It's old Internet meme, https://www.urbandictionary.com/define.php?term=Over%209000
I haven't measured it yet, but compared to my measured 115dB horn it was very noticeably louder.
I felt it like 120-125, but I am not a measurement tool and the frequency is different, so it may be anything.
But my neighbor said he heard this whistle through all doors and windows closed in his bathroom while bathing in the shower.
And then he told me it was first thing in his life that he was able to hear while bathing in the shower.