Did I see the speed of sound?

OK, so I’m in the New Orleans Superdome for a concert. I’m sitting up near the rafters to the side of the crowd. Floor is full of chairs for seating. The place is about 1/4 full with about 3-5000 on the main floor and the rest in the lower 2 levels behind and beside them. The fans were asked before the show to download an app that flashes their phone to the music (Whamlights?) At various points during the show, I noticed what can be best described as a wave front propagating from near the stage to the upper levels. This was from the flash of 15-20000 phones coming on solid and staying on for a few seconds or so. Not the twinkling effect of random flashes. The front moved through the crowd almost but not quite instantly. The lights also went out in the same order. My theory is the average phone heard and processed the sound of the music and the wave was the result of the time lag generated by the speed of sound in that venue at that time and not by the phones electronics.

If you are saying the phones are responding to the sound of the music being played in the arena, then yes, it could be that. Sound travels around 1100 feet per second, I don’t know how big the Superdome is, but if it’s around 1100 feet from the speakers to the back row there should be about 1 second difference between the front back row. There could be the effects of echoes within the arena also though, it depends on what the phone has to receive in order to turn the light on or off.

Yes, a large amphitheater is big enough for sound propagation to be noticable. In arenas where there are repeater speakers far from the stage (like Shoreline Amphitheater in Mountain View, CA), the repeater speakers need to be artificially delayed so that the sound coming from the repeater occurs at the same time that the sound from the stage speakers reaches them.


OP: a very lovely image and quite artistic moment/event you interpreted.

The speed of light is not that difficult or rare to “see”. My personal favourite are fireworks, where you can often notice the delay between seeing the flash and hearing the boom of a rocket going off. Similarly, I’ve repeatedly heard the effect, e.g. when there was a corner being kicked at the faraway end and there was a noticable delay between seeing the kick and hearing the sound it made. I guess the shortest time interval that you can easily notice with the untrained eye or ear is about a third of a second, and sound travels roughly 110 metres, or 360 feet, in that amount of time. That’s a distance you can easily have in a major arena.

By the way, for something really cool see this experiment for measuring (very roughly) the speed of light with materials available in the comfort of your home. Though you won’t get a stunning visual effect, unfortunately.

I wasn’t concerned about the time delay between sight and sound based on distance. That is thunder and lightning basically. Actually the neatest sound delay was when I was a kid and we were fencing a 20 acre field. The fence was stretched in one pull down about 3/4 of one side; around 700 feet. As they would pound the staples into the cedar posts at one end, the sound traveled the barbed steel wire faster than through the air.
Looking at the tech behind the phone app, now I am not sure if it was sound wave or a generated effect. They embed an ultrasonic signal into the music which is picked up by the phone mics. The timing does look correct enough to be sound wave propagation.

The speakers and stage would have been on the sidelines centered if it were configured as a football stadium.

This reminds me of when the Georgia Satellites played at the 2nd Woodstock festival. They performed their signature hit “Shine.” In the break, there’s a guitar riff and the singer says “Yeah.” Then another couple guitar riffs, and the singer says “Yeah.”

The audience shouted “Yeah” along with the lead singer, but there were so many people at the show, their “Yeah” got to the stage a second or two after the lead singer sang it. This means the bulk of the audience was further than 1100 ft from the stage. Isn’t that awesome?

Yes given that discription of Whamlights, the effect is to show the speed of sound.

I think you can notice the speed of propagation in the first few seconds of this video.

For a fun demonstration, see if you can find 1) an FM radio, 2) the Big Ben, and ideally 3) an adorable British person.

Collective Soul?

The best “visual” demo of the speed of sound I saw as a kid was when another kid down the street would bounce a basketball. The “thump” was way off. It seemed like I was watching one kid bounce the ball while somewhere else was another ball bouncing that I only heard.

NY’s Cathedral of St. John the Divine (aka St. John the Unfinished) is often used as a performance space. The reverb time is tremendously long (making it, frankly, terrible for most music), and musicians have often said you could sing solo simple canons, which are two-part songs with one melody, the second a certain time interval behind, eg Row Row Row Your Boat.

Never heard it done, however.