Fact or myth?

[Buckle-meister]

Dear all,

Another thread has reminded me of a question I thought to, but didn't ask a while back.

I was listening to B&W's 800Ds in a local store's listening room in which B&W's ASW855 sub was also present, though disconnected. As music was played through the towers the salesman noted that for a true audition of them, the sub should strictly be removed from the room to prevent it acting passively. After asking him to explain, he had me touch the driver of the sub and sure enough, the sound pressure created from the towers was interacting with the sub and could pysically be felt. The sub was unquestionably acting passively!

However, whilst that is not in dispute, my question to one and all is: even though the sub was acting passively, would it actually have caused an audible difference at 'normal' or 'fairly loud' listening levels? Yes, yes, I know; these are subjective terms. Just give it your best shot.

 

[corysmith01]

Maybe I'm way off here, but it moving doesn't seem like it would make one bit of difference. Wouldn't it need a signal passed through it for it to make a difference? Just because it's moving in and out doesn't mean it's making a sound...right? At high enough levels, couldn't anything in a room move in and out like a diaphram? But that wouldn't make a difference would it? Again, I may be way off, but I can't imagin it making a bit of difference unless a signal was being passed to it from a source. I'm completely open to being wrong on this one.

 

[Buckle-meister]

Wouldn't it need a signal passed through it for it to make a difference? Just because it's moving in and out doesn't mean it's making a sound...right?

Are you sure? I mean, think about it. It's the act of moving that causes a driver to create sound. The sub was definitely disconnected, no question about it, and so the driver was reacting to the sound pressure caused by the towers. It therefore follows that if the sub's driver's movement followed that of the tower's drivers (180 degrees out of phase ), then the sub must be duplicating a portion of the frequency response being created by the towers no?

 

[brendy]

A powered sub acting passively just from other speakers playing ? Sounds like one for the Mythbusters TV show.I just find that very hard to believe.

 

[Buckle-meister]

A powered sub acting passively just from other speakers playing? Sounds like one for the Mythbusters TV show. I just find that very hard to believe.

It is a powered sub, yes. I can tell you that it truly was not powered at that time, and I personally felt the driver vibrating, not a great deal mind you, but then, that's why I asked whether or not such movements would be audible. My hunch is that yes they would. As an example, play something on your stereo really quietly (though still audible) and touch the driver. You can still feel it vibrate right? Therefore it can't take much to make (audible) sound.

Also bear in mind the size of the 800D's drivers! Maybe that has something to do with it. After all, they do push a lot of air!

 

[corysmith01]

Are you sure?

Oh, I'm never sure about anything. My life is full of miscalculations.

Also bear in mind the size of the 800D's drivers! Maybe that has something to do with it. After all, they do push a lot of air!

Yes, I'd like to hear more about these. Those are some impressive speakers!

 

[WmAx]

I have measured(several measurements in each state to rule out measurement anomalies) this previously. I used 3 way main speakers, with a lower tuning of about 27 Hz, ported. I placed a medium sized unpowered subwoofer in the room, it was tuned to about 29 Hz, ported, and 10" diameter cone with 3" diameter port. No substantial difference was measured in the lower frequency band, but curiously, a null that was centered around 160 Hz was substantially alleviated with the subwoofer in the room. I wondered if it was just the physical placement of the subwoofer as an obstruction device, so I moved it to new location(from left rear corner to near front left corner of room) and the effect still existed. Is the difference audible? Perhaps in a very sensitive comparison such as ABX. But casually, I did not notice anything that I could not be sure was not imagined. I would have to record music with content around the band of the null with the subwoofer in and out of the room, time sync both recordings, then compare in a proper blind testing software system.

-Chris

 

[Nick250]

Robbie, did you place an ear withing a few inches of the sub to see if you could detect sound? A rather indelicate proposition to do in public, I grant you.

Nick

 

[PENG]

In reading your posts, you seem to know the answer already. If it vibrates, it makes sound regardless of whether there is current flowing in the voice coil or not. Depend on the magnitudes or the vibration, it may or may not affect the sound in an audible way.

