The Airpulse brand may be new to you but they're a company with some serious pedigree, as these unusual monitors prove.

I happened upon this active monitor at a hi-fi exhibition last Summer, and a few things about it struck me as unusual. Firstly, even in the typically unhelpful environment of an exhibition hotel room, it seemed to possess a noticeably explicit quality to its mid‑range. Secondly, when I read the brochure I discovered that its bass/mid driver boasts an 'underhung' voice coil; and thirdly, the brochure also revealed that it was designed by Phil Jones, who founded Acoustic Energy and was behind the original Acoustic Energy AE1, as well as the Phil Jones Bass brand.

Before I go on to explain the whats and whys of an underhung voice coil, I suspect you might be wondering who Airpulse are, and what the A300 is. The answer to the first question is that Airpulse are part of a company called American Acoustic Development. AAD have a partnership agreement with Chinese speaker company Edifier, with whom they share R&D and manufacturing facilities. Edifier market audio products in the Far East and US under their own brand, and they also own the high-end Stax electrostatic headphone company. Airpulse have had some visibility in the US market for a while, and now seem to be working to raise their profile in both the pro monitor and consumer speaker sectors in the UK and Europe. As I write, the A300 is available in the UK primarily via a well‑known online retail site named after a major South American river, but distribution through pro audio retailers is, I'm told, part of the plan.

Box Clever

As for the A300, it's a relatively large and surprisingly heavy rear‑ported, two‑way nearfield monitor comprising a 165mm anodised aluminium‑cone bass/mid driver and a horn‑loaded 65 x 12mm transformer‑loaded ribbon tweeter. The dust cap on the bass/mid driver has a conical form that's notably reminiscent of that on Phil Jones' original AE1 unit. The port is generously flared on both entry and exit, but is of relatively small diameter, which suggests a relatively low tuning frequency. The cabinet work is from unusually thick and internally braced MDF panels, finished in lacquered wood veneer and satin black paint. The cabinet feels and sounds suitably rigid under the knuckle-rap test.

Around the back of the A300, things begin to diverge a little from the norm because not only does the A300 sport an unusual range of inputs, but it incorporates all the electronics and amplification for both monitors of the pair within one enclosure — nominally the right-hand unit. The reason for doing this is probably that it makes implementation of the A300's Bluetooth connectivity easier. With all the electronics in one monitor, a connection is obviously required between the pair, and that's achieved on the A300 with a 5m DIN‑terminated cable. Now, I had no problems with the cable and its plugs, but I do think the decision to use DIN for a speaker power-amp connection is questionable. DIN sockets and plugs are a fine connection system for line‑level signals, but they were never designed to carry this level of power, and to my mind there are many better alternatives — the Neutrik Speakon for example.

The Bluetooth input enables wireless connection and audio streaming from appropriately equipped desktop, laptop and mobile devices. Clearly, Bluetooth streaming is not a make-or-break feature for music production work, but it's a useful thing to have. A perfect example for me was when my daughter came home from school one afternoon raving about a piece of music she'd found on Spotify. It was so easy for her just to connect her phone to the A300s and play it for me. Bluetooth is present on the A300 because, while being a perfectly viable pro nearfield monitor, it also has a foot in the consumer hi-fi camp (in that respect it's a little like the KEF LS50 Wireless I wrote about in the January 2018 issue), so its complement of inputs is large. Along with Bluetooth there's unbalanced phono, balanced XLR, coaxial and optical S/PDIF, and USB. The USB socket enables the A300 to connect directly to a computer and operate as a sound output device. Those inputs are routed internally, via DSP‑implemented crossover filters, to Class‑D power amplification rated at 70 Watts for the bass/mid driver and just 10 Watts for the tweeter. Clearly the transformer- and horn-loading on the tweeter does much to increase its sensitivity, so it requires far less amplifier power.

The final features of note on the A300 rear panel are detented low- and high-frequency EQ controls and a detented gain control that also functions as an input selector. The EQ controls offer a generous ±6dB shelf at 95Hz and a +2.4dB/-6dB shelf at 4kHz, and pushing the volume knob sequentially cycles through the inputs. Strangely, the LF and HF EQ controls don't function on the balanced XLR input. Input selection is displayed on the front panel of the 'master' monitor of the pair. A remote handset is supplied with the A300 and provides input selection and volume adjustment.

Well Hung?

And now I'll explain why the underhung voice coil on the A300's bass/mid driver caught my attention. Diagram 1 illustrates a comparison between an overhung and an underhung arrangement. In the overhung arrangement — the one that's used on probably 99 percent of all pro or hi-fi bass/mid drivers — the voice-coil windings extend significantly beyond the magnet top-plate gap. This means that as the voice coil moves in response to an input signal, the same length of voice coil is always immersed in the magnetic field that extends across the gap. There are three snags, however. Firstly, the overhung arrangement is wasteful, because there's always a significant length of voice coil outside the magnet gap that's dissipating power (and getting hot) but making absolutely no contribution in terms of driving the diaphragm. Secondly, the magnetic field through which voice coil moves is rarely linear at its extremities, so the arrangement has a degree of in-built nonlinearity. Finally, the inductance of an overhung voice coil will change with coil position as the mass of magnetic material within its core (the pole piece) varies. As the voice coil moves outwards, fewer of the coil windings surround the pole, so its inductance drops; and when it moves inwards, the reverse happens. Inductance varying with coil position will result in both intermodulation distortion and dynamic changes in frequency response.

