"The studio had Neumann condenser microphones that stood about six inches high. They weren't the most modern microphones, but they gave the music a certain sound quality that Elvis liked. Baffling devices and separate microphones kept the instruments, especially the drums, from overwhelming the singer's voice. For the first song, 'Hound Dog,' Elvis was set up in his booth with two microphones, one for his voice and the second for his leather-covered guitar. He didn't actually play the guitar, but he slapped the back of it. The session went something like this: 'You ain't nothing but a hound dog, crying all the time'—thump—'you ain't never caught a rabbit and you ain't no friend of mine'—thump. The thump was picked up by the second microphone."
[snip]
"When they felt as if they had something, everyone went back to their regular positions at the microphones to begin recording. 'Don't be cruel'—thump—'to a heart that's true'—thump."
The same photo also appears in "Elvis Presley: A Life in Music: The complete recording sessions" by Ernst Jorgensen, page 53.
This photo of the same session, appears in "The Official Elvis Presley Fan Clup Commemorative Album, 1935-1977" by Julie Mundy, page 16. The caption reads: July 2nd 1956, at RCA recording "Don't Be Cruel"
In my opinion, the two best songs to hear the slap are "Don't Be Cruel" and "All Shook Up".
Hear the Slap, from "Don't Be Cruel":
Supposedly it's also in "Hound Dog" (as it says above) and "Wear My Ring Around Your Neck" – but I can't hear it in either of those!
In this internet discussion one fellow says that the guitar was a "Martin D-28 acoustic with the leather covering. (The covering had some kind of funky design, along with the words 'Elvis Presley')." There are three photos of that guitar here (scroll down).
03/11/17 Tom
Parker