Page 61 - Summer 2021

FEATURED ARTICLE
Echo Classification: Statistics of Echo Fluctuations
Timothy K. Stanton, Wu-Jung Lee, and Kyungmin Baik
Widespread Use of Echo Classification
Everyone has seen images of unborn babies from medical ultrasound and weather maps of incoming storm clouds from radar. What these technologies have in common is that they involve the classification of echoes received from sensor systems that send out a signal and receive echoes from objects of interest. In each case, echoes are classified into meaningful information, much like how a sonar operator listens for echoes from enemy submarines.
The use of sending a signal and then receiving its echo is widespread and includes both man-made and biological sources (Figure 1). For example, many animals, such as dolphins, bats, and other mammals, generate and receive sounds through various anatomical mechanisms to sense objects around them. Moreover, going beyond acoustics, radars and lasers generate an electromagnetic signal rather than an acoustic one.
In all cases, whether the signals are acoustic or electro- magnetic, the echoes can be classified through various
©2021 Acoustical Society of America. All rights reserved.
https://doi.org/10.1121/AT.2021.17.2.61
quantities: the amplitude and time of return of the echo, the image created by multiple echoes as the sensor system scans an area, and the characteristics of the fluctuations of the echo across these multiple transmissions. Classifi- cation using the fluctuations is the subject of this article. These fluctuations are, in essence, the “texture” of an image formed from the echoes.
Echo Classification Serving Many Needs
Echo classification is exploited across a wide range of societal and ecological needs (Figure 1). All applications use echoes to classify objects of interest that cannot easily be seen. Uses of echo classification include
(1) Health. Through use of medical ultrasound devices, echoes are processed to produce images of the inte- rior of the human body (Ketterling and Silverman, 2017; Ruoss et al., 2020). These images can be used to assess how much a fetus has grown or to detect the presence of a tumor.
(2) Food for humans. Acoustic echoes from scientific echosounders are used to produce images of fish schools in the ocean (Stanton, 2012; Warren, 2012).
The images help fisheries managers monitor the population of the fish, with the goal of maintain- ing a sustainable source of food.
(3) Food as prey for animals. In the animal kingdom, echoes from the sounds generated by echo-locating mammals are used to navigate and hunt prey (Sim- mons, 2017; Tyack, 2017). For example, echoes are used by dolphins to hunt for small fish and squid in the ocean and by many bats to hunt for insects in the air.
(4) Defense. The echoes from sonars (underwater) and radars (in air) are used to detect and classify enemy targets such as submarines and aircraft, respec- tively (Le Chevalier, 2002; Kuperman and Lynch, 2004). A key aspect is discriminating between the echoes from these targets and from other unwanted sources (“clutter”) such as the seafloor (sonar) or
Figure 1. Echo classification has applications from biomedicine to land and sea. Copyright © 2021 Timothy K. Stanton, all rights reserved.
Volume 17, issue 2 | Summer 2021 • Acoustics Today 61