Antelope Audio Atlas i8

The green front panel of Antelope’s first monitor speaker conceals an unusual design approach.

Antelope Audio delivered a curveball at the 2023 NAMM show by launching their first professional nearfield/midfield active monitor, the Atlas i8. I now have a review pair of Atlas i8s perched on my monitor shelf and — praise be! — in contrast to almost every other monitor under the sun, the Atlas i8 is not entirely black: the front panels are an attractive (to my eyes, anyway) dark green.

At 50 x 29 x 39cm the i8 is relatively large for a nearfield monitor, and at 24kg (due in part to a substantially braced MDF enclosure) it’s not an easy lift. The i8 is also quite tall in proportions, so on my monitor shelf, its tweeter was located significantly above seated head height. However, the Atlas i8 tweeters constitute part of a coaxial compound midrange and high‑frequency driver, which means the system dispersion is notably consistent vertically and horizontally, so I was happy to mount them in landscape orientation to bring their tweeters more in line with seated head height.

Quelle Coincidence

The i8’s coaxial tweeter/midrange driver is an OEM device sourced from SICA in Italy — who, like Antelope, are another long‑established and highly regarded organisation. The midrange element is a nominally 130mm driver with a coated paper diaphragm terminated by a low‑profile, double‑roll surround. It appears closely related to the SICA bass/midrange driver of the Ashdown NFP‑1 monitor we reviewed back in SOS December 2021. One of the design issues to be managed with coaxial midrange/tweeter drivers is that the traditionally large roll surround around the periphery of the midrange diaphragm results in a significant diffraction feature for the tweeter. Consequently, a benefit of the low‑profile roll surround of the i8 midrange driver is that it reduces the tweeter diffraction effect. The detail of the roll surround on a midrange driver is also vitally important in terms of the driver’s behaviour when it is operating at frequencies above those at which the diaphragm moves as a whole. At these frequencies, typically in the mid‑hundreds of Hertz and above, the role of the surround is to dissipate the vibrational energy travelling through the diaphragm without reflecting it back, which would potentially result in audible resonant modes. Employing a double roll surround implies that SICA are looking for ways to optimise the vibrational behaviour of the diaphragm and surround, and that’s obviously a good thing.

The tweeter element of the Atlas i8 compound driver is a relatively conventional 25mm diameter soft‑dome unit. As is pretty much required by its physically restricted mounting location, it has a compact neodymium‑iron‑boron magnet and is front‑loaded by a short, fixed waveguide that blends into the profile of the midrange diaphragm. This is all pretty standard stuff as far as coaxial compound drivers go, but as ever with speaker design, the devil is in the detail.

Under Pressure

Like the i8 coaxial mid and tweeter driver, the Atlas i8 bass driver is also a SICA item. It’s a nominally 200mm‑diameter driver with a coated paper cone and a generous rubber‑roll surround. The notable feature here though isn’t really in the detail of the bass driver — it’s in the fact that there are two of them. The second driver is mounted internally, directly behind the one that’s visible on the front panel, and between the two drivers is a small, sealed, sub‑enclosure. The bass drivers are effectively configured acoustically in series, in an arrangement that’s become known in the speaker‑design world as ‘isobaric’. Now, you’ll undoubtedly know from your understanding of Greek that the term ‘isobaric’ translates as ‘constant pressure’, and in the context of an isobaric speaker system, the constant pressure is that within the sub‑enclosure between the two drivers. The pressure remains constant because the sub‑enclosure is sealed and the two driver diaphragms move equally and simultaneously in the same direction in response to the input signal. But why, I hear you ask, would that bring any benefits?

