An acoustic release is a shackle or hook that can opened remotely by sending it a coded sound signal.
The shackle is most commonly used when storing items on the seabed without the use of ROV's or saturation divers to work the disconnect. This means that a small inexpensive ship can pre-install lots of equipment on the seabed, so that the larger more expensive ship does not have to spend it's time doing surface to seabed lifts. It is also used for lifts where it's not considered vital to recover the lifting equipment.
A ship will land an object on the seabed, then signal the shackle to release via a speaker placed in the water. Each shackle is given a pre-set code, and will only open at that given signal. Upon receiving the signal, the shackle then opens, and drops the lifting bridle down onto the object. The ship can now recover it's hook and get ready to lower the next item.
The release is most often located at the top master link of the lifting bridle. This will leave the bridle in place, so that the next vessel coming in will only have to hook up to one point.
A A is the acoustic release,
/\ connecting the hook and the bridle's Master link
o________o o is an ordinary shackle
The speaker typically emits a sound around 10 kHz. This can in favourable conditions be heard at very large distances (>6 km) - but with several thermal layers or noise sources this can become much shorter.