Wotton J M, Haresign T, Simmons J A
Department of Neuroscience, Brown University, Providence, Rhode Island 02912, USA.
J Acoust Soc Am. 1995 Sep;98(3):1423-45. doi: 10.1121/1.413410.
To measure the directionality of the external ear of the echolocating bat, Eptesicus fuscus, the left or right eardrum of a dead bat was replaced by a microphone which recorded signals received from a sound source that was moved around the stationary head. The test signal was a 0.5-ms FM sweep from 100 kHz to 10 kHz (covering all frequencies in the bat's biosonar sounds). Notches and peaks in transfer functions for 7 tested ears varied systematically with changes in elevation. For the most prominent notch, center frequency decreased from about 50 kHz for elevations at or near the horizontal to 30-40 kHz for elevations 30 degrees-40 degrees below the horizontal. A second notch shifted from about 85 kHz to 70 kHz over these same elevations. Above the horizontal, a peak that flanks these notches changed in amplitude by 15 dB with changes in elevation. Removal of the tragus from the external ear disrupted the systematic movement of notch frequencies with elevation but did not disrupt changes in the peak's amplitude. Smaller changes in notch frequency also occurred with changes in azimuth, so monaural notch information alone cannot determine the position of sound sources away from the median plane. However, because bats routinely keep the head pointed at the target's azimuth, median-plane localization occurs with monaural cues delivered to the two ears. Corresponding changes with elevation occurred in the impulse-response, which consists of a series of 3-6 peaks spaced 10-20 microseconds apart. The time separation of two prominent impulse peaks systematically increased from 22-26 microseconds above the horizontal to about 36-40 microseconds below the horizontal, and removal of the tragus disrupted this time shift below the horizontal.
为了测量食虫蝙蝠棕蝠（Eptesicus fuscus）外耳的方向性，研究人员将一只死蝙蝠的左鼓膜或右鼓膜换成了麦克风，该麦克风记录从围绕静止头部移动的声源接收到的信号。测试信号是一个从100千赫到10千赫的0.5毫秒调频扫描信号（涵盖蝙蝠生物声纳声音中的所有频率）。7只受试耳朵的传递函数中的凹口和峰值随高度变化而系统地变化。对于最突出的凹口，中心频率从水平或接近水平高度时的约50千赫下降到低于水平30度至40度高度时的30 - 40千赫。在相同的这些高度范围内，第二个凹口从约85千赫移至70千赫。在水平上方，位于这些凹口两侧的一个峰值的幅度随高度变化改变了15分贝。从外耳去除耳屏会扰乱凹口频率随高度的系统移动，但不会扰乱峰值幅度的变化。凹口频率也会随着方位角的变化而发生较小的变化，所以仅单耳凹口信息无法确定声源偏离中平面的位置。然而，由于蝙蝠通常会保持头部指向目标的方位角，所以通过传递到两只耳朵的单耳线索可以实现中平面定位。脉冲响应中也出现了与高度相应的变化，脉冲响应由一系列间隔10 - 20微秒的3 - 6个峰值组成。两个突出脉冲峰值的时间间隔从水平上方的22 - 26微秒系统地增加到水平下方的约36 - 40微秒，去除耳屏会扰乱水平下方的这种时间偏移。
