Rational Acoustics

January 29th, 2009, 11:00 AM
Hey guys,
I was having a discussion today with a very credible sound engineer about sub woofers in large scale outdoor concerts and a couple of things that happen when you have to put all your subs on the ground. We were talking about the noticeable delay that sometimes occurs in the subs when there's a whole heap of people between you and the subs - and it was suggested that all those people- (we're talking 10,000+) actually slow the sound wave down. I've always had a few theory's on what's happening but this seems to make the most sense. Obviously, there's a whole bunch of stuff that happens (eg, absorption, diffraction etc etc) with low end between having an empty venue and the gig being full of people and there's a whole lot of info in the variance of sound speed in different medium- but I was wondering if anyone has heard of any real research, or answers, on what exactly is happening when all those people get between you and your subs...

Calvert Dayton
January 29th, 2009, 02:04 PM
Ummm, define "credible."

Calvert Dayton
January 29th, 2009, 02:43 PM
Oh, OK. One thing that can easily happen with outdoor events is that the temperature may change enough to mess up all your delays between the time you do setup and the time the event happens or even throughout the course of an event, since the speed of sound changes with temperature. This is why some loudspeaker processors have a provision to connect a temperature probe and/or will recalculate delay settings for you based on a temperature setting.

This phenomenon is not specific to subs though, or placement of the speakers other than that the accumulated error associated with a change in the speed of sound obviously increases with the distance between the sources you're trying to time together. And the presence of people (or not) would have little if anything to to do with the equation in this case. If you're outdoors then even a large crowd would not generate enough heat to change the temperature much.

January 29th, 2009, 04:01 PM
Hey Calvert,
Thanks for that. The phenomenon I'm talking about is a noticeble time difference between the subs and mains- that isn't apparent before all the people get in there- or apparent when the subs are flown. I've only noticed it when there's a hell of a lot of people tightly packed in between you and the subs. Im more than happy to be told Im "hearing things" and that it's not possible... or what I've heard must have been caused by something else... But my head's telling me that with that many objects in front of your subs- something must be happening that's not happening to your mains. Like I said- more than happy to be told Im being stupid and putting this one to rest.. :)

Kip Conner
January 31st, 2009, 10:27 AM
Maybe the best way to fix these absorptions is to reduce the ad budget for the event :)

Arthur Skudra
January 31st, 2009, 01:36 PM
Maybe the best way to fix these absorptions is to reduce the ad budget for the event :)
And with the money saved, give everyone headphones!!! :D

Really cool and comprehensive article on the environmental effects on sound:
http://www.rane.com/pdf/ranenotes/Enviromental%20Effects%20on%20the%20Speed%20of%20S ound.pdf

Ben Clarke
February 4th, 2009, 07:42 PM
umm - comprehensive.... YEAH

Hey Ewan hope you're well - any chance on being a bit more specific on the particular event details inc equip, trims heights, initial delays, setup temp, gig temp, etc....

dont supppose you noticed any phase response shifts on smaart/other during gig?

feed to subs (optocore etc...) losing original path and going to backup?

just thinking out loud....

more threads like this one please.....

February 5th, 2009, 11:14 PM
Sound travels faster through water, and most (all?) solids relative to air. Wikipedia quotes 1495 m/s at 25C in water. So presumably if the sheer physical presense of the audience was going to alter the propagation speed of the subs, it would it speed up?

Could it be that the system is being driven hard and some components are going into limiting / non linear behaviour? Thus altering the relationships between system elements.

February 11th, 2009, 05:11 PM
Hey Ben, Im well, hope you're well too. Been stuck out in the bush doing a festival with no phone or internet for the past week. Had no problems with subs and they were on the ground... : ) To be honest, Ive only noticed it a couple of times- I was probably dwelling on it a little too much because of two many glasses of wine after the gig!! and to be honest- when actually started to really think about it (going back through the notes from my 3 month failed attempt at university...) I started to realise there must be something else going on. The engineer I was talking to though about it had discovered it a bunch of times doing stadium rock shows (ie 50,000+) - both mixing and teching so there definitely seemed to be some merit in it. If I notice it again, Ill try and be a bit more scientific about it and really try to work out whats going on. Note to self- no more red wine before posting strange concepts on rational forum... ; )

February 11th, 2009, 08:21 PM

To take your thinking another step further...
So you're thinking that the acoustic energy in the air is being changed to vibration in a person's body, then coming out the other side, being transformed to acoustic born energy and repeating this process through the depth of the crowd? I think all that energy changing states is being transformed into heat. The body heat then raises the temperature of the air between the ground subs and the listener casusing a different arrival time. You Aussies are whack!


February 11th, 2009, 08:39 PM

Taking my thoughts a step further is not at all suggested.

I was merely making the point because it doesn't make sense to me that having an audience would slow down the arrival of the subs.

Doesn't mean we're not whack though...

February 12th, 2009, 05:16 PM
Next time anyone's doing a gig with a whole lot of people- just take an impulse of the subs before the punters get in- then take one once it's full of punters just before show.. no one would care right? ; )

Ben Clarke
February 15th, 2009, 05:01 PM
ive got it....

Aux fed sub - too early on the booze - insert delay on aux = woops

How about the ground stacked sub wavefront staying in the inversion layer of heat created by the people... while the flown array goes through the ever cooling air of an evening?