Threshold of Reflex Contractions of Muscles of Middle Ear and Recruitment of Loudness

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.1This 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. PRINCIPLE OF MEASUREMENT OF IMPEDANCE An acoustic impedance bridge2is 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 (O, B), the ends of which are closed by the impedances to be compared. The telephone membrane (M) 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