 College Physics for AP® Courses

# 17.5Sound Interference and Resonance: Standing Waves in Air Columns

College Physics for AP® Courses17.5 Sound Interference and Resonance: Standing Waves in Air Columns

### Learning Objectives

By the end of this section, you will be able to:

• Define antinode, node, fundamental, overtones, and harmonics.
• Identify instances of sound interference in everyday situations.
• Describe how sound interference occurring inside open and closed tubes changes the characteristics of the sound, and how this applies to sounds produced by musical instruments.
• Calculate the length of a tube using sound wave measurements.

The information presented in this section supports the following AP® learning objectives and science practices:

• 6.D.1.1 The student is able to use representations of individual pulses and construct representations to model the interaction of two wave pulses to analyze the superposition of two pulses. (S.P. 1.1, 1.4)
• 6.D.1.2 The student is able to design a suitable experiment and analyze data illustrating the superposition of mechanical waves (only for wave pulses or standing waves). (S.P. 4.2, 5.1)
• 6.D.1.3 The student is able to design a plan for collecting data to quantify the amplitude variations when two or more traveling waves or wave pulses interact in a given medium. (S.P. 4.2)
• 6.D.3.1 The student is able to refine a scientific question related to standing waves and design a detailed plan for the experiment that can be conducted to examine the phenomenon qualitatively or quantitatively. (S.P. 2.1, 2.2, 4.2)
• 6.D.3.2 The student is able to predict properties of standing waves that result from the addition of incident and reflected waves that are confined to a region and have nodes and antinodes. (S.P. 6.4)
• 6.D.3.3 The student is able to plan data collection strategies, predict the outcome based on the relationship under test, perform data analysis, evaluate evidence compared to the prediction, explain any discrepancy and, if necessary, revise the relationship among variables responsible for establishing standing waves on a string or in a column of air. (S.P. 3.2, 4.1, 5.1, 5.2, 5.3)
• 6.D.3.4 The student is able to describe representations and models of situations in which standing waves result from the addition of incident and reflected waves confined to a region. (S.P. 1.2)
• 6.D.4.2 The student is able to calculate wavelengths and frequencies (if given wave speed) of standing waves based on boundary conditions and length of region within which the wave is confined, and calculate numerical values of wavelengths and frequencies. Examples should include musical instruments. (S.P. 2.2)
Figure 17.22 Some types of headphones use the phenomena of constructive and destructive interference to cancel out outside noises. (credit: JVC America, Flickr)

Interference is the hallmark of waves, all of which exhibit constructive and destructive interference exactly analogous to that seen for water waves. In fact, one way to prove something “is a wave” is to observe interference effects. So, sound being a wave, we expect it to exhibit interference; we have already mentioned a few such effects, such as the beats from two similar notes played simultaneously.

Figure 17.23 shows a clever use of sound interference to cancel noise. Larger-scale applications of active noise reduction by destructive interference are contemplated for entire passenger compartments in commercial aircraft. To obtain destructive interference, a fast electronic analysis is performed, and a second sound is introduced with its maxima and minima exactly reversed from the incoming noise. Sound waves in fluids are pressure waves and consistent with Pascal’s principle; pressures from two different sources add and subtract like simple numbers; that is, positive and negative gauge pressures add to a much smaller pressure, producing a lower-intensity sound. Although completely destructive interference is possible only under the simplest conditions, it is possible to reduce noise levels by 30 dB or more using this technique.

Figure 17.23 Headphones designed to cancel noise with destructive interference create a sound wave exactly opposite to the incoming sound. These headphones can be more effective than the simple passive attenuation used in most ear protection. Such headphones were used on the record-setting, around the world nonstop flight of the Voyager aircraft to protect the pilots’ hearing from engine noise.

Where else can we observe sound interference? All sound resonances, such as in musical instruments, are due to constructive and destructive interference. Only the resonant frequencies interfere constructively to form standing waves, while others interfere destructively and are absent. From the toot made by blowing over a bottle, to the characteristic flavor of a violin’s sounding box, to the recognizability of a great singer’s voice, resonance and standing waves play a vital role.

