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.
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.