Au Whitlow W L, Kastelein Ronald A, Benoit-Bird Kelly J, Cranford Ted W, McKenna Megan F
Marine Mammal Research Program, Hawaii Institute of Marine Biology, P.O. Box 1106, Kailua, HI 96734, USA.
J Exp Biol. 2006 Jul;209(Pt 14):2726-33. doi: 10.1242/jeb.02306.
An experiment was conducted to investigate the sound pressure patterns on the melon of odontocetes by using four broadband hydrophones embedded in suction cups to measure echolocation signals on the surface of the forehead of two harbor porpoises (Phocoena phocoena). It has long been hypothesized that the special lipids found in the melon of odontocetes, and not in any other mammals, focus sounds produced in the nasal region that then propagate through the melon, producing a beam that is directional in both the horizontal and vertical planes. The results of our measurements supported the melon-focusing hypothesis, with the maximum click amplitude, representing the axis of the echolocation beam, located approximately 5.6-6.1 cm from the edge of the animal's upper lip along the midline of the melon. The focusing is not sharp but is sufficient to produce a transmission beam of about 16 degrees. Click amplitude dropped off rapidly at locations away from the location of site of maximum amplitude. Based on comparisons of forehead anatomy from similar sized porpoises, the beam axis coincided with a pathway extending from the phonic lips through the axis of the low-density/low sound velocity lipid core of the melon. The significant interaction between click number and hydrophone position suggests that the echolocation signals can take slightly different pathways through the melon, probably as a result of how the signals are launched by the production mechanism and the position of the acoustically reflective air sacs.
通过使用嵌入吸盘的四个宽带水听器来测量两只港湾鼠海豚（Phocoena phocoena）前额表面的回声定位信号，进行了一项实验以研究齿鲸类动物额隆上的声压模式。长期以来，人们一直推测，在齿鲸类动物的额隆中发现的特殊脂质，而在其他任何哺乳动物中都不存在，能够聚焦在鼻腔区域产生的声音，这些声音随后通过额隆传播，产生一个在水平和垂直平面上都具有方向性的波束。我们的测量结果支持了额隆聚焦假说，最大咔哒声振幅代表回声定位波束的轴线，沿着额隆中线距离动物上唇边缘约5.6 - 6.1厘米。这种聚焦并不尖锐，但足以产生一个约16度的发射波束。在远离最大振幅位置的地方，咔哒声振幅迅速下降。基于对类似大小鼠海豚前额解剖结构的比较，波束轴线与一条从发声唇延伸穿过额隆低密度/低声速脂质核心轴线的路径重合。咔哒声数量与水听器位置之间的显著相互作用表明，回声定位信号在通过额隆时可能会采取略有不同的路径，这可能是信号由发声机制发射的方式以及声学反射气囊位置的结果。
