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PreSonus Eris E44 & E66

Active Nearfield Monitors
Published September 2016
By Phil Ward

PreSonus Eris E44 & E66

PreSonus’ latest speakers employ an intriguing driver configuration that makes them more versatile than traditional designs.

Always striving to provide the best value in nearfield monitor reviews, this month we bring you two for the price of one: the PreSonus Eris E44 and E66. PreSonus have grown from a start-up in 1995 to carving out a significant presence across multiple pro-audio niches. One of those niches is nearfield monitoring, where PreSonus have a range of active speakers that covers all the entry-level to lower mid-range price points; the latest entries in the range are the Eris E44 and E66 reviewed here. Along with their ‘E’ prefix, the Eris monitors also have the letters ‘MTM’ attached in some PreSonus marketing material, and in contrast to the majority of snappy-sounding letter combinations pressed into service as marketing acronyms, MTM is actually significant. It stands for Mid-Tweeter-Mid and describes the physical layout of the E44 and E66 drivers — twin mid-range drivers (actually bass/mid drivers) located either side of a high-frequency driver.

The MTM format also often goes by the name D’Appolito format, after Joseph D’Appolito who, back in 1983, presented a technical paper to an Audio Engineering Society Conference in New York describing and analysing the dispersion characteristics that result from the format. It’s fair to say D’Appolito felt he was on to something significant with MTM, and the paper includes a degree of evangelism for the technique (although he wasn’t the first to arrive at the idea). I don’t know Joseph D’Appolito and it’s perfectly possible, likely even, that he’s not a man given to shameless self-promotion, but I have always wondered why he’s the only speaker designer and academic ever to have had his name attached to a design technique. Maybe it’s simply because the D’Appolito format is visually so easy to identify? I can think of quite a few engineers who’d be perhaps more deserving of having a speaker technique named after them, but I fear they are likely to remain obscure (so no ‘Harwood enclosure’, or ‘Walker electrostatic’ then). I’ll return to the implications of the D’Appolito format a little later. First though, a little more about the E44 and E66 generally.


The E44 and E66 are truly peas from the same pod. In the grand scheme of things, the only really conspicuous difference between them, apart of course from cost, is scale. The E44 measures 18 x 18 x 36.5cm and has twin 11.5cm Kevlar-cone bass/mid drivers. The E66 measures 21.5 x 25 x 46cm and has twin 16.5cm Kevlar-cone bass/mid drivers. Their 25mm soft-dome high-frequency drivers appear to be identical, as are the facilities provided on their rear connection and control panels. Those facilities are, in no particular order, balanced XLR and jack inputs, an unbalanced RCA phono input, a variable gain control, variable mid- and high-frequency EQ controls and, finally, ‘acoustic space’ and low-frequency cutoff selection switches. The amplification downstream of the input sockets is rated at 50 Watts and 35 Watts on the E44 for LF and HF drivers respectively, and similarly 80 Watts and 65 Watts for the E66. The amplifier technology appears to be conventional Class A/B, with traditional linear power supplies. Crossover frequencies are specified as 2.9kHz on the E44 and 2.4kHz on the E66, and both E44 and E66 are reflex-loaded by two narrow and minimally flared letterbox-style ports, one at each end of the enclosures. The enclosures themselves are simple, vinyl -covered, unbraced MDF boxes, finished on the front by decorative plastic mouldings.

The E44 and E66 share the same set of rear-panel EQ and acoustic space compensation controls.The E44 and E66 share the same set of rear-panel EQ and acoustic space compensation controls.

