How could one speaker be louder then two? Loving the Rect 25 mounted next to the spkr . I hope they are high power. Cajun what do you think is the loudest spkr today. I have had a couple of fed sig Dynamax spkrs blow I like Whelen better. Ive heard some say the scoops are the loudest.
It's all about acoustics, power and enclosure design. Using two speakers instead of one doesn't automatically make them louder, unless you're comparing all the same speaker. By design, a pair of thin line speakers with a small or no acoustical housing aren't going to perform any better than one speaker with a good acoustical housing. Think about Bose speakers......like the "Acoustic Wave" model. Know what's in there? A very small speaker mounted in a very well designed acoustic housing. It sounds louder than it should because of the enclosure.
This brings us back to a pair of thin line SA series Whelen speakers versus one FedSig AS124 (in a larger acoustic housing). I believe the SA124 will outperform those thin speakers every time with the same siren amp because it's in a better designed larger housing.
The FedSig Dynamax speakers did have some design problems....the smaller driver coils could not handle the lower siren frequencies and would cause the speaker to fail. Enter the redesigned ES-100......larger coil in a larger field serviceable replaceable driver but still mounted in a slim line housing. You will see the failure rate of the ES-100 go way down versus the MS-100 due to its redesign.
The loudest speakers I've heard are actually some Cast Products housings made to fit under chassis on a Tahoe. We built a 99 year model Tahoe several years ago with those Cast Products speakers for the guy that now drives that Black BatMobile Tahoe I built in the installs forum. We had a speaker mounted in front of each front tire and one in the middle under the front bumper, and actually had 3 Omega 90 amplifiers on them, one for each speaker. Theoretically running 300 watts, that truck with those Cast Products speakers was louder than his current truck, which is running a pair of Omega 90s on 4 AS124s (theoretically 400 watts). It was those Cast Product speakers that made the difference.
Those RECT25s are always Low Current (6 diode) versions when I mount them like that or externally. The only time I use Standard or High Power RECT series LEDs are when they are mounted in a filtered (through glass or tint or both) application. The LC versions are plenty bright when utilized like that with no filter.