What's with 4 ohm speakers?


If 4 ohm speakers are harder to drive, why do manufacturers keep coming out with them?
50jess
They're not hard to drive if you have a good amp. Also, 4 ohms is an average. Depending on the frequency of the music you are playing, the speakers resistance will vary.
Sensitivity.

Take a typical "8 ohm" midwoofer and double them up in parallel and you get 6 dB more. Realistically, more like 3.5 to 4 dB but also drops impedance by half. Problem is that impedance varies with frequency and the same midwoofer will have a minimum impedance, usually around 200 Hz. Some more than others.

The last drivers I used in a project had a minimum impedance of 5.6 ohms. In a MTM that's 2.8 ohms at a frequency that demands current. A 4 ohm tweeter doesn't have that current demand. Found a trick to eliminate that impedance dip but it doubles the cost.

Subwoofers are often 4 ohms because the trade-off for bass extension is sensitivity or size. So, they often use lower impedance to get more sensitivity from smaller drivers in smaller cabinets. Automotive subs are sometimes only 2 ohms. Same applies to woofers. Do you want bass? Do you want reasonably sized cabinets?

There's also the advantage of using smaller value inductors with lower impedance and less resulting phase shift.
They're not hard to drive if you have a good amp.
Depends on what you mean by good.
4 ohms is an average.
Not exactly; 4 ohm "nominal" has no commonly accepted definition. Plenty of speakers rated by manufacturer as 4 ohm nominal have very different impedance minima and maxima, which in turn will place very different demands on the amplifier that is driving it. Not to mention phase angle and sensitivity.
Depending on the frequency of the music you are playing, the speakers resistance will vary.
While true in the absolute sense, some speakers have a relatively flat impedance curve; i.e. they do not vary much w frequency. Others vary widely. And that's exactly the reason why amplifier-speaker matching is important and why there are many "good" amplifiers that will not mate well w a 4 ohm nominal speaker. And many good amplifiers that will not mate well w a 16 ohm nominal speaker.
I read a review of my 8 ohm speakers and it noted a drop to 3.something ohms here and there. Changed to the (tube) amp's 4 ohm output and huzzah...all better. Luckily there is plenty of info out there to make sure you don't mis-match yer stuff, although exceptions exist...i.e. high impedence/high efficiency things (Zu, etc.).
Some misinformation here:

They're not hard to drive if you have a good amp. Also, 4 ohms is an average.

4 ohms is not an average. Plus, you can see in the specs of all amps that its not a good thing. No matter what amp you have, the distortion will be higher driving 4 ohms. And usually its the kind of distortion you can easily hear- an amp driving 4 ohms will be harsher and less detailed than if it were driving 8 or 16 ohms.

Sensitivity.

Take a typical "8 ohm" midwoofer and double them up in parallel and you get 6 dB more. Realistically, more like 3.5 to 4 dB but also drops impedance by half. Problem is that impedance varies with frequency and the same midwoofer will have a minimum impedance, usually around 200 Hz. Some more than others.

Putting two 8 ohm speakers in parallel gets you 4 ohms, with 3, not 6 db increase in sensitivity. However the actual efficiency will be found to not have changed. It is important not to confuse efficiency, which is the actual watts that make a speaker move, as opposed to sensitivity, which has to do with the voltage on the speaker at the time. If you have 2.83 volts into the speaker and its 8 ohms, the power is 1 watt, IOW they are the same. But into 4 ohms that 2.83 volts is 2 watts- 3 db 'more'. Its not really more at all, what is happening is your amp is being asked to double its power.

Its important to understand this distinction! Not all amps double power (tube amps for example). Yet such amps can sound quite musical. Regardless in all cases, as stated earlier, amps do sound better (smoother, more detailed) driving higher impedances.

Put simply, if sound quality is your goal, your amplifier investment dollar will be best served by a speaker of 8 ohms or more, all other things being equal (and regardless of the amplifier technology). If OTOH sound *pressure* is your goal than you have a small (3db) argument for going with a 4 ohm speaker.
I don't know the answer to the OP's question but I do wonder if it has something to do with cost. Take a 4 ohm and an 8 ohm version of the same driver unit: Is it less expensive to manufacture the 4 ohm version?

