| 释义 |
impedance|ɪmˈpiːdəns|
[f. impede v. + -ance.]
1. Electr.
a. The overall opposition to an electric current, arising from the combined effect of resistance R and reactance X and measured by the ratio of the e.m.f. to the resulting current (peak or r.m.s. values); it may be represented as a scalar quantity whose value is √(R2 + X2) or as a complex number R + jX.
1886Heaviside Electr. Pap. (1892) II. 64 Let us call the ratio of the impressed force to the current in a line when electrostatic induction is ignorable the Impedance of the line, from the verb impede. Ibid. 126 The impedance may be independent of the frequency, or a constant. 1888Lodge in Rep. Brit. Assoc., Impedance of conductors to Leyden-jar discharges and to Lightning. 1923E. W. Marchant Radio Telegr. ii. 15 If the conductor is a long straight wire it offers comparatively little obstruction or ‘impedance’ to the passage of a current; while, if it is wound up into a coil it will offer a very high ‘impedance’ to the passage of the high-frequency current. 1926A. T. Dover Theory & Pract. Alternating Currents iii. 49 The sides OC, CE, OE of triangle OCE are proportional to the resistance, reactance, and impedance respectively. On account of this feature the triangle OCE, when drawn to an ohm scale, is called the impedance triangle of the circuit. Impedance is therefore a complex quantity, i.e. it is only completely specified when its magnitude and inclination, or alternatively its two perpendicular components with respect to the current, are given. 1930M. G. Malti Electr. Circuit Analysis vii. 87 A sine voltage E12 = 70 - j50 is impressed on a series circuit of impedance ZL = 15 cis 30° ohms. What is the complex expression for the current? 1931Moyer & Wostrel Radio Handbk. ii. 76 The term √(R2 + X2), known as the impedance of the circuit, takes the place in alternating-current calculations of the resistance in direct-current work. 1948A. L. Albert Radio Fund. v. 128 If the impressed voltage is divided by the input current of any line, a value of impedance is obtained, and for any line this is called the input impedance. Ibid. 129 The characteristic impedance, usually designated by Z0, is the input impedance of a line infinite in length. 1961H. Jasik Antenna Engin. Handbk. xxxi. 2 A variable load impedance connected to a source will receive the maximum possible power from the source when it is adjusted to equal the complex conjugate of the impedance of the source. 1962C. Susskind Encycl. Electronics 422/2 The impedance of dynamic, ribbon, carbon, and magnetic microphones is nearly always less than 40,000 ohms and is essentially resistive.
b. Something that has impedance and may be made part of a circuit.
1935Campbell & Childs Measurement of Inductance, Capacitance, & Frequency xiii. 260 The unknown impedance Z is put in series with the known resistance R, and across them is connected a potential divider with slider Q. 1966McGraw-Hill Encycl. Sci. & Technol. VII. 39/1 Standard resistors, capacitors, and inductors are often used as comparisons for unknown impedances.
2. Mech. and Acoustics. Any of several analogous properties of oscillatory mechanical systems that represent the force, pressure, etc., necessary to produce a given speed, rate of flow, etc., esp. mechanical impedance, the ratio of the force on an oscillating body or particle to the resulting velocity; specific acoustic impedance, in a wave, the ratio of the (excess) pressure at any point to the resulting particle velocity (i.e. the mechanical impedance per unit area of the wave-front); acoustic (or acoustical) impedance, the ratio of the average (excess) pressure over an imaginary surface in a wave to the resulting rate of volume flow across it (i.e. the specific acoustic impedance, averaged over the surface, divided by the area of the surface).
The qualifying adjs. are often omitted in contexts where there is no danger of ambiguity.
