Subs, delays and phasing.

[sleah]

This is a bit of a two parter, but related......

Large hall, about 22 wide, 30m long. 6 x loudspeakers in the grid about 18 apart (L to R).

First pair about 3m from front wall - which is also the projection screen.

Middle pair about 6m from first pair and rear pair about 7m from middle pair.

Lecturn sits roughly under the front pair.

I've set delays on the middle and rear pairs and it all sounds fine for speech and Powerpoint farts and bangs and the occasional video clip. We don't generally play loud music (such as disco) where it would matter.. However, we are planning a couple of film nights. I'm happy with the sound from the overhead speakers but would like to use a couple of subs which would be against the front wall on the floor. I wondered if there could be any issue with the delays? I'm assuming it will be fine, as sound travels at the same speed so the low frequencies will arrive at the back at the same time as the slightly delayed sound from the rear speakers. (assuming I've set it right!)

Which leads me nicely to phasing. The overhead speakers are full range 15", if the subs (also 15") were 180 out of phase then there will surely be some cancellation? I can EQ out the low frequencies from the full range speakers but an out of phase sub would still be undesirable.

I assume all drivers are created equal, i.e. the 'top' of the waveform moves them forward (or whichever way it is), however are all preamps/amps equal? Can we assume a positive going source will be a forward movement on the actual driver?

I know many mixers have phase reverse on inputs, but assumed that was for different microphone types.

I guess one's ears are the ulimate test instument in a real life situation 🙂

Sorry if I'm not making sense....

[sandall]

I would hope that decent audio gear wouldn't introduce a phase reverse, but the easy check would be to plug it all up & play a low-frequency sweep through the system, with & without a phase-reversing cable in the subs feed. If you're lucky one way will sound better 🙂

[sunray]

Define out of phase.

[sleah]

1 hour ago, sunray said:

Define out of phase.

In the context of what I'm wittering about, I mean phase reversal.

[David Morison]

Are the current speakers & the subs you hope to add active boxes (amps built in to the speakers) or passive (ie external amps in a rack somewhere)?

Assuming passive, then you'll need a crossover of some sort to split the signal going to the sub amps and the main amps.

That crossover should have at least a switch to flip the polarity, or if it's a digital system processor, it'll have the option to add any delay needed and possibly directly manipulate phase depending on how advanced a unit you use.

Assuming active speakers, then they usually again have at least a polarity switch built in.

[sleah]

Sorry I should have said.....

The six full range overheads are passive powered through 3 stereo power amps via a Symetrix DSP. The subs are active and driven direct from the desk, as all outs on the Symetrix are in use. Can't justify the cost of an add-on card to give extra analogue outs from the Symetrix!

So it's:

Source > desk main outs > Symetrix DSP > 3 x power amps > 6 x cabs

Source > desk aux out > active subs

Edited September 27, 2022 by sleah

[sunray]

20 hours ago, sleah said:

In the context of what I'm wittering about, I mean phase reversal.

The whole issue with phasing loudspeakers of different frequency bands is fairly meaning less as; for example  sine waves of say 50Hz and 5KHz go in and out of phase 100 times a second.

The crossovers we used in the nightclubs were 24dB/octave and 12dB (for what it's worth I always thought this should have been more like 6dB) down at selected frequency or so the manufacturers claimed (we usually chose 100Hz) so phasing was pretty much a moot point.

We checked the phase of the subs and the phase of the mid tops (and also the tops if tri-amped) then did this:

22 hours ago, sandall said:

I would hope that decent audio gear wouldn't introduce a phase reverse, but the easy check would be to plug it all up & play a low-frequency sweep through the system, with & without a phase-reversing cable in the subs feed. If you're lucky one way will sound better 🙂

to see if it made a difference but to be honest the most we ever detected by ear was a dip at c/o frequency 24dB rather than 6dB. Once it was music rather than tones it seemed to make no difference.

 

EDIT: And of course the speakers were never in the same place so spacial phase changes kicked in too.

Edited September 27, 2022 by sunray