I kind of liked Peter's answer... While you "could" make a 2 way 15" sound Ok, (I've heard some), all things equal it will be more challenged in the midrange than the 12". Honestly both are challenged in the midrange for hifi fidelity, the higher SPL benchmark used for live sound may trade some midrange performance for the extra bass. It is instructive to look at what combinations the manufacturers sell the most of. The marketplace over time, ultimately makes the final call on "cost effective, but doesn't suck too much" speakers. The ones that suck too much, eventually go away, and this speaker category is mature.
summarized/bolded for the impatient people.I was recently browsing selections for pole-mount/"point-source" loudspeakers, coaxial loudspeakers, "fixed curvature line array" loudspeakers, variable curvature line array loudspeakers, and also home audio loudspeakers.Long story short i noticed that alot of manufacturers usually include the following variants in their speaker lines: 2-way 1/1.75in high & 12in low, 2-way w/ 1/1.75in high & 15in low, and 3-way w/ 1/1.75in high, 6.5in mid & 15in low.These are all speakers that are made (primarily) for live sound reinforcement. By that i mean they're made to be loud as opposed to accurate. Most have peak amplifier power ratings of 1000-2000 watts (w/ mains input @ 120v). about 4/5 of the speakers i looked at had the amplifier built in to the actual cabinet housing.While i understand many of the company's decisions to produce a 3-way model for better dispersion control (and the other benefits of a 3-way design), but i'm very weary of the 2-way models that use 15" drivers, often the same ones they put in the 3-way models. It concerns me because within a 2-way speaker, the woofer must account for some of the mid range frequencies, the same frequencies that would otherwise be created by a smaller 6.5in driver that could better reproduce the wavelengths of mid-range frequencies. I like the idea of using a 12in driver (at the largest) in a 2-way speaker because, while it is smaller, it could better create the entire sound range that doesn't end up sounding worse. For example, i've noticed that in a 2-way system with a 15in LF driver, whenever someone with deeper voice speaks at the higher point of their voice, it sounds like there's something that's "holding" the sound back, almost like a muddy sound. As if someone taped a few layers of thin fabric over my ears when they speak like that. But as they progress into the deeper octaves of their voice, it sounds deeper and more direct i guess you could say. it's kind of difficult to describe the difference without something happening right in front of me, but the point is if they were to repeat the same thing in a 3-way system with a 6.5in MF & 15in LF driver, their voice sounds CONSISTENTLY more "direct", clearer, even throughout the lower sounds. I find that even the mid range has more "punch" to it, like on kick-drums, you can feel the beginning of the note have more impact & spread from the speaker. And in bass music, those long notes that dip into the sub-bass region then fade up into a higher and higher sound. You can really feel the notes "carry through" and have a fuller sound, even as they carry over from sub-bass speakers to the full-range speakers.While i'm far from an expert in sound reinforcement (and writing obviously), my personal theory is that this is because of the amount of air the driver is moving, and in relation to how fast/how often & loud it's trying to make it. Meaning 15in drivers are best for creating bass frequencies and ONLY bass frequencies, because the size of the wavelength is appropriate to the thing that's trying to create it. Just like 21in drivers are best for creating sub-bass frequencies and ONLY sub-bass frequencies. And just like 6.5in drivers are best for creating mid-range frequencies. Obviously the EXACT definitions of high/mid/bass range are all subjective to the actual speaker and various other factors (too many to list out). BUT the point remains, what is the best balance of both loud AND clear, a 2-way system with 12in driver or a 2-way system with a 15in driver?
I worked at Boston Acoustics waaaay back in the day and the engineers always said a great amp and crappy speakers will always outshine good speakers and a crappy amp.that said, since the mid 90's, speakers have come a long way in cone/surround/spider material and design, voicecoil heat dissipation and the baskets and magnets are stronger. while a big 15" driver USED to be only good for boomy bass, say for a bass guitar or hip hop low end, modern drivers can push more air faster with less deflection of the driver surface (among other mechanical limitations) so if a reputable loudspeaker mfg sells a 2-way 15" box, it's safe to say it's because they designed it to accurately recreate the signal sent to it (as spec'd), at the DB's it's designed for, with the power it requires. in other words, don't sweat it.get amps rated rms 20% OVER your cabinet's max wattage to make your loudspeakers happy (but never overdrive them or you'll cook em!!)
