Fact or myth?

[mtrycrafts]

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.

While the cones might move, it should as it is on a fine air bearing and trying to develop voltage out, like a back EMF, the question would be how much and would it keep in pace with the frequency of the other speaker and that level being audible?

 

[Buckle-meister]

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.

Hmm. Never thought of that. That is an interesting idea.

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.

This bit I don't understand though.

While the cones might move, it should as it is on a fine air bearing and trying to develop voltage out, like a back EMF, the question would be how much and would it keep in pace with the frequency of the other speaker and that level being audible?

That's the thing isn't it. HiFiHoney's independantly verified that a sub does indeed vibrate, but also finds that the motion is visible at high SPL. Whilst a sub might not be heard in its own right at those levels due to being 'drowned out' by the output from the towers, nevertheless if the sub's driver is visually moving, I can't imagine that it isn't contributing a measureable amount to the frequency response.

 

[mtrycrafts]

This bit I don't understand though. [/b]

The movement of the cone manually through the mag field, a inductor, want to create a voltage and current. Open terminals gives it a very large impedance, allowing easy movement, no current generated.
Shorting it gives a near zero impedance, more difficult to move to create voltage and current. Just verified it


but also finds that the motion is visible at high SPL. Whilst a sub might not be heard in its own right at those levels due to being 'drowned out' by the output from the towers, nevertheless if the sub's driver is visually moving, I can't imagine that it isn't contributing a measurable amount to the frequency response.

What was he seeing? Was it a passive sub? Was it on?
Yes, a meter can measure the voltage and current generated. You can see the cone move at well below its limits with test tones, yet it is not audible even by itself. So, just because you can see the cone move doesn't mean it must be audible. Be careful transferring one clue, motion, to meaning audibility.
And, even if it is drowned out, that is masking, not audible.