### Interference

Interference is such a fundamental aspect of waves that observing interference is proof that something is a wave. The wave nature of light was established by experiments showing interference. Similarly, when electrons scattered from crystals exhibited interference, their wave nature was confirmed to be exactly as predicted by symmetry with certain wave characteristics of light.

### Applying the Science Practices: Standing Wave

Figure 17.24 The standing wave pattern of a rubber tube attached to a doorknob.

Tie one end of a strip of long rubber tubing to a stable object (doorknob, fence post, etc.) and shake the other end up and down until a standing wave pattern is achieved. Devise a method to determine the frequency and wavelength generated by your arm shaking. Do your results align with the equation? Do you find that the velocity of the wave generated is consistent for each trial? If not, explain why this is the case.

This task will likely require two people. The frequency of the wave pattern can be found by timing how long it takes the student shaking the rubber tubing to move his or her hand up and down one full time. (It may be beneficial to time how long it takes the student to do this ten times, and then divide by ten to reduce error.) The wavelength of the standing wave can be measured with a meter stick by measuring the distance between two nodes and multiplying by two. This information should be gathered for standing wave patterns of multiple different wavelengths. As students collect their data, they can use the equation to determine if the wave velocity is consistent. There will likely be some error in the experiment yielding velocities of slightly different value. This error is probably due to an inaccuracy in the wavelength and/or frequency measurements.

Suppose we hold a tuning fork near the end of a tube that is closed at the other end, as shown in Figure 17.25, Figure 17.26, Figure 17.27, and Figure 17.28. If the tuning fork has just the right frequency, the air column in the tube resonates loudly, but at most frequencies it vibrates very little. This observation just means that the air column has only certain natural frequencies. The figures show how a resonance at the lowest of these natural frequencies is formed. A disturbance travels down the tube at the speed of sound and bounces off the closed end. If the tube is just the right length, the reflected sound arrives back at the tuning fork exactly half a cycle later, and it interferes constructively with the continuing sound produced by the tuning fork. The incoming and reflected sounds form a standing wave in the tube as shown.

Figure 17.25 Resonance of air in a tube closed at one end, caused by a tuning fork. A disturbance moves down the tube.
Figure 17.26 Resonance of air in a tube closed at one end, caused by a tuning fork. The disturbance reflects from the closed end of the tube.
Figure 17.27 Resonance of air in a tube closed at one end, caused by a tuning fork. If the length of the tube $LL size 12{L} {}$ is just right, the disturbance gets back to the tuning fork half a cycle later and interferes constructively with the continuing sound from the tuning fork. This interference forms a standing wave, and the air column resonates.
Figure 17.28 Resonance of air in a tube closed at one end, caused by a tuning fork. A graph of air displacement along the length of the tube shows none at the closed end, where the motion is constrained, and a maximum at the open end. This standing wave has one-fourth of its wavelength in the tube, so that $λ=4Lλ=4L size 12{λ=4L} {}$.

The standing wave formed in the tube has its maximum air displacement (an antinode) at the open end, where motion is unconstrained, and no displacement (a node) at the closed end, where air movement is halted. The distance from a node to an antinode is one-fourth of a wavelength, and this equals the length of the tube; thus, $λ=4Lλ=4L size 12{λ=4L} {}$. This same resonance can be produced by a vibration introduced at or near the closed end of the tube, as shown in Figure 17.29. It is best to consider this a natural vibration of the air column independently of how it is induced.

Figure 17.29 The same standing wave is created in the tube by a vibration introduced near its closed end.

Given that maximum air displacements are possible at the open end and none at the closed end, there are other, shorter wavelengths that can resonate in the tube, such as the one shown in Figure 17.30. Here the standing wave has three-fourths of its wavelength in the tube, or $L=(3/4)λ′L=(3/4)λ′ size 12{L= $$3/4$$ { {λ}} sup { ' }} {}$, so that $λ′=4L/3λ′=4L/3 size 12{ { {λ}} sup { ' }=4L/3} {}$. Continuing this process reveals a whole series of shorter-wavelength and higher-frequency sounds that resonate in the tube. We use specific terms for the resonances in any system. The lowest resonant frequency is called the fundamental, while all higher resonant frequencies are called overtones. All resonant frequencies are integral multiples of the fundamental, and they are collectively called harmonics. The fundamental is the first harmonic, the first overtone is the second harmonic, and so on. Figure 17.31 shows the fundamental and the first three overtones (the first four harmonics) in a tube closed at one end.

