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I live in Chicago, and lately I've been doing a fair amount of ambience recording in the various neighborhoods and downtown. So many city ambiences on SFX CDs sound like Manhattan at rush hour to me, so I'm trying to capture some variety, and some subtle variations if I can. I already have a long list of locations I plan to record. And like many folks here, I have thoughts of selling the resulting sounds as a package.
To that end: when you pull up a city sound from a library, what are you hoping to find? What are you not finding? What do you wish was available? If you don't live in or near a big city yourself, do you get everything you need from what's already available? Does this sound like a good idea?
Every city sounds different, so if these are city sounds you plan to sell then you should identify which Chicago sounds are generic (ie universal) and which are particular to Chicago.
Subways are a good example eg compare how different the subway sounds in London vs Tokyo vs New York... No one should ever use a NY (or Chicago) subway sound for Tokyo or London etc.... They just do not sound the same... Same for traffic: compare New York vs London vs Barcelona
And of course the same is true for any city sounds with people/dialogue in them
Different scales of interior and exterior pedestrian foots would be useful, but difficult to isolate without chat, which limits their use in your case to American productions
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off the top of my head:
small exterior crowds with no ambient music
tall buildings are very urban and may have some unique sounds to the city
Subway. Subways without people and with people would be very valuable. I'm thinking about eerie deserted subway ambience with the screechy train noise coming from miles down the tunnel - know what I mean?
Along the lines of Subways (good suggestion btw @Utopia), get Bridges.
One sound i just recently wished i had more off (and higher quality) were recordings of traffic from underneath bridges. big bridges. small bridges. bridges above any water source. bridges with different kinds of materials: corrugated concrete, metal grates, etc.