It would be nice to have curtains that absorb street noise.
Some custom curtain tailors offer a fabric that claims to be soundproof. It’s a little pricey. Not absurdly pricey, but it’s also a bit hard to be confident that such thin fabrics can absorb much sound (they claim 20%).
I would prefer to try hacks. I’ve heard that thick furniture moving pads absorb sound well. I’ve also heard that fiberous fabrics can be effective. For the moment, I probably want to pass on edgy ideas like egg cartons. Maybe later on those. What fabrics are decent for reducing sound? Specifically, I’m wondering about carpets or painter’s drop cloths. Not the simple white canvas drop cloths, but the thicker drop cloths may out of recycled fabrics.
I spent many years trying to sound insulate band and recording rooms on the cheap. I'm not an engineer but from my experience you need mass to stop street noise, which has a lot of low frequency power. No amount of foam or fluffy fabric will make a noticeable difference. Save your money on the 'soundproof' fabrics, they're no better than furniture pads, which can help a little but just aren't dense enough to stop much bass. Best I've found for windows is particle board with chunks of tire rubber nailed on. But you probably don't want to board up your windows so that's probably not a good solution for you
I always wanted to find a heavy sheet of rubber to see if that worked but never found anything affordable I could repurpose. Also wonder if heavy muslin dipped in some kind of rubber material might do anything, but never tried that either. Maybe something like those flaps they use in commercial coolers, those are pretty dense, but a sheet. Sorry no specific recommendations for your case but keep a look out for any dense material you could fashion into a curtain or window cover.
But also just check around your windows if it they need any sealing. A tiny opening can transmit a lot more sound than you think.
Right, no "soundproof" curtain will absorb low frequency noise (which street noise unfortunately to a large extend consists of), regardless what ads are trying to sell you. Heavy curtains can absorb high and mid range frequencies quite well and professional acoustic curtains made of heavy (≥500 g/m²) Molton or Calmuc are the best you could get (the cloth costs about 10-15 €/m²), but if honest producers will tell you they won't help against low frequency noise. Anyway, if your windows aren't too large you could try that for a few bucks..
The best bet are new insulated vacuum triple glass windows that aren't cheap at all.