Figure 17.30 Another resonance for a tube closed at one end. This has maximum air displacements at the open end, and none at the closed end. The wavelength is shorter, with three-fourths $λ′λ′ size 12{ { {λ}} sup { ' }} {}$ equaling the length of the tube, so that $λ′=4L/3λ′=4L/3 size 12{ { {λ}} sup { ' }=4L/3} {}$. This higher-frequency vibration is the first overtone.
Figure 17.31 The fundamental and three lowest overtones for a tube closed at one end. All have maximum air displacements at the open end and none at the closed end.

The fundamental and overtones can be present simultaneously in a variety of combinations. For example, middle C on a trumpet has a sound distinctively different from middle C on a clarinet, both instruments being modified versions of a tube closed at one end. The fundamental frequency is the same (and usually the most intense), but the overtones and their mix of intensities are different and subject to shading by the musician. This mix is what gives various musical instruments (and human voices) their distinctive characteristics, whether they have air columns, strings, sounding boxes, or drumheads. In fact, much of our speech is determined by shaping the cavity formed by the throat and mouth and positioning the tongue to adjust the fundamental and combination of overtones. Simple resonant cavities can be made to resonate with the sound of the vowels, for example. (See Figure 17.32.) In boys, at puberty, the larynx grows and the shape of the resonant cavity changes giving rise to the difference in predominant frequencies in speech between men and women.

Figure 17.32 The throat and mouth form an air column closed at one end that resonates in response to vibrations in the voice box. The spectrum of overtones and their intensities vary with mouth shaping and tongue position to form different sounds. The voice box can be replaced with a mechanical vibrator, and understandable speech is still possible. Variations in basic shapes make different voices recognizable.

Now let us look for a pattern in the resonant frequencies for a simple tube that is closed at one end. The fundamental has $λ=4Lλ=4L size 12{λ=4L} {}$, and frequency is related to wavelength and the speed of sound as given by:

$vw=fλ.vw=fλ. size 12{v rSub { size 8{w} } =fλ} {}$
17.28

Solving for $ff size 12{f} {}$ in this equation gives

$f=vwλ=vw4L,f=vwλ=vw4L, size 12{f= { {v rSub { size 8{w} } } over {λ} } = { {v rSub { size 8{w} } } over {4L} } } {}$
17.29

where $vwvw size 12{v rSub { size 8{w} } } {}$ is the speed of sound in air. Similarly, the first overtone has $λ′=4L/3λ′=4L/3 size 12{ { {λ}} sup { ' }=4L/3} {}$ (see Figure 17.31), so that

$f ′ = 3 v w 4L = 3f. f ′ = 3 v w 4L = 3f. size 12{f'=3 { {v rSub { size 8{w} } } over {4L} } =3f} {}$
17.30

Because $f′=3ff′=3f size 12{ { {f}} sup { ' }=3f} {}$, we call the first overtone the third harmonic. Continuing this process, we see a pattern that can be generalized in a single expression. The resonant frequencies of a tube closed at one end are

$fn=nvw4L ,n=1,3,5,fn=nvw4L ,n=1,3,5, size 12{n=1,3,5 "." "." "." } {}$
17.31

where $f1f1 size 12{f rSub { size 8{1} } } {}$ is the fundamental, $f3f3 size 12{f rSub { size 8{3} } } {}$ is the first overtone, and so on. It is interesting that the resonant frequencies depend on the speed of sound and, hence, on temperature. This dependence poses a noticeable problem for organs in old unheated cathedrals, and it is also the reason why musicians commonly bring their wind instruments to room temperature before playing them.

### Example 17.5

#### Find the Length of a Tube with a 128 Hz Fundamental

(a) What length should a tube closed at one end have on a day when the air temperature, is $22.0ºC22.0ºC$, if its fundamental frequency is to be 128 Hz (C below middle C)?

