Sound (Researched by J

SOUND- Unit 2

Sound (researched by J.Redmond-08-23-06)

Notes for Teachers of the PP&T Unit on Sound

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Sound is produced when the air is disturbed in some way, for example by a vibrating object. A speaker cone from a hi-fi system serves as a good illustration. It may be possible to see the movement of a bass speaker cone, providing it is producing very low frequency sound. As the cone moves forward the air immediately in front is compressed causing a slight increase in air pressure, it then moves back past its rest position and causes a reduction in the air pressure (rarefaction). The process continues so that a wave of alternating high and low pressure is radiated away from the speaker cone at the speed of sound.

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The speculation that sound is a wave phenomenon grew out of observations of water waves. The rudimentary notion of a wave is an oscillatory disturbance that moves away from some source and transports no noticeable amount of matter over large distances of propagation(wave building). The possibility that sound exhibits similar behavior was emphasized, for example, by the Greek philosopher Chrysippus (c. 240 B.C.), by the Roman architect and engineer Vetruvius (c. 25 B.C.), and by the Roman philosopher Boethius (A.D. 480-524). The wave interpretation was also consistent with Aristotle's (384-322 B.C.) statement to the effect that air motion is generated by a source, "thrusting forward in like manner the adjoining air, to that the sound travels unaltered in quality as far as the disturbance of the air manages to reach."

An important experimental result, inferred with reasonable conclusiveness by the early seventeenth century, with antecedents dating back to Pythagoras (c. 550 B.C.) and perhaps further, is that the air motion generated by a vibrating body sounding a single musical note is also vibratory and of the same frequency as the body. The history of this is intertwined with the development of the laws for the natural frequencies of vibrating strings and of the physical interpretation of musical consonances. Principal roles were played by Marin Mersenne (1588-1648), a French natural philosopher often referred to as the "father of acoustics," and by Galileo Galilei (1564-1642), whose Mathematical Discourses Concerning Two New Sciences (1638) contained the most clear statement and discussion given up until then of the frequency equivalence.

Mersenne's description in his Harmonic universelle (1636) of the first absolute determination of the frequency of an audible tone (at 84 Hz) implies that he already demonstrated that the absolute-frequency ratio of two vibrating strings, radiating a musical tone and its octave, is as 1 : 2. The perceived harmony (consonance) of two such notes would be explained if the ratio of the air oscillation frequencies is also 1 : 2, which in turn is consistent with the source-air-motion-frequency-equivalence hypothesis.

The comparison with water waves was strengthened by the belief that air motion associated with musical sounds is oscillatory and by the observation that sound travels with a defined speed. Another matter of common knowledge was that sound bends around corners, which suggested diffraction, a phenomenon often observed in water waves. Also, Robert Boyle's (1640) classic experiment on the sound radiation by a ticking watch in a partially evacuated (partial vacuum) glass vessel provided evidence that air is necessary, either for the production or transmission of sound.

The wave viewpoint was not unanimous, however. Gassendi (a contemporary of Mersenne and Galileo), for example, argued that sound is due to a stream of "atoms" emitted by the sounding body; velocity of sound is the speed of atoms; frequency is number emitted per unit time.

The apparent conflict between ray and wave theories played a major role in the history of the sister science optics, but the theory of sound developed almost from its beginning as a wave theory. When ray concepts were used to explain acoustic phenomena, as was done, for example, by Reynolds and Rayleigh, in the nineteenth century, they were regarded, either implicitly or explicitly, as mathematical approximations to a then well-developed wave theory; the successful incorporation of geometrical optics into a more comprehensive wave theory had demonstrated that viable approximate models of complicated wave phenomena could be expressed in terms of ray concepts. (This recognition has strongly influenced twentieth-century developments in architectural acoustics, underwater acoustics, and noise control.)

The mathematical theory of sound propagation began with Isaac Newton(1642-1727), whose Principia (1686) included a mechanical interpretation of sound as being "pressure" pulses transmitted through neighboring fluid particles. Accompanying diagrams illustrated the diverging of wave fronts after passage through a slit. The mathematical analysis was limited to waves of constant frequency, employed a number of strange devices and approximations, and suffered from an incomplete definition of terminology and concepts. It was universally acknowledged by his successors as difficult to decipher, but, once deciphered, it is recognizable as a development consistent with more modern treatments. Some textbook writers, perhaps for pedagogical reasons, stress that Newton's one quantitative result that could then be compared with experiment, i.e., the speed of sound, was too low by about 16 percent. The reason for the discrepancy and how it was resolved is discussed below (Sec. 1-4 of Pierce's book), but it is a relatively minor aspect of the overall theory, whose resolution required concepts and experimental results that came much later.

