A friend has a condition that can cause seizures when listening to frequencies over 1500Hz. To help her out I made a passive first order (RC) low pass filter with a cut-off frequency at ~1300Hz but when testing this I noticed that frequencies of up to 13kHz were still audible.
Obviously what I tried isn't the solution, but with no proper electrical engineering training, I'm struggling with what I should be looking up online regarding keywords and names of circuits.
TL;DR I'm looking for a circuit that will completely cut out frequencies over around 1450Hz with 0 wiggle room for anything over that frequency. Preferably as a DIY solution, but I'm open to all suggestions.
Any circuit names or links that can point me in the right direction is greatly appreciated.
EDIT: Also, would stacking these passive low pass filters eventually achieve what I'm after?
Update (thanks transistor): The general use case is with headphones and listening to music or audio output from a TV etc. She wears noise cancelling headphones when out in public with no music only to dampen outside noise and helps little. This is just a side project to hopefully let her be able to plug into any device with headphones and know that she wont be harmed.
Update 2: Thank you everyone for your help, the general consensus that I received is that it is harder than a simple passive filter. I found this site which lets you specify requirements on a filter and suggests (with some tweaking) a "10th order Chebyshev 0.20 dB (5 stages)". So I'll see how I go. Thanks again for the input, I really appreciate it.