To answer that question, I have to take us back for a moment to the first principles of why bass drivers need to be mounted in enclosures in the first place. It’s because, at low frequencies, the wavelength of sound is so long (around 8.5m at 40Hz) that the front and rear radiation from the diaphragm, being 180 degrees out of phase, cancel each other. The result is near silence. Trapping the rear radiation of the driver in an enclosure stops the cancellation, and so bass miraculously become audible. However, the act of putting a driver in an enclosure adds the stiffness of the air trapped inside to the stiffness of the driver suspension, and it raises the driver’s resonant frequency, below which its acoustic output will naturally roll off (a speaker driver is, in electro‑acoustic terms, analogous to a high‑pass filter). So while an enclosure makes bass possible, it also restricts low‑frequency bandwidth; and the smaller the enclosure, the greater that restriction. This trade‑off between enclosure volume and low‑frequency bandwidth is one of the immutable fundamentals of moving‑coil loudspeakers, and numerous different arrangements of drivers and enclosures (and now, more commonly, electronic equalisation) are employed by designers and manufacturers to try to optimise the results for particular loudspeaker applications and manufacturing budgets. There’s closed‑box, reflex (ported or ABR), transmission line, coupled‑cavity, horn, and some outliers that combine multiple architectures. And, just occasionally, along comes an isobaric design: a reflex‑loaded isobaric, in the case of the Atlas i8. Just in case my clumsy attempt to describe it fell short, in Diagram 1 I’ve illustrated the basic architecture of an Atlas i8‑style reflex‑loaded isobaric speaker.

Diagram 1: An illustration of the Atlas i8’s isobaric driver loading (not to scale).

Now, configuring two bass drivers isobarically has the effect of doubling both their moving mass (two diaphragms, two voice coils...) and their suspension stiffness (two suspensions in series), so their combined resonant frequency remains the same as that of a single driver. However, because the ratio of the effective driver suspension stiffness to the enclosure air stiffness is re‑cast, the enclosure has far less of an effect on the overall system resonant frequency, and less low‑frequency bandwidth is lost. In fact, all other things being equal (as if!), an isobaric system can in theory employ half the enclosure volume to achieve the same low‑frequency bandwidth as the equivalent single‑driver system can.

So isobaric sounds like a free lunch, but I’m sure you can guess what that guy with the silver platter and card payment terminal walking over to the table is carrying. Yes, there’s a bill to pay. Firstly, isobaric systems are expensive and complex to manufacture; there’s two costly bass drivers, and one of them has to be buried deep inside the enclosure. This usually means the entire rear panel has to be removable, which costs yet more in terms of manufacturing complexity and enclosure structural rigidity. There’s also a significant efficiency loss inherent to the isobaric arrangement (‑3dB, all other things being equal). Remember the immutable law of bandwidth and enclosure volume I mentioned earlier? Well, efficiency is also a factor in that sum, and greater low‑frequency bandwidth with a smaller enclosure unavoidably results in reduced efficiency. And lower efficiency means more amplifier power is required, around 98 percent of which will be dissipated as heat; so, even though the isobaric arrangement employs two voice coils to share the dissipation, the power input is doubled, meaning the potential for thermal compression is the same as for a single driver.

Furthermore, simply because low‑frequency bandwidth is more extended in an isobaric system, the drivers are required to work harder than otherwise they might. Again, this means the bass driver motor system (magnet and voice coil) is likely to be more generously specified and expensive in an isobaric system. The isobaric concept is also, I feel, more obviously beneficial to passive speakers, where the opportunity for electronic equalisation to extend low‑frequency bandwidth without increasing enclosure volume is not available, than it is to active speakers with easy access to DSP‑based EQ and generous power from Class‑D amplifiers. So it’s doubly interesting to my mind that Antelope chose to pursue the isobaric idea. They told me that during development, they found it difficult to hit their low‑frequency bandwidth targets without the enclosure size getting too large, so they tried the isobaric idea and found it did the trick. And despite the downsides I’ve described, isobaric is a perfectly valid solution to the age‑old trade‑off between bandwidth, enclosure volume and efficiency, and I’m more than pleased to see Antelope embracing it. This is genuine, old‑school electro‑acoustics in action, and I for one am happy to see it have a resurgence.