Diagram 1: Overhung (top) and underhung voice coils.By its very nature, the underhung arrangement suffers none of those overhung issues. So why, you might well ask, does the overhung arrangement dominate? Partly the reason is cost: a longer voice coil is always going to be less expensive than the much thicker magnet top-plate and the larger magnet needed to achieve the necessary flux density over the extended top-plate gap (where an overhung coil is wasteful of power, an underhung coil is wasteful of magnetic flux). And partly it's the characteristic nonlinear behaviour of the two arrangements at high volume levels that usually kills the underhung option. When an overhung voice coil reaches the limits of its linear travel the resulting increase in distortion happens relatively slowly, but when an underhung voice coil becomes nonlinear it happens suddenly, and the distortion level rises quickly and noticeably. It's like the difference between gentle compression and a brick-wall limiter. It's even possible, with an underhung arrangement, for the voice coil to get 'locked' outside the top-plate gap, and that never sounds good. It seems clear to me, however, that in choosing an underhung arrangement for the A300, a prime motivation for its designer was sound quality. Having said that, though, obviously all my evangelism for underhung voice coils doesn't mean that drivers with overhung voice coils can't be made to work extremely well.

Measuring Up

Before I describe a little of what I heard on listening to the A300, I fired up FuzzMeasure and made a few measurements to investigate a some aspects of its objective electro-acoustic performance.

Diagram 2: The A300 frequency response from 200Hz to 20kHz, measured on‑axis (red), 15 degrees off‑axis horizontally (blue), and 15 degrees off‑axis vertically (green).

The first curve, Diagram 2, illustrates the 200Hz to 20kHz axial frequency response and a couple of 15‑degree off-axis curves. There are a few things to note. Firstly, the axial response of the A300 is remarkably flat: from 200Hz to 12kHz it's within ±1.5dB. Now, while any one frequency response curve doesn't really say all that much about the likely subjective performance of a speaker, a genuinely flat response such as demonstrated by the A300 doesn't happen by accident and is impressive. The other two curves on Diagram 2 show the 15‑degree horizontal and a 15‑degree vertical (upward) off‑axis curves. These curves demonstrate that, as expected from its dimensions, the dispersion of the A300's ribbon tweeter is much wider horizontally than vertically. Having said that, it's only above 10kHz that the effect becomes really apparent — at 15 degrees, anyway. At greater off-axis angles the dispersion difference will extend further down in frequency. An interesting characteristic of the tweeter revealed by the response curves is that its horizontal dispersion at 20kHz is actually slightly wider than at 10kHz. You don't see that very often. What you do see occasionally on tweeters with relatively narrow dispersion, however, is the raised level above 10kHz that the A300 demonstrates. This could have been very easily flattened with either a simple passive Zobel network (a series capacitor and resistor in parallel with the tweeter) or an EQ stage in the active electronics, but the fact that it hasn't been is perhaps to help ensure the monitor doesn't lose its 'sparkle' in the reverberant field. You see, narrow tweeter dispersion may be no problem for head-clamped nearfield listening, but out in the reverberant field it means that there's less HF energy in the room, so a few dB of HF lift can provide some correction.

Before I leave Diagram 2, there's the so‑far unacknowledged elephant in the room in the shape of the deep suck-out around 2kHz in the vertical off‑axis curve. This is caused by the off-axis change in path length from the drivers to the microphone over the region in which the driver outputs overlap: the driver outputs go out of phase and destructively interfere. The phenomenon is typical of the majority of monitors with non-coincident drivers.

Diagram 3: The LF frequency response, measured close to the woofer, with the port open (orange) and blocked (purple).

Moving on, Diagram 3 illustrates the A300's low‑frequency response measured using a microphone positioned very close to the bass/mid driver. Without an extremely large empty space, or a similarly large anechoic chamber, the simplest option for measuring a monitor's low‑frequency characteristics is to place a measuring mic very close to the bass driver diaphragm. Unfortunately, when the monitor also has a port that's some distance from the driver, the close‑mic measurement can't accurately reflect the composite of driver and port. It can tell us a few things, however. The two curves of Diagram 3 illustrates the LF response with and without the A300 port blocked. The first thing to note is that the roll-off slope with the port blocked is around 18dB/octave. This suggests there's a 6dB/octave high-pass filter in the signal chain, because the natural electro-acoustic roll-off of a closed‑box speaker is 12dB/octave (and with the port blocked the A300 is effectively a closed box). The 'port open' curve shows clearly the reduction in driver output at the port tuning frequency (around 30Hz, which is pretty low) and the 'port closed' curve shows that, measured in such a manner, the A300 has a -3dB LF cutoff at around 45Hz, so its published spec (with the port open) of 40Hz is perfectly believable. In either case this is impressive bandwidth from a nearfield monitor. However, in the 'port blocked' state, the output shows a few cycles of low‑frequency overhang in its step response, and relatively high group delay (low‑frequency latency) of around 15ms at 40Hz (a figure that's more typical of ported monitors). Without that huge room or anechoic chamber I'm unable accurately to measure the group delay with the port open, but there's little doubt it will increase. As with pretty much every moving-coil‑based speaker, the A300's extended LF bandwidth gains appear to be accompanied by 'losses' in the time domain.

