruments that are…
Open tubes or tubes with one closed end.
How do standing waves form in instruments?
A sound wave travels down the tube. It is reflected at the end. A wave travels in the opposite direction and the two waves superpose to form one standing wave.
In flutes what’s the longest wavelength?
1 wavelength = 2 * length (A N A)
In closed end clarinets what’s the longest wavelength?
1 wavelength = 4 * length (A N)
On the open ends of instruments, what are found?
Antinodes.
How do waves superpose when a string is plucked?
Progressive waves travel to the end of the string. They are reflected. When they meet waves of the same frequency travelling in the opposite direction, they superpose to form a standing wave.
What is fundamental frequency in guitar strings?
The lowest frequency with longest wavelength. WL = 2L
What are nodes?
When waves superpose in antiphase minimum displacement occurs as the waves cancel each other out.
What are antinodes?
In phase, displacement occurs at maximum as the amplitudes add to produce greatest displacement. Antinodes have the highest amplitude of oscillation.
How can rubber bands be used to measure wavelength and so velocity of standing waves?
Fix the end of a string and use a signal generator at 10Hz. Gradually increase the frequency. When it’s doubled, the amount of antinodes will double. Measure the wavelength by multiplying distance between nodes by two, and multiply this by frequency to find wave velocity.
How can resonance tubes be used to find the speed of sound?
Use a source of sound such as a tuning fork or a loudspeaker connected to a signal generator. This should be known frequency. For the first tube, the length plus end correction ‘k’ should be equal to a quarter of the wavelength. Increase depth until the amplitude increases again at 3/4 the wavelength. Take the second wavelength and subtract the first, before multiplying by two. Multiply this by the frequency to obtain the speed of sound.
How can the wavelengths of microwaves be found?
Place a microwave detector in the middle of a microwave and the reflector. The progressive waves of equal frequency will travel in opposite directions and superpose, forming nodes in antiphase. Move the detector to find this minimum signal. Then, move the detector until the signal is found again. Measure the distance with a metre rule and multiply by two to find the wavelength.
What allows for comparison between small and large values? (ie- a car speed and the speed of a wave)
The car’s speed may be smaller but providing the frequency of the wave is precise, this will not affect the speed of the wave significantly, allowing the two to be compared.
