Calling on B&W experts...?

[flm]

Hi,

I am considering purchasing the 802D. I read something interesting on the AVS forum where someone said the 6" FST Kevlar driver B&W uses has a tendency to resonate at levels higher than 95 db causing the speaker to sound strained. Can anyone here further elaborate on this issue?

Whenever I have heard the N801 or N802 I did not notice this but the level were under 95 db.

Thanks!

BTW I impressed that the 800 Series is using Mundorf Supreme capacitors in thier crossovers!

[audioqueso]

This is going to sound bad, but most of the people over at AVS don't know what they're talking about. Without exaggerating some people will say something like 'don't waste your money buying anything but Home Depot speaker wire for your B&W 802... it's all the same', or to that degree. I stopped posting there in regards to speakers and amps cause of the silly responses I was receiving. That's not to say I'm bias towards everyone there cause there was some people that really know their stuff, but the majority of the common members have given me that impression.

Might be true about the 6" FST driver, and it might not. I would feel more assure asking something like that here in HTG. I don't know the answer for that (I'm a 805 guy... not up to 802 level yet lol) but hope my advice helps.

[js24]

maybe you've noticed this already but it is quite scary world out there for B&W users
the person who "bashes" B&W in the link below is an NHT owner... funny.. another infamous B&W basher at the audioasylum (Mr. John Ashman?) owns/deals(?) the NHT speakers...

look at the link below: there is a review of 800D, which the reviwer also mentioned the midrange as the weakest link of the new 800 series, and the battle between good and evil

http://www.avsforum.com/avs-vb/showthread.php?t=539251&highlight=800D

as for me, I thought I loved the mid/upper mid of my 703 in the beginning, but kinda getting tired of them... what I figured out is though.. the infamous harshness of 703 comes from the bright tweeter AND the kevlar(given it goes all the way to 4khz)
..
I might switch to another brand or give B&W another chance with the 803Ds... I dunno ... there is something about B&W speakers that makes me want to own.. just like Apple computers...

there are some *knowledgeable* pple in the avsforum- i.e. kal Rubinson of stereophile.. you just have to filter out dummmies

[flm]

I appreciate the comments!

Thanks!

[RobP]

Hello FLM, I have never heard of this nor have I actually heard this happen, I have a pair of 803's and that has never happened. It makes you wonder what type of power that they were using and what they were hearing was actual clipping???

[PewterTA]

I'd bet a set of 802Ds that the person that made those comments was using an under powered source and it was not the 802Ds mid range that was causing the problem.

You can really crank those speakers and they don't sound strained, now I've never listened to them at 95dbs for sure, but I'd say I hit pretty close to reference and it was pure bliss (and really loud too)...

But my guess is that the amp wasn't suited to drive the speakers.

[Pieter]

Most of this is, at bottom, based on some sound physical principal, which is often boldly stated, “proving” the validity of the “correct” opinion.

Trouble is, there’s little understanding of either the principal or more importantly, its application and the extent of its validity.

Yes, Kevlar is rigid and may be prone to resonate. But resonance is also affected by geometry of the cone, weave pattern of the fibers, etc.

What is the primary resonating frequency? Its amplitude? Does the surround dampen some of this? Is there a surround? Does the weave pattern help in breaking up the resonance? Is it “just” Kevlar?

Is it audible?

Are you listening to the speakers or the opinion of others?

There’s danger in the previous question too.

[csuzor]

I have read all these posts on the problems with the FST in avsforum, and the B&W development paper on the 800s...

There is some truth in what is being said negatively. According to B&W engineers themselves, the constraint that there is only 1 midrange driver from 350-4kHz, implies that stiffer materials + purer designs (such as piston drivers) cannot be used to cover the whole range without undesirable effects either at the lower or upper frequencies. Thus the semi-stiff kevlar, with it's non-radial distortions, coupled with the matched damping of the edge of the FST design, is the best trade-off.

However, the reason for the choice of just 1 midrange driver instead of 2 driver (4-way speaker), is not given, and may be historic, may be esthetic, may be marketing, may be audio quality / coherent soundstage related... does anyone want to suggest which?

edit: maybe the answer lies within B&W research on Aluminium Diaphragms (see below). Ultimately stiff materials would suffer directivity, so only the sweet spot listeners would benefit, and off-axis listeners would get lower quality.


Now, at what point would 2 drivers (and maybe another technology, pistonic drivers) present an acoustic advantage relative to the FST? If this is only at high volumes, then the choice of FST is clear for most persons. Did B&W try 2 drivers, and what was their conclusion?

As for me, I compared the FST on 703 Vs the normal kevlar design on 704, and the mid-range was so much more detailed, I couldn't believe it... FST is my preferred choice for now. And B&W gets my trust.


