Originally Posted by datranz
lets say, i want to build a mtm speakrs, my mids are zaph audio 5", rated at 87db/1w. If i use 2 in a mtm config, would the sensitivity of the system increase to 90 db/1w? I ask because i am hoping to use this w/ a ribbon cross at 2.5K. The ribbons at 96db/1w.
Amplifiers are essentially voltage sourcing devices so when you put two speakers in parallel, the impedance halves and you get double the power for the same input voltage.
So, if your speakers are 8ohm nominal, then your sensitivity spec is for a 2.83V input. When you hook the speakers in parallel, your amplifier will put out 2W, and thus so you will end up with 93dB SPL. So if your ribbon is 96dB for a 2.83V input, then your woofers will only be 3dB down.
Ribbons tend to beam quite a bit as you go higher in frequency, so you might consider going with a gentle shelf filter to pad down the lower levels of the ribbon by ~3dB and leave the highs tipped up a bit (to get back to a flat power response in the room). If you're clever with the xover design, then you might be able to do this with just one additional cap...so not a huge cost. Worst case you'd need to add a capacitor and a resistor.
Another thing to keep in mind is that in order to get more sensitivity from another driver, either the cabinet needs to get larger, or you need to accept less low frequency extension...Hoffman's Iron Law.