Let me clarify. I simplified the question. The speaker would stay the same. It's the ohms that are changing. Here's a more detailed scenario. I have 4 speakers that are 4 ohm and 150 watts RMS. I have an 2 chanel amp that can produce 75 watts RMS at 4 ohms per channel or 150 watts RMS at 2 ohms. I can connect the 4 speakers to this one amp by running a pair of speakers in parallel to each channel. This would produce 150 watts RMS per speaker at 2 ohms. I could also get another amp and run each speaker at 4 ohms 150 watts RMS. What would be the difference in sound quality and loudness? PS the amps are rated to handle playing at 2 ohm.
Unlikely to hear much of a difference but amps generally prefer higher impedances.
Sound quality is not determined directly by the impedance of the speaker. It just plays differently loud, provided your amp can do it. Yes you can have scenarios where low impedance deteriorates sound quality but not vice versa.
So in the scenario I described, it might be a little louder if I used 2 amps vs 1, but not noticeably?
I think you need to take some time to learn what speaker wattage ratings mean (little) and how impedance/load from various speakers and what amps are best suited for your needs, etc are and how they relate but gotta do the homework to have a better understanding in this case as the answer isn't particularly simple.
Ok, so now I'm supper confused. Because most of the time the watts RMS and ohms are the only parameters that are listed on speakers. If I shouldn't be looking at that, then what should I be looking at to determine the quality and loudness of the speaker? What should someone be looking at for the matching amp if not watts RMS/channel and the rated ohms? The guides and youtube videos I've looked at talk basically about these things. Is there some other knowledge document, videos, or educational material you would suggest?
Here are a couple of youtube videos from a speaker manufacturer, a good starting point; impedence, power handling.
The ratted power (Watts) and the resistance (Ohms) are not published to tell you about the loudness or the quality.
There is an IEC standard that makes some standardized assumptions about the "fuzzy" parts but I almost never see "IEC" in the specifications.My situation is a little difference because the speakers are going on a boat. It's a fairly open environment, and there's engine noise, so the speakers will be turned up either all the way, or close to it fairly often. I'm also still confused about the other comments because they say that watts and ohms don't mean anything but your are referencing watts and ohms to match speakers to amps, and you said that watts plays a role in I'm how loud a speaker sounds in several sections of your response.Some of this might be redundant... I just quickly scanned the comments above...
Loudness depends sensitivity (usually rated as X-dB SPL at 1 Watt and 1 meter). Then of course more wattage means louder, up to whatever the speaker can handle. Sensitive is unrelated to power rating.
If you want to calculate the maximum SPL, there are different dB formulas for voltage & power so make sure to use the power version. And the dB calculation will give you the difference relative to the reference... If the calculation gives you "10dB", that's 10dB louder than the speaker's 1W@1M rating.
This stuff gets a little "fuzzy" but a speaker rated for 100W is supposed to be safe with an amplifier playing regular program material and putting-out 100W on the peaks.
You can't really trust manufacturer's speaker ratings or amplifier power ratings!
With regular program material the average power from the amplifier will be a fraction of the peaks so you might fry the speaker with continuous test-tones.
Speakers are rarely fried "at home". We are usually listening at a few watts and in most cases you can use an amplifier with a lot more power than the speaker rating because you don't have it cranked-up all the way and only running a few watts before it gets uncomfortably loud. Speakers usually get fried at parties when a drunk person has access to the volume control, or by teenagers...
On the other hand... Once you get "loud" it takes exponentially more power to get more loudness so once you exceed the speaker's power rating, things get dangerous fast. +6dB is 4X the power and +10dB is 10X the power.... So if 100W isn't loud enough you're probably going to want 500W or more, and in most case that means new/different speakers.
In "pro" environments like with live music or in a dance club you have to be more careful to make sure the speakers can handle the amplifier power.
Or if you drive the amplifier into clipping, most amplifiers can put-out more than their rated power when clipping. A "worst case" clipped sine wave turns into a square wave and a square wave has twice the power of a sine wave of the same voltage.
Also, normal program material has more low-frequencies energy than high-frequency energy. It's easier to fry a tweeter with test-tones than a woofer.
And clipping generates harmonics so that's more high-frequency energy to potentially damage the tweeter. There is a myth that it's safer to run a high-power amp than to drive a lower-power amp into clipping. But in reality a 400W amp running at full power (not clipping) is sending more power to the tweeter than a 100W amp with 6db of clipping. Both are dangerous (for the speaker) but the high-power amp is more dangerous than the clipping lower-power amp. Plus, you are likely to turn it up higher if it's not clipping.
It's rare to fry a speaker but you can't really be 100% safe unless your speakers are way over-rated, like maybe 100W speakers and a 10W amplifier.
With impedance you can usually "assume" the speaker impedance is constant at it's rated value. ...An amplifier rated for 4-Ohms is usually fine with a speaker rated at 4-Ohms even if the actual speaker impedance drops to 2 Ohms, etc. Of course there are exceptions to everything.
...I actually have an amplifier rated for 8-Ohm speakers driving my 4-Ohm subwoofers... So I could burn-up the amp but I'm not pushing it that hard and It's been OK for a few years. I could buy a properly-rated amp now or I can wait 'till mine dies and then buy one, if that ever happens. I'm not really risking anything since it's an old amp and I wouldn't have a use for it if I replaced it.
Ok, what I understood from these videos is that the impedance/ohms is dynamic through the various frequencies. But he still comes back to recommending matching ohms of speaker to Ohms of amp in order not to overload the amp and fry it. And his last recommendation of just listening to it makes sense, but it's not a realistic way of putting a speaker system together. You can't just buy a ton of equipment to compare all of them and then return everything that you don't end up using.
It's more having an amp capable of lowest impedances/current needed if you have very low impedance speakers, especially if of low sensitivity and you want it loud. Sensitivity of speakers is often available, though, or at least with good ones....Klipsch overstates theirs compared to others as they use a basis that makes the number larger (I'd knock off 4-6dB off the sensitivity of a Klipsch accordingly). Try using this calculator to get an idea of the relationship of distance/sensitivity/spl. http://myhometheater.homestead.com/splcalculator.html
The 400W loudspeaker may have beefier components and voicecoils etc. If it's very inefficient, it will be quieter than a very efficient 100W speaker. Consider a loudhailer - it is horn-coupled and bandwidth limited, so can go very loud on only a battery. So wattage of a loudspeaker is generally not much help.