The input gain and mid- and high-frequency controls frustrate a little in being variable so that, unless they are at their centre-detent positions, it’s not easy to be sure they are set identically across the pair of speakers. They offer a generous ±6dB spread, too, so control is relatively coarse. The acoustic space control provides optional -2dB and -4dB LF attenuation at a corner frequency of 800Hz, and while this type of EQ can’t really fully compensate for monitor locations in different acoustic spaces, it is quite useful for tweaking the broad tonal balance for different rooms and locations — although even the -2dB setting represents a really significant tonal change. Finally, the E44 and E66 provide 80Hz and 100Hz low-frequency cutoff options in addition to their flat response. The LF cutoff options can be used to enable the E44 and E66 either to integrate with a subwoofer or, at a stretch, to mimic smaller monitors with more restricted low-frequency bandwidth — although the result of adding the 12dB/octave slope of the filters to the 24dB/octave slope inherent in the reflex-loaded E44 and E66 will result in a compound system characteristic that won’t really bear much resemblance to many small monitors.Diagram 1. Diagram 1.

The Drivers

Perhaps it’s not rocket science, considering the physical layout, but the D’Appolito format is mostly about symmetry. With a conventional two-way speaker, where an HF driver is adjacent to a single LF/MF driver with a crossover frequency between 2kHz and 3kHz, there will be an asymmetry in its vertical dispersion (or horizontal dispersion if the monitor is oriented in landscape mode). The asymmetry occurs over the frequency band through which the outputs of the LF/MF and HF drivers overlap and, depending on the steepness of the crossover filter slopes, the effective overlap band can be anything up to, say, three octaves. Over those octaves the output of the two drivers will interfere, sometimes constructively and sometimes destructively, and locations of constructive or destructive interference in front of the speaker will depend on the relative time delays between the output of the drivers at the listening (or measuring) position. The regions of constructive interference create lobes in the forward dispersion and — back to that asymmetry — in a typical conventional speaker, the primary forward lobe will point downwards (this is because the acoustic centre of the LF/MF driver is almost always behind that of the HF driver), usually by around 20 degrees. Diagrams 1 and 2 illustrate the vertical dispersion around the crossover region typical of a conventional two-way speaker and a D’Appolito-format speaker.Diagram 2.Diagram 2.

By introducing the second, symmetrically located LF/MF driver, Joe D’Appolito primarily employed driver layout to ensure vertical dispersion symmetry (or horizontal dispersion symmetry if the monitor is oriented in landscape mode). However, there’s a bit more to D’Appolito, because the ‘price’ for that symmetry is that the forward lobe will tend to be narrower and extend to a lower frequency than would be the case for the ‘equivalent’ lobe in a conventional speaker (mainly because the outputs of the two LF/MF drivers begin to interfere as frequency rises). Furthermore, the greater the distance between the two LF/MF drivers, the narrower the primary forward lobe will be — which implies another subtle difference between the E44 and E66 (because the drivers are closer together on the E44 than on the E66).

In the previous paragraph I wrote that there is a ‘price’ to be paid for the symmetrical dispersion of the D’Appolito format. However, in implying a negative, ‘price’ may well be the wrong word, because in nearfield monitoring applications I think there’s a reasonable argument to be made that narrower dispersion (ie. a tighter forward lobe) through the all-important mid range might actually be a positive. This is because, to a certain extent, it removes nearby reflective surfaces from the equation: the side walls if the monitor is oriented in landscape orientation and the desk and ceiling if it’s in portrait orientation.Diagram 3.Diagram 3.

We’ll see how that idea plays out in practice on the the E44 and E66, but a few frequency response curves generated with FuzzMeasure illustrate some of the D’Appolito effects. Diagrams 3 and 4 each show three frequency response curves (from 300Hz to 20kHz). The blue curve in each case is the on-axis frequency response (on-axis means the speaker is pointing directly at the microphone). The green curve in each case is the 30-degree vertical off-axis curve with the speaker in landscape orientation, and the red curve in each case is the 30-degree vertical off-axis curve with the speaker in portrait orientation. Green and blue curves are pretty much as expected. Both the E44 and E66 display competent-looking frequency responses that raise no obvious questions. The two red curves are interesting, though, because they illustrate clearly the effect of the D’Apollito configuration on dispersion. The dispersion through the crossover region, and even lower, is significantly tighter vertically (in portrait orientation) than it is horizontally, on both the E44 and E66.

Diagram 4.Diagram 4.

