Soundproofing Floor Between Upstairs / Downstairs Units

[Spencer Holland]

I recently purchased a home built in 1964 in a great location in Denver (LoHi). We have a separate entrance that allows us to turn our 4 bedroom into two 2 bedrooms. We have just begun Airbnbing out the lower unit and my largest concern is the noise from walking upstairs. This isn't a huge deal for Airbnb, but I'm thinking long-term about a solution for when I move out and rent out these 2 units. This is a higher-rent area, so those tenants would have higher expectations. I'm willing to make a decent-sized investment here, but how can I stop the noise that comes from the hardwood floors upstairs shaking when we walk on them? The bathroom and kitchen also both have tile which is about 1/3 of the upstairs