Microphone Benchmark Tests

 

Every element is different, we all know that. Some are "hotter" than others, some "dirtier", some "cleaner". But it's frustrating to try an A/B mic elements against each other with just the ear. For a while now, I've been trying to think of a way to test and compare mic elements in some sort of logical way. I'm not there yet, but this is my first attempt at getting something concrete to measure. Here's my method (and feel free to suggest better ways to do this).

  1. I found a sound sample of white noise on the Internet, and played the sample through the best speakers I had.
  2. I set up my mics and plugged them into a mixer, fed directly to another computer.
  3. I recorded samples into Sound Forge and used the spectrum analyzer to show graphically what the mics were recording.
  4. I measured the impedance (resistance?) on a meter with the mic unhooked, no load was applied to the elements.

Again, this is not perfect, and I'm certainly not an electrical engineer. There are flaws in the system. Surely the speakers are not perfectly flat, and there's probably some interaction between the mic and speaker affecting the sound, as well as the room itself. In addition, there's the issue of cupping the mic, which has it's own way of affecting the mic's response. Finally, there is no harmonica in this equation! So take all this with a grain of salt and think of it as more of a comparison than any kind of concrete evidence. For me it's just a fun little exercise.

Below is a series of screen shots of all the mics and elements I had on hand at the time.

 

White Noise

This is the raw sound file. Notice how even the frequency distribution is.

 

"Control" Mic - EV N/D 257B (Low Impedance)

This is the nicest vocal mic I had on hand. It has a full-range response, with a steady roll-off of highs. This is common with cardioid mics at close range, which tend to accentuate bass. This is called "proximity effect", and it's why we all like to eat vocal mics when we sing - it makes us all sound better.

 

Vintage Shure 99a86 Controlled-Magnetic Element in a 520 Bullet Shell, 1059 Ohms

Honestly, this is the hottest CM I have had so far and sounds great (thanks Retro-Specs). I'm guessing it to be a mid-60s vintage. The frequency range is the widest of the elements I tested, about 20 hz - 10 khz or so, with some frequencies extending as far as 13khz. This one has a little bump around 9k. Note the prominent frequency at 172 hz - nice and beefy.

 

Vintage Shure White-Label Controlled-Reluctance Element in a T-3 Shell, 1045 Ohms

I don't remember the model number of the mic and I didn't take the time to peek in the shell. A more narrow frequency response, with less mids. The prominent frequency is quite a bit higher at 517 hz.

 

1950 99H86 Black-Label CR in a T-3 Shell, 1276 ohms

Definitely a hot element. The frequency response is much more narrow and concentrated on the bottom end. But the most prominent frequency is in the 1k range. It's a honker!

 

1950 99H86 Black-Label CR in a JT-30 Shell, 1358 Ohms

This one had a slightly wider frequency response, and a nice little peak in the lowest frequencies around 20-100 hz, with the most prominent frequency at 65 hz. Balls, baby!

 

Astatic MC-127 Ceramic in a T-3 Shell, Impedance ?

I could not get an ohm measurementon the meter, but these are high-Z elements. This one was among the quietest I tested, comparable to the white CR above. The frequency response is wider and flatter than the magnetics, meaning more highs and less lows. It's also a bit cleaner sounding element.

 

Conclusions

To my surprise, the CM did very nicely against the black-label CR elements. Although the prestige factor is there with older black and white labels, you can't discount the CM's simply because they don't have the word "reluctance" in the title. I will say though, that in my experience, the elements from the 50s and 60s are definitely louder than the newer variety. Unfortunately, I didn't have any newer CM's here to bench test to qualify this.

A higher impedance does not mean a hotter mic. Additionally, what is shown on the meter is not a true measurement of impedance, as a load has to be applied to the mic. Again, I'm not qualified to get into this. Perhaps what the meter sees is really a measure of simple resistance (?). Somebody set me straight on this.

**A recent customer, Derek Lark writes: "You were correct in saying you only have the simple DC resistance rather than the AC reactance and hence impedance of the microphone.  Impedance is the sum of the resistance and reactance at any given frequency so you would have to inject say 500Hz at a very low current (accurately known) and measure the voltage across the element to get an approximation of the impedance.  It is why you can't get a resistance reading across a crystal or ceramic cartridge, they don't have a DC continuity at all due to the design."

Numbers aside, I can not tell very much difference between the CR/CM elements. There is a very obvious difference between the ceramic and the magnetics, with the CM/CR winning out for loudness and crunch. At the time I did this test, I didn't have an MC-151 on hand, but my ear tells me the ceramic and the crystal are pretty close in tone.