A Sonar Phenomena: Sound Reflection
Sound reflects off smooth surfaces such as sides and bottoms of a swimming pool and a calm water surface. This can give you multiple images of an object as a mirror can give multiple images with reflected light. We show three examples of reflections.
Figure 1 is a movie showing direct and reflected images. Initially the sonar aims directly at the cinder block (shown in Figure 4a), then tilts upward to aim at the water surface imaging a reflection of the block (shown in Figure 4b). A swimmer makes waves which distort the reflected image.
Figure 2 shows a reflection which looks like an extension of the back wall to the left of the swimming pool back-left corner. The extension is formed by the sound echoing off the back wall and reflected off the side wall as indicated by the yellow lines marking the path of the reflected sound. The beams receiving the reflected sound map these returns as from the extended back wall from the direction of the dotted line. The bright arc intersecting the back corner is crosstalk from the strong return from the pool corner.
The third example (Figure 3) is a triplet of fish images. In this case the sonar is low and pointing toward the surface. A fish is swimming near the surface and provides the three images. The image at the closest range is from sound traveling directly from the sonar to the fish and directly back to the sonar, the direct-direct image. The next closest image is made from sound that has one direct leg and one leg reflected from the surface, the direct-bounce image. The third and farthest image in range is made from sound that has two reflected legs. Both the transmit and received paths bounce off the surface, the bounce-bounce image. These three paths are shown graphically in Figure 5.
Being aware that reflections can and do happen will help you solve mysterious multiple images. Look for smooth surfaces in the field of view and move the sonar or object to different locations. The repositioning of the reflected targets will give you clues to what is going on.
 Figure 1: Moving the aim of the sonar from the object at the bottom of the pool to the water's surface provides a reflection of the object. |
 Figure 2: The main pulse leaves the sonar, hits the back wall, reflects off the side wall therefore mapping the reflected sound beams as an extended back wall. |
 Figure 3: A sonar pointed toward the surface provides triplet reflective images of fish. |
 Figure 4a: Aiming the sonar correctly at the target; Figure 4b: The target's image is reflected on the surface. |
 Figure 5: Triplet Sound Reflection Diagram |
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