I'm using an Elk M1G control for security, fire alarm, and a few other things. It will be "connected" to my HE.
I'm trying to decide on speakers for voice announcements generated by the M1G. I'm using an ELK-M1TWA add-on card to power and control the speakers. the M1TWA has three speaker "zones", each of which can handle multiple speakers as long as the impedance on a zone is between four and 32 ohms. Each zone has its own amplifier and volume control.
I want to use quite a few ELK-SP12F speakers spread around the house to keep from having any of them needing to be turned up really loud. They are 32 ohm speakers, so I can have up to eight per M1TWA zone, for a total of 24 (which is way more than I need).
I would like more flexibility setting the volume, i.e., possibly an inline volume control for each speaker. My question... does an inline volume control affect the impedance of a speaker? If so, how do I take that into consideration when calculating the total impedance of an amplifier zone?
When using multiple speakers, the main consideration is whether they are going to be installed in serial or in parallel. If you install two 32 ohm speakers in series, the impedance will be additive...64 ohms. If you place speakers in parallel, the resistance is calculated by summing the reciprocals. If two speakers are in parallel, the overall resistance will be 16 ohms. Three speakers in parallel would be 10.7 ohms. Four speakers in parallel would be 8 ohms. Eight speakers in parallel would be 4 ohms. Thus, if you install speakers in parallel, you could use anywhere from one to eight speakers per zone and stay withing the specs of the amplifier.
Speaker volume typically controls work by placing a variable resistor in series with the speaker. When the variable impedance is low, the impedance seen by the amplifier will be similar to that of the speaker alone and the volume will be at maximum. When the variable resistance is high, the resistance will be added to the speaker impedance so the impedance seen by the amplifier will be high. The current flow will drop, some of the voltage drop will occur across the resistor and the speaker volume will be low. However, having a higher impedance won't be problematic for the amplifier. It is low impedance that cause a problem by causing too much current to flow, overheating the final amplifier stage.
A L-pad contains 2 variable resistors that are controlled by a single knob. As one resistance increases to lower the volume of the speaker, the other resistor decreases to maintain the same load on the amplifier.
Although you could vary the volume of a speaker with a simple variable resistor, it could result in the volume of the other speakers increasing when you reduce the power going to a single speaker.