Substantial progress toward the development of a viable theory of sound propagation resting on firmer mathematical and physical concepts was made during the eighteenth century by Euler (1707-1783), Lagrange (1736-1813), and d'Alembert (1717-1783). During this era, continuum physics, or field theory, began to receive a definite mathematical structure. The wave equation emerged in a number of contexts, including the propagation of sound in air. The theory ultimately proposed for sound in the eighteenth century was incomplete from many standpoints, but modern theories of today can be regarded for the most part as refinements of that developed by Euler and his contemporaries.

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Acoustics is the science of sound, including its production, transmission, and effects. In present usage, the term sound implies not only phenomena in air responsible for the sensation of hearing but also whatever else is governed by analogous physical principles. Thus, disturbances with frequencies too low (infrasound) or too high (ultrasound) to be heard by a normal person are also regarded as sound. One may speak of underwater sound, sound in solids, or structure-borne sound. Acoustics is distinguished from optics in that sound is a mechanical, rather than an electromagnetic, wave motion.

The broad scope of acoustics as an area of interest and endeavor can be ascribed to a variety of reasons. First, there is the ubiquitous nature of mechanical radiation, generated by natural causes and by human activity. Then, there is the existence of the sensation of hearing, of the human vocal ability, of communication via sound, along with the variety of psychological influences sound has on those who hear it. Such areas as speech, music, sound recording and reproduction, telephony, sound reinforcement, audiology, architectural acoustics, and noise control have strong association with the sensation of hearing. That sound is a means of transmitting information, irrespective of our natural ability to hear, is also a significant factor, especially in underwater acoustics. A variety of applications, in basic research and in technology, exploit the fact that the transmission of sound is affected by, and consequently gives information concerning, the medium through which it passes and intervening bodies and inhomogeneities. The physical effects of sound on substances and bodies with which it interacts present other areas of concern and of technical application.

Some indication of the scope of acoustics and of the disciplines with which it is associated can be found in The first annular ring depicts the traditional subdivisions of acoustics, and the outer ring names technical and artistic fields to which acoustics may be applied. (The chart is not intended to be complete, nor should any rigid interpretation be placed on the depicted proximity of any subdivision to a technical field. A detailed listing of acoustical topics can be found in the index classification scheme reprinted with the index of each volume of the Journal of the Acoustical Society of America.)

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Sound is the quickly varying pressure wave within a medium. We usually mean audible sound, which is the sensation (as detected by the ear) of very small rapid changes in the air pressure above and below a static value. This “static” value is atmospheric pressure (about 100,000 Pascals) which does nevertheless vary slowly, as shown on a barometer. Associated with the sound pressure wave is a flow of energy.

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Hewitt, Paul - Conceptual Physics , Prentice Hall (2006)

Sound- Unit IV-Chapter 26 (Pgs. 390 to 403)

26.1 - The Origin of sound

26.2 - Sound in Air

26.3 - Media That Transmit Sound

26.4 - Speed of Sound

26.5 - Loudness

26.6 - Forced Vibration

26.7 - Natural Frequency

26.8 - Resonance

26.9 - Interference

26.10 - Beats

STS - Noise and Your Health

Tacoma Narrows Bridge

Ultrasound Imaging

Robertson, William - Stop Faking It... Sound NSTA Press (2003)

Chapter 1 - Stop Children, What's That Sound"

Chapter 2 - Waving Strings

Chapter 3 - How sound Gets Around

Chapter 4 - Harmonic Convergence

Chapter 5 - Waves Do Basic Math - Adding and Subtracting

Chapter 6 - The Hills Are Alive

Chapter 7 - Listening Devices

PP&T - Activities List for Sound

12.01 - Auditory System

12.02 - Introduction to Sound

12.03 - Resonance and Interference

12.04 - Doppler Effect

Operation Physics - Sound (Donald Kirwan et.al) 1992

See Binder

Operation Primary Physical Science (Donald and Gayle Kirwan -LSU)

Sound and Music

1. Sound is produced by vibrating matter.

2. Sound is the propagation of longitudinal waves through matter.

3. The speed of sound varies depending on the medium through which it is traveling.

4. A variety of different methods are used to cause sound vibrations in musical instruments: striking, plucking, stroking, or blowing.

5. Pitch (highness or lowness of sound) is related to the frequency of vibrations.

6. Loudness is related to amplitude of vibrations, size of vibrating object, and/or number of vibrating objects.

7. Musical sound has tone (harmonic content, quality).

8. Resonance is the inducing of vibrations of a natural rate by a vibrating source having the same frequency. (In a like sense, interference is inducing vibrations of a natural rate by a vibrating source having a different frequency)

Student Sound Lessons

Sound - Unit 2

2.01 - Speed of Sound

Background

We will start our work on sound by working with speed, a concept you should be familiar with. In the history of sound study (sometimes called acoustics) the speed of sound took quite a while to determine. We are going to present you with two methods for finding the speed of sound in air. We say, "in air", because this is how we normally sense sound. We can definitely hear sound through other media like water or various metals. We'll work with these other media later on in the unit. We get bombarded by sound almost every minute of every day. To isolate one sound source from another and determine its speed, may pose a challenge. But challenges are the forces that move scientists to discover new things. If you already know the speed of sound in air, forget it for the time being and work on finding sound's speed through experiment.

