The Endless Debate:
An addendum to the dilemma: low efficiency speakers are paired with high power amplifiers not simply to play loud enough, but to WAKE THEM UP to sound good (have detail level, have bass, have clear midrange). As such, low efficiency speakers are not all the same: some low efficiency speakers DEMAND high power gear to wake them up. Others SING with a mere 2W amplifier. (For example, the Ryan 610 booskshelf speaker is magnificent driven by 2W zero feedback SET - with both small scale and orchestral music). So, there is no hard line between low efficiency and high efficiency, especially as companies treat sensitivity ratings rather liberally, often GROSSLY overspecifying sensitivity ratings. A lowish efficiency rating might simply mean that the manufacturer is being honest.
An addendum to the dilemma: low efficiency speakers are paired with high power amplifiers not simply to play loud enough, but to WAKE THEM UP.
These terms are treated very liberally in general, so just be aware. Both reflect on how well power is translated to acoustic energy, that is, information transmitted to the air. Efficiency expresses it in percentage format. 100% efficiency is equivalent to 112dB/Wm sensitivity. Each 10dB drop in sensitivity is equivalent to 10 times lower efficiency. That is, 102dB/Wm is 10% efficient, 92dB/Wm is 1% efficient, and 82dB/Wm is 0.1% efficient.
Efficiency tells what percentage of the amplifiers power is turned into sound. 1% means that 1% of the energy is converted to sound, while 99% is wasted as heat and mechanical loss. So, when a 1000W amplifier is driving a 1% efficient speaker (92dB/Wm), only 10W are used to generate sound AT PEAK POWER OUTPUT. 99% of the wattage, 990W, is wasted as heat by the loudspeaker. You are listening only to 10 watts.
High efficiency speakers definitively sound more dynamic in my experience. Dynamic means the range between loudest and softest perceptible sounds. At least to my ears, with the speakers I heard. Which is a lot of speakers and lot of amp/speaker combos. This experience might be different from yours, because I have experience with high quality efficient speakers mated to optimal amplification, while most commercially available efficient speakers show quite a bit of compromise, especially while driven by amplifiers that do not support them. While mathematics is insufficient to support higher dynamic range based on efficiency x power, experience shows that a more efficient speaker digs out a wider range of dynamic information from the same source.
The math says that less efficient speakers SHOULD have SAME dynamic range when played at EQUAL SPL. However, I hear that it's not the case. Lower efficiency translates to a compressed experience, where the average SPL rides louder, and the softest sound still appears quite loud, all the while the dynamic peaks are quite compressed.
Listening to a low efficiency speaker at high volume: it is boomy, loud and fatiguing in comparison to an ultra efficient speaker playing at the same perceived volume. While I have heard of a report that stated otherwise, I have not had any such experience myself.
Price: the materials cost of a commercial speaker is 10-30% of its total cost. So, spending 2000$ on parts, your speakers will be comparable (in competent builder scenario) to the commercial price range of 6000-20000$ speakers. (The 2K$ is just an arbitrary number, and I chose it because it is a realistic price for all the parts for an excellent very high level home built loudspeaker - yet you can pick any number, and the 10%-30% rule will also apply from 10$ to a million.)
Of course, 2000$ in parts will get you that comparative value (in sonic performance!! - not looks, design, nor resale value!) when you have the experience. That IS HOW FAR YOU CAN REACH! Dreaming of 20K$ dream speakers of brand X? Yup, for a much smaller budget you can build SIMILAR SOUND QUALITY LEVEL speakers for yourself, provided you put effort in learning how to build it. Will it sound EXACTLY like Model X? NO!!! It will be different - and you may like it even better than your coveted Model X. (If you want something to sound EXACTLY like Model X, you should buy Model X, no way around it!)
All in all, you either put the effort to earn the 20K$ for the commercial speakers, or put effort to learn how to DIY it.... both routes require effort, NO FREE LUNCH.
Do not be put off. Even though your first try might not be perfect, and your 2K$ MIGHT NOT stand up to the 20K$ dream speakers shoulder to shoulder. Yet, likely it will stand its ground against the 5K$ product range, and will easily give you so much more than the same 2K$ would buy you in the range of commercial speakers.
Exploring why Low and High sensitivity speakers could have wider or narrower dynamic ranges 2/14/2023
Information retrieval. From a packet of 1W energy a specific amount of acoustic information (air pressure change) is retrieved. Retrieving 100dB information content versus 90dB (/Wm) means we read out ten times the information. Think of your camera rendering ten times the pixels in a camera, that is the equivalent.
Compromise to source information: 10dB less sensitive loudspeaker requires x10 watts for equivalent SPL. The amplification process compromises signal integrity. More amplification = higher power amplifier required => higher power needs additional amplicifation stage(s) => additional stages increase noise floor & ERASE LOW LEVEL DETAIL, ERASING LOW LEVEL DYNAMIC CONTENT.
Back EMF. High sensitivity & high impedance speakers have low back EMF, as the cone moves little (low speed) and little current in the voice coil translates to low back EMF. Low back EMF means low compromise to dynamic range. Low sensitivity and high current speakers have massive back EMF due to large cone excursion and arc-welder level currents. The massive back EMF erases fine, low level dynamic content.
High sensitivity speakers have low cone excursions. Half the cone excursion translates to 1/4 accelerating and decelerating force required. That is, to keep the same level of control over a cone that moves twice as much, we need x2 power to extend the cone, but on top of that, x4 additional power / damping to keep it under control. Hence, low sensitivity speakers require immense damping factor from the amplifiers to keep their cones under control. This damping requirement is massively exacerbated by the back EMF, so the damping factor demand is even higher for the low sensitivity speakers.