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Genelec 8351B & W371A

Active Monitors & Woofer System By Phil Ward
Published November 2021

Genelec 8351B & W371A

This impressive setup combines point‑source technology, advanced loudspeaker management DSP and a powerful new bass extension system. Is this the state of the art in studio monitoring?

The 8351A was the first of the technologically advanced Genelec The Ones series of three‑way, dual‑coincident, ‘point‑source’ monitors to be introduced, and Bob Thomas gave it a glowing review in SOS August 2015. Bob has also more recently (in SOS September 2021) written, again in glowing terms, about the larger 8361A. Since the launch of the 8351A Genelec have expanded the series to include both smaller and larger models, and some of the know‑how and newer technology developed for those products has been fed into the first subject of this review; the 8351B.

There’s more than just a revised and updated monitor to describe here, so the plan is to cover quite a big parcel of Genelec land along with the 8351B. That’s because Genelec have significantly refreshed and upgrared their GLM (Genelec Loudspeaker Manager) optimisation and SAM (Smart Active Monitor) control technology, and also introduced the W371A Woofer System. There are, by the way, some very good and technically fascinating reasons why I didn’t, and Genelec don’t, describe the W371A as a subwoofer. I’ll get around to explaining that later on. First I’ll offer a few paragraphs on the electro‑acoustic fundamentals of the 8351B.

To The Point

As with a few other dual‑coincident monitor designs of recent years, Genelec’s opportunity to design and introduce dual‑coincident technology came about with the lapse of KEF Electronics’ original patents, which effectively protected the idea of a small tweeter mounted concentrically at the apex of a substantially conical bass/midrange diaphragm. With The Ones series, however, Genelec chose not just to borrow the KEF idea and apply it to an otherwise conventional two‑way monitor design, but to develop it much further and extend the idea to a three‑way ‘point source’ (meaning that low, mid and tweeter elements all effectively radiate from the same location in space) monitor format, and to integrate the mid/tweeter compound driver with the curved form of the enclosure front panel. At a quick glance it’s not really obvious where the midrange driver diaphragm ends and the enclosure front panel begins. And as I’m mentioning the front panel, it’s probably a good time to say that the whole 8351B enclosure is created from two immensely rigid aluminium die‑cast components.

Also cleverly incorporated within the enclosure design is Genelec’s unique three‑way ‘point‑source’ solution to integrating low‑frequency reproduction with dual‑coincident mid and high frequencies. A fundamental issue with low frequencies and dual‑coincident drivers is that the output of a tweeter mounted at the apex of a bass/midrange diaphragm will suffer significant frequency modulation distortion if that diaphragm is required to move beyond a millimeter or two. This is because the bass/midrange diaphragm effectively has a secondary role as a tweeter waveguide, and the last thing you need in that scenario is the waveguide moving significantly when the bass player hits a low B, just as the drummer’s playing some delicate brushed ride cymbal. The obvious solution is to take the responsibility for bass reproduction away from the midrange driver so that its diaphragm is not required to move significantly — but then, where do you locate the bass driver and still retain the point‑source principles?

Genelec’s solution is to go to a three‑way format and split responsibility for low frequencies between a pair of bass drivers located symmetrically about the compound mid/tweeter axis and loaded by a single rear‑panel reflex port. The bass drivers are oval in form and all but hidden behind the profiled front panel. They radiate through slots formed between the front panel and the top and bottom surfaces of the enclosure. With the bass‑to‑midrange crossover frequency at 320Hz, the physical spacing from the bass driver slots to the mid/tweeter driver is small enough a proportion of the one‑metre wavelength for the point‑source principle to remain valid.

