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Ether 9th August 2011 06:21 AM

Conference room PA system - N00b needs advice
Dear all,

First of all please let me tell you that I am a complete newb on this matters, i worked as DJ in my youth but never got too much into the technical aspect of the business. Years passed and now I do work in a totally different area which is the hospitality (hotel business to be more exact). One of our hotels is experiencing an expansion and the owner asked me to design the audio system for a new mini-ballroom that they're going to use as conference/speech room.

Naive as I am, accepted and now I am quite in trouble, hehe...

Will jump straight away to the basics. The room is a carpeted floor, gypsum wall/ceiling area of 11m width, 24m length and 4m high. I have used some program to calculate what kind of (ceiling) speakers should I install to have a decent sound distribution, and i ended up with 10 units of 8" speakers evenly installed through the area.

The cable im using for these is just generic (don't ask, i have budget limitations and some material stock that i had to use), and every of the speakers leads to the sound control cabin, which is equipped with a TOA ZA2120 amplifier and a Yamaha MC82CX mixing table.

We have two possible locations for the stage, which are represented in the pic below. Since the use is going to be simply speeches or conferences, i just passed one cable directly from each of the locations till the sound control cabin.

Now the problem is, i have two batches of 5 cables (divided in L/R groups) and I don't really know how to attach them to the amp, should all them just go in the rear panel squished as one or should i use some kind of box to combine them first? What is the standard procedure for this case?

Detail of the sound control cabin below:

Also, I passed just one cable that is going to end in a Jack-in outlet in every stage position just to install a fixed stand microphone for the MC, the rest of mics will go wireless, the point is... I know it can be done (way) better, what's your advice, audio wizards?

One more thing, presenting to the owners the plan in Sketchup looks not right from my point of view, what program should i use to present this kind of design in a professional way?

A BUNCH of thanks in advance, I don't mind being flamed as long as I can learn, so fire away!

Thanks again,


mickeymoose 10th August 2011 02:23 PM

One parameter I did not notice is the ceiling heigth. To get away with only 10 speakers the ceiling has to be quite high and I do not believe 8" units would have the power handling capacity required.
If you google "distributed pa system design" you should get some good info. E

Ether 10th August 2011 02:30 PM

I mentioned is 4 meters high, mickey :)

I used the TOA program for ceiling speaker calculation, the results were using 8 units, I added 2 just in case.

Link to the program: TOA Speaker System Design Software


Pano 10th August 2011 03:33 PM

Your basic plan looks good, and I don't see any problems with the Sketch-up, it looks nice.
A few points:
  • I can't find that amp on the TOA site. Got a link? Is it a 70V amp?
  • With speaker cable runs like this, you should go to a 70V system.
  • Forget stereo, run it all mono or use zones if you want
  • If you can arrange a switch or control to turn off the speakers over the stage, you should. It will sound better and help avoid feedback.
  • Don't expect a distributed speaker system like this to sound great, they rarely do. If done right you should get enough vocal intelligibility to make it worth your while.
  • One mic line at each position is probably OK, but if you have the budget, do a few more. Remember, a lot of presenters will want sound from their laptop on stage.

Ive worked in 100s of ballrooms that have ceiling speakers. Generally they don't work well, so I always brought my own. I can remember only 1 exception to this over many years. So don't expect too much. It will work, if you plan correctly.

Ouroboros 10th August 2011 03:39 PM

If you are using 100V (or 70V if you are in the USA sphere of influence) speakers, then it is a good rule of thumb to set the speaker tappings to give a total load on the amplifier to no more than 80% of its rated power. This way you will have enough headroom in the amp to allow you to increase the individual power settings on some of the speakers if it is found that the SPL is not high enough in some parts of the building.

Pano 10th August 2011 03:45 PM

Good point!

Ether 10th August 2011 04:45 PM

Thanks a lot for the answers.


Regarding the "stereo" thing, yeah we could call them zones as well, as the speakers are grouped in two areas as well. Yeah stereo definition didn't make too much sense after all...

I separated a bit the speakers from the stages position on purpose, hope its enough! :)

Yeah, I know is not gonna be a miracle of sound quality, but at least and as you say Pano, I hope at least to make it useful for people to understand the speeches :)

Regarding putting more inputs in the stage positions, now is too late since the walling is closed already and would be messy... couldn't I fix that with a stage box?


The amplifier is a 120V one, IIRC, the speakers are 7.2V so total power needed would be 72V, I applied some margin and got a 120V. (Is what you guys mean, right?)

Thanks a lot again, I really appreciate your input.


Pano 10th August 2011 05:10 PM

Hi Ether. No, not the mains. The voltage of the speakers. See this:
Constant voltage speaker system - Wikipedia, the free encyclopedia
Important for systems like this. It's the best way to do it.

I don't know what you mean by a "stage box". Is this a multichannel snake you plan to run back to the mixer?

Enzo 10th August 2011 11:57 PM

I agree with Pano, unless you have some need to send individual signals to each speaker, forget the wires out to each one. Study that "constant voltage" speaker distribution article. In the USA, that tends to be "70.7 volt" distribution. Most commercial PA amplifiers include a 70.7v output or some other CV output. Then all speakers are in parallel running from the same pair of wires. At each speaker will be a small matching transformer with adjustable taps. They are not expensive. You could run all the speakers together, or you could connect them in groups - "zones" - so the room can be divided in use.

CV speaker distribution will sound OK, especially for this sort of application. I used to install them for jukebox systems in restaurants and such, and no one complained it wasn't hifi enough.

The suggestion about turning off the speaker over the stage was good. Or just don't put one over the stages in the first place.

CV transformers have power taps. The signal is sent down the wires, and a ceiling speaker will usually have something like taps for 0.5w, 1w, 2w, 5w, 10w. Or whatever. In a bar or restaurant, for example, if there is a speaker over the waitress station, they would tap it lower. In other words if all the ceiling speakers were set at 5 watts, the one over the waitress station would be set at 1 watt. That way for a given loudness that one area would be less loud, so they can hear their orders.

Ouroboros 11th August 2011 06:52 AM

The ZA2120 has both 100V and 70V outputs according to the data sheet.

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