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Old 6th December 2011, 07:12 AM   #1
MrChips is offline MrChips  United Kingdom
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Default Connecting 8 speakers?

Firstly, please excuse my terminology ... it has been many years since I got involved in Audio.

Problem.
In a small village hall, the public complain that those nearest the speakers get the sound too loud, and those at the back can't hear what is going on!

Equipment in use.
Portable power amp with two speakers, costing in the range of 200-300. I do not have the make and details at the moment.

Proposed solution.
To purchase 8 x 30.00 loudspeakers and put them in the ceiling.
Once again, I don't know what the specs are.

Concerns.
I do not want the Hall Management to waste their money on this project, (if indeed they are).

A number of questions arise in my mind.
1. Is there any possibility that such a proposal might damage the PA system?
2. How could you connect 8 speakers and retain impedance matching (is that important).
3. Is it likely that the speakers on full volume would resemble an Osram light bulb?

Is it worth me looking deeper into the specs of the various bits making up the proposed system, or do you think it might be better just to forget the idea.
If so, is there any solution that you could suggest as an alternative, considering an extremely low budget.
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Old 6th December 2011, 08:53 AM   #2
Enzo is offline Enzo  United States
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Ceiling speakers are a common way to distribute sound in large conference rooms, restaurant dining rooms, even airport announcement systems.

The way multiple speakers like that is often done is with "constant voltage" distribution. Probably the most common of several standards is the 70.7 volt system.

If you just connect speakers in combination to the amp, then you have to calculate involved impedance results. But consider the mains distribution in your home. Every table lamp you plug into the wall and turn on lowers the total impedance to that mains suply, but no one ever considers inpedance, it is a constant voltage system. All you need do is add up all the table lamps, and don;t let the total exceed what the mains breaker can handle. That is how constant voltage distribution works.

The essence of CV speakers is that the amplifier output drives a transformer that steps the voltage up to a specified amount, then a smaller transformer sits with each ceiling speaker and steps that voltage back down to what the speaker needs. The typical small speaker transformers will have multiple secondary taps on them, usually in watts. SO a typical 8" ceiling speaker might have taps at 10-5-2.5-1.25-.75 watts. COnnect the speaker wires to the secondary tap of your choice and the 70.7v distribution wires to the primary. SImply add up all the watts of your taps, and keep the total within the amplifier power rating. (Some larger amplifiers will drive a 70.7v line directly) For example you could have a 50 watt anplifier, and connect 5 speakers to it set for 10 watts each, or 10 speakers set to 5 watts each or 20 speakers set to 2.5W each. No impedance worries, just total wattage.

A nice feature of this is that all the speakers need not be the same loudness. For example, in a tavern, the majority of the speakers might be set at 10 watts, but the speaker over the witress station might be set at 1 watt. That way, for any given volume setting, the speaker over the waitress station will be a lot less loud, so they can order drinks from the bartender and be heard. When I did jukebox installations we'd tune the ceiling speakers all to 5 or 10 watts and the speakers in the box itself down to 1 or 2 watts, that way people sitting right in front of the box didn;t get blasted, but the room got covered well. In churches, they may have multiple speakers at relatively high volume, and another speaker tapped down to low power for cry rooms or other small rooms off the main hall, a row of small classrooms perhaps.

And one wire pair can be run to all the speakers one after another, just as multiple outlets in your wall can be wired from one run of cable sequentially. Of course you may also break the system down into "zones." A mutipurpose room in a hotel might have ceiling speakers grouped into zones so they can split a large room into several smaller rooms without the sound of one room being sent to the remaining areas unwanted.

Portable systems made for rock and roll bands usually will not have a 70.7v output. But amplifiers called "PA amplifiers", or "Public Address" amplifiers usually do. The sort of thing Bogen makes.

Look up "constant voltage" speakers. WHat may be somewhat complex to explain is just daily fare for a sound professional. You might consult with a sound system company in your area.


There are other options. If your system can handle it, adding more speakers part way back from the front of the house can provide more sound to the rear of the room. And the front speakers can then be less loud, because they no longer have to reach the rear.
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Old 6th December 2011, 09:47 AM   #3
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Great , Enzo !
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Old 6th December 2011, 12:10 PM   #4
MrChips is offline MrChips  United Kingdom
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Thank you Enzo.

You have opened up an entirely new world to my gaze.

Very interesting, and a good, practical, solution to their problems.

I have a good look around the internet, trying to find the transformers that would be needed.
I assume I need one for the Power Amp end, and multiple others (8. one for each speaker.)

Would you be kind enough to direct me to a website where these are sold ... preferably one in the UK.

Thank you for the time spent on this, it is much appreciated.
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Old 6th December 2011, 10:13 PM   #5
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Constant-Voltage Audio Distribution Systems: 25, 70.7 & 100 Volts
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Old 6th December 2011, 11:27 PM   #6
Enzo is offline Enzo  United States
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I don't know who sells such stuff in the UK, but try doing a search for "ceiling speakers" and you will likely come up with people selling them.

COmmercial ceiling speakers often come with the small transformer attached to the frame already. Otherwise "speaker matching transformer" ought to get you to those parts. The remaining issue would be to get the amplifier end stepped up or a different amplifier that had 70v outputs already.


And that link from SPeedskater looks like a great place to start. The Rane folks have some very interesting and informative papers. You might look through the rest of the Rane library while there.
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Old 7th December 2011, 08:48 AM   #7
MrChips is offline MrChips  United Kingdom
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Speedskater.

Many thanks for that paper, the points relating to the serial/parallel connections look quite interesting .. and may be in their price range!
I imagine that heavy mains cable would be ideal for the interconnections.
If they have a 500watt amp, and if they had 10 speakers, would that equal 50 watts per speaker, or are their heavy losses involved?

Enzo.
I will get into Google, as you suggest.

Thank you both for your help with this.
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Old 7th December 2011, 07:05 PM   #8
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Old 8th December 2011, 10:09 AM   #9
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The trouble with distributed systems is for the people who are hearing multiple speakers (often met in airports, and far too frequently in conference halls) of varied delay from the different sources (the electricity driving the speakers moves effectively at light speed, the sound a lot slower) which can adversely effect the comprehension. (think of the garble you get on railway stations with the sequence of re-entrant horns – that's not lousy loudspeakers as you might suspect, but multiple delays). Ceiling speakers alone are generally not too catastrophic, as when you're between sources delays tend to be fairly balanced, but a combination of direct and distributed speakers tends to leave large regions in the room where there is no shortage of level, but information transfer is essentially zero.

I've always handled the problem by trying to lift the main speakers up high enough that the front rows are not in their direct line, and get the off axis leakage, so are not blasted out, while the back rows get their attenuation by d squared, and if fill speakers are absolutely necessary, used directional units aimed away from the stage and nasty complicated digital delays and extra power amplifiers (100 V line or low impedance and placed in the hall). This has occasionally required tall ladders and temporary rigging points in ceiling beams, but if this is to be a permanent installation (as the ceiling speakers imply) this is not a major problem (unless the present sound system doubles as a portable unit).
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