 

[Audioholics]

I think you have to remove yourself from the room for them to sound their best as well - after all, YOU might vibrate. And there should be absolutely no diplays - or furniture. And the walls need to be concrete not drywall less they resonate at lower frequencies.

Which brings up the age old question: If a tree falls in the forest and a passive subwoofer is nearby...

 

[gene]

I was listening to B&W's 800Ds in a local store's listening room in which B&W's ASW855 sub was also present, though disconnected. As music was played through the towers the salesman noted that for a true audition of them, the sub should strictly be removed from the room to prevent it acting passively. After asking him to explain, he had me touch the driver of the sub and sure enough, the sound pressure created from the towers was interacting with the sub and could pysically be felt. The sub was unquestionably acting passively!

Its called sympathetic vibrations and they are insignificant to the sound you are hearing compared to the powered speakers. The salesman needs to lay off the crack pipe while he is on the clock

 

[Sheep]

I think you have to remove yourself from the room for them to sound their best as well - after all, YOU might vibrate. And there should be absolutely no diplays - or furniture. And the walls need to be concrete not drywall less they resonate at lower frequencies.

Which brings up the age old question: If a tree falls in the forest and a passive subwoofer is nearby...

Wicked Post.

SheepStar

 

[Buckle-meister]

...I'd like to hear more about these. Those are some impressive speakers!

They're some size right enough. I'd just happened to be in the shop on another matter, I wasn't there specifically to audition the speakers (I wish!), but seen them in the shop's largest listening room, still not that big (for those speakers) at, I'd say, approximately 5m x 6m.

In fact even though you could hear how clear and neutral sounding the 800Ds were, I wasn't overly impressed by them which I totally put down to the fact that:

• The room is totally untreated (unsurprisingly)
• As previously noted, given the sheer physical size of these speakers, I tend to think that they might've benefitted from placement in a larger room in order to better 'breathe'

...did you place an ear withing a few inches of the sub to see if you could detect sound? A rather indelicate proposition to do in public, I grant you.

No, I never thought to do so at the time. I remember being surprised that it was vibrating even a little, though it seems obvious now that it would do.

In reading your posts, you seem to know the answer already...

Not at all Peng. My question wasn't whether it was making sound but whether its effect would be audible.

I think you have to remove yourself from the room for them to sound their best as well - after all, YOU might vibrate. And there should be absolutely no diplays - or furniture. And the walls need to be concrete not drywall less they resonate at lower frequencies...

It's funny you say that because (I'm probably setting myself up for ridicule here) there've been times when I've been sitting at the listening position at home with one leg crossed over another in an American '4' shape and wondered just what 'damage' I'm doing to those higher frequencies ricocheting off my 'barrier' legs!

Ridicule may now begin.

Its called sympathetic vibrations and they are insignificant to the sound you are hearing compared to the powered speakers. The salesman needs to lay off the crack pipe while he is on the clock

Actually, the salesmen in the shop in question here in Glasgow are without a doubt the most knowledgeable I've ever come across (most are total incompetants), that is to say the least dishonest.

 

[PENG]

Actually, the salesmen in the shop in question here in Glasgow are without a doubt the most knowledgeable I've ever come across (most are total incompetants), that is to say the least dishonest.

At least the guy knows the theory, he just overstated the effect. I would give him some credit for the theory part.

Just out of curiosity, what amp did they use to demo the B&W?

When I started shopping (just mentally) for a pair of 802D a few months ago, one dealer told me they must be paired with Classe or Krell monoblocks, and that with anything less, even Bryston, will cause the diamond tweeter to blow due to clipping. Well I didn't go visit that store. Where I went they first powered the 802 with a small Musical Fidelity system (200WX2 I think) and it sounded so good that my mind was made up right there and then. The sales rep then switched the amp to a Bryston 14B SST hoping that I would hear much improved sound. Well it didn't do much better for me except that he cranked the volume way up, yet the speakers still sounded the best I have ever heard with no audible sign of distortion. I can't imagine how the top model 800D would sound. You're so lucky to be able to hear a pair locally.

 

[Buckle-meister]

...out of curiosity, what amp did they use to demo the B&W?

