I come across this sort of rear mounted speaker thing once in a while in my job as a church system installer. I assume the thinking is, more = better. This almost never works out well, (the "almost" back there, is me being polite). If they could hear clearly with multiple speakers around the room, they wouldn't have called me in to clean up their mess in the first place. The only people who benefit from this shotgun approach are the 1 or 2 members with hearing loss, who happen to be sitting right in the splash-zone of the one speaker nearest to them. For everyone else in the room, it usually just makes things worse.

This is mostly about how we perceive sound. Our brains do an incredible job of processing sounds from competing sources and locations, but there are limits.

Problem #1 for the folks in the back pews is, the amplified sound arrives to the speakers at the back of the sanctuary via wire, at nearly the speed of light (which for all practical purposes here = instantaneously). The natural acoustical energy generated from the front of the sanctuary (clergy, song leaders, musicians, etc.), and the amplified sound from front speakers arrives at the back of the sanctuary at (you guessed it) the speed of sound. The speed of sound through air is quick-ish (1130 ft/sec = 1 ft / .89sec ), but nothing compared to the speed of light. So now, depending on the length of the sanctuary, there can be significant delay between when those two sounds arrive to my ears in the back row (with the amplified sound from the rear speakers arriving first).

For example; if I'm sitting in the back pew and it's 50ft from the pulpit / front speakers to the back pew, and the rear speakers are another 10ft. behind me, it takes the sound from the front approximately 44.25 ms to get from the pulpit to me through the air, and only 8.9 ms (approximately) for the sound to travel by wire to the rear speakers (10ft away) and then through the air from the rear speakers to my ears. Not only is the sound coming from behind me out of phase with the front speakers, it's hitting my ears from behind - which is not ideal, and it's arriving about 35ms too soon - relative to the other natural sound emanating from the pulpit and front speakers. If the time between these two (now separate) sounds is too great, our brain cannot process it as one sound and it's perceived as an echo. Compound that by the reflections you will likely get from each of the 4 speakers. More often than not, the result is a smearing effect that actually hurts audio clarity throughout the entire room much more than it helps.

If you have a lot of older folks in the congregation, (who usually own the back pews by virtue of seniority), those two separate sounds are distinctly amplified by their hearing aids and often come across as a echo to them. Hopefully, you have some sort of assistive listening system in the church for the severely hard of hearing. Newly constructed, or altered public spaces should have 4 systems (2 of which include an inductive loop system) per 100 seats to be ADA compliant if you're in the US. Older churches may be grandfathered-in, or get a pass on ADA compliance, but they should still have an interest in making sure everyone can hear.

Since sound pressure levels decrease by 6dB for every doubling of the distance, and our hearing usually loses sensitivity of time - churches should make the youngest members sit in the back, and then make you move forward a row or two with every milestone birthday. Unfortunately, that'll never happened.

Of course there is a proper way to do this, but I've never once seen it done correctly in a church. The correct way to combat this difference between the speed of sound through the air and the speed of the signal traveling by wire, is to use an electronic delay to realign the sound from the rear satellite speakers to the front speakers to compensate for the distance. In the example above, you'd delay the rear speakers by between 40-50 milliseconds.

A reasonable and common rule of thumb is 1 millisecond / foot, it isn't perfect, but it really simplifies things and will generally get you within the range to be perceived as one sound. Some modern power amps have delay circuitry in their on-board DSP for this purpose, and some digital mixers have this functionality built-in too.

Problem #2 is, this isn't a 5.1 Surround-Sound application, where each speaker has a distinctly unique soundtrack of ambient and background sounds coming from it to enhance the illusion of being immersed in the movie. Churches are going to have the exact same signal going to all of the speakers, with little or no variance. So not only is the sound from the rear speaker 35ms too soon in our 50ft+10ft example, it's probably too loud and will be glaringly obvious the sound is not coming from the front of the room. So it will sound like there's another church behind you having the exact same service a fraction of a second ahead of yours.

In the real world, unless the room an enormous mega-church, it's always much more beneficial to have one good speaker cluster at the front of the room to provide a unified focal point for both the sound and visual. When smaller churches start scattering speakers around the room in a last-ditch effort to hear, it usually guarantees that every single seat in the place is going to sound different - generally ranging from bad to worse, based on the distances from 4 or more speakers, and the chancel area. A single center-cluster, or wider stereo pair will be much more focused, and uniform side-to-side, front-to-back.

There are always weird nulls where there's an overlap in speaker coverage. If the church has a center aisle, you can have speakers left/right and the anomaly will be in the aisle where nobody will notice, or care. If there is no center aisle, I would encourage them to use a center-cluster of speakers. And if there are aisles to the side (not against the sidewall), there may be side-fill speakers involved to cover the seating area(s) outside the aisle.

Our ears are tilted toward the front for a reason, so our eyes and ears can work together to detect distance / direction. We have two eyes facing forward, with a little distance between them to provide depth perception. Our ears are tilted forward on opposite sides of our head to allow us to triangulate the direction and distance of a sound source, and also to help us sense hard boundaries by picking up those slight echos and reflections.

The easiest thing to do is tell a church, 'there's a reason God made our ears point the same way our eyes are looking.' They're not likely to argue with that. They may wonder why it doesn't work like home theater surround-sound, but that's going to get all "sciencey" and you can bore them with some of the details, as I'm sure I've done here.

Best of luck.

AUD10, post: 446810, member: 23483 wrote: I have come across a setup in a medium size House of Worship hall where two sets of stereo speakers are facing each other. The front two facing the congregation and the rear two facing forward from behind the congregation.

Is this likely to result in sound dead spots or is it acceptable if the speakers are phased correctly?

dvdhawk has given you a really good description of the type of problems you can encounter with corner-sited inward-facing loudspeakers in a relatively large auditorium space. Most of the problems are due to time delay (not phasing). Setting an electronic delay on the feed to the rear speakers can improve matters over having no delay, but is still only useful for listeners seated at a defined distance from the two sets of speakers. Those seated, for example, half-way back will be getting the front sound with half a building delay and also the rear sound with the other half building delay plus the compensation delay.

The only way that I have found to make this sort of setup work reasonably in a venue like a church is to have sets of speakers along the depth of the space, facing the listeners. Each set then has a delay added in its feed that is the sum of the sonic delay measured from the front speakers and some small additional time to make the directional effect work better. With only two sets of speakers, the second set would have to be positioned a little more than half way back. It's far from ideal, but should work better than having non-delayed rear speakers facing the backs of the listeners' heads.

^^^tl:dr

Multiple arrival times will cause intelligibility problems.