Expert review and test of the B&W 702 S2 Loudspeakers by Australian Hi-Fi Magazine. Free pdf download included.

A lot has been happening at B&W in recent times, with the company increasing the size of its R&D team by 40 per cent, which entailed closing its old research facility in Steyning and re-locating it to a new, larger, and better-equipped facility in West Sussex, England… and also opening a new factory in Zhuhai, China, where all the 700 Series 2 (700 S2) loudspeakers are built. (Which means that as of now, the only B&W speakers still manufactured in Great Britain are the 800D3s and the Nautilus.)

The Equipment

B&W pioneered the ‘curvy’ loudspeaker cabinet, so in some ways, the fact that the cabinet of the 702 S2 is conventional is a bit sad, but curvy cabinets cost money, which is why you’ll mostly find curves only on the more expensive offerings from loudspeaker manufacturers… no matter where the cabinets are made. However, as you’ll find later in this review, B&W has put a lot of the money it’s saved by building conventional rectangular cabinets into ensuring there is a lot of ‘high-tech’ driver technology in the 702 S2.

The most obvious of these high-tech drivers is the tweeter, which is truly ‘nose on your face’ obvious, since it’s affixed to the top of the cabinet… a technique rather uninspiringly described by B&W as its ‘tweeter on top’ technology. The idea of mounting the tweeter separately and on top of the main speaker cabinet is one that was, like the curvy cabinets, also pioneered by B&W, with the first ‘tweeter on top’ appearing on the B&W DM7, released in 1977. John Bowers and his team started doing this after their measurements using newly-developed test equipment revealed the frequency response errors and phase irregularities that were introduced when a tweeter was mounted directly into the front baffle.

The jet-engine-like black housing in which the tweeter is enclosed on the 702 S2 is different from the zinc housing used on more expensive ‘tweeter on top’ models in B&W’s range, being instead milled from aluminium. Despite being about one-third the mass of zinc, the 702 S2 tweeter body still weighs a touch more than one kilogram, so it’s completely rigid and totally non-resonant. The metal also acts as a heatsink so the tweeter’s magnet always runs cool. When used without a heatsink, tweeters are exceptionally prone to dynamic compression caused by their magnets overheating and becoming less efficient as a result. Many tweeters have heatsinks to counteract his, but few are as large and efficient as the one on the 702 S2.

The 25mm dome diaphragm contained at the front of the housing is a brand-new design for B&W that uses what the company calls a ‘carbon’ dome. The front portion of the dome is formed from a 30-micron-thick sheet of aluminium that has an ultra-thin carbon coating applied by vapour deposition. This is attached to an inner section of carbon material that’s 300-microns thick and profiled to match the curve of the main dome.

B&W says the performance that results falls midway between that of the aluminium double-dome used in the lower-priced 600 Series speakers and the Diamond domes used in the company’s top-line 800 Series speakers. Despite being artificially hardened, the carbon dome’s first break-up mode is said to not occur until 47kHz.

The driver that B&W 702 S2 uses to produce the midrange frequencies should be familiar to all: it’s B&W’s famous FST driver. The FST cone is a perfect example of the benefits of ‘trickle-down’ technology, because it was originally developed for—and first appeared on—one of B&W’s top-line 800 Series speakers.

The cone on an FST driver is unlike almost all other midrange drivers ever made because it does not have a roll surround around its periphery to assist with cone movement. Instead its suspension is ‘fixed’, which is why B&W refers to this driver type as a ‘Fixed Suspension Transducer’ or ‘FST’. In reality, the suspension is not really ‘fixed’ at all, but instead of connecting to a flexible ring of rubber or foam that moves up and down in sync with the cone, the cone is instead terminated by a very narrow polymer ring that stretches and contracts to accommodate the cone’s movement. This trick is made possible because B&W’s FSTs operate over only a very narrow band of frequencies that does not require much in the way of cone movement.

The advantages B&W claim for its FSTs include improved transient response, improved damping and a reduction in dynamic distortion (this last because the foam ring does not reflect energy back across the cone surface in the same manner as a roll surround). B&W also says that this suspension extends the frequency response of the driver higher than it would be for an equal-sized cone with a conventional roll surround, which means that the crossover point to the tweeter can be at a higher frequency than it would otherwise need to be. The cone itself (150-mm in diameter in the case of the 702 S2) is made from a material about which B&W is very secretive, saying only that it’s a ‘woven composite’ fabric it calls ‘Continuum’. Although I don’t know what Continuum is, I certainly know what it is not, and that’s Kevlar, because Kevlar was the material first used in B&W’s FST designs. I therefore feel fairly safe in assuming that for use as a cone material in a midrange driver, Continuum must be superior to Kevlar.

