Tor Halmrast: Sam Phillips: Slap Back Echo, Luckily in Mono
Abstract: “Slap back echo” was created by Sam Phillips for Elvis Presley´s early Memphis recordings. Using cepstrum and autocorrelation, we find that the tape delay used in Sun Studios was 134-137 ms, which is so long that the echo is perceived as a single, distinct echo in the time domain, and not the comb filter coloration of timbre in the frequency domain defined as Box-Klangfarbe. Such coloration would be perceived if a distinct, separate, reflection gave a comb filter with a distance between the teeth (CBTB: Comb-Between-Teeth-Bandwidth) comparable to the critical bandwidth along the basilar membrane in the cochlea. When Elvis changed to RCA Victor´s studio in Nashville, “RCA was anxious to recreate the “slapback” echo…To add them to Elvis’ vocals Chet [Atkins] and engineer Bob Farris created a pseudo “echo chamber” by setting up a speaker at one end of a long hallway and a microphone at the other end and recording the echo live”. Analysis of these recordings gives that the echo is somewhat shorter (114 ms and 82 ms), and much more diffuse, so “slap echo” was not actually recreated. The main findings is that even though the delay time of the Sun Studio “slap tape echo” is long, the echo is still perceived as rather “close”, because the echo is in mono. Panned in stereo, the feeling of being inside a small room would disappear. In addition, we analysed also a shorter delay, as for a possible reflection from the floor of the studio back to the singer´s microphone. These results are more unclear, but we found that such shorter delay would have given Box-Klangfarbe, but if this actually was a floor reflection, the measured deviation of the delay time must mean that the singer moved his head during the recordings (a highly reasonable assumption for Elvis!)
[JB note: Tor’s presentation was outstanding, but it was also extremely technical in terms of physics and data, so I’m not sure I fully did it justice with this live blog post. With this limitation in mind, I’ve posted several of his slides to help the more technical reader].
Tor begins (after a disclaimer that he is not an Elvis fan) with some background about Sun Studios and their recording environment, and some technical analyses of slapback parameters – comb filtering, phase, delay and frequency. We hear the delay from Heartbreak Hotel, leading into a more detailed discussion of how a very short delay creates comb filtering. If you are 1.751m from a wall, ou get a time delay of 10ms, and a Comb Between Teaath Bandwidth (CBTB) of 100Hz. Importantly it is not possible to get rif of this effect with EQ. So if you put a source/mic this close to a wall you will hear this artefact.