They were driven by Densen pre and power amplifiers, though I couldn't tell you how powerful they were.

It was the first I'd ever heard of Densen (Danish company) so I can't tell you if they're any good, though with the shop also selling Classe and other uber-expensive gear, I suspect that they are. They're certainly visually very appealing to me as I like that 'minimal' look to my gear.

Where I went they first powered the 802 with a small Musical Fidelity system (200WX2 I think) and it sounded so good that my mind was made up right there and then.

Yeah, I do like MF gear. My first Pre/Power amps were MF.

You're so lucky to be able to hear a pair locally.

Just to make you sick , some time ago the shop put out invites (I think upon winning the dealership for Classe gear) for customers to attend an evening audition of the 800Ds paired with a full complement of Classe amplification.

I declined.

Well, I mean come on! The event was going to be in some hotel function suite they'd hired for the day. As such it'd potentially be far too big even for the 800Ds and certainly wouldn't have had any form of room treatment in place.

Cheers.

 

[jaxvon]

It's not surprising that Classe gear was present as B&W and Classe are owned by the same company.

 

[billnchristy]

Yeah, just because they move doesn't mean anything is really happening there.

Besides, its not like it is a perpetual motion machine, you could not possibly make any SPL out of it because you would be using the driven speakers power to make the passive one work.

I built a huge bandpass for a Kicker 18" at work one time. We hooked it up to a Linear power 800W amp (biggest we had unfortunately) and cranked it up.

Our sound room had a wall of subwoofers in it, probably at least 4 of each size 8" to 15". My box was driving most of those to full excursion with just the pressure being made. The SPL did not indicate there were 20+ drivers being used.

I guess worst case you might lose some bass because of the effort being put forth to vibrate things is using energy but jeez what would it be a fraction of a dB. And if it wasnt shaking the sub wouldnt it be shaking something else?

 

[highfihoney]

I thought i had heard it all from hifi salesmen over the years but this one was a new one on me,after reading this thread i got thinking about the whole theory & decided to try it out.

I have a passive sub in my system & sure enough when the sub is out of the system i can feel the driver move & if i turn the system up loud enough i can see the driver move,i would think this effect is simular to having a passive radiator within the main speakers & would add to bass response even if the effect is small.

 

[Buckle-meister]

I thought i had heard it all from hifi salesmen over the years but this one was a new one on me...

I wouldn't be too hard on the poor guy. He wasn't trying to flog me the speakers. He knew I wasn't there to buy and was just letting me have a listen for a while.

I have a passive sub in my system & sure enough when the sub is out of the system i can feel the driver move & if i turn the system up loud enough i can see the driver move, i would think this effect is simular to having a passive radiator within the main speakers & would add to bass response even if the effect is small.

Exactly. The sub effectively acting as a passive radiator is how I see it as well, though from WmAx's post, it sounds almost like the sub's acting as a Helmholtz resonator. I suppose this makes sense too, as anything (non-powered) causing the sub to move must be removing energy from the air in the room.

 

[mtrycrafts]

I have measured(several measurements in each state to rule out measurement anomalies) this previously. I used 3 way main speakers, with a lower tuning of about 27 Hz, ported. I placed a medium sized unpowered subwoofer in the room, it was tuned to about 29 Hz, ported, and 10" diameter cone with 3" diameter port. No substantial difference was measured in the lower frequency band, but curiously, a null that was centered around 160 Hz was substantially alleviated with the subwoofer in the room. I wondered if it was just the physical placement of the subwoofer as an obstruction device, so I moved it to new location(from left rear corner to near front left corner of room) and the effect still existed. Is the difference audible? Perhaps in a very sensitive comparison such as ABX. But casually, I did not notice anything that I could not be sure was not imagined. I would have to record music with content around the band of the null with the subwoofer in and out of the room, time sync both recordings, then compare in a proper blind testing software system.

-Chris

It might be interesting to placing a voltage recorder on the terminals to see what values are generated being a passive driver and maybe the frequency too.

I forgot. See if by shorting the terminals on the back will do the same. The cone would have a much more difficult time moving.