Everyone knows how a motion of small amplitude over a sufficiently large area may give rise to tremendous sound disturbances. It is a very broad principle in physics. When the two vectors are in the positions shown in Fig. There is an interesting distinction between what might be called mathematical and audible beats. There is a theorem, due to Fourier, so powerful in its ability to analyze such a repeating function into its separate component frequencies that it deserves considerable attention in any discussion of vibration and sound. We shall make frequent use of the Superposition Theorem throughout this book.
. With ordinary sound intensities a real difference frequency is never observed, that is, a third musical note is never evident. Plane Waves in Air 3. The frequency, within the body of the wave disturbance, may be defined as the number of crests passing any one point in space per unit time, and is ordinarily the same as the frequency of vibration of the source of the wave disturbance. Summary No branch of classical physics is older in its origins yet more modern in its applications than acoustics.
Reflection and Absorption of Sound Waves9. This is a fact of fundamental practical importance in the production of music. The introduction of the velocity potential, into the differential equation for space waves -- 2. We are using loudness here in the purely qualitative sense. Since and since the integration will always be over an integral number of cycles, the result of all integrations on the right-hand side of Eq. It is of considerable assistance in the design of aperiodic radiators, like radio loudspeakers, where the problem is too difficult for complete analysis by means of the classical wave equations.
Most musical instruments, fortunately, vibrate in such a way as to give rise to a fundamental tone and overtones, all of which bear whole number ratios to one another, and consequently the over-all vibration is a repeating function. It is one peculiarity of a fluid like air, with little or no resistance to shear, that only longitudinal waves may be propagated. Courses on acoustics very naturally begin with a study of vibrations, as a preliminary to the introduction of the wave equations. Nevertheless, a consideration of particle vibration theory is basic to the understanding, of the more complicated motions of extended bodies such as strings, bars, plates, etc. Fundamental particle vibration theory -- 2.
Simple harmonic motion originates, in mechanics, because of the existence of some kind of unbalanced elastic force. At the receiving end, whether it be at the ear or at a microphone, amplitudes may be unbelievably small. In addition, if the whole number relation does second, when they have executed 203 and 202 cycles respectively, they will be in phase again. Reflection and absorption of sound waves -- 9. The design of such equipment is difficult, as we shall see, and it is only recently that any considerable success has been achieved.
Both vibrations and waves, of course, are vastly important to all branches of physics and engineering. From a purely objective point of view, it has been common to explain quality as due solely to the number and prominence of the steady-state harmonic overtones. The paper cone of a radio loudspeaker, fed with the same energy at a variety of frequencies, will have imperceptible amplitudes at the high audible frequencies, whereas at low frequencies, visible amplitudes of as much as a millimeter or two may easily occur. Vibrations, whether connected with strings and diaphragms or with subatomic oscillators radiating electromagnetic waves, are all of a kind, and to understand the one type is a great help towards understanding the other. Both vibrations and waves, of course, are vastly important to all branches of physics and engineering. The audible effect of beats contains none of the subtleties discussed above. It is true that thus far the instruments born of modern science, such as the electronic organ and the like, have aped the older traditional instruments.
With the refinements achieved in the electrical circuit and electronic fields, the importance of improving the acoustical features of such reproducing systems has become more and more apparent. Since no dissipative force is being considered, the total energy of the system must remain constant. Reflection and Absorption of Sound Waves 9. Acoustics plays an important part in the reproduction of speech and music through the radio and the phonograph. The total energy of the system may obviously be taken as either the maximum potential energy or the maximum kinetic energy. The results of this process might appear to be crude in many cases, but the student should appreciate that the ear itself is, fortunately for the analyst, a rather crude device, incapable under ordinary conditions of detecting discrepancies of less than 10% to 20%.
But it is very helpful to students to gain an understanding of mechanical waves before trying to comprehend the more subtle and abstract electromagnetic ones. Subsequent topics include longitudinal waves in different gases and waves in liquids and solids; stationary waves and vibrating sources, as demonstrated by musical instruments; reflection and absorption of sound waves; speech and hearing; sound measurements and experimental acoustics; reproduction of sound; and miscellaneous applied acoustics. Only the constant term will remain, and solving for A0, 1—15 To evaluate A0 it is necessary, of course, to have the expression for x as a function of time. Synopsis No branch of classical physics is older in its origins yet more modern in its applications than acoustics. In only a few cases, in particular for the piano, is the mathematics capable of predicting the intensity of some of the more important harmonics that are so essential to the quality of the emitted sound.
Miscellaneous applied acoustics -- Appendices: -- 1. Longitudinal waves in different gases. While it is somewhat in the nature of a digression in the logical development of the subject, the discussion of sound waves along classical lines will be followed by a brief introduction to the electrical analog method as applied to acoustics, with chief emphasis upon the concept of acoustic radiation impedance. Fortunately, this is usually so in acoustics. This differential equation completely defines the type of motion and from it all other properties of simple harmonic motion may be obtained. Whether the cosine or the sine function appears in Eq.