Quadrophonic system

[marsonpee]

Hello

I have been presented with a sound issue for an upcoming project I have. I have a fairly large area and have deemed it to be unsuitable to jus have one set of speakers from the front so wanted to have a second set of speakers at the rear which are aimed on the audience area. What I wanted to know is would I have to invert the phase on the second pair so as to minimise comb filtering? The speakers would in effect be aiming from 4 corners onto the audience.

 

Thanks Guys Andy

[Albatross]

It sounds like you want to have more even coverage, or lower volume throughout the large room.

This is typically done with delayed Stacks, i.e. Speakers part way down the room, time delayed so that the sound wave front co-incides with the sound from the man speakers.

 

Having a 2nd set of speakers pointed back toward the stage/main speakers will create more problems than it will solve. Inverting the polarity ( as you've called changing phase) will not solve the comb filtering & cancellations that will occur. All that will do is shift where in the room they occur.

 

To do this correctly, both sets of speakers should ideally point in the same direction.

[Simon Lewis]

Just to add to Albatross's useful comments....

 

Running four sets of loudspeakers does not constitute a quadrophonic system....

 

Inverting phase/polarity rarely solves comb filtering issues

 

Pointing loudspeakers at people's backs tends to ruin the sense of where the sound should be coming from.

 

 

 

Try searching for loudspeaker delay systems, there should be some useful material here.

 

Good luck!

 

Simon

[Bobbsy]

Good advice based on the original question, but....

 

Use of a second set of delayed speakers tends to be the solution where distance is the problem, i.e. a long, relatively narrow room. However, it it's simply a large room of normal proportions, you may also need to just look at using selected professional speakers which allow you to control the coverage. To overly simplify, you'd use cabinets with a fairly wide coverage pointed at the audience near the stage, and cabs with a narrower beam pointed at people farther back.

 

Obviously, there's more to it than this, but hopefully this gives you the basics to start your search with!

 

The one universal here is that you DON'T want speakers at the back of the room...other than for special surround sound effects.

 

Bob

[mackerr]

Inverting the polarity ( as you've called changing phase) will not solve the comb filtering & cancellations that will occur.

Inverting the polarity involves changing the plus and minus voltages of a balanced signal, such as pins 2 & 3 of an XLR, or the hot and cold outputs of an amplifier. Phase is a parameter that describes the time and frequency relationship between 2 signals.

 

Mac

[Albatross]

will not solve the comb filtering & cancellations that will occur.

Inverting the polarity involves changing the plus and minus voltages of a balanced signal, such as pins 2 & 3 of an XLR, or the hot and cold outputs of an amplifier. Phase is a parameter that describes the time and frequency relationship between 2 signals.

 

Mac

 

Correct!

I meant to edit my own post but rushed out the door to a gig today.

I wrote -

Inverting the polarity ( as you've called changing phase)

but meant to write

Inverting the PHASE ( as you've called what is actually changing POLARITY)

 

Most desks have a button for each channel on microphone INPUTS that change/invert polarity, but is often incorrectly labelled phase. This is what I took the OP to mean.

 

I'm not aware of any desks that change the polarity by button press to the outputs, but some amplifiers do, or it could be achieved by rewiring the outputs to the speakers.

Regardless of swapping polarity, this will not solve any problems for the original posters configuration.

 

Adjusting the phase would require a time shift of some sort, usually a delay device, and this will again not solve the problems, unless the speakers are all pointed in the same direction, allowing time alignment between spatially separated speakers.

Inverting the phase would only be possible on some frequencies ( and their multiples) as it is a time based function.

 

Perhaps Andy could give some more detail of what his gig is, so that we could offer more helpful advice?

[marsonpee]

Thanks all for the recent advice. The reason I called it phase was because on my system controller it has a function where you can rotate the 'phase' from 0 to 180 degree's. What I am actually doing is a music event in a large sports hall, I wanted to 'home' the sound in onto the audience rather than blasting it at them from the front. My main problem with this is going to be the hard walls and the sound reflections inherent with this.

[Bobbsy]

The phase/polarity thing is a very common topic for mistake and debate...not helped by the fact that a fair number of mixers have buttons marked "invert phase" or similar, when they really mean "invert polarity". It's no wonder that wrong useage is very common!