Four ohm versions are usually more sensitive. A good amplifier properly mated to a 4 ohm speaker will usually deliver more output than it would into an 8 ohm load. With more output, you can play louder with less distortion and more headroom.
Well, I guess I should have checked the thread again before posting because Atmasphere practically wrote just the opposite and I trust his knowledge about such things far more than mine. He didn't answer the OP's question, though, so somewhere along the value chain, I still wonder if it has something to do with cost.
It's easier to design a speakers with a steady low impedance than a steady high impedance, something that some amps seem to have an easier time with. Historically there have been more speakers that can produce wave form fidelity with a lower impedance than a higher impedance. IME, I typically prefer systems that have speakers with a low impedance rather than a high impedance, YMMV.
It's easier to design a speakers with a steady low impedance than a steady high impedance, something that some amps seem to have an easier time with. Historically there have been more speakers that can produce wave form fidelity with a lower impedance than a higher impedance.

This is entirely false, plain and simple.

To answer the OP question, I suspect the reason is that many designers don't know how amplifiers work, so we often see crazy loads that are 'hard' to drive.

Folks, there is a reason such speakers are considered hard to drive- the amp has to work harder to do the job. You can always see it in the specs of any amplifier- the harder you make it work, the more distortion it makes. Unfortunately the distortion we are talking about is the kind that makes a system harsher and brighter- the odd ordered harmonics, to which the human ear/brain system is very sensitive.

Part of the problem is that speaker designers often confuse Sensitivity with Efficiency. I can point to examples if anyone is interested. You don't get something for nothing in this world. In electronics, this idea is known as the Law of Energy Conservation, or Kirchoff's Law.

But many speaker designers don't understand this. They think that if they put two drivers in parallel, that the speaker gets easier to drive (sensitivity increases). It does not! It gets *harder* to drive, and the amount of power to make it play a certain sound pressure does not change at all!

If lowering the impedance was actually helpful, why not 1 ohm instead of four? Then the sensitivity would be increased by 9 db! Nearly a 10:1 improvement... but of course that would violate Kirchoff's Law. If you are able to violate Kirchoff's Law, FWIW, you will have created a Free Energy Device which, as far as we know, does not exist.

Yet many speaker designers persist in trying to do exactly that, and many audiophiles that don't understand how this works (its really just math when you boil it down, FWIW) follow along in the fantasy.

So here is the bottom line: higher impedance speakers cause amps to make less distortion (smoother, more detailed). Increasing the Sensitivity of a speaker by decreasing its impedance does not affect Efficiency, but it does make the speaker harder to drive (amp will sound harsher, less detailed).

Put another way: If Sound *quality* is your goal, your amplifier investment dollar will be best served by a speaker of higher impedance, all other things being equal. If sound **pressure** is your goal and you have a transistor amp up to the task, then there is an argument for lower impedances.
Using solid state amplifiers without output transformers a 4 Ohm nominal impedance allows peaks 3dB louder than 8 Ohm speakers with the same cabinet size and low frequency cut-off which ultimately limit efficiency per Hoffman's Iron Law. This is generally a better engineering choice than doubling cabinet size (thus halving the spousal acceptance factor ) or choosing a low frequency cut-off 1/3 octave higher.

People don't go too over-board with lower impedance in the home market because of

1. How the FTC requires manufacturers to rate stereo and mono home amplifiers - they must be "pre conditioned" at 1/3 of rated output power and the power dissipated into low impedance loads would make the numbers look bad so the capability to run 2 Ohm loads isn't usually advertised and consumers would be leery of buying such speakers to go with their "4 and 8 Ohm compatible" electronics.

2. Some amplifiers are unstable (they start to oscillate) driving low impedances; and starting with a 2Ohm nominal impedance minimums of 1 Ohm aren't unreasonable.

3. Some audiophile amplifiers have silly high output impedances which interact with the speaker's varying impedance to change the frequency response and this is exacerbated with low load impedances. Output Transformer Less Tube amps are especially bad although single ended triodes without global feedback can also have problems.