(b) What is the frequency of its fourth overtone?

#### Strategy

The length $LL size 12{L} {}$ can be found from the relationship in $fn=nvw4Lfn=nvw4L size 12{f rSub { size 8{n} } =n { {v rSub { size 8{w} } } over {4L} } } {}$, but we will first need to find the speed of sound $vwvw size 12{v rSub { size 8{w} } } {}$.

#### Solution for (a)

(1) Identify knowns:

• the fundamental frequency is 128 Hz
• the air temperature is $22.0ºC22.0ºC$

(2) Use $fn=nvw4Lfn=nvw4L size 12{f rSub { size 8{n} } =n { {v rSub { size 8{w} } } over {4L} } } {}$ to find the fundamental frequency ($n=1n=1$).

$f1=vw4Lf1=vw4L size 12{f rSub { size 8{1} } = { {v rSub { size 8{w} } } over {4L} } } {}$
17.32

(3) Solve this equation for length.

$L=vw4f1L=vw4f1 size 12{L= { {v rSub { size 8{w} } } over {4f rSub { size 8{1} } } } } {}$
17.33

(4) Find the speed of sound using $vw=331 m/sT273 Kvw=331 m/sT273 K size 12{v rSub { size 8{w} } = left ("331"" m/s" right ) sqrt { { {T} over {"273 K"} } } } {}$ .

$vw=331 m/s295 K273 K=344 m/svw=331 m/s295 K273 K=344 m/s size 12{v rSub { size 8{w} } = left ("331"" m/s" right ) sqrt { { {T} over {"273 K"} } } = left ("331"" m/s" right ) sqrt { { {"295 K"} over {"273 K"} } } ="344"" m/s"} {}$
17.34

(5) Enter the values of the speed of sound and frequency into the expression for $LL$.

$L=vw4f1=344 m/s4128 Hz=0.672 mL=vw4f1=344 m/s4128 Hz=0.672 m$
17.35

#### Discussion on (a)

Many wind instruments are modified tubes that have finger holes, valves, and other devices for changing the length of the resonating air column and hence, the frequency of the note played. Horns producing very low frequencies, such as tubas, require tubes so long that they are coiled into loops.

#### Solution for (b)

(1) Identify knowns:

• the first overtone has $n = 3n = 3$
• the second overtone has $n = 5n = 5$
• the third overtone has $n = 7n = 7$
• the fourth overtone has $n = 9n = 9$

(2) Enter the value for the fourth overtone into $fn=nvw4Lfn=nvw4L size 12{f rSub { size 8{n} } =n { {v rSub { size 8{w} } } over {4L} } } {}$.

$f9=9vw4L=9f1=1.15 kHzf9=9vw4L=9f1=1.15 kHz size 12{f rSub { size 8{9} } =9 { {v rSub { size 8{w} } } over {4L} } =9f rSub { size 8{1} } ="1150"" Hz"} {}$
17.36

#### Discussion on (b)

Whether this overtone occurs in a simple tube or a musical instrument depends on how it is stimulated to vibrate and the details of its shape. The trombone, for example, does not produce its fundamental frequency and only makes overtones.

Another type of tube is one that is open at both ends. Examples are some organ pipes, flutes, and oboes. The resonances of tubes open at both ends can be analyzed in a very similar fashion to those for tubes closed at one end. The air columns in tubes open at both ends have maximum air displacements at both ends, as illustrated in Figure 17.33. Standing waves form as shown.

Figure 17.33 The resonant frequencies of a tube open at both ends are shown, including the fundamental and the first three overtones. In all cases the maximum air displacements occur at both ends of the tube, giving it different natural frequencies than a tube closed at one end.

