Imagine you are the CEO of one of the largest banks in the world. Imagine you just spent a boatload of money on a speech reinforcement system in the cavernous lobby of your headquarters building so you can host events in the space. Imagine that the walls directly across from the line array loudspeakers are 40′ high slabs of imported Italian marble. Finally, imagine that at the inaugural event, out of nowhere, you find yourself uncontrollably marble-mouthed during your address. You’d be none-too-happy, right?
That’s the power of slap echo. The sound from the line arrays hit the marble slabs directly across from them. Because of the high reflectivity of marble, a very significant and distinct echo arrived back at the presenter space after about 55ms. This caused the presenter to no longer speak intelligibly. When a person hears themselves through a sound system within 10ms, it can act as positive reinforcement or a type of foldback to let them know that “Yes, this thing is on.” If a person hears themselves delayed by 40ms or more through the sound system, the brain gets very confused and loses the ability to speak clearly. This is funny if you’re messing around with friends. It is not funny at all if you are the captain of a multi-trillion dollar financial institution.
I use the story of Speech Jammer in my classes to drive this point home. Inventors in Japan use this principle to discourage cell phone use at events. Using a focused shotgun mic, they pick up the speech of an annoying cell phone user during a movie, delay the sound about 50ms and play the person back to themselves with a very focused loudspeaker. The cell phone talker loses the ability to talk and, hopefully, ends the conversation. It’s brilliant, unobtrusive to the rest of the event and amusing as all get-out to the operator.
But don’t take my word for it. Take a look at Rhett and Link from Good Mythical Morning demonstrating this effect with GarageBand. Not only are they hilarious (IMHO), but they demonstrate the point perfectly. Feel free to watch the whole thing, but the fun starts at 2:47:
Like I said before, slap echoes can be fun, educational and psychologically interesting when you’re with friends. However, they are not so fun when you are presenting to some of your top performers, where many of their bonuses are in the seven figures, in the lobby of your flagship office building. (These were not people who wanted to see their fearless leader speak like he was in one of those post-dentist videos. “Is this real life?”)
There are many things that can be done to combat this, of course. You can steer the loudspeakers away from the reflective surface. You can introduce true foldback loudspeakers in the presentation area to combat the slap echo’s effects. You can acoustically treat the walls. However, it’s usually preferred to take care of these things prior to the first use. It’s less sweaty for everyone involved.