The Drivers

While I’m on the subject of the bass drivers, not only is their architecture and location unusual in the 8351B, they also display an interesting constructional feature in that their rubber roll surround is corrugated around the curved section of the oval. I asked Genelec’s R&D Director Aki Mäkivirta about the surround and he explained that its corrugations and curved sections work together to optimise the force distribution around the perimeter of the glass‑fibre‑reinforced paper driver diaphragm. Diaphragm stability is important in enabling the driver to retain linerarity at high excursions, and to achieve that, the surround force needs to be equal around the perimeter of the driver. Genelec optimise the surround in this respect by using variation in its height as a factor to adjust its flexibility, hence the corrugations. Finally on the bass drivers, they are loaded by a shared reflex port located around the back of the enclosure that’s very generously flared at its exit. The flare will help keep the airflow laminar and so delay the onset of obvious chuffing noises or more insidious port compression effects.

I wrote earlier that it’s not really obvious where the midrange driver diaphragm ends and the enclosure front panel starts, and that’s because, relieved of the need to play any bass, the midrange driver of the 8351B (and the other The Ones‑series monitors) can be fitted with an almost flat surround. It doesn’t need the generous roll that would otherwise enable the diaphragm to move more significantly. Designing a surround that needs only to deal with midrange energy also means that its material properties can be optimised to work over a much narrower band of frequencies. Dual‑coincident or not, this is one of the big wins that can come with the decision to go with a three‑way monitor format: the midrange driver can be designed specifically for midrange and doesn’t suffer compromise by also having to play bass. With a dual‑coincident driver, however, a flat midrange surround brings a second benefit: it doesn’t result in a significant high‑frequency diffraction feature.

Before I leave my basic description of the 8351B, I’ll just mention that the mid driver diaphragm not only features an unusual two‑piece aluminium construction, it also has an inner surround along with the outer surround that I’ve already described. The inner surround joins the apex of the midrange diaphragm to the outer part of the central tweeter housing, and in doing so cleans up a few potential sources of diffraction. And again, it’s the three‑way format and the fact that the midrange diaphragm isn’t required to move significantly that makes the inner surround feasible. Finally, the 8351B tweeter, nestled down at the apex of the midrange diaphragm, is a 25mm aluminium dome. It’s quite possibly the most conventional‑looking element of the 8351B.

From A To B

A simple change of codicil letter from A to B, and the fact that the fundamental architecture and external enclosure design of the 8351B remains the same, perhaps suggests that not too much has changed since the 8351A. The reality is that there are numerous revisions. One of the most significant is that Genelec have moved from a conventional, linear internal power supply to a switched‑mode device. This not only results in a monitor that’s a significant 4.7kg lighter, but enables a substantial increase in amplifier power. Where the 8351A was rated at 150W, 120W and 90W for bass, midrange and tweeter respectively, the 8351B is rated at 250W, 150W and 150W. This increase in amplifier power helps the 8351B achieve a 2dB increase in maximum output level. However, it’s not just more power that results in increased maximum volume; all the 8351B drivers are slightly larger than their 8351A predecessors. The slightly larger midrange driver has also made possible a drop in the bass‑to‑midrange crossover frequency: from 470Hz to 320Hz. This ought to help a little with vertical off‑axis consistency at large angles.

A further refinement on the 8351B over the 8351A is that its DSP incorporates phase equalisation from around 500Hz upwards. Phase equalisation corrects for the frequency‑dependent delay effects that are fundamental to the processes of electro‑acoustics, and although it results in slightly increased overall in/out latency, Genelec say it can provide a subtle but worthwhile gain in subjective performance.

...perhaps even more significant DSP revision in the 8351B is that it incorporates significantly more filter ‘horsepower’ than the 8351A in terms of the number of individual notch and shelf filters...

One perhaps even more significant DSP revision in the 8351B is that it incorporates significantly more filter ‘horsepower’ than the 8351A in terms of the number of individual notch and shelf filters than can be accessed and deployed by Genelec’s GLM room optimisation package. Where the 8351A offered a total of 10 filters (six notch and four shelf), the 8351B increases the total to 20 (16 notch and four shelf). The notch and shelf filters are accessible for manual selection through banks of DIP switches on the 8351B’s rear panel, adjacent to its balanced XLR analogue and AES3 digital input sockets, but to my mind, manually selecting EQ options for the 8351B borders on the daft when the power of Genelec’s GLM app is so easily and, in the context of the cost of the 8351B, relatively inexpensively, available.

