I need some pointers on playing "slap" acoustic guitar. I've seen people do it, it seems easy (don't they all), but it's probably a lot trickier than on bass and I can't seem to get it right.

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well its far harder than on bass simply because the strings are closer together! bbut it is still very possible. youtube john mayer playing 'neon' on his acoustic guitar, that might help, and its a great song, and he has a very good technique. basically i find that the 'popping' element of slap (bass) on acoustic is easier, but probably becasue i play a lot of fingerstyle, so you may find thumb the easier part. play resonably near the bridge depending on what sound you want, make you have a strong right hand, and watch videos and read books on how its done on the bass, because the hand position at least is very similar if not identicle for most people. its a cool technique and can add great dynamics to a song, so check out that john mayer vid, and maybe get some tabs depending on your level, and free time and give it a try.

good luck

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now im not sure how specific you are on "slapping" or if you want to include tapping as well. go listen (immediately) to "Close Your Eyes and You'll Burst Into Flames" by Kaki King. she is one of the foremost tap/slappers around right now. all of her stuff is instrumental and when i first heard this song it made me want to burn all my guitars and become a monk. using tapping and slapping, she plays the beat, the bass line and the melody all on one ovation. and she does it with her hands over the top of the neck. but i guess you're asking for tips, eh? well i've been studying her work a lot lately and here's what i can say:
1. detune-- the slinkiness provides for more rythmic response. she uses drop C on that particular one
2. alternate tunings--this matters more depending on what you're trying to achieve, but if you can get easy-to-access chords at your disposal, you can concentrate a lot more on rythmic devices
3. use the body--the body is a drum and can produce many different timbres to accentuate your lines
4. the strings are too-- strings don't only make notes, they get scraped slapped and make harmonics that can be great rythm tools
5. use both hands--sounds stupid, but 10 fingers are better than 5, and you have palms and wrists too. watch kaki, you'll understand
6. low action--you're not going to do this style so well ona martin or gibson. in this case, what ovations and the like lack in tone, they make up for in playability
7. get amped-- if you're plugged in, more of your subtle nuances (when they cease becoming mistakes) will shine through

i'd say thats more helpful than people usually get around here, so good luck!