High efficiency speakers, low efficiency speakers


1 Like

Well, hearing Sean talk about this doesn’t inspire much confidence about him knowing the “why” or being able to discuss the positives and negatives.

The physical parameters driving efficiency in a woofer are cone area, moving mass and motor force (BL).

Yes, all other things being equal, a single larger diameter woofer will have higher efficiency than a single smaller diameter woofer.

Also, assuming the same excursion limits etc., the larger woofer will be able to displace more air at low frequencies or conversely at a given SPL have lower excursion. So, why is it more common to use a pair of 7" woofers versus a ~10" woofer or a pair of 8" woofers versus a ~12" woofer when midrange reproduction is also taken into account?

The issue with using a single larger cone is that, as sound wavelengths approach the size of the cone, it starts becoming directional because of phase cancellation. There is a dimensionless measure of this called “ka” - circumference of the cone divided by the wavelength of sound it producing. Relatively smooth off-axis response is maintained until ka=2. In the case of Sean’s 10" eminence woofers, the circumference is ~65 cm, so ka=2 is about 1 Khz. A 7" woofer puts this at about 1.65 Khz. As such, a 10" woofer needs to be crossed over nearly an octave lower to achieve smooth off-axis response, which is critical to sound quality. Using a typical hifi tweeter, this isn’t practical (most dome tweeters perform best crossed over in the 2-3 Khz range) and so, in most domestic speakers, you’ll see a pair of 7" woofer instead of a 10" or 12" as a midwoofer. In pro audio, this is different, where a 1" or 1.4" exit compression driver can cross over much lower (though has other issues) so a 12" 2-way is very standard.

Zu compounds this by crossing over the woofer very high in frequency and so the off-axis response isn’t consistent with the on-axis and the performance in this respect is not good.

The primary factors driving distortion at low frequencies (where displacement is higher) are non-linear motor for versus excursion, compliance versus excursion and inductance versus excursion. There is also current related distortions (flux modulation/force factor modulation, AC eddy currents in the steel, Le vs i).

The issue with high efficiency is that to get a low mass, you end up with a thinner cone and shorter voice coil and so typically, motor force versus excursion is worse (shorter coil causing less BL vs X). Also, a thinner cone is less stiff and has more break-up and midrange issues. So this is a place where the low efficiency speaker tends to do better (it’s common to have around double the excursion on a hifi midwoofer as a pro midwoofer). As such, it’s not a case of “all things being equal” and the smaller, lower efficiency woofer can have the same displacement and bass output (cone area times excursion) as a larger, high efficiency one.

However, high efficiency speakers have less current based distortion because you’re putting less current into the speaker for a given output. They also tend to have fewer turns of voice coil wire (to keep mass low) and so inductance and it’s related distortions are lower. Also, high efficiency woofers tend to have big magnets for high motor force (and efficiency) and an optimized motor with high magnetic flux density pushes the steel into saturation where the flux modulation/force factor modulation is less. The main take-away is getting more of your motor force from the magnet and less form the number of turns of the coil is “a good thing”: Also, much of this current based distortion can be improved with proper inductance treatment in the motor (copper or aluminum faraday rings or sleeves).

However, this high motor force increases electrical damping (lower Qes) and actually is cutting bass response and so many high efficiency speakers aren’t moving much at very low frequencies because output is low there (not very good extension). Remember that a 3 db increase in sensitivity takes 8X the enclosure size for a given low frequency cutoff and so most high efficiency systems aren’t playing as low as a lower sensitivity one (unless they have huge boxes).

Anyway, the answer is complex and doesn’t make a quick sound byte but there are positives and negatives to high sensitivity systems or those using single, large woofers but those are some of the reasons that you don’t see them as much in hifi.

That being said, I have been putting thought into doing a more budget larger format woofer and compression driver because performance versus cost can be quite good (a single larger woofer is less expensive than two smaller woofers of equivalent performance). However, I don’t agree with the bunch of the design decisions that Zu makes and the system would have to have good even directivity and off-axis response, no audible cone breakup issues and “hifi” sound.


Thank you, Chris. Great stuff.

There are clearly more than a few trade-offs.

1 Like

Thanks Chris. That’s exactly what I was hoping for when I saw this and posted it.

Hi RonP,

Ok, cool. The thing that I like about audio is that a simple question like this is very fertile ground for discussion.