Before I move on from giving Joe D’Appolito yet more free press coverage, there’s actually one respect in which the two E44 and E66 monitors are not quite of true D’Appolito format. This is that their HF drivers are not located on the centre line between the twin LF/MF drivers but displaced laterally (well, laterally in portrait orientation). The effect of this will be two-fold: firstly it enables the two LF/MF drivers to be located slightly closer together (which is good), and secondly it will impose a slight lateral skew (away from the HF driver) to the forward dispersion lobe at the upper end of the LF/MF and HF overlap (which probably isn’t all that significant).

So as promised, no more about Joe D’Appolito, but before I describe how the E44 and E66 perform, there’s one more subject to raise, and it’s all about size. If you ask any speaker designer they’ll probably agree that large speakers are often more difficult to get right than small speakers. Speakers get bigger fundamentally in order that they can reproduce audio down to a lower frequency at a higher volume level. But getting bigger to go lower and louder has implications, and one of them is that a bigger enclosure is more likely to play along with the music, because its larger panels are more resonant. (As a youthful hi-fi speaker engineer at Mordaunt-Short in the 1980s, I quickly discovered how difficult it was to make big, floor-standing speakers sound right, often for this very reason.) This issue on the Eris E44 and E66 is revealed simply by tapping the sides of their enclosures: where the E44 side panel sounds reasonably high-pitched and rigid, the much larger E66 side panel has a more hollow-sounding ring. We’ll see how that plays out in a paragraph or two.

Listening Tests

So, beginning with the E44, I installed the monitors, initially in landscape orientation, on the wall brackets either side of my DAW and began to run through the usual range of familiar material — some Pro Tools sessions and some CDs. First impressions were positive. The tonal balance was workably neutral, if a little bright, and the sense of detail was good, although I was sometimes aware of a slight nasal tendency in the upper mid range. Immediately, however, I had some doubts about the E44 bass. On louder and busier material it seemed to lose a little definition and composure, although of course in the context of compact, inexpensive nearfield monitors this kind of phenomenon is not unusual. The E44 was definitely at its best on simple material with relatively limited low-frequency energy, although even then, I was slightly disappointed by the stereo imaging. I hadn’t adjusted any rear-panel EQ settings at this stage though, and doing so offered some useful tonal tailoring to my room and preferences (although of course I cussed a bit about the variable EQ knobs), but the most interesting change I made was to use the monitors in portrait mode.

The way my workstation is set up in my studio room means that the side walls are a fair distance away, so not only are the first reflections from them subdued, but they are also relatively delayed at the listening position relative to the direct sound (which gives my brain a chance to ‘ignore’ them). The workstation does, however, incorporate a reasonably large desk, and any reflection from that will be pretty strong and quite close in time to the direct sound. So, used in landscape mode, the E44 will generate a strong reflection from the desk because its vertical dispersion is wide. Used in portrait orientation, however, the D’Appolito format results in vertical dispersion that is restricted (especially in the vital mid range, as illustrated in the frequency response curves) so the reflection from the desk is reduced — and it really shows. With the E44 now in portrait orientation, the stereo image that I was a little disappointed by in landscape orientation snapped more into focus, and the way the E44 generally presented mid-range detail was made more explicit. It wasn’t possible for me to try the E44 in a listening environment with no desk and closer side walls, but I wouldn’t be surprised if landscape orientation were to prove more appropriate in that kind of room.

And the E66? Well, first, perhaps it’s no surprise that it too displays the ‘D’Appolito effect’ and worked best for me in portrait orientation. The effect was perhaps even more pronounced than with the E44. Like the E44, the E66 also displays a sensibly neutral tonal balance that on a basic level enables reasonable confidence about the chances of a mix transferring. As befits its increased size, the E66’s low-frequency performance is usefully more extended than the E44’s, and it doesn’t suffer quite so much from the E44’s ‘congestion’ as things get louder and more busy. There was still an occasional sense of low-frequency inconsistency and delay, however, that troubled me a little when concentrating on bass guitar and kick drum balance. The E66 is by no means unusual at its price point in this respect, but simply reflects that being price-competitive means compromises are made.

