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Understanding High-Efficiency Full-Range Drivers Feb 2004 by Steve Deckert
With great sounding low power tube amplifiers becoming integrated into the hi-fi consciousness there is more then a casual interest in high-efficiency full-range drivers. As a result there have been many speakers manufactured based on these types of drivers. Unfortunately from a realistic perspective many of them have a sound that is somewhat questionable. Odds are not in one's favor when it comes to designing cabinets for these drivers unless they are fully understood and properly implemented. As a result odds are not in one's favor that there will be any long term satisfaction from owning a pair. At least not unless they're really good. Of course everyone will claim theirs are really good so where does that leave us? This paper is a collection of observations and or insights that will help you understand why these drivers sound the way they do. Hopefully with a better understanding of these potentially wonderful little beasts you'll be able to buy a good one, or tweak something you may already own to a more satisfying level of performance.
Voice Coil Design The first thing to understand about these drivers is what makes them efficient and what differentiates them from a more conventional audiophile driver. The efficiency comes from the voice coil design and stronger magnets combined with lighter cones and spiders. Higher magnetic force controlling a less then normal moving mass. A visual picture of what this means can be easily seen by looking at the differences in voice coil designs between the two types of drivers. The high efficiency driver will have a thin or even flat voice coil wire wound to a width that is close in size to the magnetic field that surrounds it. This magnetic field exists between a metal pole piece that fits inside the voice coil bobbin and a steel ring that surrounds the outside of the voice coil. The magnetic gap (or flux) is typically between ¼ and ½ inch long in the majority of drivers made today. The large differences between high and low efficiency drivers can be found in the width of the gap. The other thing that varies between the two types of drivers is the diameter of the voice coil wire and the winding width. Many popular audiophile drivers and especially long-throw woofers have wind widths that exceed the magnetic gap by as much as 400% or more. This is called an “overhung” voice coil. High Efficiency drivers typically have the opposite type of coil design referred to as “underhung”. The gap between the voice coil and pole in a good high efficiency driver will usually be so close that a dollar bill will just fit between the two. In contrast a low efficiency driver (like the overwhelming majority of drivers used today) will accommodate 1 or 2 business cards or more. Now, if you’re building a driver from scratch and your first attempt is to use a high efficiency approach, you’ll measure it and see a frequency response that is full of peaks and dips that can exceed 12 dB. This is usually handled by weighting the response recorder or reducing the frequency sample rate so that your response graph is plotted from less points. This averages out the response and hides the spikes - on paper. It won’t change the way it sounds however, so once the obsession to get a smoother and flatter response takes over you find yourself with the same driver using a wider gap and longer coil. Now the response peaks are 5 or 6 dB instead of 12. So should you jump up and down claiming victory? Considering your driver has (as a result of your meddling) now lost 6 dB of efficiency and will need 4 times the power to reach the same output it’s debatable whether the result is a cop-out or a success. No matter how you compare the two, the high efficiency driver will always have 6dB more detail, and be 4 times faster making it FAR more linear and for lack of a better term – accurate. The low efficiency driver will always look smoother on a frequency response plot.
Phase Response You might find it interesting to know that because of the resistance and capacitance and inductance created by moving the voice coil inside the magnetic gap (also known as Impedance) there is a phase shift that manifests itself across the frequency response of the driver. For example, if you started at 100 Hz and moved up the scale you would find that absolute phase does not exist. By the time you reached 5 or 6 octaves the output of the driver can be delayed in time by as much as 180 degrees or more. Boy that sucks, you think to yourself, and yea it sorta does. The trick is to make it work for you rather then against you but that’s another paper. You wonder why it wouldn’t be possible to make a driver that maintained at least a nearly flat phase angle across its bandwidth, and actually some companies do. The problem is this – on a linear phase angle as you go up in frequency the efficiency follows. That means that a driver designed with linear phase response would get louder and louder as it travels into the midrange and treble frequencies. This of course wouldn’t sound too good, and it certainly doesn’t look attractive on paper so manufactures design a phase shift into the drivers to prevent the efficiency from rising too much at higher frequencies, or put another way - the keep the frequency response relatively flat. In a perfect world, such a linear-phase driver would be used in a full size straight horn where the horn itself would exponentially raise the efficiency as the music goes down in frequency. This can nicely offset the response of the driver and end with a result that has both flat frequency response and a minimal phase shift. When you start downsizing and or folding (usually both) the perfect horn you open a can of worms that will include phasing problems. I think I’ve just outlined a substantial reason for why Lowther drivers in particular sound the way they do.