The technology has come a long way, yes. The downside is that the technology has followed into the smaller elements as well. Meaning that the ratio is probably still the same today as it was 20 years ago in terms of difference between a larger element and a smaller one? In other words a smaller element is still going to sound better in a 2 way design. The only limiting factor is frequency range and potential level. The larger elements will produce more volume and go deeper into the lower octave. This would be the same 20 years ago. A modern box should sound better today than even a higher end box did 20 years ago, but that is not always the case. I simply believe that modern technology has allowed us more volume, linearity and lighter construction, more so than better sounding boxes. As for the size of the amp you suggest, I think your a little over zealous?!?!?!? I am large proponent of larger is better, but feel that rms +20% larger than peak is WAY overkill. Even a modest speaker rated for 500 watts peak would then require an amp capable of about 2,400 watts peak based on your guideline!!!!! Most amps these days are rated for peak power. Many don't even list rms anymore. Rms is usually 1/4 of peak power ( at least for speaker ratings anyway ). Could you imagine buying an amp for a sub rated for 4,000 watts peak, or even a main rated for 1,000!!!!! You would need a 20,000 and 5,000 watt peak amp respectively!!!! Even if rms was half of peak power you would still need way too much amp...... The way it works is pretty simple. A speaker has a sensitivity rating and a theoretical SPL at peak wattage rating. If you power a speaker with it's peak rated wattage, you should in theory acquire the rated peak SPL. Highly doubtful though as power compression will eat several DB's and you can only run at peak power for a short period of time. An amplifiers peak power is actually doubled when you start going into solid clip!!!! You should be roughly 3db down just before the lights start to flicker. You should never desire to power a speaker at it's peak power ( for long anyway ) and you should never desire to clip the amp. Which means that you should spec the system that goes out to acquire the needed level without having to go to that distance with the speakers and amps. The program rating of a speaker is usually about as far as you should ever wish to run it for any length of time. The rms rating ( AKA continuous rating ) is the more ideal range to run at if at all possible. Which means you really only need an amp that is rated for about 75-100% of the speakers peak rating. This should give you enough power to run up to the program power rating of the speaker with a little bit of headroom left. Buying an amp that is rated for between the 75-100% mark may be the limiting factor? The cost difference for an amp rated for 4,000 watts and 6,000 watts may be too much? The 4,000 watt amp which may sit right at the 75% mark of your speakers peak power should be more than enough in theory. The extra cost to acquire the other 2,000 watts may be several hundred dollars and only net you a theoretical 1-2 db's of potential gain. That gain is negligible for the added cost and doesn't add any protection for the speakers at all. Running the speakers at their peak rated power can only be done for a short period of time ( possibly only minutes )!M.I level gear isn't always the most robust and probably not the most honestly marketed stuff either. Other higher end stuff usually comes with it's own required amplification or at least a recommended list of amps. I can guarantee that a D&B rig ( which is required to run with Lab Gruppen amps ) is not running at RMS+20% of peak speaker wattage! The last D&B rig I worked with used the larger D80 Lab amps and powered 2 V8 boxes per channel at 4,000 watts. Each V8 is rated for 2,000 watts peak and 500 watts rms. In other words one of the top companies out there is running their system so that each speaker see's roughly 100% of it's peak rated power. Guess what? There is a lower power version of the same thing! You can run the same rig with the Lab Gruppen D12 amp which is rated for 1,200 watts per channel. The only difference is less potential output. The 1,200 watt output is right about the 60% mark. Which puts it just above the program rating of those speakers no doubt.
This discussion reminds me of why the Dave Rat designed the Rat Trap 5 they way he did. It's a 4-way design which in and of itself introduces problems associated with multiple drivers/bandpasses in a single box. But for it's day it solved some problems. The trick was to get the drivers as close together as possible which wan't easy until EV came out with horns that could worked in tight spaces. The 4 way design is 15" x2, 10" x2, 4" compression driver with 2" exit, 2" compression driver with 1" exit. The 15 cross over to the 10s at 250Hz, the 10s crossover to the 2" horns at 800Hz, and the 1" horns kick in at 8KHz. The biggest problem with the design is that the 10s aren't that close together. So you have some problems in that area. But they're jammed in tight with the 2" horn and the drivers are all pretty close together so they behave pretty well considering. Interestingly, even the 10s start to get pretty beamy in the upper end of their range. This can be both an advantage and a disadvantage. When hanging large clusters of those boxes, the beaming counteracts the combing in the vocal range to some degree and you've still got good coverage from all the boxes pointing everywhere. But if you're using a smaller number of boxes, it can be an issue.
On the placement issue how far apart 10" cone driver is from the mid horn in the crossover region is a big problem if you are using them for Near Field.
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