Speaker Wiring / Many Speakers in a systems
NOTE: We are not discussing 70 volt systems in this article. Some offices and buildings are wired this way. The giveaway is a transformer on the speaker, or near it, marked with taps for the speaker (usually 8 ohm) and taps for the 70 volt supply that is used to deliver the music to the speakers. If your power amp has only a 70 volt output for its audio systems, then this article is not for your set up. See 70 Volt systems instead
Your effort will involve some combination of series and parallel speaker wiring. Please see Speaker Wiring/Loading Examples as a reference.
The First rule is that of consistency - each one of the speaker drivers (or cabinets) you use should be the same as the others. Mixing and matching different speakers (or cabinets) into a large network of speakers will give you uncontrollable results - some will be quite a bit louder than others and unless you also want to buy faders for each of them (which could get quite expensive), its best to work with drivers that have consistent characteristics (SPL level, for example - also called Sensitivity).
The Second rule is to share the power as equally as possible across each speaker. This means that they all should be the same general impedance - ie. 4, 8 or 16 ohms. In some cases the speakers will be coaxial or a cabinet (having a Woofer and a separate tweeter, possibly also a midrange speaker and a crossover network built in), and in other instances each speaker will simply be a single full range driver. For these examples, All speakers (or cabinets) will be listed as 8 ohm units (I do list in Blue the results for using 4 ohm speakers in place of the 8 ohm speakers). Your system needs may differ. Common Speaker impedances are 4, 6, 8 and 16 ohms. The most common is non-car audio speaker is 8 ohms (car audio tends to use 4 and 6 ohm speakers). You will most likely see 4 or 8 ohm setups where you need to connect up large networks of speakers.
It basically turns 4 speakers into the equivalent of a single load, allowing me to add 4 of these together (now 16 speakers) in exactly the same way and still maintain exactly the same impedance load, Also if any one driver fails, only that 1 speaker will be out and it will be a simple task to identify the problem speakers.
Alternately, you can wire the speakers up with many more speakers in a series, then connect them together, such as
It gives the exact same loading, however, if one driver fails, then the entire string connected in series with it also goes out - leaving you guessing which of the 4 is bad. This could be very bad if you have to be raised 40 feet in the air to verify the operation of each individual speaker (or cabinet).
Here are the same concepts with 64 speakers.
Using 32 speakers.
This example shows using 9 speakers (an array of 3 by 3). If they are all 4 ohm speakers, then
the resulting load will be 4 ohms. If they are all 8 ohm speakers, the resulting load is
8 ohms. The magic trick here is that if you square the number of speakers (3X3, 4X4, 5X5, 6X6, 7X7, etc.)
and you wire in this fashion, the end result impedance is the same as if it was a single speaker.
If you look at the 16 and 64 speaker configurations (above), you will see that we already mentioned this design. Now you know that it can be extended to cover other combinations (the 4 speaker Series/Parallel is this pattern as well).
The same holds true for 4, 8, 16, 32 or 64 speakers wired in the above combination of Series, Parallel, Series/Parallel or Parallel/Series.
For sizing an office sound system, 1 to 5 watts per speaker is pretty reasonable, since people will be trying to have conversations while the music is playing. Most likely for an office sound system, the music content will not require a power amp that can deliver more than 1 watt per speaker. Having more wattage is good, less is bad. If you also use this as a public address system that cuts in for occasional messages, 3 to 5 watts per speaker would be a better fit. As an example, for a 16 speaker system, 20 watts RMS at 8 ohms would be around the minimum. If also used as a PA system, 50 to 100 watts RMS at 8 ohms would be sufficient. The logic is the same for a 64 speaker office sound system, 80 watts at 8 ohms would be a minimal setup. To add a PA capability, 200 to 400 watts RMS at 8 ohms would be a minimal setup. Speakers used for this purpose should be able to handle 4 to 10 watts.
For Pro-Audio/PA systems, you will need higher volume levels, and your audio drivers will probably be rated at 50 watts or more. You will be sizing the power amplifier nearer to the wattage levels of the speakers. If each driver is capable of 100 watts, and you have 16 speakers to drive, you will want to be able to push approximately 1600 watts at 8 ohms into this system. Its unlikely that you would build a Pro-Audio/PA system that used 64 speakers with a single power amp, however, if you did, the logic would be the same (just use some very heavy copper cables), since you would need to push 6400 watts at 8 ohms to drive all 64 speakers at 100 watt levels - its likely that you will break these into clusters of 4, 8 or 16 speakers and drive them with separate power amplifiers. Typically, stadiums have banks of power amplifiers driving sets of speakers in different areas, where the power amplifiers are located close to the speakers to keep the runs of copper cable as short as possible to reduce the losses. This is usually not an issue with office wiring - the speaker output level requirements are substantially different.
When you are running higher powered Pro-Audio systems, you may find that the power amplifier cannot provide adequate voltage and current for large numbers of speakers. I can't give you any guidelines here, other than if you run out of headroom with your power amp, it will probably be in the lower (Bass) frequencies. You might look into a bi-amped system if low frequency at high volume levels are a requirement.
NOTE: There are other ways to wire speakers and attain a specific impedance, however there are instances when you will not be able to get the exact impedance that you desire. Never go below your power amplifiers rated load capability.
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