Standing Waves and Resonance
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Acoustic Standing Waves and resonance Leacture Dr.khitam Y. Elwasife Acoustics is the science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound . A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics technology may be called an acoustical engineer. The application of acoustics is present in almost all aspects of modern society with the most obvious being the audio and noise control industries Quantization When waves are combined in systems with boundary conditions, only certain allowed frequencies can exist. We say the frequencies are quantized. Quantization is at the heart of quantum mechanics. The analysis of waves under boundary conditions explains many quantum phenomena. Quantization can be used to understand the behavior of the wide array of musical instruments that are based on strings and air columns. Waves can also combine when they have different frequencies. Waves vs. Particles Waves are very different from particles. Particles have zero size. Waves have a characteristic size – their wavelength. Multiple particles must exist at Multiple waves can combine at one point in different locations. the same medium – they can be present at the same location. Introduction Superposition Principle Waves can be combined in the same location in space. To analyze these wave combinations, use the superposition principle: If two or more traveling waves are moving through a medium, the resultant value of the wave function at any point is the algebraic sum of the values of the wave functions of the individual waves. Waves that obey the superposition principle are linear waves. For mechanical waves, linear waves have amplitudes much smaller than their wavelengths. Superposition and Interference Two traveling waves can pass through each other without being destroyed or altered. A consequence of the superposition principle. The combination of separate waves in the same region of space to produce a resultant wave is called interference. The term means waves pass through each other. Superposition Example Two pulses are traveling in opposite directions (a). The wave function of the pulse moving to the right is y1 and for the one moving to the left is y2. The pulses have the same speed but different shapes. The displacement of the elements is positive for both. When the waves start to overlap (b), the resultant wave function is y1 + y2. Superposition Example, When crest meets crest (c) the resultant wave has a larger amplitude than either of the original waves. The two pulses separate (d). They continue moving in their original directions. The shapes of the pulses remain unchanged. This type of superposition is called constructive interference. Destructive Interference Example Two pulses traveling in opposite directions. Their displacements are inverted with respect to each other. When these pulses overlap, the resultant pulse is y1 + y2. This type of superposition is called destructive interference. Types of Interference Constructive interference occurs when the displacements caused by the two pulses are in the same direction. The amplitude of the resultant pulse is greater than either individual pulse. Destructive interference occurs when the displacements caused by the two pulses are in opposite directions. The amplitude of the resultant pulse is less than either individual pulse. Radiation from a monopole source A monopole is a source which radiates sound equally well in all directions. The simplest example of a monopole source would be a sphere whose radius alternately expands and contracts sinusoidally. The monopole source creates a sound wave by alternately introducing and removing fluid into the surrounding area. A boxed loudspeaker at low frequencies acts as a monopole. The pattern for a monopole source is shown in the figure at right. Radiation from a dipole source A dipole source consists of two monopole sources of equal strength but opposite phase and separated by a small distance compared with the wavelength of sound. While one source expands the other source contracts. The result is that the fluid (air) near the two sources sloshes back and forth to produce the sound. A sphere which oscillates back and forth acts like a dipole source, as does an unboxed loudspeaker . A dipole source does not radiate sound in all directions equally. The pattern shown at right; there are two regions where sound is radiated very well, and two regions where sound cancels. Radiation from a r quadrupole source Superposition of Sinusoidal Waves Assume two waves are traveling in the same direction in a linear medium, with the same frequency, wavelength and amplitude. The waves differ only in phase: . y1 = A sin (kx - wt) . y2 = A sin (kx - wt + f) . y = y1+y2 = 2A cos (f /2) sin (kx - wt + f /2) The resultant wave function, y, is also sinusoida.l The resultant wave has the same frequency and wavelength as the original waves. The amplitude of the resultant wave is 2A cos (f / 2) . The phase of the resultant wave is f / 2. Sinusoidal Waves with Constructive-destructive Interference When f = 0, then cos (f/2) = 1 The amplitude of the resultant wave is 2A. The crests of the two waves are at the same location in space. The waves are everywhere in phase. The waves interfere constructively. In general, constructive interference occurs when cos (Φ/2) = ± 1. That is, when Φ = 0, 2π, 4π, … rad . When Φ is an even multiple of π When f = p, then cos (f/2) = 0 . Also any odd multiple of p The amplitude of the resultant wave is 0. See the straight red-brown line in the figure. The waves interfere destructively Sinusoidal Waves, General Interference When f is other than 0 or an even multiple of p, the amplitude of the resultant is between 0 and 2A. The wave functions still add The interference is neither constructive nor destructive. Constructive interference occurs when f = np where n is an even integer (including 0). Amplitude of the resultant is 2A Destructive interference occurs when f = np where n is an odd integer. Amplitude is 0 General interference occurs when 0 < f < np . Amplitude is 0 < Aresultant < 2A Interference in Sound Waves Sound from S can reach R by two different paths. The distance along any path from speaker to receiver is called the path length, r. The lower path length, r1, is fixed. The upper path length, r2, can be varied. Whenever Dr = |r2 – r1| = n l, constructive interference occurs.a maximum in sound intensity is detected at the receiver. Whenever Dr = |r2 – r1| = (nl)/2 (n is odd), destructive interference occurs. No sound is detected at the receiver. A phase difference may arise between two waves generated by the same source when they travel along paths of unequal lengths. Standing wave When a sound wave hits a wall, it is partially absorbed and partially reflected. A person far enough from the wall will hear the sound twice. This is an echo. In a small room the sound is also heard more than once, but the time differences are so small. This is known as reverberation. Music is the Sound that is produced by instruments or voices. To play most musical instruments you have to create standing waves on a string or in a tube or pipe. The pitch of the sound is equal to the frequency of the wave. The higher the frequency, the higher is the pitch. Wind instruments produce sounds by means of vibrating air columns. To play a wind instrument you push the air in a tube with your mouth. The air in the tube starts to vibrate with the same frequency as your lips. Resonance increases the amplitude of the vibrations, which can form standing waves in the tube. The length of the air column determines the resonant frequencies. The mouth or the reed produces a mixture if different frequencies, but the resonating air column amplifies only the natural frequencies. The shorter the tube the higher is the pitch. Many instruments have holes, whose opening and closing controls the effective pitch. Standing Waves--Free/Open Ends This shows longitudinal standing waves, such as in sound waves in a pipe open at both ends. L The top wave is the fundamental or first harmonic. It has displacement antinodes at each end, and a displacement node in the middle. When the displacement is large and to the right at one end, it is large and to the left at the other, and they're separated by one node. In other words, it is one- half wavelength from one end to the other. The next two waves show the next two harmonics, the second and the third. The pipe holds, respectively, two and three half-wavelengths, and shorter Type of air column wavelengths correspond to higher frequencies--the Closed at both ends frequencies are two and three times the Closed at one end open at the other fundamental. Open at both ends The longest standing wave in a tube of length L with two open ends has displacement antinodes (pressure nodes) at both ends. It is called the fundamental. The next longest standing wave in a tube The longest standing wave in a tube of length L with of length L with two open ends is the two open ends has displacement antinodes second harmonic. (pressure nodes) at both ends. It is called the fundamental. It also has displacement antinodes at each end.An integer number of half wavelength have to fit into the tube of length L. L = nλ/2, λ = 2L/n, f = v/λ = nv/(2L). For a tube with two open ends all frequencies fn = nv/(2L) = nf1, with n equal to an integer, are natural frequencies.