GLM 4.1

We’ve written about GLM a few times since its launch in 2006 so I’ll not spend too many words covering old ground, but to recap a little, there are two fundamental elements to it: the hardware, comprising a measuring microphone, a compact USB interface and a bunch of RJ45 cables to connect the monitors to the interface; and the software, comprising a Mac OS or Windows app that handles both monitor and room optimisation and monitor system configuration and management. Once a monitoring system is up and running, GLM can also operate as a very handy on‑screen monitor controller too.

The recently launched 64‑bit architecture, 4.1 version of GLM marks the 15th anniversary of Genelec’s room optimisation technology, and incorporates a re‑engineered AutoCal 2 routine that, Genelec say, results in quicker and more accurate optimisation data. In particular, AutoCal 2 is claimed to produce better optimisation results in rooms that are more reverberant in character, and also in setups that employ extreme nearfield monitoring (as close as, say, 50cm), either necessarily due to an ultra‑compact listening space, or through a desire to emulate a more headphone‑like feel to monitoring. GLM 4.1 also adds further to the 500Hz and above phase compensation inherent to the 8351B that I mentioned a few paragraphs ago. The new Extended Phase Linearity option reduces low‑frequency group delay (at the cost of an additional 7.5ms overall system latency) such that, for example, an 8351B displays group delay that’s comparable to that typical of closed‑box monitors: less than 5ms down to 70Hz.

The decision to implement positive‑gain EQ in GLM 4.1 was partly, say Genelec, a result of the data collected since GLM 3 began to employ the Genelec cloud server for AutoCal processing.

Another new development in GLM 4.1 is that it allows for localised regions of positive gain EQ to be applied where a significant notch in the in‑room response can be feasibly equalised without the monitor running into headroom problems. Equalising notches by adding localised gain is something that previous versions of GLM were not designed to do, for two reasons. Firstly, any notch in a room response that’s primarily caused by destructive interference between two signals (direct and back wall reflected sound for example) fundamentally cannot be filled in by adding localised gain. The destructive interference between two signals will always occur, never mind how much extra local gain is applied. The second reason for avoiding extra localised gain is simply that it can potentially result in a monitor running out of amplifier headroom. Every 3dB of gain demands twice the amplifier power, so even relatively modest positive‑gain EQ can quickly ask significant questions of an amp. The decision to implement positive‑gain EQ in GLM 4.1 was partly, say Genelec, a result of the data collected since GLM 3 began to employ the Genelec cloud server for AutoCal processing. The resulting dataset of 20,000 examples of monitor optimisation curves gave Genelec the confidence to know in which frequency bands, and to what degree, positive EQ could be both safely and effectively implemented.

In use, GLM 4.1 is fundamentally straightforward and intuitive, but its ability to manage and optimise systems comprising very large numbers of monitors and channels, and to create optimisation profiles for multiple listening positions and multiple monitoring setups, means things can quickly become complex. To provide a flavour of just how complex, GLM 4.1 can manage systems comprising over 80 monitors and subwoofers, creating an individual response optimisation profile for each one while also enabling monitors to be grouped in different format arrangements (stereo, 5.1 or Dolby Atmos, for example) and also offering multiple optimised listening positions for each group. GLM 4.1 imposes very few limitations in terms of the practical application of even the largest monitoring systems. The decision I think to endow GLM 4.1 with the capability to optimise and manage so many monitor and subwoofer channels reflects Genelec’s increasing success in equipping very high‑channel‑count immersive performance and production spaces.

A significant element of GLM 4.1’s ability to manage such complex systems is the combined architecture of the software and monitors, which puts the DSP hardware needed to handle the room compensation EQ in the monitor. The GLM software processes no audio itself, so in terms of doing the maths it cares little how many monitors are involved — it ‘simply’ manages the optimisation process and uploads the response optimisation data to the appropriate monitor. Similarly, the remote cloud server location of the AutoCal 2 optimisation calculations helps GLM manage extremely complex systems without imposing any significant processing load on the local computer (which quite possibly already has its hands full running a massive, multi‑output DAW session).