For instance an Atmasphere M-60 Mk.II.2 has a 4.1 Ohm output impedance.

Driving a 3-way speaker with impedance varying from 16 to 64 Ohms this would cause a 1.4dB output difference between the minimum and maximum impedances.

With 4 to 16 Ohm impedance the difference would be 4dB. This is not atypical for a 3-way - the reactive components for a Zobel network to counter the bass driver's resonant peak would be too big and expensive so the best you can do is bring it down with a resistor in parallel.

At 2 to 8 Ohms it'd be 6dB.

Apart from this edge case the effects on distortion aren't interesting compared to what the speaker is adding to the sound.

If you do want to run such an amplifier you'll do well going out of your way to buy speakers with high (16 Ohm nominal) and intentionally flat impedance.
I wrote:
Apart from this edge case the effects on distortion aren't interesting compared to what the speaker is adding to the sound.

That's not quite right - my power amplifier biases run towards push-pull solid state class AB for practical reasons although I like building with tubes where I can actually see how a negative charge on the control grid surrounding the cathode limits electron flow to the plate on the outside.

There are output stage device, topology, and biasing combinations which won't play nice with low load impedance.
Point:

Just because a loudspeaker says 4 ohms on it's back panel does not mean it's actually 4 ohms..Perhaps it's 5 or 6 ohms or 2.5 ohms?? Or an 8 ohms loudspeaker is actually 6 ohms simply stating that 4 ohms is hard to drive is not the answer as the answer is all over the board. Speaker builders don't want to post 5 ohms in there stats they use 4 or 8 ohms.

Sony's new reference speaker is considered very hard to drive and it's rated at 4 ohms but my bet is that it's 3 ohms or less..again it depends on the Frequency as the ohm loads change by the frequency. 16 ohms loudspeaker? good luck... perhaps Avante-garde acoustic: several of there models are rated 16 ohms but they have there own problems such as sounding quacky and having poor bass... no simple answer here.
Drew Eckert wrote:

"Some audiophile amplifiers have silly high output impedances which interact with the speaker's varying impedance to change the frequency response and this is exacerbated with low load impedances. Output Transformer Less Tube amps are especially bad although single ended triodes without global feedback can also have problems.

"For instance an Atmasphere M-60 Mk.II.2 has a 4.1 Ohm output impedance.

"Driving a 3-way speaker with impedance varying from 16 to 64 Ohms this would cause a 1.4dB output difference between the minimum and maximum impedances.

"With 4 to 16 Ohm impedance the difference would be 4dB. This is not atypical for a 3-way...."

In reply, I'd like to point out that, with the same 4 to 16 ohm impedance difference described here, the power that a transistor amp puts out varies by 6 dB, because it is putting out constant voltage rather than constant wattage. Why does the audio world accept this without a blink, and yet think there's a problem when a tube amp exhibits less variance in power output into the same load??

It is because the audio world is accustomed to the way transistor amps behave, and most speakers are designed to work well with transistor amps. The designer designs the speaker to sound right when driven by an amplifier that puts out 1 watt into the 8-ohm portion of its curve, 2 watts into the 4-ohm portion of its curve, and 1/2 watt into the 16-ohm portion of its curve, all at the same time (2.83 volts).

Now, what if the designer's goal was a speaker that works great with an amplifier that puts out approximately constant wattage, regardless of the impedance curve (within reason)? Well that can be done just as easily, but there are fewer amplifiers designed that way out there, so his potential market is smaller.

These two approaches to amplifier and speaker design have a name: Voltage paradigm, and power paradigm. You can read more about the subject here:

http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php

Okay, what kind of amplifier sounds best? Boy that's a long debate for another day, but a lot of people familiar with many types of amps prefer the sound of a good OTL or SET amp, assuming a good speaker pairing, and that includes yours truly. Some speaker manufacturers give priority to building speakers that will work best with power paradigm amps because they believe that combination sounds best, and just accept that they are fishing in a smaller pond.