Cone Mass I strongly believe that this is the most overlooked part of a high efficiency full range driver. In a nut shell, the cone is always going to be very THIN. A business card is thicker then most of these cones. It kills me when I look at some of the cabinet designs being used with these drivers today. It’s a PUSH PULL device folks. AC – Alternating. For every forward motion there will be an equal and opposite rearward motion. Why do so many people focus the majority of their attention on the sound that comes off the front of the driver? There is just as much coming off the rear of the driver. When you cram that sound into a box of some sort what exactly do you think is going to happen to it? On a low efficiency driver with a thick pulp cone you can wait for the sound to ring around in there until the insulation and dense pulp of the cone eventually absorb it. With high efficiency full range drivers you don’t have that luxury. At least 90% of the sound that came off the back of the driver will reflect back right through the cone and out into the air if steps are not taken to deal with it. To illustrate this you can remove the high efficiency full range driver from it’s cabinet and take a battery operated portable radio and drop inside the cabinet. Make sure the radio is turned on and playing at a normal listening level. Now re-install the driver in the cabinet and sit back and listen. You will hear the radio perfectly and hopefully after the horror wears off you’ll have an epiphany and start modifying the cabinet. This alone accounts for most of the “Bad Sound” you hear from these types of speakers.
Whizzer Cones These are not perfect devises. However many people blame them for sins they simply don’t commit. For example the famous or infamous rather – Lowther “Shout” is often blamed on the whizzer cone when in fact it is usually the problem described in the paragraphs above. The whizzer cones on 8 inch drivers extend response above 5 or 8 kHz. They do not effect the sound in the 2K area where the “shout” is perceived. Don’t believe it, cut one off and measure the response. The biggest way to improve the performance of a whizzer cone is the application and type of phase plug used. Fixing a bullet shaped plug onto the pole so that it sticks out past the voice coil instead of using a dust cap will usually correct or at least improve many of the phase conflicts that exist between the two cones. This in my opinion will make or break a drivers sound quality. Of course there are many types of phase plug designs. Treating the whizzer by sticking small pieces of foam under it actually does little to the whizzer cone, but rather damps the top end response of the main cone.
Warm up
People I’ve noticed also do not realize that a high efficiency speaker because of it’s light weight voice coil and tight voice gap must warm up before it sounds good. This is because the diameter of voice coil increases as it heats up and reduces the gap between it and the plate. True in all speakers, but when the gap is huge the tiny increase in diameter is by percentages far less meaningful. In a high efficiency gap the percentage of coil growth is significant. In such a sensitive device, operating temperature absolutely WILL change the sound and even the measured Theil and Small parameters. I find it takes about 30 minutes of playing at a normal listening level for voice coil to stabilize and that’s about when things start really sounding right.
Set-up With any speaker, where it sets in a room will determine it’s frequency response and the more experimentation one does with placement the better the odds will be for long term satisfaction. Many full range drivers will sound best between 5 and 25 degrees off axis because the frequency response will be flatter. This is partially because of the whizzer cone and since most all full range drivers have one I thought it worth mentioning.
Cables High efficiency speakers should be called high-resolution speakers. People don’t comprehend how high this resolution actually is. No system will ever sound better then it’s weakest link. You will not be able to tolerate these types of speakers without finding a high quality speaker cable. Same thing goes for interconnects. Generic interconnects like those packaged with generic CD players and lamp cord is simply not going to cut it. If you can’t do the cables right you’re better off to wait to get into this type of speaker until such a time that you can.
Amplifiers Same thing applies to amplifiers. Stay away from high power amps and if you don’t like to gamble with odds, stick with tubes. Your system can’t sound any better then your amplifier. If the amp isn’t as good as ( or better then) the speakers and cables you ARE WASTING your time.
Preamp/Sources Same thing also applies to preamps and your source. Recently a customer came by with a wonderful sounding DAC. It was well above average – I would place it in the top 10% at any price. We listened to a good pair of high efficiency full range single driver speakers with it for a few hours. Then I put my reference vinyl rig on as the source. The customer's exact words were “Are the same speakers playing?” The difference was obvious. The sound took on a size and weight that actually did make it sound like far better speakers were playing. The moral is that even if you have the best source in the world it will probably always be the weakest link so long as all the other ducks are in a row. This is of course unless you can listen to 2-inch master tapes directly off the mastering deck, then you have the best possible source. In the real world that’s not likely to happen for most of us, so as far as sources go it’s always a balance of compromise.
Conclusion As a general rule you don't put a rocket in you're ass and light it unless your standing outside.
Alternate Conclusion High efficiency full range drivers if done right can have wonderful frequency balance with spot on clarity. They can have full bass and nicely extended highs without midrange glare if they are implemented in a good cabinet design. There is no perfect cabinet design, or anything else for that matter so success is usually found by those who do the best job of balancing all the pros and cons. No one said it was going to be easy but then if you were satisfied with your SUV you shouldn't be looking at race cars.
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