[Boatanchors] Microphone Output

[Richard Knoppow]

----- Original Message ----- 
From: "David C. Hallam" <dhallam at knology.net>
To: <Boatanchors at mailman.qth.net>
Sent: Friday, January 11, 2013 7:17 AM
Subject: [Boatanchors] Microphone Output


> When a microphone output is given at say -52db, what 
> exactly does that mean?  -52db referenced to what?
>
> David
> KW4DH
>     There have been several standards and its different 
> for
high impedance than low impedance.  However, the most common
standard for low impedance is db with reference to 1
milliwatt for a sound pressure of 10 dynes per centimeter
square at the diaphragm. By memory, and I will have to check
it, this is about 87 dp SPL which is about right for close
talking.
    For high impedance microphones the output is often in db
below 1 volt for a pressure of 10 dynes per centimeter
square.  I think at present the physical constants have
different names but the numbers come out the same.
    The reason for using a _power_ unit for output is that
it remains constant with impedance.  That is, a microphone
with an output of -52 will be the same regardless of what
its source impedance is where a _voltage_ rating would
change.
    Despite being rated in power most microphones are
voltage sources and should be operated into a high
impedance, this includes low impedance microphones. A
matched impedance will reduce the output by half. This can
be confusing but again there is a reason for using a power
rather than a voltage level for indicating the output level.
    If you look at very old literature you will find several
other reference levels for both the output and the sound
pressure but the one above became pretty much a standard the
mid 1950s.


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