On Thursday, the Associated Press released the first audio recording of the sound that some diplomats say they heard during mystery attacks in Havana, Cuba. Those attacks have so far left 22 Americans with a puzzling range of symptoms, from brain injuries to hearing loss.

The sound is high-pitched and grating. You can listen to it here (but beware: it’s unpleasant).

The noise is composed of 20 or more different frequencies, all around about 7,000 Hz and 8,000 Hz. It reportedly came in abrupt pulses of varying lengths.

Not all those attacked heard the noise. Some heard nothing, and others heard variations. But several individuals involved told the AP that the recorded noise was consistent with their experience.

The recording is being examined by US Intelligence services and the US Navy, which has the capabilities to perform advanced acoustical analysis. The version released by the AP was enhanced to increase volume and reduce background noise but is otherwise unaltered.

When and where the recordings were taken are unclear, but the AP reports that the recordings have “not significantly advanced US knowledge about what is harming diplomats.”

As the AP notes, the recording may not be a full picture of what attack victims experienced. Standard recording equipment may not pick up very low or very high frequencies. And investigators may be exploring whether ultrasound or infrasound is involved.

The recording is being used for training, however. To become aware of what to listen for in case of another attack, employees at the US embassy, where some of the attacks took place, have heard the recordings. If they hear it, officials advise them to move quickly to a new location, as the targeted attacks are unlikely to be able to follow them. In the past attacks, victims said that the noise was confined to a room or part of a room.

The most recent attack occurred in late August.

Investigators are said to be exploring a range of possibilities from malfunctioning, surreptitious recording devices, to sonic weapons, electromagnetic pulses, or some other unknown, high-tech device.

Last month, the US State Department pulled more than half the staff from the embassy over safety concerns.