 

With regard to your gig, is it live or a DJ-type thing? Blasting the audience from 4 corners would be a major no-no for a live act, but I have seen it done for playback in clubs. It's still not good, but at least you're not worried about losing directional information that tells the audience where the band is. In the "4 corners" scenario, switching polarity isn't going to do anything for you though...you WILL have peaks and troughs where the sound drifts in and out of phase whichever polarity is started with.

 

The big question is: do you have a budget for this? It IS possible to get decent sound in a sports hall, but, in simple terms, the way to do this is to use an array of speakers with tight dispersion angles, flown, and carefully aimed at the audience (not the walls) so that each covers it's own section of audience and nothing more. It can work pretty well...but won't be a cheap solution.

 

Bob

[marsonpee]

The budget is to be as low as possible to make it work effecitvely, if it means flying multiple cabinets and so forth then thats what would have to be done. The gig is a mixture between live acts and DJ, the maximum audience would be approx 500 people, at a guess I would say the room is approx 30 metres or so, the rear of the room is to be a chillout area so wouldnt need to have high spl.

[Bobbsy]

Well, with live acts (in my opinion at least) you really have to have the sound coming from the front of the room only. Think of the effect of people watching the lead singer on stage but hearing his voice come from behind you!

 

A room 30M long holding 500 people isn't particularly large by SR standards...certainly you don't need to get into delay stacks or anything like that. Decent speakers up at the front should give you the coverage you need, particularly if you want a "chill out" area at the back.

 

Ideally you'd have the sort of ultra-professional setup I mentioned above...but the most important thing in your situation is to get the speakers as high as possible and angle them to point at the audience (i.e. nice soft bodies) to avoid, as far as possible, reflections off the walls and ceiling.

 

Bob

[marsonpee]

Ah, I am again going total overkill into this as usual ** laughs out loud **. When you mention 'decent' speakers, what type do you mean? horn loaded, line array or a standard good bass reflex cab? Thanks :D

[Chris Hinds]

When you mention 'decent' speakers, what type do you mean? horn loaded, line array or a standard good bass reflex cab?

One that is a known, professional grade system suitable for arraying, flying (this will help with coverage but isn't essential), high power output. It could concieveably be any of the types you have listed, but if a line array it should come with a company like Wigwam or similar attached to do the setup. Something like Nexo AlphaE, D&B C7 not something from the local music shop.

 

Regards

 

Chris

[marsonpee]

So would I be able to get away with something like the JBL SRX700 series? After a bit of research and drawing out the dispersions etc I found that I could do it with some SRX712M's, a pair splayed at each side, suspended from flying yokes. Then a pair of the SRX718S subs at each side. Im looking for the system to be fairly compact so Ideally I dont want to venture to the line array end of things. Cheers everybody for your help.

[Matt Riley]

Hi Andy,

 

You may find SRX series difficult to hire in the UK, and when you've been doing calculations you need to remember that the 90*50 angle for the cabinet is probably only for the horns - it loses a lot of its directivity below the 1.2 kHz crossover point. Also if you are using a pair splayed each side - then that means that you will have 180 degree coverage on each side, which is too much. If you use the JBLs, then use 1 each side slightly tilted in so the horns aren't firing onto the walls.

 

Assuming that the room is about 20m wide I would lean towards a flown C7 system each side, like Chris suggested with matching subs, as this will provide better performance and is easier to hire.

 

If you do go down the SRX route, then just use a double 18" sub on each side - its probably cheaper and easier! And it may help to put the subs together, on top of each other in the middle as this will reduce comb filtering.

 

To any hardened D&B types out there - would using the CSA mode with Qsubs/B2s in this situation help with getting good sound out there - I guess the directivity would mean that the bass energy is kept off the walls and therefore room modes are less excited?

 

Regards,

 

Matt

[Bobbsy]

Ah, I am again going total overkill into this as usual ** laughs out loud **. When you mention 'decent' speakers, what type do you mean? horn loaded, line array or a standard good bass reflex cab? Thanks :D

 

Chris Hinds got in before me with the answer, but I couldn't have said it better. There's lots of ways to design a good cabinet and you could end up using any of the above depending on the brand. The important thing is to have a known, controlled coverage and a suitably high power output level.

 

I've never used the SRX series so can't really comment on their suitability but if the coverage is 90/50 then Matt's comments about a splayed setup sound sensible to me.

 

Count me as another d&b fan btw...they'd do your job very nicely indeed.

 

Bob