Ambient noise apps that feature the sounds of human interaction are surprisingly rare on the App Store, but Ambiance ($2.99, Universal) goes a long way toward filling this gap by offering a staggering collection of almost 300 tracks from urban environments, in addition to the more popular nature sounds. You’ll even find several specimens of the elusive coffee shop track, the most of any ambient noise app.

When it comes to customization, few ambient noise apps compete with WhiteNoise ($1.99, iPhone). Core options include static noise and the sounds of crowds or running water, but you can mix up to five tracks from a library of 40 to create the concentration environment that’s best for you. As an extra topping on an already generous helping, you can mix in your own ambient sounds and loop them in.

SleepMachine ($1.99, iPhone) might not have the range of customization options found in WhiteNoise, but its stereo recordings allow it to emerge as a superior audio experience. As a bonus, you can adjust the strength of each track. It’s a pity that the nature-focused SleepMachine doesn’t have the crowd selections found in WhiteNoise, but the quality of the sounds largely makes up for the loss.

The great outdoors have never felt so close in urban settings than with Naturespace (free, universal), which uses 3D “holophonic” recordings to recapture sensations such as standing in an empty forest, or sailing wind-tossed seas. The free app comes with a small assortment of quality tracks, with more available for purchase within. Its only shortcoming? You can’t play iPod music with it running.

In addition to 43 ambient recordings, Relax Melodies (free, iPhone) also features two “binaural beats” for either concentration or relaxation, which play in the background of the tracks you’re listening to. Relax Melodies’ many musical tracks also make it a smart choice for music lovers, although the way the sounds stop when you open another app with the free version can be a drag.

Sleep Bug: White Noise Soundscapes (free, universal) offers similar customization options to WhiteNoise and SleepMachine, but it handles them differently. Here, each core track has several auxiliary tracks that you can enable or disable at will, allowing you to, say, add lightning strikes to the mix. Be prepared for some odd options, though, such as a disco riff in the pro version’s airport track.

If it’s simplicity you want, you can’t get much simpler than White Noise Box (free, universal). It gives you five options from the spectrum of white noise, and it provides five selections of nature-based stereo audio tracks that roughly mirror the spectrum selections. That’s it, aside from a volume control. An optional charge of $0.99 removes the relatively unobtrusive ads and the store icon.

If you liked the simplicity of White Noise Box but wish it had a wider selection and deeper customization options, then you’ll enjoy using White Noise +($1.99, iPhone). The restaurant and market tracks satisfy that coffee shop itch, and underneath the effective minimalistic menu, you’ll find a four-track mixer, plus playlist and timer options. From keyboards to Zen gongs it’s got a little bit of everything.