xkcd

Maybe, I don’t know. I mean for a single cycle, you could probably do low frequencies easily.

Two black holes 1ly apart would take at least pi ly to complete a full circular arc around their barycenter. That would give you a frequency of one full cycle of the gravitational wave every 3years. (Actually it might be 1.5 years since I think the peaks of the wave are caused by either body and the troughs are the gaps between them so a half circle is a full cycle of the wave assuming both holes are the same size). Point is, thats a really low frequency which is “easy” to setup.

But yes, trying to prevent the decay of the spiral long enough to get a few good pulses at roughly the same frequency would require some work and might not be possible past a certain high frequency.

However, this might only limit how many of the increasingly higher frequencies you can get before it’s not really feasible. But hey you only really need the biggest one to cause weird visual effects. It just wont be as sharp of an effect since the geodesics light takes will smoothly change their relative angles to you rather than doing so almost instantly.

So if you can get a pair of supermassive blackholes to orbit with a period of ~1sec without immediately spiraling faster and colliding, your prank on earthlings would still mostly succeed. (Your prank would likely succeed even with the collision since as the frequency increased, it would eventually hit the audible range and that would honestly be more distressing in some ways (to be audible would of course require your ears to be oriented such that the spatial disturbances happen along the right axis and are strong enough to move your inner ear bones without being strong enough to cook you from the induced vibration in your whole body))