B&W has also changed the material from which the FST chassis is cast. Whereas the FSTs on the CM Series (which the 700 Series replaces) used zinc chassis, the chassis of the 702 S2’s FSTs is an aluminium casting. It’s therefore lighter in weight but according to B&W it is still, thanks to Finite Element Analysis-Optimisation, stiffer than the zinc chassis used in the CM Series. To ensure the aluminium chassis does not resonate, B&W has added a tuned mass damper (TMD)—cleverly disguised as a ‘trim ring’—around the periphery of the chassis where it meets the front baffle. The TMD is colour-matched to the cabinet finish, so it’s light-coloured in the case of models finished in white, and dark-coloured in the case of models finished in black and rosenut. Despite appearing to be solidly mounted to the front baffle, the 702 S2’s FST is isolated from it by isolastic mounting hardware, so cabinet vibrations cannot affect its performance.

Yet another example of trickle-down technology can be found in the three 165mm-diameter bass drivers fitted to the 702 S2. The cones on the three drivers are made using a triple-layer ‘sandwich’ technique B&W originally developed for its 800 Series, where two outer layers enclose an inner layer. In the 800 Series cones, the outer layers were made from carbon-fibre and the inner layer from syntactic foam—a technique B&W called ‘aerofoil’. These materials were obviously too expensive to use on the 702 S2, so although B&W still uses the same sandwich construction technique, the outer layers on the cones of the 702 S2 drivers are made from paper and the inner layers are made from expanded polystyrene (EPS).

The bass reflex port is located on the rear baffle. Although the port tubing is made from a smooth-surfaced material, the flared exit is made from a dimpled polypropylene material that minimises turbulence as the air exits (and enters!) the port. B&W calls this port design ‘Flowport.’ Below the Flowport is the speaker terminal plate which has four large, multi-way terminals to allow bi-amping or bi-wiring. If you don’t do either, you’d leave the flat bus-bar connecting plates in place under the terminals. Each B&W 702 S2 speaker measures 1087×200×364mm (HWD) and weighs 29.5kg.

In Use and Listening Sessions
Tall, heavy speakers with fairly narrow cabinet dimensions have a relatively high centre-of-gravity, which makes them fairly easy to knock over if given a relatively healthy shove. B&W has solved this issue by supplying a very large base-plate which should be attached to the speakers before installation. Note that although the Owners’ Manual gives the impression that fitting the base plate is optional, by using the word ‘may’ (the exact wording being ‘You may attach the plinth during the unpacking process, following the diagrams on the top flap of the carton’) the speakers are so tall and narrow that you will need to fit the plates to avoid accidentally toppling the speakers.

This will be particularly important if you have small children and/or household pets.

The rear-firing port means that speakers’ bass response will be more affected by the cabinets’ proximity to the rear walls than if the ports were front-firing, so you should not place the speakers too close to rear walls (the exception being if you plug both ports, in which case you can push them right back against a rear wall… but more about these port plugs in a moment). B&W’s Owner’s Manual advises: ‘Moving the speakers further from the walls will generally reduce the volume of bass. Space behind the speakers will also help to create an aural impression of depth. Conversely, moving the speakers closer to the walls will increase the volume of bass. If you want to reduce the volume of bass without moving the speakers further from the wall, fit the foam plugs or, for less severe bass reduction, the foam rings in the port tubes.’

The foam plugs supplied are two-stage types with an inner plug within an outer ring, so you can block off the port completely by using both plug and ring, or half-block the port by removing the central plug and leaving the outer ring. The Owner’s Manual has a good description of the effects these configurations will have on the bass response, plus three very good—and very accurate—graphs showing how the low-frequency response will be affected.

My first impression of the bass response from the B&W 702 S2 was that it was quite forward and quite warm-sounding, so I moved the speakers further away from the walls than where I had initially positioned them. This certainly reduced the level of the bass, but the warmth was still there, so I then closed off the ports with the bungs which took away most of the warmth, which I thought then resulted in a rather more ‘natural-sounding’ bass and also allowed me to move the speakers back closer to the rear wall to where I’d originally positioned them. But either with or without the ports blocked the bass was certainly extended, with the B&W 702 S2s having no problems reproducing the lowest notes you’re ever likely to run across when listening to music, right down to and including the lowest note on the piano keyboard (a ‘G’ with a frequency of 27Hz).

