. Elements of acoustical engineering. Electro-acoustics; Sound. 272 MEASUREMENTS The loud speakers are connected to two pipes, one of which is variable in length and the other equipped with a means for attaching the unknown impedance. At some distance beyond this point the two pipes are joined by a small pipe which, in turn, is connected to another pipe leading to a microphone and amplifier. Standing waves in the pipes are reduced by the introduction of small tufts of felt. With the branch closed the voltage across the two loud speaker units ,TUFTS or FELTv UNKNOWN IMPEDANCE lQ jS^Kth m &&
. Elements of acoustical engineering. Electro-acoustics; Sound. 272 MEASUREMENTS The loud speakers are connected to two pipes, one of which is variable in length and the other equipped with a means for attaching the unknown impedance. At some distance beyond this point the two pipes are joined by a small pipe which, in turn, is connected to another pipe leading to a microphone and amplifier. Standing waves in the pipes are reduced by the introduction of small tufts of felt. With the branch closed the voltage across the two loud speaker units ,TUFTS or FELTv UNKNOWN IMPEDANCE lQ jS^Kth m && ©"ffl ^ tS"'fe> Q <i9 &T!i t-S "^ eg ^ (MW' ^Sfc'ml^ ATTENUATORS . :ri)= 8 AMPLIFIER ~S?ETERJ OSCILLATOR MICROPHONE TELESCOPIC TUBING I iii'Tiii. °nM LOUD SPEAKER UNIT Fig. Schematic arrangement of an acoustic impedance bridge for measuring imped- ance. (After Stewart.) and the length of the variable tube are adjusted until a minimum reading is obtained in the output meter. The unknown impedance is now attached and the process repeated. The unknown impedance can be obtained from the following equation 2zau _ po Izau + Ta P^ ; (cos 6 + j sin d) where Zau = unknown impedance, in acoustic ohms, fA = pc/^ acoustic resistance of the damped pipe, in acoustic ohms, ^ = area of the pipe to which the branch is attached, in square centimeters, p = density of air, in grams per cubic centimeter, and c = velocity of sound, in centimeters per second. The ratio po/po' can be determined from the following equation 1^ £l£2_ where ei and ^2 = the voltages applied to the loud speaker without the branch, and ei and ^2' = the voltages with the branch Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Olson, Harry Ferdinand, 1901-. New York, D. Van Nostrand company, inc
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