Contains reports on two research projects.National Science Foundation (Grant G-16526)National Institutes of Health (Grant MH-04737-03)National Aeronautics and Space Administration (Grant NsG-496

Durlach, Nathaniel I.

Moller, A. R.

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XVI. COMMUNICATIONS BIOPHYSICSProf. M. Eden Dr. G. P. Moore Patricia KirkpatrickProf. J. L. Hall II Dr. R. H. Wendt K. C. KoerberProf. W. T. Peaket R. M. Brownt D. LangbeinProf. R. R. Pfeiffert S. K. Burns R. G. MarkProf. W. A. Rosenblith R. R. Capranica M. J. MurrayProf. W. M. Siebert R. J. Clayton Ann M. O'RourkeProf. T. F. Weiss A. H. Cristl Cynthia M. PyleDr. J. S. Barlow$ N. I. Durlach M. B. SachsDr. E. Giberman** J. L. Elliot I. H. ThomaeDr. R. D. Hall P. R. Gray J. R. WelchDr. N. Y-S. Kiangf J. J. Guinan, Jr. M. L. WiederholdF. N. JordanA. ACOUSTIC STIMULATOR FOR USE IN ACUTE EXPERIMENTS ON CATSUniform frequency response of an acoustic stimulator is not very important as longas the signals are sinusoidal; irregular frequency response can then be compensatedfor on the basis of measurements of the sound pressure close to the eardrum. Whencomplex signals are used, however, such compensation cannot be made easily and thewaveform of the acoustic signal may be highly dependent on the frequency response ofthe stimulator.The device described here was developed to achieve an impulse response with neg-ligible ringing, together with a flat response characteristic over a broad frequencyrange. The assembly must also be easy to apply to the ear of the experimental animal(cat).A conventional condenser microphone (1-inch diameter Bruel and Kjaer ) was chosenas sound source. The arrangement is shown in Fig. XVI-1. The right end of the assem-bly fits into the ear bar of the "Horsley-Clark" headholder, and the left end of theassembly is placed in the external auditory meatus. It is possible to inspect the eardrumthrough the center tube without removing the assembly from the cat's ear. The shorttube from the 1-inch microphone to the main tube is filled with damping material. Loosecotton is normally used, but steel wool was tried with similar effect. The sound pres-sure near the eardrum can be measured by a microphone (in this case a 0. 5-inch B & Kcondenser microphone) at the end of a narrow probe tube ending close to the position ofthe eardrum as shown in Fig. XVI-1. With the frequency response of the microphone,This work was supported in part by the National Science Foundation (Grant G-165Z6);in part by the National Institutes of Health (Grant MH-04737-03); and in part by theNational Aeronautics and Space Administration (Grant NsG-496).Also at Massachusetts Eye and Ear Infirmary, Boston, Massachusetts.lResearch Associate in Communication Sciences from the Neurophysiological Labor-atory of the Neurology Service of the Massachusetts General Hospital.From the Department of Physics, Weizmann Institute of Science, Israel.QPR No. 73 1812 19 6 3588K MICROPHONE 413419 AL L kENSK)NS IN MLU N*TERSFig. XVI-1. Arrangement of the stimulator.-30.0.1FREQUENCY (KCPS)Fig. XVI-2. Characteristic of the probe tube with and withouta washer, which decreases the air volume in frontof the diaphragm. The microphone was a B & Ktype 4134 (0. 5-inch diameter).QPR No. 73 182(XVI. COMMUNICATIONS BIOPHYSICS)and probe tube known, the sound-pressure level at the tip of the probe tube can be cal-culated from the measured output of the microphone. The frequency characteristic ofthe microphone and probe tube is shown in Fig. XVI-2. The characteristic, for this con-figuration, is far from uniform.By appropriate damping of this probe tube with steel wool it is possible to get almostflat response up to a certain frequency and, above this, a response that declines withalmost constant slope. The device is used in acute experiments, however, in which theprobe may become filled with blood, which is difficult to remove from the steel woolwithout changing the characteristic. Hence, calibration of the probe microphone wouldbe necessary before each experiment. An undamped probe tube can be cleaned easilywithout changing its characteristic. The drawback to the use of an undamped probe isthe nonuniform calibration.Fig. XVI-3.Impulse response of the assembly shown in Fig. XVI-1equipped with a dynamic earphone (selected PermafluxPDR-10 with better impulse response than that of anaverage earphone) compared with (right) a condensermicrophone (B & K 4131) used as sound source. Inboth cases the electrical input to the earphones was a50-psec rectangular pulse.Figure XVI-3 shows the impulse response of the assembly with a dynamic earphoneas sound source (left) and with a condenser microphone as sound source (right). Theresponses were measured in a 0. 6-cc rigid cavity connected to the assembly in the placeof an ear. The bottom of the 0.6-cc cavity was a 1-inch condenser microphone(B & K 4131). The sound pressure was measured with that microphone. When the assem-bly is terminated with the ear of a cat it is not possible to measure the soundoutput in the same way as for the case in which the assembly is terminatedwith a rigid cavity. The probe microphone is not suitable for measuring theimpulse response because of the distortion introduced by the probe tube. It isseen from Fig. XVI-3 that the impulse response of the assembly with a conden-ser sound source (right) is narrower than it is if the sound source is a dynamicearphone.QPR No. 73 183Fig. XVI-4.S9590-j 0 85I 804o SOUNDS75- MEASUI-70 TO THEI N A CAS65-. 60-o 55SO 50= 45Second-harmonic content in the sound outputof a condenser earphone (B & K 4131). Themeasurements were performed in a 1. 