If, like me, you are fond of turning the volume of your hi-fi system up to ‘real-life’ levels, you will find—as I did—the B&W 702 S2s can oblige… big-time! I turned up the wick on Jean Guillou’s ‘Great Organ of Saint Eustache’ recording (Dorian DOR-90134) and was amazed at how loudly I could play back the music without any audible distortion. Being able to play very loudly is an immediate advantage bestowed by having three bass drivers, because you get triple the cone area, triple the voice-coil power and triple the cooling, so there’s no dynamic compression. As you’d expect, Bach’s wonderful Toccata and Fugue in D Minor (BWV 565) is on this disc (the very first track, in fact) and it sounded magnificent when played through the B&W 702 S2s, with room-shaking pedal notes in particular, but also a beautifully rich-sounding ‘D’ at the bottom of the runs and a resonating chordal sound at the conclusion of the arpeggios.

The rich bottom-end sound of the B&W 702 S2s also worked wonderfully well for playing rock, jazz and popular music—and when playing any music genre, actually—with the kick-drum sound being full-fledged and meaty, and the sound of both double-bass and electric bass rendered rich and full, with a nicely detailed string sound, particularly when listening to bowed double-bass, but also an excellent transient bass sound when the strings were plucked.

I couldn’t hear any overhang to bass notes at all, and it was easy to hear the fundamental frequency being played, so obviously there was very little doubling either. I found no better example of this than when listening to Nicola Vernuccio’s double-bass when backing Claudio Tellini on Mountain O’Things (from Stazioni Sonore – Who’s Got Its Own). When he plucks the string so hard it hits the fingerboard, it sounded just like he’d done it live in my listening room.

The richness of the bottom-end sound of the B&W 702 S2 speakers was perfectly married to the equally sonically-rich midrange sound and the two synergistically combine into an overall soundfield that presented aurally as a gloriously full sound-stage extending right across the space between the left and right speakers, as if I was listening to a ‘wall of sound’. To get a perfect illustration of the clarity and precision of the B&W 702 S2s’ midrange abilities I’d advise flicking forward to Track 3 on this same disc, which has Tellini singing her own multi-tracked interpretation of Roger Waters’ classic Pink Floyd hit, Money. It has everything: solo vocal, spoken vocal, scatted vocal, and ‘harmony’ vocal via the miracle of multi-track. The FST delivered the sound of her voice to perfection, allowing me to admire the perfection of her articulation and pitching. When she switches from one vocal range to another the B&W 702 S2 follows just as fast as she switches… and she can switch flawlessly and faultlessly very fast indeed. A few tracks further along, where the two musicians cover Thelonious Monk’s Extra, the B&W 702 S2 put me inside the studio itself, so I could hear the exhaled breaths as Vernuccio works hard on the double-bass, plus Tellini trying to be being ‘silent’ (obviously not well enough!) as she waits for her cue to come in. Great stuff!

I’m a great fan of getting tweeters away from front baffles, and also a great fan of isolating tweeters from anything that could possibly affect the motion of their domes—and thus their sound quality—so B&W’s 702 S2 already had me at ‘hello’. But the incredibly sweet, pure and uncoloured treble sound I heard from the B&W 702 S2s wasn’t just down to the facts that the tweeter is physically isolated and radiates its sound without any interference from the main front baffle: It’s also down to that new carbon-dome tweeter, which is so much better than the tweeter B&W used on its CM Series. And while it’s also not quite up to the level of performance of the diamond dome tweeter fitted to the 800 Series models, it certainly approaches it, and when you consider the price differential, that’s an achievement in itself. So how did it sound? I’ve already told you: it sounds sweet, pure and uncoloured.

A great way to tax the ability of any tweeter is to play ‘Day Breaks’ (Norah Jones), particularly the title track where the tweeter has to simultaneously handle cymbals, violin, pedal steel guitar and Hammond B-3 organ along with the harmonics of the soprano sax… it can get very busy and, if the tweeter is not up to the task, will sound congested and mushy. The B&W 702 S2’s carbon-dome tweeter handled all these sounds in its stride, whilst still maintaining that sense of ‘air’ around the highs that only the finest tweeters can manage. The resolving power allowed me to hear each sound in its own acoustic, with the tonalities of the individual instruments separated from each other. If you listen, make sure you pay attention to the sound of the harmonics of the higher notes of Jones’ piano on the following track (Peace) as well as the higher harmonics of Wayne Shorter’s soprano sax sound, which never become harsh.

Conclusion
What I liked best about the B&W 702 S2 loudspeakers was that everything I played through them sounded great!

