ACOUSTICAL IMPEDANCE, AND THE THEORY OF HORNS AND OF THE PHONOGRAPH

BY ARTHUR GORDON WEBSTER

DEPARTMENT OF PHYSICS, CLARK UNIVERSITY

Communicated, May 8, 1919*

The introduction more than thirty years ago of the term 'impedance' by Mr. Oliver Heaviside has been productive of very great convenience in the theory of alternating currents of electricity. Unfortunately, engineers have not seemed to notice that the idea may be made as useful in mechanics and acoustics as in electricity. In fact, in such apparatus as the telephone one may combine the notions of electrical and mechanical impedance with great advantage. Whenever we have permanent vibrations of a single given frequency, which is here denoted, as usual, by n/2π, the notion of impedance is valuable in replacing all the quantities involved in the reactions of the system by a single complex number. If we follow the convenient practice of denoting an oscillating quantity by ein and taking its real part (as introduced by Cauchy) all the derivatives of ein are obtained by multiplication by powers of in, or graphically by advancing the representative vector by the proper number of right angles.

If we have any oscillating system into which a volume of air X periodically enters under an excess pressure p, I propose to define the impedance by the complex ratio Z = p/X. If we call dX/dt = I the current as in electricity, if we followed electrical analogy we should write Z pl so that the definition as given above makes our impedance lead by a right angle the usual definition. I believe this to be more convenient for our purposes than the usual definition and it need cause no confusion.

=

If we have a vibrating piston of area S as in the phonometer, we shall refer its motion to the volume S it carries with it and the force acting on it to the pressure, so that F Sp. The differential equation of the motion is

=

where m is the mass, « the damping, f the stiffness. The real part of S2Z, f - mn2, is the uncompensated stiffness, which is positive in a system tuned too high, when the displacement lags behind the force, by an angle between zero and one right angle, negative when the system is tuned too low, when the

* This article was read in December 1914 at the meeting of the American Physical Society at Philadelphia, and has been held back because of the continual development of the experimental apparatus described in a previous paper in these PROCEEDINGS.

lag is between one and two right angles, as shown in figure 1. If we force air into a chamber of volume V, the compression s = X/V will be related to the excess pressure p by the relation p = es, where e is the modulus of elasticity of the air e = pa2, p being the density and a the velocity of sound. Consequently we have

and the analogy is to a condenser. If we have air passing through an orifice or short tube of conductivity c its inertia gives an apparent mass p/c, and if it

escapes from a circular hole in an infinite plane it dissipates energy so that the whole impedance credited to the hole is

These three typical impedances will be at constant use in acoustics. It is to be remembered that systems in series have their impedances added and in parallel have the reciprocals of impedance added. Also that the free vibrations of a system are obtained by equating the impedances to zero.

As a simple example consider the phone described in the previous article, figure 3.

Let X1 = St be the volume introduced by the piston X2 that entering by

Disregarding phase by taking the modulus and putting k = n/a we have the phone formula for the strength of source.

If instead of sending the air out through a hole it goes into a cone or any other horn, we must use for the impedance Z2 that given below, and we arrive at the theory of the phonograph, and are thus able to answer the question as to the function of the horn in persuading the sound to come out of the phonograph when the motion of the diaphragm is given (it is well known that very little sound is emitted by the phonograph or the telephone with the horn taken off, although in the former case the motion of the diaphragm is exactly the same).

The phonometer was formerly arranged with the back of the diaphragm protected from the sound, figure 3. Let P be the external pressure, then, as before,

giving the formula for the measurement of the pressure,

and may be termed the sensitiveness of the phonometer. Where

As described in my recent article the back of the piston is exposed to the sound, figure 4. Then

Tubes and Horns.-Beside the above described phone and phonometer, the theory of which assumed a resonator so small that the pressure is supposed to be the same at every internal point, I have made use of many arrangements employing tubes or cones, in which we must take account of wave-motion. The familiar theory of cylindrical pipes may be included in the following generalized theory, which I have found experimentally to serve well.

Let us consider a tube of infinitesimal cross section σ varying as a function of the distance x from the end of the tube. Then if q is the displacement of the air, p the pressure, s the compression, we have the fundamental equations

For a simple periodic motion we put p, q proportional to eint, and obtain

Both these linear equations may be solved by means of series, and if we call u(kx), v(kx) two independent solutions we have

p = Au + Bu, Bq = Au' + Bv', B = pa1k,

where the accents signify differentiation according to kx. If we'denote values at one end x = x, and at the other end x = x2 by suffixes 1, 2, respectively, and form the determinants

we may determine the constants A, B in terms of any two out of p1, 91, P2, 92, so that we obtain

As it is more convenient to deal with the volumes X1

= 0191, X2

have in general

= 0292 we shall

(24)

and for the impedances belonging to the ends of the tube

so that the impedance at either end of the tube is a linear fractional function of the other. According to the apparatus attached to an end the impedance attached to that end is known. A tube for which a, b, c, d are given may be replaced by any other tube having the same constants. Examples.-Cylindrical tube, a constant. Put x2x1 = Z1,