Abstract
THE CONTRACTION of the intratympanic muscles brings about a change in the tension of the tympanic membrane and thus in the acoustic resistance or impedance of the ear.<sup>1</sup>This change in impedance may be utilized for examination also of the muscular contraction in ears with normal tympanic membranes—as, for instance, in determination of the threshold value. <h3>PRINCIPLE OF MEASUREMENT OF IMPEDANCE</h3> An acoustic impedance bridge<sup>2</sup>is employed, the principle of which is very simple: measurement of acoustic impedance by comparison. The impedance to be determined—here, that of the ear—is compared (balanced) with a variable acoustic impedance standard. The bridge (Fig. 1) consists of a tube (<i>O, B</i>), the ends of which are closed by the impedances to be compared. The telephone membrane (<i>M</i>) sounds to either side. When the impedance is the same in two planes lying symmetrically in relation to the telephone membrane, a pressure node will arise at
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