I think that Purifi has done a good job breaking down the cone driver distortion mechanisms on their website. As a final point, they specifically say:

" Cleaner than large-cone short-stroke drivers: The combination of the above characteristics results in a compact long-stroke driver that delivers complex sound with a clarity and lack of effort that was previously the exclusive province of large-cone short-stroke drivers. Additionally, thanks to the small cone size, the driver has excellent midrange reproduction. It is highly suited for two-way systems."


Chris, slightly off topic but…
As sound wavelengths approach four times the size of the cone, is there a cancellation effect at that frequency due to the the cone being a quarter wavelength of that frequency?

Chris, is the sensibility to toe in and the kind of recommended (and best suited) toe in (without using it as tone control) depending on the kind of off-axis response (any coarse rule available?)? If not, what is it depending on?

Very misleading and oversimplified.

Woofer displacement does not indicate anything about efficiency. For any given SPL and frequency in a sealed, direct radiator enclosure the only thing that determines woofer excursion is the size of the cone. Ports can affect the excursion but will result in decreased excursion at some frequencies and (dramatically) increased excursion at others.

Lower excursion does relate to lower distortion when everything else is equal. Of course, everything equal is as rare as perfect spheres with perfect inelasticity moving on a frictionless surface in a vacuum.

Hi All
I’m not a professional in sound engineering but have a passion for my music rooms to sound great. One of my sound rooms I like to call it my party room (10 metres by 6 metres) the objective to play loud with great bass to dance to, nice even extended bass, not one note bass. 10 years ago I bought PA/DJ set up using 4 active subwoofers (2x 18 inch and 2x 15 inch) placed in each corner of the room. Ideally I would have liked 4x 18 inch but I just bought everything 2nd hand at the right price. I use a calibrated microphone and a DSP parametric eq to setup the bass response. I like to setup a house curve eq response I love low end bass. With all the tweaking over the last 9 years I could never get the low end bass how I wanted compared to my other rooms I have (hifi and home cinema equipment in my other rooms). To cut a long story short I blew up the 15 inch subs and amps at my last Houseparty, I caught covid in a bad way one year ago and was on furlough for around 5 months. I wanted a furlough project to keep me busy. I wanted to replace all subwoofer drivers. I measured out my loudspeaker enclosure and started to look for drivers with the bass response I was looking for my requirements, the bass must play down to 20 Hz flat. I just could not find any PA/DJ replacement driver with the frequency response I was looking for. I started to look at car subwoofers and found a company which make very heavy, high end, 3 to 4 inch voice coils, massive magnet assembly, very high excursion subwoofers but extremely low sensitivity compared to PA/DJ drivers. I know a lot of experts would say, do not use car subwoofers for home use they do not work, but this is my furlough project, I wanted to take the risk and just experiment. The 18 in subwoofers I decided to run vented with 2x port and use port bungs to tune the box, 25hz, 19hz, sealed. 15 inch subwoofers 4x ports, tuning 34hz, 29hz, 24hz, 17hz, sealed. The loudspeakers enclosure are no longer active, I have now used separate amplifiers, I just found the most powerful amps I could afford that could run the low impedance subwoofers. Once tuned in with the calibrated mic and dsp parametric eq, boy… do these subs sound good. I have the crossover set at 60 hz and they go down to -3db at 18Hz. The old setup struggles anything below 30hz. I’ve read on many forums for music you do not need to go so low in frequency response it is more for home cinema but with the house curve I’m using I hear and feel so much more detail. The old high sensitive subs leaned towards efficiency above 50 hz, but my house curve eq made that efficiency pointless. The only con’s to this system, it is very power hungry I think the amps I am using would pull 240v 60+amps easily at higher volumes but I have the electrical grid in place for that. Everything I did with reading forums etc was against the standard practice for PA systems, maybe my room size is too small for PA but I am loving the results and cannot wait until lockdown ends for my next Houseparty. My system probably would not sell to the masses but as a bespoke system I am enjoying every time I switch on my subwoofer amplification.

