So why would a designer go the other direction?
Lower impedances have appeared as a means to get more power out of solid state designs. But this is very different from getting lower distortion! So a lot depends on what your goals are!
I think the trend towards speakers with lower impedance corresponds to
the trend for speakers to be smaller yet more full range to fit into
people’s lifestyle.
The size of the speaker has nothing to do with its impedance. The two are unrelated design aspects.
Impedance at port frequencies is always low. Check any ported speaker impedance curve and see.
This statement is mostly false. Ports are usually placed in the peak of a cabinet resonance (which will be seen by a peak in the impedance) as a means of reducing the peak and spreading it out. If properly placed, two lessor peaks with a dip in between will be seen, but overall usually represents a higher than nominal impedance.
Usually it comes down to approach: picking the speaker you love and then finding the right amp
I don't buy that this works! Often people have a preference about tubes and transistors- the speaker **must** be chosen to take that preference into account!! Otherwise you may never get satisfaction and a lot of money down the loo.
^ Lest anyone get the idea that the increase/decrease of power output
by ss amps into decreased/increased impedances would suggest that
frequency linearity would be compromised, the opposite is actually the
case, they actually provide better frequency linearity. Most typical
speakers will decrease/increase their sensitivity in direct proportion
to the increased/decreased impedance changes.
In the specific case to which this quote refers, a Quad ESL was the speaker and most definitely does ***not*** fit this rule! The Quad has impedance curve arising from capacitance and not resonance of a driver in a box (for starters, there is no box). Planars in general don't fit that rule either.
I feel like several points need clarification and at least one needs to be made. First the clarification:
If sound quality is your goal, your amplifier investment dollar will be best served by a speaker of 8 ohms or more, all other variables being equal (and unfortunately they almost never are).
By this I mean that no matter what amp you have, its not in your best interest to make it work hard if you want the best sound out of it that is possible! It does not matter what sort of amp- tubes, transistors or class D. The problem is the same- as you decrease impedance the distortion in the amp goes up. Unfortunately the kind of distortion that is going up is the kind you really don't want- because its the kind that is pretty audible! You don't hear it as breakup or crackle though, you hear it as tonality. Usually this means that the sound will be harsher and less detailed. This is because the distortion components are usually recognized by the ear as harshness and it take a vanishingly small amount of distortion to do this!
This is because the human ear uses higher ordered harmonics to gauge how loud sounds are, and as a result is evolved to be far more sensitive to them. This is in fact why distortion can be so low as to be hard to measure, yet it can still manifest as brightness and hardness.
So to reduce distortion and make the amp sound smoother, use a higher impedance load. Steve McCormick, a well known solid state amplifier manufacturer, sent a note to Paul Speltz, who is known for the 'anticables' but also for an autoformer known as the ZERO. The ZERO allows you to drive a 4 ohm speaker while the amp is loaded at 16 ohms. What Steve said in his letter was that even though his amps had no worries doubling power into 4 ohm loads (and BTW are very well built and designed IME) that the fact of the matter is they sound better driving 4 ohms via the ZEROs. This is simply due to reduced distortion.
One other point not previously mentioned is the effect of speaker cables! At lower impedances they tend to be far more critical, where shorter distances and much larger gauges are required to prevent loss of definition and impact. This can be a pretty big deal as the series resistance of the speaker cable can have a pretty dramatic effect on the effective output impedance of the amplifier! By contrast the speaker cables are far less critical at 16 ohms- which is why a lot of us kids got by with hardware store zip cord in the old days.
The reason 4 ohms is such a big deal has more to do with transistors than anything else. Back when tubes were the only game in town, speakers were usually either 8 or 16 ohms. They were often a lot more efficient too, as acoustic suspension hadn't been invented (Henry Kloss, who was the inventor of that was a co-founder of Acoustic Research, who in turn made the AR-1, the world's first acoustic suspension speaker about 1958).
Obviously several things at once were happening at the end of the 1950s. Acoustic suspension (far less efficient) was getting started, and so were transistors. At first the two were not significant. But as silicon transistors (and consequently higher power) became more available (which really started in the late 60s or early 70s) the need to get a bit more output out of the amp started to take off. BTW, high efficiency loudspeakers are a **lot** harder to build, by nearly a factor of 10 over acoustic suspension, so you can see that speaker manufacturers sensed a profit motive (as did the amp manufacturers by going solid state- a similar motivation is occurring now with class D). However by getting a possible 3 db more output out of an amplifier suddenly became a big deal; acoustic suspension allowed a smaller size but paid the price in efficiency. Anything that might make them appear easier to drive was helpful.
But in the world of high end audio, where tiny little things can make a difference, the need for 4 ohms is almost non-existent (at least not if sound quality is the goal). There really isn't a speaker technology (like planars for example) that actually **need** to be 4 ohms as opposed to a higher impedance. In most cases, this simply happens because the speaker manufacturer does not realize the smoothness and transparency benefits that are possible with the same amp if the impedance is increased!