Is it possible to build a speaker that works well with both types of amps?

Yes, by keeping the impedance curve as smooth as possible, the speaker will work well with both types of amps. And with such a speaker, you can really make an apples-to-apples comparison of the different amplifier types, rather than actually evaluating whether a (typically roller-coaster impedance) speaker synergizes best with a voltage paradigm amp or a power paradigm amp.

Drew's numbers above illustrate an argument in favor of using a high impedance speaker with a low-output-impedance amp: The amp's output is approximately constant-power when the speaker's impedance is varying between 16 and 64 ohms, but the amp's power output changes significantly when the speaker's impedance varies between 4 and 16 ohms (same 4-to-1 variance in both cases). Also note, in both cases the amp's power output change vs speaker impedance is in the opposite direction of what happens with a voltage-paradigm (solid state) amp, and this is probably the main reason why simply dropping a specialty tube amp into your current system is a roll of the dice (and the odds are against you) unless you already know your current speaker is a good match.

Back to the original question, what's with 4 ohm speakers if they're harder to drive, well in general they can play louder with a solid state amp, and most people have solid state amps, so they get more sound per dollar with 4 ohm speakers (quantity outsells quality). Also, most woofers are 8 ohms, so if the designer wants to use two such woofers, he has to choose between series connection (16 ohms) and parallel connection (4 ohms). Most choose parallel connection because 4 ohm speakers outsell 16 ohm speakers. I believe that most amps - tube or solid state - sound better into a 16 ohm load, so my home audio two-woofer designs are 16 ohm loads (whereas my prosound two-woofer designs are 4 ohm loads, because there we're trying to maximize available SPL with solid state amps). What do I do about the typical halving of maxium power from solid state amps when driving a 16 ohm load? I start out with speakers that are about 3 dB more efficient, and of course pay a corresponding price in box size vs bass extension. However there is one "free lunch" to using power paradign amps: They generate equal or even increased, rather than reduced, power into the speaker's virtually inevitable bass impedance peaks, and if we keep this in mind and design our box accordingly, we get back most of the bass extension we otherwise would have lost when we traded off in the direction of higher efficiency.

So I take the position that, all else being equal, 16 ohm speakers sound better than 4 or 8 ohm speakers, and that power-paradigm amps are well worth seeking out matching speakers for.

Duke
dealer/manufacturer/power paradigm groupie
Well said Duke.
I'm just now reading through all this and have to apologize for my 1st post. Its really a half ass response, but I did it for a reason. As far as 4 ohms not being an average, thats my error. I always thought nominal meant average (in context of speaker impedance.) I also should have put an exception in for most tube amps and a few SS ones, as well.

When I read the OP, I couldn't make up my mind if it was an attempt by someone who doesn't like high end audio for some reason and just wants to start an argument, or possibly a beginner who just doesn't know. Its just a vague statement with no equipment or examples listed. Usually, when someone really wants to find out something, they do a little better with the specifics. Thats why I made the if you have a good amp comment. I just pictured a beginner with something like a Sony receiver possibly looking to move up. In that context, a typical entry level SS amp shouldn't have too much problem driving a 4 ohm load.

Unfortunately, it looks like I may be right. Everyone here is giving some really high quality answers and the OP is no where to be found. If it makes any difference, I read through all the posts and thought they were truly excellent.
For modern amps 4 ohms is a doodle. Its not an issue.

Thanks
Bill
Tell you what I'm going to do. Got another couple projects coming up this summer. I'll make up a 'quick and dirty' mono cabinet with 4 small, induvidually sealed midwoofers. Each will have it's own terminals that I'll hook up individual, parallel pair, series pair and series/parallel, measuring amperage, voltage and frequency response. Don't have state-of-the-art testing gear but should be consistent. No crossovers. Bottom woofer will be at least 1.5 feet above floor. Planned drivers are Dayton DS135-8's, which are fairly conventional except for small size and required cabinet volume.

Since the question was never about percieved sound quality or matching amps, (tube or SS) I'll just use what I have.

