>
> Have you ever listened to PMC ? <http://www.pmc-speakers.com/>
> There are active (self powered) models, this one is the biggest
> brother <http://www.pmc-speakers.com/product.php?mode=view&pid=15> ;
> and passive, consumer oriented, like the MB2i and other bigger models:
> <http://www.pmc-speakers.com/product.php?mode=view&pid=36> is what I
> hope to have at home on Monday ....
>
>
Oooo. Nice!!! No, I haven't personally experienced these monitors. The
only thing that jumps out at me as a design compromise, however, is the flat
face. Normally not a concern, but it does affect off-axis cohesiveness of
the sound. Three approaches have been pretty common to this, though:
First approach is to contour the approaches to the tweeter to get the
wavefront to bond properly without the wave ripping apart at the edge of the
speaker cone. This is where a study of fluid dynamics will show that a
speaker with a flat interface to the surface will spin off vortexes
off-axis. If the speaker is small, the percentage of vortex to primary
wave-front is higher. A larger speaker surface will generate a larger
vortex, but typically at a smaller percentage because of the greater surface
area is moving a greater amount of air with less distance.
The second approach is to minimise the front surround and make the face of
the speaker cabinet as small as possible. This causes the vortexes to spin
off almost directly to the side and if the housing is a teardrop behind the
face, the vortex will follow the speaker housing rearward. If the speaker
cone were to not have any surround whatsoever, there is usually no vortex,
but unless you are using horns this is not possible.
The third approach is to make the face of the speaker cabinet as big as
possible. If you can make the face at least 1/2 wavelength as large as the
frequencies you wish to control the distortion of, this will cause the
vortexes to spin off directly to the sides and will render them harmless.
This obviously has no bearing on low frequencies as they go wide anyway, but
higher frequencies are more directional and highly susceptable to this
distortion as a vortex reaching the listener position can actually invert
the sound wave at certain frequencies or when the audio is subject to
certain combinations of low and high frequency sounds.
Genelecs follow the first approach. Bose follows the second.
In studying the pictures of the PMC speakers, I'm seeing a lot of the third
approach. The squarish corners of the cabinet are not necessarily helpful,
but as a percentage of the interior volume, they shouldn't be a problem.
These speakers on-axis should be absolutely terrific. I'm personally more a
fan of the first approach, but given sufficient engineering, one can make an
outstanding, world-class speaker out of any of the three.
I learned about phase coherency and various approaches to it and some people
choose to address it with cabinet design others with cone design and others
with electronics. The cabinet design method actually has proven to be
problematic because as you move off-axis the image shift becomes rather
severe. The PMC speakers look to address phase coherency with cone design
AND electronics, so off-axis performance should be decent without imaging
shift.
Fernando, I'm expecting you to invite us all over for a listen.
:)
AG
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