Acoustic Hailing is the process of using sound to get attention. This process occurs routinely in nature as a communications mechanism for virtually all species. The distance at which acoustic hailing can be effective varies based on several factors including the sound level, directionality, and frequency of the acoustic source, the sensitivity and directionality of the receiver, and the transmission channel environment. The sound level diminishes or attenuates with distance. Consequently, as a general rule, higher source levels have greater range.
The acoustic level of the source is commonly expressed in terms of Sound Pressure Level or SPL. SPL is a logarithmic measure of the rms sound pressure of a sound relative to a reference value. It is measured in decibels (dB) above a standard reference level. For reference, at a distance of 1 meter, a normal talking voice is approximately 50 dB and a jet engine at 30 meters is 150 dB.
An Acoustic Hailing Device (AHD) is a device used to extend the effective range of the acoustic hailing process. Loudspeakers and horns are common examples of products that have been around a long time that extend the acoustic range through sound amplification. The term Acoustic Hailing Devices came into common use following the suicide attack on the USS Cole while it was at port in Yemen in 2000. Following this attack, the United States Navy established a requirement for an Acoustic Hailing Device. The intent of this AHD was to provide the Navy with a means to establish the intent of an approaching vessel at a distance such that defensive measures could be taken should the vessel not heed a warning. One unique aspect of this requirement was that the sound needed to be focused so that it could be clearly directed at the approaching vessel.