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Calculating a delay without an RTA?

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junoprobelaunch View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote junoprobelaunch Quote  Post ReplyReply Direct Link To This Post Topic: Calculating a delay without an RTA?
    Posted: 19 June 2018 at 9:22pm
Hi 
Am soon going to be setting up my very first soundsystem but am a bit confused as to how I could calculate and set delay without an RTA mic or if it's even possible. Can anyone share a method I could use ?
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Post Options Post Options   Thanks (1) Thanks(1)   Quote I-shen Soundboy Quote  Post ReplyReply Direct Link To This Post Posted: 19 June 2018 at 11:33pm
Mobile phone? Use the microphone in that. 
Get a free sound app (I use sound analyser by Droid dev. For Android, Plenty others about)
Learn how to set up and read a waterfall plot.  This shows amplitude by frequency over time.  Set up the scale so it shows a second or so.

Now get a burst track. There's probably one on the void website, or use something off YouTube.  That's full spectrum (all.frequencies) noise for a short period of time (milliseconds).  Play that through your system. On loop.  Stand well back and measure the burst track - use the hold on the  waterfall to see how the lower frequencies arrive (at the microphone/phone) milliseconds after the higher frequencies.

Add delay to the highest frequency bands on the LMS and test until all frequencies arrive approximately the same time (wherever your measuring point is.... you can only get perfect delay alignment at one distance).

If you don't have a delay line/LMS, then you can compensate physically by offsetting the drivers: the higher frequency drivers are progressively further away from the measurement point, usually a couple of inches further back.





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Post Options Post Options   Thanks (1) Thanks(1)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 20 June 2018 at 1:02am
An RTA is not the right tool for alignment anyways. Nor really, is a waterfall plot done with a small mic on a mobile phone in a hugely reflective setup.

If you have access to measured group delay plots of all of the boxes (unlikely) then you can technically get into a ballpark from that, but again it won't factor in any EQ filters, which includes your chosen highpass and lowpass filtering on each passband.

If you don't have access to a dual channel FFT tool, then your best bet is to play warbles or second best, sine wave at the crossover frequency, polarity invert one passband and delay until it is quietest, then flip polarity back. This will get you into the right city, so to speak - the right ballpark is often way further down the line.

In all honesty, REW is free, it works even with the basic soundcard in any machine if you can make a stereo 3.5mm adapter to provide separate left and right channels for loopback, and has a comprehensive manual. Mic wise, even a basic dynamic handheld vocal mic will do to get you started. It won't have a flat response but you're looking for relative time offsets anyways so as long as its kind of the same and has SOME low end info present around say 100Hz you can use it for rough alignment and you learn something in the process.

The REW manual is nice and comprehensive and the interface is easy. Do it in your garden.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Timebomb Quote  Post ReplyReply Direct Link To This Post Posted: 20 June 2018 at 11:14am
Its true a free copy of REW and any basic mic is all the kit you need to get 2 cabinets roughly aligned at crossover point, there are some budget measurement mics that are reasonably accurate frequency response wise and some even come with calibration files.  
James Secker          facebook.com/soundgearuk
James@soundgear.co.uk               www.soundgear.co.uk
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sypa Quote  Post ReplyReply Direct Link To This Post Posted: 20 June 2018 at 12:16pm
How does eq affect time? Through additional processing lag or some funky phase bussines  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Timebomb Quote  Post ReplyReply Direct Link To This Post Posted: 20 June 2018 at 12:34pm
EQs and high / low pass filters introduce phase distortion, the steeper the filter the steeper the shift, there are some FIR phase linear filters on some units but the vast majority will cause phase shift.  
James Secker          facebook.com/soundgearuk
James@soundgear.co.uk               www.soundgear.co.uk
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Post Options Post Options   Thanks (0) Thanks(0)   Quote fatfreddiescat Quote  Post ReplyReply Direct Link To This Post Posted: 20 June 2018 at 1:00pm
Eq will affect phase, not sure I would call it distortion as it can do the opposite and also correct the phase response if used to eq a minimum phase area of the response.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote opus jody Quote  Post ReplyReply Direct Link To This Post Posted: 22 June 2018 at 6:00am
There's a great way of doing delays with just a pink noise generator, and and SPL meter.
Wire one speaker reverse polarity, then mess about with the delays to get the biggest sound reduction (from phase cancellation) possible. 6-8 dB reduction is perfect.
then correct the polarity of the wiring.
Voila.

With mid & top delays, you really need an RTA to see the cancellation notch, but the same thing works.
I guess if you could isolate a noise range around the crossover point, then the same would work.
 
beware of sound reflections though. SPL includes all reflected sound.
Improvised Hardware Music http://vimeo.com/user9389813/videos
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