[Boatanchors] EV Microphone

[Richard Knoppow]

----- Original Message ----- 
From: "Lee Craner" <leecraner at aol.com>
To: <dhallam at knology.net>; <boatanchors at mailman.qth.net>
Sent: Sunday, February 05, 2012 12:25 PM
Subject: Re: [Boatanchors] EV Microphone


>
> David,
>
> Absolutely.  The 676 is a great mic.  I've used one for 
> years.  Being dual impedance is especially handy when you 
> run modern and boatanchor rigs and want to use the same 
> mic.  The 676 uses, supposedly, an improved Variable D 
> cardiod system over the venerable 664.
>
> 73
> Lee WB6SSW
>
     The difference is in the method of making the 
acoustical delay line that is used to produce the cardioid 
pattern. The earlier EV "Variable-D" mics, beginning with 
the famous 666 and including the 664, used three separate 
lumped constant delays each with a separate sound entrance. 
The 676 and later mics use a transmission line behind the 
diaphragm, that is the strip along the spine of the mic with 
the slots in it. It is designed so that the higher 
frequencies can enter at the slots nearest the diaphragm and 
lower frequencies move progressively back. As a result the 
effective part of the sound wave that activates the 
diaphragm remains constant with frequency over a quite wide 
range. This gives a constant driving force on the diaphragm. 
Since the distance varies with frequency and remains 
effectively a constant part of the wave it is not affected 
by the variation in differential pressure between plane 
waves (that is those originating at a considerable distance) 
and spherical wave front waves (as found in sounds 
originating at a distance comparable to a wave length). 
While the use of frequency sensitive acoustical transmission 
lines long predates the Electro-Voice 666 they were not used 
for producing the differential drive necessary for cardiod 
microphones. Perhaps the first transmission line was one 
developed at Bell Telephone Laboratories and made to fit on 
the Western Electric type 618 dynamic mic. This made it very 
directional but by a wave interference principle for sounds 
entering the front rather than by the differential principle 
used for bi-directional and cardioid microphones. 
Electro-Voice uses slotted transmission lines for both 
purposes in some microphones.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com