Never actually seen this done before, just math and theory.
Atmasphere...+1

Excellent post.
Ngjockey, Pipedreams has done something like that. I've seen Pipedreams that were 4 ohms, and the exact same model in 16 ohms.

Seems to me the ZU Definition, normally a 6 ohm speaker, is also available as a 30 ohm speaker.
Drew + Duke + Ralph = lots of good info.

The thing with voltage paradigm is, it represents the vast majority of products out there today whereas power is perhaps having a renaissance in high end circles but remains distinctively as more of a niche approach these days, for better or for worse.

You need to be well advised or know what you are doing to get the best results out of either. Lots of ways to hit or miss either way. Excellent results can be achieved either way, as long as all the parts fit together well. Each has its unique ups and downs. Having a choice is a good thing. I hope it stays that way!
AudioKinesis writes:
>In reply, I'd like to point out that, with the same 4 to 16 ohm impedance difference described here, the power that a transistor amp puts out varies by 6 dB, because it is putting out constant voltage rather than constant wattage. Why does the audio world accept this without a blink, and yet think there's a problem when a tube amp exhibits less variance in power output into the same load??

Those of us with a little technical knowledge accept and expect it because at lower frequencies dynamic loudspeaker driver output is proportional to voltage.

Individual drivers together with their enclosures form spring-mass systems where oscillating movements at frequencies approaching system resonance take less energy for a given excursion and therefore less power since that distance is traversed in a fixed time interval like the five milliseconds for a half wave of a 100Hz tone.

Even order acoustic cross-over networks with their drivers in-phase (or to be pedantic a multiple of 180 degrees out of phase with the polarity of one driver flipped for odd multiples) have an efficiency peak at the cross-over point due to mutual coupling which is 3dB assuming the two drivers have matching efficiency.

To apply audiophile pseudo-science dictating simpler is better you want to go with this flow (implying negligible output impedances like the .01 - .02 Ohms of many transistor amplifiers) instead of adding electrical circuits and their errors to mash things together.

This is completely orthogonal to the maximum output an amplifier may deliver where some technologies (output transformer equipped amplifiers with taps optimized for various load impedances and some digital amplifiers) allow more similar limits regardless of load impedance.

I'm also ignoring that you can design an amplifier with output current proportional to the input signal and combine it with an appropriate cross-over network to yield better performance because audio output is no longer reduced by voice coil heating (which increases resistance and therefore decreases current flow with a constant voltage source) and affected by inductance changes as the voice coil moves through its range (this causes IM and harmonic distortion) since such combinations are uncommon and would be a hard sell to the audiophile market that could no longer make arbitrary amplifier and speaker swaps. DSP cross-overs with current/voltage programs and relays to switch amplifier configuration would make a fun Burning Amp presentation.

> Back to the original question, what's with 4 ohm speakers if they're harder to drive, well in general they can play louder with a solid state amp, and most people have solid state amps, so they get more sound per dollar with 4 ohm speakers (quantity outsells quality).

Quantity becomes quality when it avoids clipping.

When I play a nice jazz recording at a less than live but realistic sounding 85dB average SPL with 20dB peaks that can be pushing 108dB 1 meter from a speaker.

With 86dB / 2.83V / 1 meter speakers having a 4 Ohm bass compliment I'm just going to miss clipping an amplifier built to meet a 100W into 8 Ohm FTC rating (they measure with sine waves that have a 3dB crest factor).

Keeping the same efficiency but increasing impedance to 8 Ohms takes a 200W amplifier.

This fits well with the desire for single box speakers especially where last octave extension is desired (example - the Revel Salon 2). Keep the size, loose the bottom octave or move it to a sub-woofer, and you could have another 9dB of efficiency for a 92dB sensitive/efficient 8 Ohm speaker.

The two approaches work for different market segments and I'm aware that yours is the latter.
"Quantity becomes quality when it avoids clipping."

Excellent point, Drew. That's why I go with 4-ohm cabs for prosound, where systems get pushed hard, and solid state (voltage paradigm) amps dominate. And even in the bass cab world where some players still use transformer-coupled tube amps, those amps are optimized for 4-ohm loads.
"Quantity becomes quality when it avoids clipping"

I like that and would have to wholeheartedly agree!
Those of us with a little technical knowledge accept and expect it because at lower frequencies dynamic loudspeaker driver output is proportional to voltage.

