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When an echolocating bat approaches a target, its outgoing sounds return as echoes, which are Doppler shifted upward in frequency. In certain species of bats, which produce constant frequency (CF) echolocation calls, the bats compensate for the Doppler shift by lowering their call frequency as they approach a target. This keeps the returning echo in the same frequency range of the normal echolocation call. This dynamic frequency modulation is called the Doppler Shift Compensation (DSC), and was discovered by Hans Schnitzler in 1968.〔Schnitzler, H.U. 1968. Die Ultraschallortungslaute der Hufeisennasen- Fledermäuse (Chiroptera, Rhinolophidae) in verschiedenen. Z. Vergl. Physiol. 57, 376–408〕 CF bats employ the DSC mechanism to maintain the echo frequency within a narrow frequency range.〔Hiryu, S. Shiori, Y. Hosokawa, T. Riquimaroux, H. Wantanabe, Y. 2008. On Board Telemetry of Emitted Sounds from free-flying bats: Compensation for velocity and distance stabilize echo frequency. Journal of Comparative Physiology A. 194: 841–851.〕 This narrow frequency range is referred to as the acoustic fovea. By modulating the frequency of the outgoing calls, the bats can ensure that the returning echoes stay nearly constant within this range of optimal sensitivity. Ultimately, by keeping the echoes in this optimal range, the bats can quickly ascertain certain properties (such as distance and velocity) about the target. This behavior appears to have evolved independently in several species of the Rhinolophidae and Mormoopidae families.〔Jones, G., & Teeling, E. (2006). The evolution of echolocation in bats. Trends in Ecology & Evolution, 21(3), 149–156. .〕 The common features shared by bats with DSC are that they produce CF sounds, and that they have a specialized cochlea that is adapted to receiving a narrow range of frequencies with high resolution.〔 DSC allows these bats to utilize these features to optimize the echolocation behavior. == Description == All bats, when there is some nonzero relative velocity between itself and the target (the object that the call rebounds off of, which produces an echo), will hear Doppler shifted echoes of the pulses they produce. If the bat and the target are approaching each other, the bat will hear an echo that is higher in frequency than the call it produced. If they are moving away from each other, the bat will hear an echo that is lower in frequency than the call it originally produced. It is important that the bat be able to detect and be extremely sensitive to these echoes, so that it can determine properties about the target object. For CF bats, who possess an acoustic fovea, the Doppler shifted echo will fall outside of the narrow range of frequencies to which the bat is optimally responsive. This problem can be avoided if the frequency of the calls produced is altered. As the bat speeds up and approaches a target, the echoes it hears will be of increasingly high pitch, outside of the auditory fovea. In order to compensate for the Doppler shift during an approach toward an object of interest, the bat will lower the frequency of the calls it produces.〔 The overall effect is that the echo frequency maintains nearly constant, and remains steadily within the auditory fovea.〔 By lowering pulse frequency by the same increment that the Doppler shift will raise the echo frequency, the bat can maintain the frequency of the echoes around a constant value, within the auditory fovea.〔 This was measured using a small, portable microphone (the Telemike) on the top of the bat’s head to record the echo frequencies heard by the bat.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Doppler Shift Compensation」の詳細全文を読む スポンサード リンク
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