Perhaps another compromise made in order to hit the price point is the E66’s unbraced enclosure, and to my ears, this imparts a slight character to the lower mid range. Part of me wanted to disassemble the cabinet and glue a wooden brace between the two largest cabinet panels — like the sound-post that’s wedged between the front and back of a double bass. The coloration is audible particularly on male voices and ‘cellos, where it adds a touch of unnatural warmth and body. But again, the vast majority of monitors around the E66 price point display their own foibles and character, so perhaps a little enclosure coloration isn’t a deal-breaker in the grand scheme of things. It is a shame, though, that PreSonus couldn’t find the budget to pay much attention to the cabinet.

Final Thoughts

I’m not sure that even PreSonus really appreciate that perhaps the greatest value of the D’Appolito format in a nearfield monitoring context is that it can provide dispersion options to suit different installations. I think this is quite a valuable characteristic for entry-level products such as the E44 and E66, which are most likely to be used in smaller spaces. Considered in more conventional terms, neither the E44 or E66 is entirely without flaws, but then, nor are any other monitors in the same price band. So if you can live with a little coloration and slightly inconsistent bass, but like the idea of D’Appolito and its dispersion options, they are definitely worth considering. If pushed, I’d plump for the E44 rather than the E66 as the slightly more rounded package, but to some extent that perhaps reflects my interest in less complex music as much as it does the qualities of the two monitors. If you work with music that tends to be louder and busier, the E66 may well be the better option.


There’s no shortage of active nearfield monitors around the price of the E44 and E66. Models from Focal, Fostex, Adam, Eve and Genelec, for example, would all be worth considering.

Tweeter On Top?

You might think that the downward-pointing forward lobe of the conventional HF-above-LF/MF driver layout isn’t ideal, and I wouldn’t disagree — especially for hi-fi speakers that are almost invariably positioned at, or just below, the listening height. The convention of locating the HF driver above the LF/MF driver goes back to the beginning of hi-fi speaker design, and I guess it happened simply because the consensus was that it looked right. There have been quite a few speakers over the years that have bucked the trend and gone for a layout with the HF driver below the LF/MF driver (I‘ve been involved in a couple of them). Experience shows, however, that, for whatever reason, ‘upside-down’ is harder to sell!

Before D’Appolito

As tends to be the way with speaker design, whatever idea you’ve come up with, you can be reasonably certain that somebody will have got there before you. The D’Appolito format is no different. For example (because there are probably others), the Meridian M2, a British hi-fi speaker designed by Bob Stuart (these days one of the brains behind the MQA audio codec — see last month’s issue), pre-dates the D’Appolito AES paper by three years but was undoubtedly a ‘D’Appolito format’ design. Unusually for the time also, the M2 was active — and it also happens to be a remarkable speaker. See

Panel Show

The significance of enclosure panel resonance can be appreciated simply by comparing the radiating area of, say, a 180mm bass/mid driver with the potential radiating area of the cabinet walls enclosing it. The ratio is typically at least 30:1 in favour of the cabinet walls. And then if you factor in a modest musical dynamic range of, say, 60dB (which in ratio terms is 1000:1), it’s pretty obvious that the enclosure walls don’t have to move much to make a significant contribution to the sound of a speaker. There’s some more on this subject on my blog at

About The Author: Phil Ward’s loudspeaker career began in 1982 when he joined UK hi-fi company Mordaunt-Short in a junior design role. After leaving Mordaunt-Short in 1987 for a spell in audio PR, Phil joined Canon as Design Manager for the Japanese multinational’s range of consumer and custom install speakers, and then Naim Audio as speaker design and project manager. Since 2001 Phil has worked as a freelance consultant and writer across both the pro and consumer audio sectors. Phil plays electric and double bass and has recorded, produced and mixed numerous bands and artists. Phil's blog can be found at  

Published September 2016