Good morning Chris!
Some 8 years ago, I read about Zoo Audio in the March 2013 issue of Sound And Vision.
I lifted the phone number to Zoo Audio, from the magazine.
At the time, they were offering their sound cubes.
The magazine didn’t go in to very much details about the sound cubes, so I had to call Zoo Audio to get that information.
What they told me about them, is that each cube uses a 10ench full range driver with a 1.5ench tweeter in the throat of the driver.
When I asked how much bass could I get out of them, I was told about their powered subwoofer that uses a down firing 12ench driver, with a 500 watt RMS amplifier.
I thought about ordering 4 cubes, and 2 subs from them.
But then, I stopped and thought about it again.
And I decided that, "for the kind of money that their asking for that speaker system, that I could either buy or build one myself for a small fraction of the price that Zoo Audio is charging for theirs.
I am willing to bet, that you could put together a speaker system that would be a little more affordable, and would give me everything that Zoo Audio’s sound cubes were lacking.
This is probley why Paul has you working for him.
I say this because, you know a lot of stuff about speakers, and how they work.
My hat is off to you man!

hi Chris, You could be the next Jim Thiel and that is the supreme compliment in my book. Good luck to you and to your work at PS. I know that when the speakers are ready, they are going to be “STELLAR”.

1 Like


I would rather compare old tech with new tech. Your founder and PS Audio has Infinity IRS Vs and some others like myself have Infinity IRS Betas. The V woofers have a larger magnet than the Betas, both have carbon fibre reinforced polyprop … with foam surrounds. Only 1 woofer of the Vs 6 woofers and the Beta’s 4 woofers are servo controlled. So, it is my understanding that you replaced the Vs woofers with some other make. 12 12" dia woofers MUST have had enough surface area for the largest of rooms. So, what was the deciding factor? And do you think PS Audio can create a line of subs? The M1200 could surely then power them. You can then incrementally bring in mid/tweeter panels or sats and see what sells like hot cakes.

Thanks for the detailed explanations Chris! You didn’t mention the whizzer cone…

Does the whizzer cone do anything to block the high frequency break up of the 10.3 cone? If not there would be two separate break up patterns from both cones?

Thank you!

Well, if you’re looking at building something like this, Zu is using some eminence drivers, based off of their beta10CX woofer. Almost 20 years ago, I used to work for a kit speaker and subwoofer design company in Seattle called Adire Audio and we made a kit with the Beta10CX and Eminence APT-50 tweeter that is documented here and can be made for a couple hundred dollars each.

A newer/slightly better version of this is available at diysoundgroup.com as the Volt-10.

However, I don’t generally recommend these coaxes as there is a large mismatch in the horn flare between the pole piece and the cone and this causes a lot of diffraction and response errors. The B&C coaxes are certainly better (and a lot more expensive) but I think a separate horn/waveguide is the way to go for more hifi applications unless you put a ton of work into optimizing things (like KEF has for 30 years).

As a company, if we offered something like this, it would certainly have to be at a higher price point than a kit, as we’d need to offer higher performance and you’re paying for the enclosure/furniture aspect of things and enough profit to support it and keep the lights on. Still, I think that more cost effective products are exciting engineering challenges, as engineering at it’s heart is doing the most with the least.

1 Like

Thanks for sharing about your project. You might check out a site like data-bass.com where they test high output subwoofer drivers (both car audio and pro audio) and include a lot of projects on the level of crazy that you’re talking about :grin:

1 Like

Your opening statement flies nicely after the video. I doubt if that “technical” exchange would fly even in a pre-school environment.

OOOPS. My intention was to critique the video, not your comments.

Sorry, I misunderstood. That’s what I get for writing forum posts late at night :slight_smile:

Well, with round woofers and tweeters etc. you will get these cancelations where there are discontinuities in the baffle edge or mounting details (even very small “lips”).

Also, on woofers you can often get an edge resonance of where the cone terminates to the surround edge because of the mechanical impedance mismatch between the materials, though there are ways to combat this.

So, yes, can see cancellations/resonances but it’s for a different reason.

Good morning Chris!

Thank you for the tech info man!

Some of those drivers you spoke of, I’ve heard them before.

Perhaps it’s the way my ears hear things.

You know, me being a blind audiophile and all.

I get better bass response from a pare of Jensen P10N alnico speakers then the beta10’s.

And also, you’re correct about the speakers that Kef makes.

The floor standing towers have bass drivers built in to them.

Although, you can’t see them, but you know that they’re there because you can hear them.

That’s one of the many things I love about Kef speakers.

But if I had to put you up to building me a pare of speakers that are tube amp friendly, and getting the bass too, what would you come up with?