Atlas Plugged

Around the back of the Atlas i8, you’ll find the usual combination of configuration controls and connection sockets. On the latter front, the Atlas i8 offers a balanced XLR/TRS combi socket for analogue signals, plus two further XLR sockets for AES3 digital input and output. There’s also, naturally, an IEC mains inlet, and a USB Type B socket. Currently, this is used only for firmware updates, but in the relatively near future, Antelope plan to make it possible for a USB‑connected computer to configure the Atlas monitors using a special app.

The Atlas i8’s configuration options are set using a colour screen and rotary encoder.

In terms of configurable features, the Atlas i8 offers a range of operational mode, EQ, delay, input sensitivity and sample rate options, accessed using a rotary encoder/switch in combination with a small colour display. In more detail, the i8’s configuration options comprise:

• Input Source: selects between analogue, AES 1 (left) and AES 2 (right).
• Sample Rate: switches the internal sample rate of the Atlas i8 between 176.4kHz and 192kHz to suit the AES input sample rate. For example, if the incoming sample rate is 48kHz, you should use the 192kHz option, as 192 is a multiple of 48.
• Analogue Input Level: enables the input sensitivity of the Atlas i8 to be matched to the output level of the driving interface.
• Two‑way Mode: introduces a high‑pass filter at 80Hz, 100Hz or 120Hz for use with a subwoofer, or to mimic a monitor of more limited bandwidth.
• Bass Extension: applies an IIR (Infinite Impulse Response) filter to the natural roll‑off of the isobaric system to extend the Atlas i8 low‑frequency bandwidth.
• Equalisation: four bands of ±6dB EQ comprising a low‑frequency shelf at 100Hz (Q=0.7), a high‑frequency shelf at 2.5kHz (Q=0.7), a low‑mid parametric boost or cut between 100‑250 Hz (Q=3) and a high‑mid parametric boost or cut between 250Hz‑2.5kHz (Q=3).
• Delay: up to 10 milliseconds to be engaged, either to compensate for unequal listening distances between speakers, or for video delay.
• Presets: four configurations can be saved and recalled.

DSP

Antelope have employed their audio DSP expertise in the Atlas i8 to engineer not only a linear amplitude/frequency response but also to create a time‑aligned, linear‑phase characteristic. This characteristic is partly made possible, and gains some extra validity, from the use of a coaxial tweeter/midrange driver, which ensures that respective signal arrival times from the two drivers don’t vary with listening position. The benefits, say Antelope, are tightly focused stereo images and good depth detail. Downstream of the inputs and DSP is a Class‑D amplification stage comprising amps rated at 200W for the low band and 100W each for the mid and high bands. Unusually, the Class‑D amps, rather than being based on commercially available modules from companies such as Hypex, Purifi or Pascal, are an Antelope in‑house design developed specifically for the Atlas i8. Antelope’s well known and admired expertise in digital audio signal processing has clearly played a significant role in making the Atlas i8 both possible and unique.

The result of the Atlas i8’s DSP, its 400 Watts per speaker, the isobaric bass loading and the complement of drivers is that Antelope claim some impressive acoustic specs: in particular, low‑frequency bandwidth to 35Hz and maximum continuous sound pressure level of 117dB at 1m. These are specs somewhat beyond those of a typical compact nearfield monitor, and nudge up against even main‑monitor territory. But, you’re probably asking, how does it sound? Well, I’ll get to that in a paragraph or two, but first...

Testing, Testing...

I fired up FuzzMeasure Pro in my usual measuring space and proceeded firstly to check the Atlas i8 axial frequency response and harmonic distortion. The measuring axis I chose was that of the coaxial mid/tweeter and the result is illustrated in Diagram 2. The frequency response isn’t the flattest I’ve ever measured, but it’s very competent, and bar the odd minor aberration, fits into ±1.5dB limits over the important 100Hz to 10kHz band (note that the curve isn’t fully representative of reality below 100Hz). The distortion performance, measured at 90dB SPL, is impressive, with third harmonics in particular remaining predominantly at least 55dB down above 100Hz. That’s a good result. Along with the axial response measurements, I checked the Atlas i8’s off‑axis performance both vertically and horizontally and found, as expected, nothing of note to report, so haven’t included any curves. This is one of the great benefits of coaxial drivers — off‑axis speaker measurements become much less interesting. The off‑axis performance of the Atlas i8 is well controlled and managed.

