In underwater acoustic networks (UANs), an acoustic modem is one of the key elements that governs underwater communications. The major demerit of the existing surveys conducted on acoustic modems is that such surveys do not comprise of the novel and state-of-the-art modems. In addition, the described characteristics do not specify the full potential and capabilities of the designed modems. This, in consequence, makes it difficult to identify the parameters for improvement in future investigation. Also, it does not provide the optimal choice of a modem for various underwater applications. To overcome these challenges, this paper considers the latest acoustic modems designed in the last five years. Further, the modems are classified in terms of their suitability for the various applications, which helps the selection of the appropriate modem for a particular application scenario. Furthermore, the specification of every modem is described in a manner so that it is completely analyzed in highlighting its promising performance parameters as well exposing its region of compromised performance. Besides, acoustic modems developed by commercial manufacturers are also described. These strategies help researchers, scientists and engineers to compare to opt for the right modem for the right application. They also provide knowledge of the margin for improvement in the characteristics of these modems. Finally, perspectives are given for future investigation to make these modems more efficient, robust, smart and effective.
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