Based on the fact that a tube open at both ends has maximum air displacements at both ends, and using Figure 17.33 as a guide, we can see that the resonant frequencies of a tube open at both ends are:

$fn=nvw2L,n=1, 2, 3...,fn=nvw2L,n=1, 2, 3...,$
17.37

where $f1f1 size 12{f rSub { size 8{1} } } {}$ is the fundamental, $f2f2 size 12{f rSub { size 8{2} } } {}$ is the first overtone, $f3f3 size 12{f rSub { size 8{3} } } {}$ is the second overtone, and so on. Note that a tube open at both ends has a fundamental frequency twice what it would have if closed at one end. It also has a different spectrum of overtones than a tube closed at one end. So if you had two tubes with the same fundamental frequency but one was open at both ends and the other was closed at one end, they would sound different when played because they have different overtones. Middle C, for example, would sound richer played on an open tube, because it has even multiples of the fundamental as well as odd. A closed tube has only odd multiples.

### Applying the Science Practices: Closed- and Open-Ended Tubes

Strike an open-ended length of plastic pipe while holding it in the air. Now place one end of the pipe on a hard surface, sealing one opening, and strike it again. How does the sound change? Further investigate the sound created by the pipe by striking pipes of different lengths and composition.

When the pipe is placed on the ground, the standing wave within the pipe changes from being open on both ends to being closed on one end. As a result, the fundamental frequency will change from $f= v 2L f= v 2L$ to $f= v 4L f= v 4L$. This decrease in frequency results in a decrease in observed pitch.

### Real-World Applications: Resonance in Everyday Systems

Resonance occurs in many different systems, including strings, air columns, and atoms. Resonance is the driven or forced oscillation of a system at its natural frequency. At resonance, energy is transferred rapidly to the oscillating system, and the amplitude of its oscillations grows until the system can no longer be described by Hooke’s law. An example of this is the distorted sound intentionally produced in certain types of rock music.

Wind instruments use resonance in air columns to amplify tones made by lips or vibrating reeds. Other instruments also use air resonance in clever ways to amplify sound. Figure 17.34 shows a violin and a guitar, both of which have sounding boxes but with different shapes, resulting in different overtone structures. The vibrating string creates a sound that resonates in the sounding box, greatly amplifying the sound and creating overtones that give the instrument its characteristic flavor. The more complex the shape of the sounding box, the greater its ability to resonate over a wide range of frequencies. The marimba, like the one shown in Figure 17.35 uses pots or gourds below the wooden slats to amplify their tones. The resonance of the pot can be adjusted by adding water.

Figure 17.34 String instruments such as violins and guitars use resonance in their sounding boxes to amplify and enrich the sound created by their vibrating strings. The bridge and supports couple the string vibrations to the sounding boxes and air within. (credits: guitar, Feliciano Guimares, Fotopedia; violin, Steve Snodgrass, Flickr)
Figure 17.35 Resonance has been used in musical instruments since prehistoric times. This marimba uses gourds as resonance chambers to amplify its sound. (credit: APC Events, Flickr)

We have emphasized sound applications in our discussions of resonance and standing waves, but these ideas apply to any system that has wave characteristics. Vibrating strings, for example, are actually resonating and have fundamentals and overtones similar to those for air columns. More subtle are the resonances in atoms due to the wave character of their electrons. Their orbitals can be viewed as standing waves, which have a fundamental (ground state) and overtones (excited states). It is fascinating that wave characteristics apply to such a wide range of physical systems.

Describe how noise-canceling headphones differ from standard headphones used to block outside sounds.

How is it possible to use a standing wave's node and antinode to determine the length of a closed-end tube?

Sound

This simulation lets you see sound waves. Adjust the frequency or volume and you can see and hear how the wave changes. Move the listener around and hear what she hears.

Figure 17.36

### Applying the Science Practices: Variables Affecting Superposition

In the PhET Interactive Simulation above, select the tab titled ‘Two Source Interference.’ Within this tab, manipulate the variables present (frequency, amplitude, and speaker separation) to investigate the relationship the variables have with the superposition pattern constructed on the screen. Record all observations.

As frequency is increased, the wavelength within the standing wave pattern will decrease. This results in an increase in nodes and antinodes, as represented in the applet by black and white surfaces. As amplitude is decreased, the contrast between black and white surfaces decreases, in demonstration of the decrease in sound level. There is no impact on sound wavelength, or number of nodes and antinodes shown. Increasing the speaker separation will affect the wavelength constructed. However, by separating the speakers, the number of nodes and antinodes within the applet will increase, as the waves are able to interfere over a greater distance.

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