lAB­o­RA­ToRy TEsT RE­PoRT

Australian HIFI - - ON TEST - Steve Hold­ing

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 suck­out 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.

Graph 1. Av­er­aged fre­quency re­sponse us­ing pink noise test stim­u­lus with cap­ture un­smoothed. Trace is are the av­er­aged re­sults of nine in­di­vid­ual fre­quency sweeps mea­sured at three me­tres, with the cen­tral grid point mid­way between tweeter and midrange.Graph 2. High-fre­quency re­sponse, ex­panded view. Test stim­u­lus gated sine. Mi­cro­phone placed at three me­tres mid­way between midrange and tweeter. Lower mea­sure­ment limit 500Hz.Graph 3. Low fre­quency re­sponse of front-fir­ing bass re­flex port (red trace) and woofer with no bung (blue trace) and with port com­pletely blocked (black trace). Nearfield ac­qui­si­tion. Port/woofer lev­els not com­pen­sated for dif­fer­ences in ra­di­at­ing ar­eas.Graph 4. Im­ped­ance mod­u­lus show­ing phase (blue), high-pass sec­tion (green), low-pass sec­tion (pink), port open (black dashed), port blocked (black solid)Graph 5. Com­pos­ite re­sponse plot. Red trace is out­put of bass re­flex port. Dark blue trace is ane­choic re­sponse of bass driver. Light blue trace is sine re­sponse of midrange driver. Black trace is av­er­aged in-room pink noise re­sponse (from Graph 1) Dashed black trace is high-fre­quency re­sponse (from Graph 2).

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