From the development paper appendix 2:
At first sight 'perfect piston' drive units, (ie those that move rigidly without bending and with a total freedom from resonances), would appear to satisfy the ultimate requirement for perfect sound reproduction. However, there are two limitations to this approach. Eventually, even very stiff materials exhibit break-up and, when they do, the resonances tend to be very severe due to the low inherent damping of stiff materials. Thus, one must ensure that the lowpass filter of the crossover to the next higher drive unit can be set at least 1.5 and preferably 2 octaves below the first resonance frequency.

One must also have regard to the fact that the loudspeaker has to convert a one-dimensional electrical signal into a three dimensional sound field. Beaming or directivity effects limit the useful bandwidth of the drive unit. Much research has shown that a wide and uniform directivity pattern is important in creating a more realistic sound image and enabling off-centre listeners perceive a correct balance. In order to maintain a more uniform off-axis response, one must normally restrict the unit’s bandwidth to below that frequency where the wavelength is equal to the circumference of the diaphragm.

In the 800D, a single midrange drive unit is required to cover the range 350Hz to 4kHz, with useful output outside this bandwidth. At the lower limit, the cone must be large enough to radiate high sound pressure levels without an excessive amount of excursion that would compromise non-linear distortion. A very stiff cone of that size would then exhibit the resonance and beaming limitations outlined above at the top end of the range. One must therefore opt for the controlled break-up approach and use a more flexible cone material.

The break-up pattern of woven Kevlar® has proved beneficial for use in midrange and bass/midrange units, being superior to many other materials, not only because of the inherent properties of Kevlar, but also because the woven cone is not axi-symmetric.

Aluminium Diaphragms
Another potential problem with stiff diaphragms concerns directivity – how much the off-axis response differs from the on-axis response. The broadness of sound dispersion depends on the ratio of the wavelength of sound to the diameter of the diaphragm. The higher the frequency, the shorter the wavelength, and the narrower the beam of sound becomes. Excessive variations in the spread of sound with frequency will lead to listeners sitting away from the central 'hot spot' position hearing a different sound balance and a change in the character of the various instruments. It will also impair the sound image. In severe cases, the position of an instrument may appear to change with frequency.

[Pieter]

Thanks Chris, this allows me to take some of my own medicine I've been dishing out saying Kevlar was "rigid".

[flm]

Thank you again for the informative comments!

I know nothing is perfect, I was just curious.

I will soon audition the 802D. If I do eventually end up with them I'll have to buy a new listening chair?

[turbokuo]

To csuzor comments. Rarely are B&W techs vague about the technical architecture and decisions. Therefore many individuals suspect that the choice to stick with the single Kevlar FST, was due to influence from marketing intelligence.

[Pieter]

Roger, there may be more than a hint of truth in what you're saying.

When friends ask me what the driver is made of in the Nautilus 805s I own, the reply, "Kevlar" always manages to elicit a sorta', "Wow!" effect.

That's after they try to wrestle the tweet and housing off the top of the speaker for an impromto Karaoke performance. "Sorry, thought it was a mike."

[csuzor]

Even admitting that 2 separate drivers, appropriately designed, would not suffer from the distortions the kevlar FST suffers from when measured with laser interferometry and modelled on computers... could mere mortals like us really hear the acoustic advantage?

The reviewer of the 800D who found:
there is still a hint of a quality that was also apparent with the Nautilus 800, namely a slight unevenness, a suggestion of coloration that changes as the pitch of the music changes. There is a hint of boxiness in the midband, and of a loss of image depth and differentiation, albeit very minor.

then naturally goes on to explain the technical reasons, which he of course knew when he wrote those words... I would bet, he read those technical findings from B&W's engineers, and then convinced himself he could actually hear the distortion they were talking about.

The reviewers get paid to hear what others can't, the problems us mere mortals cannot hear. But in reality, on these internet forums, how can we be sure some of us really exist?

Christophe

[turbokuo]

Roger, there may be more than a hint of truth in what you're saying.

When friends ask me what the driver is made of in the Nautilus 805s I own, the reply, "Kevlar" always manages to elicit a sorta', "Wow!" effect.

That's after they try to wrestle the tweet and housing off the top of the speaker for an impromto Karaoke performance. "Sorry, thought it was a mike."

Pieter,

Haha, my 802 get similar comments. "Wow, yellow Kevlar and that bowling ball head."

[Claude D D]

The best thing you can do is go listen to a pair of 802D's and judge for yourself.I always say "show me don't tell me!" :T

[Pieter]

Roger, you're fortunate. That bowling ball impresses even me, and I'm "used" to B&Ws!

Claude, you've driven that nail straight through core of this pursuit of ours!