1919A. G. Webster in Proc. Nat. Acad. Sci. V. 275 (heading) Acoustical impedance, and the theory of horns and of the phonograph. Ibid., The term ‘impedance’..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. Ibid., 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. 1927I. B. Crandall Theory Vibrating Syst. ii. 71 In most parts of the text it is convenient to take the mechanical impedance as the ratio of maximum force to maximum velocity. Strictly, the velocity is analogous to current density rather than to total current... The acoustic impedance per unit area, divided by the area, is what corresponds most closely to the electrical impedance. 1934N. W. McLachlan Loud Speakers i. 4 The difference between acoustical and mechanical impedance must not be confused. In the one case waves travel in some form of conduit or channel which impedes their progress. The greater the area the smaller the impedance. In the mechanical form something is being driven, so the greater the area driving the medium the greater the impedance opposing motion. 1936P. M. Morse Vibration & Sound vi. 192 A consideration of the behavior of the specific acoustic impedance of a plane wave will enable us to work out the details of its interaction with various mechanical systems. 1953Sci. News XXIX. 11 Whenever in the layers lying below the bed of the sea there is a sudden change in the acoustic impedance (that is in the product of the density of the material and the velocity of sound waves in it), there is a reflecting boundary. 1955Hueter & Bolt Sonics ii. 33 The concept of acoustic impedance is useful in the analysis of lumped systems, such as cavity resonators, sirens, jets, etc. 1960R. B. Lindsay Mech. Radiation ix. 218 The excess pressure in a sound wave is taken to correspond with the electromotive force, and the volume current (the product of the particle velocity ξ and the area of the wavefront S) is assumed to be analogous to the electric current. Hence it is natural to define the acoustic impedance of a wave as Z = Pe/ X. , where X. = volume current = Sξ. With the use of the complex notation for wave quantities the impedance will usually be complex. We first examine..the case of a plane harmonic wave of angular frequency w... The impedance of such a wave is then Z = ρ0V/S, a real quantity. The numerator is called the specific acoustic impedance of a plane wave and denoted by Zs. It is the impedance for a unit area of wavefront. 1961Bickley & Talbot Introd. Theory Vibrating Syst. xi. 139 The example is that of torsional waves on a shaft... The [angular] velocity is proportional to the transmitted torque... Now in the electrical analogy the ratio of force to velocity is that of e.m.f. to current, a ratio which the electrical engineer calls impedance. It is therefore convenient to call the ratio of transmitted torque to angular velocity (and the analogous quantity in other cases) the transmission impedance. 1963C. T. Morrow Shock & Vibration Engin. I. vi. 127 A high impedance indicates that even a large force results in little motion; it suggests massiveness and rigidity. A low impedance indicates that motion is easy to produce; it suggests lightness and flexibility. A resistive impedance indicates that the energy supplied at the point is absorbed, as in a dashpot... A reactive impedance indicates that energy supplied is stored, as in..a compressed spring.
3. Special Comb.: impedance bond, a kind of rail bond used to connect electrified rails in adjoining signalling sections, having a low resistance (so that the direct traction current can pass unhindered) and a high inductance (so that the alternating signalling currents are confined to their respective sections); impedance-matching, the adjustment of impedances in such a way as to minimize the power reflected or the reduction in the power transferred that occurs when an oscillatory current or other wave meets a change in impedance; also attrib.
1926Harding & Ewing Electr. Railway Engineering (ed. 3) xix. 258 The development of the two-rail signal system making use of impedance bonds..marked the beginning of a new era in the development of the new railway signal systems. Practically all of the automatic block signals which are being installed on both alternating- and direct-current railways at present are of this general type. 1967G. F. Fiennes I tried to run Railway v. 49 Installing impedance bonds in the track.
1929E. Mallett Telegr. & Telephony vii. 160 Impedance matching is necessary in order that the maximum available power may be absorbed where it is required. 1934Jrnl. Inst. Electr. Engin. LXXV. 803/2 Impedance-matching transformers have to be used at both ends of the screened lead-in feeder. 1968Listener 22 Feb. 235/3 The bony levers [of the middle ear] reduce the amplitude of vibration and so serve as what an engineer would call an ‘impedance-matching transformer’. This gives efficient transfer of energy from the air to the denser fluid in the cochlea of the inner ear. |