Don Pearce wrote:
On Sat, 31 Mar 2012 17:05:50 +0100,
lid (Adrian Tuddenham) wrote:
Sure, I understand that interposing the obstacles causes a phase shift
between the two, resulting in poor cancellation. And the 6dB per
octave also makes sense, up to the point where a half wave is being
approached and the effect starts to turn over.
But that isn't the same as a velocity-gradient induced pressure
difference between two points a thousandth of an inch apart.
They are only a thousandth of an inch apart "as the crow flies" (through
the thickness of the ribbon), but the pathway which the sound takes
around the pole pieces is much longer. As an example, in the STC 4038
the path difference through the air between one side of the ribbon and
the other is almost an inch. The designers also interposed resistive
screens in that pathway to correct the frequency response.
http://downloads.bbc.co.uk/rd/pubs/a...raphs/bbc_mono
graph_04.pdf
The formula you quoted may be adequate for one particular microphone,
but it needs to include a term which accounts for the change in
sensitivity with change in path difference if it is to explain all the
observed aspects of the phenomenon.
The formula isn't for any kind of microphone. It is for air velocity.
Even a zero width, zero thickness ribbon suspended in free air will
suffer this bass lift.
If it is to usefully give the relationship between the acoustic
conditions and practical microphone output, you need to include another
term which will account for the relationship between path length and
sensitivity. As an explanation for microphone behaviour (as opposed to
air behaviour) it needs at least one more term, possibly more.
I also find it is much easier to understand bass-lift if it is explained
in terms of two different causes of pressure difference across the
ribbon.
Like the Bohr atom, maybe easier to understand, but wrong.
I think you will find it is as valid as the velocity concept. These are
just two different ways of explaining the same observed and measured
phenomena. My concern was not that the velocity calculation was
'wrong' but that it did not tell the whole story to someone who had
originally enquired about microphone responses.
Published textbooks and research papers show that the BBC and STC have
been designing their ribbon microphones on the basis of path length for
many years; if the concept was wrong, they would not have continued to
use it. There may be other manufacturers who use the velocity concept -
and I'm sure their products work equally well.
--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk