Although I’ve been working in the field of Power Quality since my first job out of college (er….that would be 1983) I actually majored in analog electronic design with an eye towards working in the electronic music / audio field. While I’ve never formally worked in that arena; I’ve always kept a toe in (through making music, sound reinforcement, and working for corporate productions) and on July 25th, I’m back behind the sound board for the 5th annual Om Street: Yoga on LaSalle event.
So how do math and electronics figure in to a yoga class?
First, we needed to figure out how much space we’d need for the class. We’ve got about 800′ of city street to play with, and expect 1500-2000 people (we exceeded 1000 last year). I calculated the number of yoga mats (standard 24″ x 68″) we can fit into a 60′ x 400′ area, given a 12″ spacing left / right and front/back. Answer: 1200. So if we’re going to get 1500-2000, we’re pretty clearly going to be reaching towards the back of that 800′ space, and will need additional sound systems.
In this photo from last year’s event, you can see a satellite PA system (large black speaker, set up on a speaker stand), set up approximately 200′ from the front of the space. This coming year, we’re adding a 3rd system, set up about 400′ back. However, when reinforcing sound like this, one needs to account for the speed of sound in air (relatively slow, 1126 feet / second) as compared to the speed of electricity in cables (practically instantaneous)
So, at the 200′ speakers, one would hear an echo, roughly 175 msec as the sound from the front speakers propagate through the space. At the 400′ speakers, there are two echos – 175 msec and 350 msec.
The solution is to delay the signal to the satellite speakers – 175 msec to the first set, and 350 msec to the second set – resulting in matched sound throughout the space.
Since we have minimal time to set up (the class starts at 8:00 am, the street closes at 7:00 am) I pre-mounted and wired a mixer / delay units for the satellite system. The main feed (from the front of house) comes in to the mixer, where we can tweak the sound a bit (level and tone) if needed – then we feed left and right channels to two delay units (Behringer Shark FBQ100), one for the mid speakers, one for the far speakers, that we can program individually.
The Shark delay units allow a delay setup based on distance (feet or meters) or milliseconds – we’ll be using the “feet” adjustment, after measuring the exact distance using one of those little rolling distance measurement tools that I picked up last year.