Chris,

Here's a bit more about that 100W rating.  Looking at a spec sheet for an attenuator, ATS001063D-Commercial-Attenuators-Datasheet.pdf (atlasied.com), we can see that the power rating is really the maximum power or current the transformer can pass, without over-heating and burning up.

The question then goes to, how much power needs to go to the loudspeakers that are fed from the attenuator?  A simple calculation is to add up your tap settings on the loudspeakers and make sure you are below the rating of the attenuator.  In practice, you will probably far less power than the tap settings.

Here is a video on how to calculate how much power you need for your loudspeakers.
https://www.youtube.com/watch?v=jQaYxLEchLk

Paul

Chris, This is an interesting question and how this can be answered is first knowing what your use case is an what level of SPL you are trying to achieve. For example, if you have a background music application using distributed audio based on 70 V distribution then the Volume Controls could be from 10 to 35 W or more. Using your example with parallel wiring for each speaker (assuming 8 ohm spkr with a stepped transfomer) and a 100 Watt amp with 70v output the calculation would be: Zt = Z1 + N Zt = total impedance of speaker system  Z1 = the impedance of one speaker N = Number of speakers in the distributed system wired in parallel. From what I could find there is a very small insertion loss of only .5 dB on the Volume Control that uses a stepped tranformer design. So, I would not be too concerned about a insertion value for the Volume Control however the rating for wattage needs consideration.

So, the headroom on the amplifier should be selected on the basis of having at least 2-4 times the Continuous Power rating per channel. However this headroom need will depend on the music or voice application so the multiplier of output power would vary if the program is background music application all the way up to a rock band application. 

Below is a article that may help the overall understanding of distributed audio and amp selection.

I hope this helps... 

How Much | Crown Audio - Professional Power Amplifiers | English

Hi Chris - Good question; and the way you've phrased it shows you have a good understanding of the topic.  As you reference it's about accounting for max dB loss from source to listener(s).  Assuming uniform coverage pattern and listener positions your second choice - "...where the neighboring loudspeaker provides equal coverage" makes sense.  But for listener positions (if any) around outside edges of the coverage pattern area it could be your first choice; "..edge of dispersion cone".  Given headroom is also part of the equation, can't hurt to use that as the larger of the two.

Great question!  The short answer is to make up for the losses in the transformers.

Many years ago, I was doing the calculations for distributed loudspeaker systems and then doing physical measurements to see the accuracy of the methods.  It turned out the measurements were not very close to the calculations.  The measure impedance at the amplifier was always less than the calculated amount.  It turns out the part missing was the transformer specifications.  Looking at loudspeaker specifications, many times there is no mention of the transformer.  Every transformer has loss.

I'm a bit concerned about your comment about 1.2x multiplier as the math sheet that is given at the CTS-D test offers a 1.5x multiplier.  I will have to review the Study Guide and see if we need to these into alignment.

I multiplier of 1.2 is really subtracting about 0.8dB and a multiplier of 1.5 is subtracting about 1.8dB from the output.  Or, put the other way, the amplifier needs an additional 0.8 or 1.8dB gain to make up for the transformer loss.  Based on my measurements, I would opt for the larger number.  Remember, a 3dB change is doubling the wattage of the amplifier.  

In practice, under sizing an amplifier can cause real problems.  An under sized amplifier will clip and distort the audio signal.  If clipping continues over an extended period, the loudspeakers can overheat and damage the coils.  The amplifier will be drawing as much current as it can, overheating the amplifier circuits which can cause premature failure.  The audio quality coming from the system will not sound very good.

I hope that helps answer your question.

Hi Christopher - interesting question.  Per CTS-D Exam Guide 2nd Edition starting ~page 145, it's an approximation of speaker spacing (creating overlap) delta of the distance (not area) between two speakers from otherwise "edge to edge" coverage distance.