Diagram 4: Waterfall plot of the A300's low-frequency response.

One slightly unusual feature of the A300's LF response is the bump around 65Hz. Such a feature is unusual, so I generated a waterfall curve (technically known as a cumulative spectral decay plot) from the FuzzMeasure data to see if the bump has any time-domain implications. Diagram 4 illustrates that it does. A waterfall (CSD) plot illustrates how the output of a monitor decays with frequency after a wide-band signal stops (imagine instantly muting pink noise). The decay of the 65Hz bump is clearly visible — persisting right out to to past 50ms at -30dB. The waterfall plot also illustrates the overhang of the A300's (port closed) LF response. The fact that the 65Hz bump persists after the signal has stopped suggests it's a resonance of some kind, and the fact that it persists for so long tells us that it's not well damped, but neither of those facts tells us what its root cause is. The 65Hz bump is a bit of a mystery but it may well contribute to the A300's subjective bass performance.

Diagram 5: The port's low‑frequency response.

Diagram 5 illustrates the port output measured with the microphone positioned right at the port exit. The expected peak in output can be seen between 30Hz and 50Hz (the A300 port has an unusually wide bandwidth) but also revealed is that the port has a significant organ-pipe resonance at just over 700Hz. As I've described in previous reviews, organ-pipe resonances aren't uncommon in ported monitors, and that's because they are not easy to suppress without damping the desired port resonance at the same time. The A300 organ-pipe resonance is a potentially audible one, however, because its peak is only around 5dB below the fundamental port output. I'd expect that resonance to make a small contribution to the A300's subjective character.

Listening In

Speaking of character, first the good news: playing my usual variety of Pro Tools sessions and CDs, I wasn't disappointed by the A300's mid‑range. The sense of explicit clarity and uncoloured detail I thought I heard back at the hi-fi exhibition is definitely all there. It's a revealing, natural, seductive character that had me reaching for all sorts of material just to hear what it would sound like. But more than just being seduced and entertained, in a mix context, the level of mid‑range detail the A300 reveals is seriously useful and I think it's up there with monitors that cost very much more. The mid‑range clarity is accompanied by a really tightly focused stereo image that contributes to a sense of being able to hear deep into a mix. The classy, smooth and detailed character of the ribbon tweeter also helps, although its narrow vertical dispersion is noticeable. I ended up using the A300s upside-down to align the tweeters with my head height.

While it's by no means a complete fall to earth, however, I was mildly disappointed by the A300's bass performance — especially in a mix context. There's no doubt that the measured low‑frequency bandwidth extension is audible and impressive, but to my ears the A300's bass is slightly slow and imprecise. I listened with the ports blocked (which will limit the A300's maximum level) too and preferred that option to ports open, but still felt the bass was a little overblown. Reducing bass level using the rear panel LF control also helped a little (although it doesn't work on the XLR inputs), but still there seemed to me to a be a somewhat sluggish character and slight uncertainty of pitch to the bass. It is partly a matter of need and taste, however, and if your work often includes material that relies on very low bass, a pair of surprisingly inexpensive nearfield monitors that can make any kind of stab at reproducing it, especially combined with such a fabulous mid‑range and HF performance, might be just the ticket. One last observation before I conclude: blocking the ports did appear subjectively to suppress a slight edge in the mid‑range a little. I have no proof, obviously, but the reason might just be that the port organ-pipe resonance is audible.

One of the attractions for me of reviewing the A300 was that it is idiosyncratic and unusual. The idiosyncrasies partly arise from the A300 having feet in both hi-fi and consumer camps. For example, a Bluetooth input is needed for the consumer market, but that means the A300 can't follow the usual pro active‑monitor configuration of an amplifier in each box. And the A300's LF performance is also, I suspect, a result of its need to sell in the ultra-competitive US consumer market, where bass quantity often trumps bass quality. However, one idiosyncrasy that could I think have been avoided is that decision to connect the two A300 units with DIN plugs and sockets. File under "What were they thinking?" and move on. Listen to the A300 however and, especially considering its price, the idiosyncrasies suddenly become rather less of an issue, because there are fundamental elements of its subjective performance that are genuinely exceptional. If you can live with its quirks, the A300 will reward you with remarkable mid‑range performance that's so good it may well be enough.