Diagram 2: The Atlas i8’s on‑axis frequency response (red trace), and second and third harmonic distortion levels (green and blue, respectively).

A somewhat academic but nonetheless interesting result came from examining the Atlas i8’s front‑mounted, slot‑style reflex port output, and that’s illustrated in Diagram 3. This test records the output of the port captured by a measuring mic placed right at the port mouth. The resulting curve shows the peak in port output, as expected, at just under 40Hz, and a very well‑controlled and resonance‑free roll‑off thereafter. But then at 500Hz there’s a high‑Q peak that I believe is either the result of an organ‑pipe resonance in the port, or a standing‑wave mode within the cabinet escaping through the port. Either way, if you look carefully at Diagram 2 again, you’ll see a tell‑tale wiggle in the system frequency response at the same frequency. This, I think, is the resonance contributing to the overall output. However, I wrote that the phenomenon is academic because, in reality, the resonance is far enough down in level and so localised that it is probably not significantly audible.

Diagram 3: A close‑mic measurement of the Atlas i8’s reflex port.

The Atlas i8 is one of those monitors that sounds impressively sorted and capable from the start.

Listening In

The Atlas i8 is one of those monitors that sounds impressively sorted and capable from the start. The extended low‑frequency bandwidth is immediately apparent and the stereo quality is notably well focused, with really tight imaging and very good overall retrieval of recorded detail. For example, Laura Marling’s track ‘Soothing’, from the Semper Femina album of 2017, has become a significant reference for me recently. Part of the reason for that (apart from the fact that it’s fabulous recording of a wonderful song and arrangement) is that there’s a subtle, buzzy distorted noise from something on the left channel early on in the piece. When I first noticed the noise a while ago I thought it was that of a failing monitor bass driver, but it’s not, it’s on the recording — but not every monitor makes it audible. Some I’ve heard miss it completely (it’s also far less audible on MP3/compressed renders of the recording). Not the Atlas i8, however, which made the Laura Marling buzz very clearly audible.

I wrote earlier that the Atlas i8 midrange driver appears to be closely related to that of the Ashdown NFP‑1, and one aspect of that monitor’s performance was its clean and uncoloured way with voices and acoustic instruments. The Atlas i8 is very much cut from the same cloth. It presents voices with a natural tonality and perspective and timing between vowels and consonants. Not only does that point to a classy midrange driver, it suggests the tweeter is pretty handy too. And it is: detailed and revealing without being shouty. I did once or twice feel that the Atlas i8 overall balance was a little light in the low midrange, not quite warm enough on male voices, but ‘correcting’ for such personal preferences is what the EQ facilities are for.

Atlas i8 bass performance is impressive in its bandwidth extension and, on the whole, highly satisfying in terms of reproducing dynamics and pitch. It doesn’t sound particularly like a reflex‑loaded speaker, coming across much more closed‑box in character. Only at very low frequencies and at higher volume levels did the Atlas i8 to my ears begin to sound a little less in control. And I preferred the default Atlas i8 low‑frequency setting over the Bass Extension option for this reason (your mileage may of course vary). However, the Atlas i8 is generally extremely capable of playing loud. I occasionally find myself writing in my reviews, especially of smaller monitors, that they suit small‑scale, acoustic music. Not the Atlas i8; its wide bandwidth, general competence, neutral tonal character and fine stereo imaging remain very much intact to higher volume levels than I would be likely to use other than very occasionally.

Finally, it’s not at all easy for a new manufacturer to enter the active monitor market, especially with a product that offers something out of the ordinary. Antelope have pulled it off with the Atlas i8, and with no little style. They have created an unusual and highly capable wide‑bandwidth monitor that, without doubt, can play the role of a useful and accurate mix tool.