2-ccrigid cavity with a B & K condenser micro-phone type 4134 at 1000 cps.B 8 K I' MICROPHONEAS SOURCE,MEASUREIN A 12CC COUPLER(WITH A 1/2' B 8 K MIKEB 8 K + COUPLER/ + TUBEFREQUENCY (KCPS)Fig. XVI-5.QPR No. 73Frequency response of the stimulus device. Solid circlesindicate measurements performed with the B & K micro-phone (1 inch) as sound source and measured with a 0. 5-inch microphone in a 1. 2-cc cavity. The open circlesshow the response of the same transducer in connectionwith the tube (Fig. XVI-1) measured in a 1.2-cc cavity;the triangles show the response of the same system whenconnected to the ear of a cat and measured at the end ofthe tube. The electrical input to the condenser earphoneis constant throughout.18420-500(0-3 -2 -10 D8 INPUT RE 31V RMS95 10DB SPL(XVI. COMMUNICATIONS BIOPHYSICS)1. LinearityDistortion is introduced in the output of a condenser microphone used as a soundsource when the movement of the diaphragm is an appreciable fraction of the distance95- o K 295- K 292*K 29085 -.z8 075 -0 P 70gL)65o -600551 10 50FREQUENCY (K CPS)Fig. XVI-6. Sound pressure measured close to the eardrum ofthree different cats with the stimulator equippedwith a B & K condenser microphone (4131) as soundsource with constant electrical input.between the diaphragm and the stationary electrode. Figure XVI-4 shows the contentsof the second-harmonic sound-pressure level (SPL) for different polarization voltages.The sound pressure was measured in a rigid cavity with a volume of 1. 2 cc. It is seenthat it is possible to obtain a level of 105 db SPL with approximately 3 per cent distor-tion.Figure XVI-5 shows the frequency response of the stimulus devices measured in dif-ferent ways. The response of the stimulator system is much better when applied to anear (triangles) than when connected to a rigid cavity (solid circles). The peak in theresponse near 20 kc,2 which is most pronounced in the response measured in a rigidcavity, is a transverse resonance in the measuring cavity. The cavity in front of thecondenser earphone (see Fig. XVI-1) has the same diameter as the measuring cavity.Therefore, a similar peak, although not as pronounced, is seen in the sound pressuremeasured near the eardrum of a cat.Figure XVI-6 shows the response of the assembly measured close to the eardrumon three different cats.A. R. M0ller(Mr. Aage R. MOller is now at the Fysiologiska Institutionen, Karolinska Institutet,Stockholm. Mr. MOller visited this laboratory from September to December 1961,during which time he carried out the work reported here in cooperation with the staff ofthe Eaton-Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston.)QPR No. 73 185(XVI. COMMUNICATIONS BIOPHYSICS)References1. Bruel and Kjaer Instruments Inc., 3044 West 106 Street, Cleveland 11, Ohio,hereafter referred to as B & K.2. The measuring cavity is cylindrical and the diameter is 1. 7 cm. The measuredpeak appears at 21. 5 kc and the calculated natural frequency is 22. 7 kc (see P. M. Morse,Vibration and Sound (McGraw-Hill Book Company, Inc., New York, 2d edition, 1948), p. 398.)B. BINAURAL UNMASKING AND "ECHOLOCATION"In recent months, new projects have been initiated in two areas of hearing that werenot included in our Research Objectives given in Quarterly Progress Report No. 72(pages 223-224): (1) psychoacoustics of binaural unmasking and (2) "echolocation"behavior.2 The work in both these areas constitutes a continuation of previous work atLincoln Laboratory, M. I. T. 3In the area of binaural unmasking, experiments have been started on the followingtopics: localization in the presence of masking noise; binaural unmasking as a functionof the angular dispersion of the masking noise; time-intensity trading with a detectioncriterion; binaural unmasking as a function of interaural phase shifts in the maskingnoise; binaural unmasking as a function of interaural time delays in the masking noise;adaptation to magnified interaural time delays. Also, a mathematical model has beenconstructed for describing the data on binaural unmasking at high frequencies.4In the area of echolocation, work has been continued in collaboration withF. A. Webster on the ability of the bat Myotis lucifugus to discriminate between targetsof various shapes. Also, new experiments have been started on the ability of Myotisto detect targets in a background of clutter and on the ability of humans to echolocatewith the aid of artificial sonic generators.The work in both of these areas will be reported on when results are available.N. I. DurlachReferences1. The phrase "binaural unmasking" refers to the advantages of two ears over oneear with respect to the detection of a signal in a background of interference.2. The term "echolocation" is the biologist's equivalent of the term "sonar." Fora summary of all but the most recent work on echolocation, see D. R. Griffin, Listeningin the Dark (Yale University Press, New Haven, Conn., 1958).3. N. I. Durlach, Note on the creation of pitch through binaural interaction, J. Acoust.Soc. Am. 34, 1096-1099 (1962); N. I. Durlach, Equalization and cancellation theory ofbinaural masking level differences, J. Acoust. Soc. Am. 35, 1206-1218 (1963);F. A. Webster and N. I. Durlach, Echolocation Systems of the Bat; Progress Reporton Experimental Studies Conducted at F. A. Webster's Laboratory (1 November 1961-1 September 1962), Report No. 41G-3, Lincoln Laboratory, M. I. T., 14 August 1963.QPR No. 73 186(XVI. COMMUNICATIONS BIOPHYSICS)4. N. I. Durlach, Note on binaural masking-level differences at high frequencies,J. Acoust. Soc. Am. (in press).QPR No. 73 187

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