The first activity will be done outside with a starter's pistol firing a blank round. Make sure your group takes the temperature of the air at the position where the starter's pistol will be fired. You will be taking time readings at 50, 100, 150, 200 m away from the starter's pistol. Your job will be to start the watch when you see the smoke and stop it when you hear the sound. This may take some practice and a few trials at each distance. Each lab group will take its own times and we'll compare with the other groups later, back in the classroom.

Measuring the Speed of Sound in Air Using a Starter’s Pistol

Aim

To calculate the speed of sound in air by collecting and graphing the average time taken for sound to travel a systematically varied distance.

Apparatus

Starter’s pistol, at least 12 ‘caps’ and ear muffs for starter/s,

Trundle wheel (or measuring tape), Stop watches (x 10), Thermometer (-10 to 100 oC)

Method

1. Use a trundle wheel to measure out and mark distances of 50 m, 100m, 150m, and 200m (and beyond if possible) in an appropriate area.
2. The starter stands on the zero marker, gives a pre-arranged signal to ensure that the timers are ready and then fires the pistol.
3. The students use their stopwatches to measure the time between seeing the pistol smoke and hearing the sound.
4. Repeat this procedure twice at this distance.
5. Repeat all of the above steps at the other distances specified in step 1.

Analysis

1. Record your individual results in the table similar to that shown below.

Air Temperature: oC

1. Collate the class data when you get back to class and identify and discard outliers.
2. Use the class data to produce an Excel chart of time (s) versus distance (m). Time is the dependent variable and should be plotted on the Y-axis.
3. Determine the gradient of this graph and manipulate it to determine the speed of sound in air.

Questions

1. What uncertainty is associated with your distance measurements? Does this vary, as the distance gets greater? Why? Calculate the average percentage uncertainty for your distance measurements.
2. Why do you take an average of the timing data? What uncertainty is associated with your timing measurements? Does the uncertainty vary with distance? Why? Calculate the average percentage uncertainty of your timing measurements.
3. Use the average percentage uncertainties to calculate the percentage uncertainty associated with your calculated value for the speed of sound in air.
4. Why is the temperature of the air when you did the prac recorded? What effect does it have?

Conclusion (write a conclusion for the experiment)

Please note that this document is based on one prepared by Phil Noonan from St. Bedes. The original is available on the STAV-AIP Conference Proceedings 2000 CD-ROM or at

Activity 2, finding the speed of sound in the class room with a computer and a microphone. Follow directions from the experiment write-up.

Speed of Sound

Compared to most things you study in the physics lab, sound waves travel very fast. It is fast enough that measuring the speed of sound is a technical challenge. One method you could use would be to time an echo. For example, if you were in an open field with a large building a quarter of a kilometer away, you could start a stopwatch when a loud noise was made and stop it when you heard the echo. You could then calculate the speed of sound.

To use the same technique over short distances, you need a faster timing system, such as a computer. In this experiment you will use this technique with a Microphone connected to a computer to determine the speed of sound at room temperature. The Microphone will be placed next to the opening of a hollow tube. When you make a sound by snapping your fingers next to the opening, the computer will begin collecting data. After the sound reflects off the opposite end of the tube, a graph will be displayed showing the initial sound and the echo. You will then be able to determine the round trip time and calculate the speed of sound.

Figure 1

OBJECTIVES

• Measure how long it takes sound to travel down and back in a long tube.
• Determine the speed of sound.
• Compare the speed of sound in air to the accepted value.

MATERIALS

PRELIMINARY QUESTION

1.A common way to measure the distance to lightning is to start counting, one count per second, as soon as you see the flash. Stop counting when you hear the thunder and divide by five to get the distance in miles. Use this information to estimate the speed of sound in m/s.

PROCEDURE

1.Connect the Vernier Microphone to Channel 1 of the interface.

2.Use a thermometer or temperature probe to measure the air temperature of the classroom and record the value in the data table.

3.Open the file “24 Speed of Sound” in the Physics with Computers folder. A graph of sound level vs. time will be displayed.

4.Close the end of the tube. This can be done by inserting a plug or standing a book against the end so it is sealed. Measure and record the length of the tube in your data table.

5.Place the Microphone as close to the end of the long tube as possible, as shown in Figure 2. Position it so that it can detect the initial sound and the echo coming back down the tube.

Figure 2

6.Click to begin data collection. Snap your fingers near the opening of the tube. You can instead clap your hands or strike two pieces of wood together. This sharp sound will trigger the interface to begin collecting data.

7.If you are successful, the graph will resemble the one below. Repeat your run if necessary. The second set of vibrations with appreciable amplitude marks the echo. Click the Examine button, . Move the mouse and determine the time interval between the start of the first vibration and the start of the echo vibration. Record this time interval in the data table.