My wife and I went to a Crosby, Stills, Nash & Young concert the other evening at the Saratoga Performing Arts Center. It was a great show in spite of a little rain. You can read a local review of it here. What does this have to do with photons and pressure waves? I’ll get to that in a minute.
First, a little about the show. This concert was part of their 2006 “Freedom of Speech” tour. They did two sets, each around 90 minutes long. We had lawn seats (if you’re not familiar with SPAC, the facility is on a hillside with a portion of the seating on a grassy slope). The first half was dominated by material from Neil Young’s new “Living with War” CD while the second set featured some of their quieter material, but eventually got quite raucous with tunes like “Ohio” and Young’s new “Let’s Impeach the President”, complete with sing-along lyrics presented on the big view screens. It was one of our favorites. The concert had a clear protest/anti-administration vibe to it, but that shouldn’t come as any surprise to long time CSNY fans (although it did, apparently, if you check out the comment posted at 11:08 am on August 20 which follows the review link above- the responses to it are even better). It wasn’t 1969, but the crowd certainly had its share of grey hairs, and I’d guess that a fair number of them even remember (parts of) 1969 (I was eleven at that time). Some songs, like Deja vu, had a sense of, well, deja vu about them. Unfortunately, “We have all been here before” wasn’t referring to a concert, but more a time of war and anger over it.
From our (lack of) vantage point, it was sometimes difficult to see the stage. This was not helped by those in front of us using large umbrellas or tallish chairs (we came prepared with our usual inches-off-the-ground beach chairs along with ponchos to shed the rain). Fortunately, we had clear sight of one of the large view screens, and while you’re not looking directly at the stage, the result is certainly more appealing than peering through wet binoculars.
My wife made an interesting comment after the show, namely that it seemed that the musicians (Neil Young in particular) seemed to be “out of sync” with the music. That is, the body movements seemed ill-timed. While in younger days I might attribute this to an ingestion of “trendy chemical amusement aid”, as Frank Zappa might say, on the part of audience members, musicians, or both, I knew there was more to it than this. What my wife had noticed was a multiple propagation delay effect.
As a drummer, I have developed a tendency to note whether or not things are out of sync, sonically speaking, by relatively small amounts. As an engineer, I have studied the subjects of audio and acoustics, and I remember being fascinated when first introduced to the subject of psycho-acoustics and items such as the Haas Effect. Yes, I was one of those kids who used to count the seconds between lightening flashes and thunder crashes. Anyway, back to the concert. For all practical purposes on a human scale, the speed of light is fast enough to be considered instantaneous. At approximately 3×10 to the 8th meters per second, the images of the band on the view screen (the photons that augmented those directly from the stage) were reaching us in under a microsecond. There’s no way normal human vision would notice any timing discrepancy between the stage and screen. Unfortunately, sound pressure waves in air propagate much more slowly, at about 340 meters per second. Consequently, the sound was arriving at our location perhaps 100 to 150 milliseconds after the corresponding visuals. That’s enough to notice if you know what to look and listen for, and of course, if you’re not inebriated. I have a tendency to watch a drummers’ hands as they play, and given the rapid wrist flicking and stick movement, along with my own sense of playing the instrument, a strange sense of disconnect comes over me when the audio and video are not in sync. Sometimes it can be so bad that I simply cannot watch further. I don’t seem to have as much difficulty with guitar players, particularly while they flail through a solo, perhaps because most rock players tend to toss in a wholesale crate of tortuous body bending and associated facial contortions while they tweedle the top frets. Rather distracting actually. Consequently, I found it interesting that my wife noticed the variance with the guitar playing, but then, she’s not a drummer, and well, Neil was on camera a whole lot.
In any case, propagation delays or not, like Mr. Young, I think the president should be impeached.