Rather than aiming for studio monitor-like accuracy, B&W instead seems to have engineered this design to make whatever music you play through them sound richer, more musical and more exciting… in a way almost larger than life, yet at the same time not. The result is that the B&W 702 S2 speakers will be able to fill even the largest room with great sound at high volume levels! And if you don’t have a large room and/or you don’t listen at high volume levels, B&W has another new model, the 703 S2, which uses identical drivers (but this model has only two bass drivers, rather than three), in a slightly smaller cabinet to which a rather smaller price tag is attached. So the best of both worlds then…  # Nicholas Bossi

Laboratory Test Report
Newport Test Labs measured the frequency response of the B&W 702 S2 speakers as 38Hz to 38kHz ±3.5dB, which is an excellent result. A smaller section of this response (between 20Hz and 10kHz) is shown in Graph 1, and you can see that in addition to the superb linearity, there’s also no spectral skew, which if there had been would have resulted in incorrect sound balance, irrespective of the ±dB variation. The graph shows a very slight suck-out in the response between 1.5kHz and 3.5kHz that I suspect is more related to a microphone positioning issue than to the speakers themselves, but it’s so minor it would not be audible. The low-frequency roll-off that starts at 100Hz is controlled, even and extended, suggesting superior bass response.

An expanded view of the high-frequency response of the B&W 702 S2 showing performance from 500Hz all the way up to 40kHz is shown in Graph 2. It was measured using a gating technique that simulates the response that would be obtained if the speakers had been measured in an anechoic chamber, and allows extremely precise frequency vs. level measurements. You can see that the response is quite ‘lumpy’ with the aforementioned dip between 1.5kHz and 3.5kHz followed by another response dip between 5kHz and 8kHz, then another shallow dip above 12kHz that also includes the natural high-frequency roll-off of the tweeter. This response was measured with the grille off, but another measurement with the grille on (not shown) proved that B&W’s grille is acoustically transparent, so the speakers will sound the same irrespective of whether you use them with their grilles on or off, so I’d recommend leaving the grilles on. Note that despite the ‘lumpy’ appearance of the trace, all the variations in the response are within a ±3.5dB envelope, as you can see from looking at the dB scale at the left of the graph.

Graph 3 shows the low-frequency response of the B&W 702 S2 as measured by Newport Test Labs. The output of the port (red trace) is at its maximum at around 22Hz, but there’s good output from the port all the way from 14Hz all the way up to 80Hz. The output of the three bass driver(s) is shown by the black and blue traces on the graph. The black trace shows the response when the bass reflex port is completely blocked by the foam bungs. It rolls off steadily below 120Hz at around 12dB/octave. The blue trace shows the output of the bass driver when the port completely open. The roll-off is much steeper, at around 18dB/octave, but of course the output of the port compensates for the loss.

The overall system impedance of the B&W 702 S2 is shown in Graph 4 for both the bass reflex configuration (black dashed trace), and totally sealed configuration (solid black trace). You can see that in both configurations the minimum impedance is just 3Ω at around 120Hz and the impedance dips below 4Ω between 90Hz and 190Hz, 500 and 850Hz and from 13kHz to more than 40kHz. This means that the B&W 702 S2 only just scrapes in as being able to be classified as ‘nominally 4Ω’ under IEC 60268-5. I would definitely recommend using an amplifier that was completely comfortable driving 4Ω loads, not least because of the current it will draw, particularly at 80Hz, where you can see a 4.7Ω impedance combined with a –68 degree phase angle (phase angle is light blue trace). The electrical crossover between the ‘LF’ and ‘HF’ parts of the crossover network takes place at around 420Hz, as you can see from the pink and green traces on Graph 4.

Graph 5 is a composite response plot where the red trace shows the output of bass reflex port and the dark blue trace the anechoic response of the bass driver(s). The light blue trace shows the frequency response of the midrange driver which rolls off below 550Hz and above 1.3kHz. The solid black trace is the averaged in-room pink noise response (from Graph 1) and the dashed black trace is the anechoic high-frequency response (from Graph 2).

Newport Test Labs measured the sensitivity of the B&W 702 S2 as being 90dBSPL at 1m for 2.83Veq under its standard test conditions, which means this design has well above average sensitivity and also confirms B&W’s own specification of 90dBSPL for this parameter.

Although its frequency response is not as flat as some B&W speakers Newport Test Labs has measured in the past, the B&W 702 S2’s frequency response is still admirably flat and linear and Newport Test Labs’ measurement of it exceeded B&W’s own specification by a good margin. The B&W 702 S2 also met its specification for sensitivity, which is a rare achievement for any loudspeaker. # Steve Holding