BME 403 Auditory Section Exam (100pts max) --Answers -- 10-29-1997 n Closed book; all calculated results can be rounded to 2 decimal places. n Work must be shown in order to receive partial credit. 1. {10} A. There are two separate 1000-Hz tones: tone1 has pressure amplitude A1 and duration of 100 ms, tone2 has pressure amplitude (2*A1) and duration of 50 ms. If root-mean-square (rms) values of both tones have been computed, pick an answer from following choices (circle one): (a) (rms of tone1) > (rms of tone2) (b) (rms of tone1) < (rms of tone2) (c) (rms of tone1) = (rms of tone2). A. Now, if the two tones are added together (in phase), and if tone1 is at level of 60 dB SPL, find the overall level for this combined tone in dB SPL. Answers: A. (b). B. , P2 = 2*P1, and Pt=P1+P2=3*P1. Now, . Therefore, dB SPL(Pt) = 9.54+60 = 69.54 (dB SPL). 1. {10} The resonance property of the outer ear (a resonator) sets the first processing stage for the input of sound wave to the auditory system. A. If a complex sound wave, which consists of 100 Hz and 3000 Hz components with equal amplitudes, is applied to the ear, what will happen to the two components at the tympanic membrane? Answers: A. Since the resonance frequency of the outer ear resonator is about 3000 Hz, the amplitude of the 3000-Hz component will be amplified, so it will be bigger than 100-Hz component’s amplitude. B. Name the three major parts of the outer ear which form this resonator. Answer: Auricle (or concha), external (ear) canal (or meatus), tympanic membrane. 1. {15} Through an impedance matching process in the middle ear, air pressure vibration at the tympanic membrane is transformed into fluid traveling wave in the cochlea at the oval window. A. Name the 3 ossicles in the middle ear, which perform this transformation. Which one of them is not involved in the impedance matching process? Answer: Malleus, incus, and stapes. Stepes is not involved. B. The area of the tympanic membrane is equal to A1 and the area of the oval window is equal to A2, and (A1/A2)=80. Now in order for a sound to have 100 dB SPL at the oval window, what sound input level should it have at the tympanic membrane (in dB SPL)? Answer: Pow=(A1/A2)*Ptm. 100 dB SPL = 20*Log(Pow/P0) = 20*Log[(A1/A2)*Ptm/P0)}. That is 100 dB SPL = 20*Log(A1/A2) + 20*Log(Ptm/ P0) = 20*Log(80) + Ptm (dB SPL). Therefore, Ptm (dB SPL) = 100 – 38.06 = 61.94 (dB SPL). 1. {5} A. During a cochlear implant surgery, if the round window is opened for electrode insertion, which fluid may come out? Answer: Perilymph. B. If this fluid comes out, will it affect the fluid in the Scala Vestibuli? Why? Answer: Yes. Because it is continuous from the Scala Vestibuli through helicotrema. 1. {15} The pressure vibration applied at the oval window will set the basilar membrane (BM) in motion from the base to the apex along the cochlear duct. A. Which 2 types of hair cells on top of the BM are the sound detectors, and which one is the main detector? Answer: Inner and outer hair cells. Inner hair cell is the main detector. B. If the stereocilia of the hairs cells near the base of the cochlea are damaged, would it affect the normal detection of a 100-Hz tone? Why? Answer: NO. Because the 100-Hz tone is mainly detected by the hair cells at apex of the cochlea. C. Is tectorial membrane involved in sound detection with the hair cells? If yes, how? Answer: Yes. When the BM moves up and down, the tectorial membrane will deflect the stereocilia of the hair cells. This action will initiate the mechanical-to- electrical signal transduction. 1. {5} The hair cells convert the mechanical motion of the BM into intracellular electrical potential change. Through synaptic connections with auditory neurons, the hair cells communicate with the central auditory cortex. Give names of following types of neurons: A. The neurons which transmit the detected signals to the central auditory cortex Answer: The afferent neurons. B. The neurons which transmit commanding signals from the central auditory cortex to the hair cells Answer: The efferent neurons. 1. {10} A. Why is the loudness function L = k*(I)q (where k and q are constants, I is the sound intensity) called a psychophysical function? Answer: Because the function describes the relationship between a physical stimulus (the independent variable, I) and a subjective sensation (the dependent variable, L). B. If a subject’s loudness function is measured, and in the test, the subject him/her self controls the variation of the stimulus, name this testing method. Answer: Method of adjustment. C. For this subject, is it possible that the function obtained today may be different tomorrow? Answer: Yes. 1. {5} One measure of the auditory system function is to test a subject’s minimum detectable sound pressure levels among a set of pure tones in quiet. A. Is this called difference threshold or absolute threshold testing? Answer: Absolute threshold testing. B. For a normal subject, would you expect his/her threshold (in dB SPL) for a 250-Hz tone to be the same as for a 3000-Hz tone? Answer: No. 1. {10} A. If a 500-Hz tone at 55 dB SPL is judged by a subject to be equally loud as a 1000-Hz tone, which is at 40 dB SPL, how many phon is this 500-Hz tone? Answer: 40 phon. B. The same subject is now given the same 500-Hz tone at 55 dB SPL, to which he assigns a value of 1 sone. He then starts to increase the level of the tone. When the level of the tone reaches 65 dB SPL, he reports that it is about twice loud as it was at the 55 dB SPL level. How many sone is the tone now? Answer: 2 sone. 1. {15} A. Why is measurement of masking effect more closely related to the real hearing situation? Answer: Because in daily life, we often need to detect a sound of masked component (i.e. hear someone talking in a restaurant) at the presence of a sound of masker component (i.e. background noise). B. In a simultaneous masking experiment with a probe tone, which is at the center of a noise masker’s frequency bandwidth, describe the relationship between the probe tone’s threshold and the masker’s bandwidth when the masker’s frequency band is symmetrically widened while its power density level is kept constant. (Hint: is concept of “critical band” involved in this experiment?) Answer: When the bandwidth of the masking noise is widened, the threshold level of the probe tone will have to be increased accordingly. However, as the bandwidth of the masking noise reaches a critical value and beyond, the level of the probe tone’s threshold will not increase any more. This critical value of the masker bandwidth is called critical band. (Some drawing will be helpful.) End of the Examination 5/6