[ProAudio] Microphones question

Bill Whitlock engineer_bill at verizon.net
Fri Jun 11 22:03:45 PDT 2021

I'll be showing my age here, but the EIA or Electronic Industries Association set standards for components used in consumer gear back in the 50s, when I was young repair tech. They defined speaker impedance as the value on the impedance vs frequency curve as the first minimum after the first maximum as frequency is increased ... and the most common standard was 3.2 ohms.  In most cases, it represented the lowest impedance over the audio frequency range.
And I think you're right that I published both the equivalent circuit and the impedance curve for the SM57 in the Ballou book.  Today, I only checked the version posted at the Jensen website, which I think is the 3rd edition.  I'll check my manuscripts for the piece in the 4th or 5th edition.  If I find it, I'll post it here.  I'm guessing that there are a lot of impedance plots that are never published by mic manufacturers.

-----Original Message-----
From: Jim Brown via ProAudio <proaudio at bach.pgm.com>
To: proaudio at bach.pgm.com
Sent: Fri, Jun 11, 2021 8:25 pm
Subject: Re: [ProAudio] Microphones question

On 6/11/2021 6:47 PM, Bill Whitlock via ProAudio wrote:
> As I recall from my tests of the SM57, its impedance varied from under 
> 150 Ω at very low frequencies to over 300 Ω at resonance - and continued 
> to rise at higher frequencies.  I'll try to find the data - I did the 
> tests as research before writing Jensen AN-005 about mic splitters.

I remember seeing that data, perhaps in your chapter in the Ballou 
Handbook for Sound Engineers.

To others -- it's important to realize that the impedance of a dynamic 
mic is complex, because it's equivalent circuit is complex. Remember 
that a dynamic mic is the analog of a single driver loudspeaker. If I 
remember correctly, the nominal impedance of a mic is defined by the 
manufacturer as 1/5 of the minimum load impedance. Someone will correct 
me if my memory has failed me. For loudspeakers, it's the minimum value 
of its impedance when plotted vs frequency, and it's impedance typically 
varies by at least two orders of magnitude over its operating range.

Jim Brown

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