4.4 - Waves pt 2 Flashcards
What is a stationary/standing wave?
A stationary wave is the superposition of two progressive waves with the same wavelength, moving in opposite directions. It STORES energy unlike progressive waves.
How are stationary waves formed?
You get a stationary wave when a progressive wave is reflected off a fixed boundary, producing a wave in anti-phase to the other (with the same frequency and amplitude). The two waves travel in opposite directions, interfere, and superpose producing a wave that is stationary and ‘bops’ at these resonant frequencies.
What is a node?
A point on the standing/stationary wave where the amplitude is zero.
What is an antinode?
A point on the standing/stationary wave where the amplitude is at a maximum.
What do you get at a fixed boundary in terms of standing waves? (e.g., closed end of tube)
A NODE
What do you get at an open boundary in terms of standing waves? (e.g., open end of tube)
An ANTINODE
Outline an experiment you can perform to measure the speed of sound.
You can create a closed-end pipe by placing a hollow tube into a measuring cylinder of water. Choose a tuning fork and note down the frequency of the sound it produces (it’ll be stamped on the side of it). Gently tap the tuning fork and hold it just above the hollow tube. The sound waves produced by the fork travel down the tube and get reflected (and form a node) at the air/water boundary. Move the tube up and down until you find the shortest distance between the top and the water level that the sound from the fork resonates at (when sound is loudest). This distance is a quarter of the wavelength (λ/4). Multiply this by 4 and use v = fλ to work out the speed of sound in air.
What is a harmonic?
A wave whose frequency is an integral (whole-number) multiple of the frequency of some reference wave, fundamental then 1st harmonic, 2nd harmonic, 3rd, and so on…
At resonant frequencies how many wavelengths fit onto the string?
An exact/whole number of HALF wavelengths fits onto the string.
What is the rule for working out the harmonic when there is a node at each end? (two fixed ends)
The harmonic number = number of antinodes or just count the full circles.
What is the rule for working out the harmonic when there is an antinode at each end? (two open ends)
The harmonic number = the number of nodes or just count the full circles.
What is the rule for working out the harmonic when there is a node at one end and an antinode at the other? (one fixed end and one open end)
The harmonic number = (number of nodes + antinodes) - 1 or just count the full circles and multiply by 2.
What types of waves are produced in stringed instruments like violins and guitars?
Transverse stationary waves. Your finger or the bow sets the string vibrating at the point of contact, waves are sent out in both directions and are reflected back at both ends.
What types of waves are produced in a wind instrument like a flute or oboe (or other air column)?
Longitudinal stationary waves. If the instrument has a closed end, a node will form here. You will get the lowest resonant frequency when the length (L) of the pipe is a quarter of the wavelength. If the instrument has an open end, an antinode will form here meaning you will get the lowest resonant frequency when the length (L) of the pipe is half the wavelength.
What is the distance in λ between adjacent nodes or antinodes?
λ/2, half the wavelength.
What is the distance in λ between an adjacent node and antinode?
λ/4, a quarter of the wavelength.
What are the key differences between stationary and progressive waves?
Progressive waves transfer energy through space while stationary waves only store energy.