The statement is ambiguous. We know that doubling power is 3db, and that there is or should be a direct correlation with driver output. Since this is so then driver output is also proportional to power.

Quantity becomes quality when it avoids clipping

This is a bit of a strawman. If the amp is clipping get a speaker with more efficiency or a more powerful amp. Sound quality is not served by quantity if that quantity is also containing annoying distortion products, which are common with a lot of amps without clipping coming into the equation!
"The statement is ambiguous. We know that doubling power is 3db, and that there is or should be a direct correlation with driver output. Since this is so then driver output is also proportional to power."

Wouldn't you have to take other factors into consideration like how the room interacts with the speaker, or possibly, the distance from the speaker to where the measurement is taken? Or is there some type of industry standard and everyone measures these type of things the same way?
Atmasphere writes
> I wrote:
> Those of us with a little technical knowledge accept and expect it because at lower frequencies dynamic loudspeaker driver output is proportional to voltage.

>The statement is ambiguous. We know that doubling power is 3db, and that there is or should be a direct correlation with driver output. Since this is so then driver output is also proportional to power.

Nope. You're confusing voltage and power where power is voltage squared divided by impedance.

Reactive impedance varies with frequency. Inductive impedance magnitude is 2 pi f L with f ferquency in Hz and L inductance in Henries. Capacitive impedance magnitude is 1 / 2 pi f C with capacitance in Farads.

Driver + enclosure combinations are reactive loads with their mechanical parameters reflected in the electrical characteristics at the driver terminals. The driver's in-enclosure compliance Cms shows up as an inductive reactance Lces. The moving mass Mms works as a capacitive inductance Cmes.

Driver voice coils are inductive which also causes impedance to increase with frequency.

You can have a 40 Ohm maximum impedance at driver + enclosure resonance with impedance around a driver's 6 Ohm voice coil resistance as you move through its pass-band before its voice coil inductance becomes significant and increases to 20 Ohms before leaving the audible spectrum.

Over the same range the same voltage can yield the same output. 2.83V might be 90dB SPL although that varies somewhere between 1/5W and 1 1/3W electrical.

As noted complete speakers complicate things more. You get increased output as the speaker transitions from full to half space radiation (baffle step) and can have rising response as driver directivity increases with frequency. When cross-over designers compensate for that with a series load power impedance increases at those frequencies and power dissipated decreases.

Amplifiers which don't accommodate these physical realities with terminal voltage that's a fixed multiple of input voltage regardless of load impedance aren't universally useful in high-fidelity applications for speakers having impedances that are otherwise compatible causing neither instability nor power dissipation issues.
I should probably proof-read better before posting
>The moving mass Mms works as a capacitive inductance Cmes.

capacitive impedance

>When cross-over designers compensate for that with a series load power impedance increases at those frequencies and power dissipated decreases.

series load impedance increases at those frequencies and power dissipated decreases.
Or is there some type of industry standard and everyone measures these type of things the same way?

They are supposed to, anyway. Of course the room is important, about 1/2 of the total system sound, IME.

>The statement is ambiguous. We know that doubling power is 3db, and that there is or should be a direct correlation with driver output. Since this is so then driver output is also proportional to power.

Nope. You're confusing voltage and power where power is voltage squared divided by impedance.

Sorry - not confused. I am quite literal though. If you are saying there is not a proportion then you are contradicting yourself. But I suspect we are arguing semantics. To clarify I was simply stating that +3 db more watts is twice as many watts.

Amplifiers which don't accommodate these physical realities with terminal voltage that's a fixed multiple of input voltage regardless of load impedance aren't universally useful in high-fidelity applications for speakers having impedances that are otherwise compatible causing neither instability nor power dissipation issues.

