30th IBO English (Official) V1-31 Theory 2 HUNGARY 2019 Q15 An audiometry exam tests a subject's ability to hear sounds at different frequencies. Sound waves travel to the inner ear through the ear canal, eardrum, and the bones of the middle ear (air conduction, empty circles (o) or alternatively, through the bones around and behind the ear (bone conduction, full dots (•). Figure 1. shows the lowest sound intensity (in dB) at a given frequency (Hz) the test subject can still perceive, where a 0 value is the population average and any positive value suggests that a higher than average intensity is required for the subject to hear the sound. dB -20 dB -20 -10 -10 10 10 30 30 40 40 50 60 70 60 70 80 80 90 90 100 100 110 110 120 60 120 60 125 250 500 1000 2000 4000 8000 Hz 125 250 500 1000 2000 4000 8000 Hz dB -20 -10 10 20 40 50 60 70 80 90 100 110 120 60 125 250 500 1000 2000 4000 8000 Hz 60 125 250 500 1000 2000 4000 8000 Hz Fig.1 Q.15.1 What is the highest frequency at which bone conduction is tested? (Hz) Match the following conditions with the test results in Figure 1 (A-D). Indicate your answer by putting an X in the appropriate box on the answer sheet. Q.15.2 Old age related reduced sensitivity to high frequency. Q.15.3 Middle ear infection impeding the movement of the auditory bones. Q.15.4 Neurological hearing damage causing overall reduction in hearing acuity. Q.15.5 Damage caused by an air horn. 용유우ㅇ은8유유8 8888을28 ------ Q16 The Hungarian biophysicist, György Békésy was awarded the 1961 Nobel Prize for his research on the function of the mammalian cochlea. He showed that each frequency of sound causes a specific part of the basilar membrane to vibrate the most. This point is determined by the resonant properties of the membrane, with narrower parts vibrating more at higher frequencies. Neurons sense this vibration and encode the sound with two different mechanisms: 1. Place code: Each neuron connects to the membrane in one place and signals the presence of the narrow frequency band that causes maximal vibration in that part of the membrane. 2. Periodicity code: If the frequency is below 4 kHz, the timing of action potentials in the cochlear nerve is synchronised with individual cycles of the stimulus (this is called phase-locking). The intensities of audible sounds are expressed in decibel sound pressure level (dB SPL). Pressure ofsound in Pa dB SPL= 20 log,o 2. 10-5 Legend 2 130 |--- 120 120 100 80 60 50 40 20 Legend 1 20 100 1000 10000 Axis 1 Fig.1. Axis 1 = Frequency (Hz). Axis 2 = Intensity (dB). Legend 1 = Threshold of audibility. Legend 2 = Threshold of pain Based on the information above, indicate with an X if each of the following statements is true (T) or false (F). Q.16.1 Sounds detected by hair cells towards the narrowest part of the cochlea are coded in a phase-locked manner. Q.16.2 For sounds detected near the base of the membrane, the number of fibres firing is proportional to the amplitude of the displacement of the basilar membrane. Q.16.3 What is the absolute refractory period of fibres in the cochlear nerve? Give your answer in milliseconds (ms) in the appropriate box on the answer sheet. Q.16.4 At 50 Hz, how many times is the pressure of the sound that is painful greater than that of the sound that is just audible? Give your answer in the appropriate box on the answer sheet. Axis 2