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Audio over Long Distances

Discussion in 'Sound, Music, and Intercom' started by brb8910, Jun 12, 2017.

  1. brb8910

    brb8910 Member

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    We recently held an outdoor event at a football field with an in house audio system. How the event worked was that there was a stage on the football field with some vocal mics. On the opposite side of the field, (short way across) and at the top of the bleachers was where the in house mixer and PA system was located. In order to send some of the inputs for the event announcements to the PA system, we used a Wireless SLX4 body pack with a 1/4inch connector patched into one of the AUX outputs on the stage mixer. We had the mic receiver on the other side of the field plugged into the in-house PA system. We had some speakers that we brought in for the event, but the organizers wanted all of the announcements to go through the main PA system, while the vocals went through the speakers we brought in.

    There was a significant lag between when the speaker spoke and when it went through the PA system due to the long distance between the mic and the receiver. When we do this event again, I wanted to get an idea on the best way to transmit the audio from the board on one side of the field to the other, either via wireless or something that could help reduce the lag time between the mic and the speakers. (It was very distracting for the people making the announcements to hear their voice coming back at them with a delay.)

    Thanks in advance.
     
  2. MNicolai

    MNicolai Well-Known Member Fight Leukemia

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    The audible delay is not a function of the wireless transmission distance. Wireless transmission is effectively instantaneous. What you were noticing was probably a significant distance between the loudspeakers and the talking heads.

    The speed of sound in air is actually pretty slow. About 1140 ft/s on a mild summer day. In general, anything within the first 40ms of hearing a sound gets combined together with it without a problem. Anything after 40ms gets perceived as an echo. In this case, that translates out to 45 ft (1140ft/s * .040 seconds = 45.6 ft).

    It's not a big deal if you're in the crowd because you only hear the sound of the PA. Everything you hear in the crowd falls into the 40ms window from the first bit of sound you hear to the last. If you're the presenter, you're hearing your voice 3 times though. Acoustically from your mouth through the air and into your ears, through conduction from your jaw into bones around your inner ears, and from the PA. This means if you have 150' between you and the PA and you're talking into a mic, you have 2 sources of your voice that are hitting you instantaneously and while the sound from the main PA is arriving at your ears 132 milliseconds later -- this is well above that 40ms threshold and is perceived as an echo, throwing any presenter off their game.

    While I'm not a fan of spreading the sound system out across multiple sets of PA for something like this, you could use this same rig again and solve the problem by giving your presenters a monitor wedge at their feet. It has to be turned up loud enough though to cut through the sound coming off the main PA. Depending on how the main PA is designed, that could be an uphill battle and it might be easier just to put all of the signal through the portable speakers pointed toward the stands instead of using the house system pointed toward the field.

    This is the same reason you try to avoid having a flat rear wall to a theater. The slapback path of sound from the stage hitting that wall and arriving back at the stage could be 100-200ms and is more than enough to throw your drummer or vocalist off rhythm. Thus, we try to put angles or diffusion or absorption on those rear walls to either minimize the echoes back to the stage or scatter them up into a bunch of quieter echoes that can actually provide a little warmth to the room.
     
    Last edited: Jun 12, 2017
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  3. brin831

    brin831 Member

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    Hence one of the biggest reasons to spec a large enough pa to cover the space ... and also one of the biggest problems with stadium installs or large venues... what to time align your sources to??? center of the field ? One sideline? End zone? Etc every person is hearing a different delay from a different source ... again why you need a large enough pa to cover your expected audience from a single point ... and then if more coverage is required You time align in that plane form that one source, not various sources all facing eachother.
     
  4. JD

    JD Well-Known Member

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    As was mentioned, air is very slow. It is why you want your core PA to be at the location where the sound is sourced (usually the stage.) Any additional downstream speakers need to be put on a delay line and the delay matched to the air delay. I am reminded of these large events down in DC on the mall, where there are multiple delay lines set up for each sound tower and then aligned so that the stream of sound is in sync.
    On a playing field where there is already a dispersed speaker system, it is rare for any effort to be made to sync the sound. For game announcements, it is simply not a priority. It is best NOT to tie into such a system for that reason.
     
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