The speaker is a device used to play audio. Nigel Marven used a speaker to attract the attention of a Basilosaurus.
In Sea Monsters[]
Into the Jaws of Death[]
Nigel Marven, after recording the calls of a Basilosaurus, lowered an underwater speaker into the Eocene seas and played the recorded audio.
Soon later, The Ancient Mariner was attacked by a male Basilosaurus who had heard the sound coming from the speaker. Whilst diving with the whale, Nigel watched as the distressed predator tore the speaker off.