Certainly this is true. However not all speakers have this requirement of an amplifier- such speakers can be incompatible with amps that are capable of Constant Voltage behavior. Rather than repeat myself ad naseum I invite you to go back to an earlier point in this thread an look for a link I dropped to an article on the Voltage and Power paradigms, so you can catch up on the conversation.
Whats' with 4 ohm speaker's? Its the damn drivers. Many of the tweeter's on the market from RAAL, ScanSpeak and other's have a 4 ohm impedance.
In tweeters the mass of the voice coil becomes an issue so I do not doubt that they would use 4 ohms over 8. All things being equal that would cut the voice coil weight in half.
I was incorrect on the RAAL ribbon's. They are all 8ohm impedance. ScanSpeak has 36 different tweeter model's in production. 23 of those models have 4ohm impedance. Vifa currently has 18 tweeter model's that have 4ohm impedance.
Ribbons are often 8 ohms as the ribbon itself is the equivalent of a voice coil.
Speaking of tweeters, Morel offers nearly identical MDT29's in both 4 and 8 ohm versions. As rated, the 4 ohm is 92 dB sensitivity and the 8 ohm is 89 dB. For a MTM, the 4 ohm would be more likely.

So, just by halving impedance and not doubling driver area and motors, you get +3 dB.
Speaking of tweeters, Morel offers nearly identical MDT29's in both 4 and 8 ohm versions. As rated, the 4 ohm is 92 dB sensitivity and the 8 ohm is 89 dB. For a MTM, the 4 ohm would be more likely.

So, just by halving impedance and not doubling driver area and motors, you get +3 dB.

I may be wrong but FWIU halving the impedance will not increase efficiency by 3db. If you look at the spec's more closely you will notice that Morel spec's the 8 ohm at 89 db at 1W/1M which is 2.83 volts into 8 ohms. If you look at the 4 ohm spec sheet they stayed with a 2.83v input which is actually 2 watts into 4 ohms. So the doubling of the wattage is where the 3db comes from.

It seems to be fairly common practice among driver manufacturers to spec a driver that is available in both 4 and 8 ohm versions in this manner. Can't say as I blame them since they believe that the majority of there drivers will be used on a "constant voltage paradigm" type of amp. Thus the amp is going to try to keep the voltage output the same regardless of impedance.
Jjrenman, you are absolutely correct. People commonly assume that Efficiency and Sensitivity are the same but that is only true into exactly 8 ohms.
Finally got around to that test I mentioned earlier.

So I built a single test speaker with 5 individually sealed Dayton DS135-8's (5"). Not normally a choice for a sealed enclosure but fairly conventional except for a high Qms,low volume requirement and low price. These are nominal 8 ohm rated drivers. They were arranged vertically with 9 inches center to center and a 9 inch wide baffle. That approximates a typical MTM setup but without the T. I kept the volume and the microphone fixed. This is an Omnimic USB microphone. I tested with both "pseudo-noise" and short sine sweeps and measured max SPL. The amp used was a bridged Plinius SA-100.

For a starting point, I measured a single driver at 78 dB C weighted. (+- 0.1 dB for either test)

Two in parallel (4 ohms) measured +4 dB higher than a single. A touch lower on the short sine sweep.

Two in series (16 ohms) measured -1.5 dB lower than a single.

Four in parallel/series (8 ohms) measured +1.7 dB higher than a single. In earlier tests with this setup, I did notice a comb filtering effect and a dip around 1 KHz at the same mic location.

Won't draw any conclusions but I encourage anybody else to try a similar experiment. Results do go against standard convention. With closer spacing, or other factors, results may vary.

This test speaker was for more than just this. It also proved that I would need five of these drivers in a parallel/series 2.5-way setup to achieve some BSC and a fairly flat in-room response.
Won't draw any conclusions but I encourage anybody else to try a similar experiment. Results do go against standard convention. With closer spacing, or other factors, results may vary.

Actually this sounds pretty predictable to me. The Plinius can double power when impedance is halved, so it would make 3 db more output. Depending on where the mic is placed, you will find out about beaming and line source effect as well.

If you ran the same tests with a tube amp you would get different readings- likely more output with the drivers in series rather than parallel.