My understanding of microphones and audio says this myth is not true. I’ve been told by knowledgeable experts that mic sensitivity is linear below physical clipping regardless of the transducer technology. I actually asked this question some time ago over on rec.pro.audio – a usenet discussion forum where professional audio engineers hang out. Some of their responses were rude, but they basically established that preamp gain is all that is required to match sensitivity between a dynamic and a condenser mic. Here’s a link to that thread.

Now I believe those guys. I thought it might be interesting to devise a demonstration of sorts, by plugging in a dynamic and condenser mic, playing a reference tone through a speaker in front of the mics, then adjusting preamp gains so the levels match. Then I could generate a quieter and quieter test signal by walking away from the mics making noise. Recording this diminishing sound with both mics would tell me if one could “hear things” the other could not.

Oh No!! Is the Myth True?

To my amazement, when I did conduct a demonstration for myself, I could hear more distant, quieter sounds on the condenser mic. It seemed as if the myth was true.

I knew the error was not the physics or engineering of mics, but rather something I had overlooked in setting up my test. After a day or two of research and pondering the light came on. I realized that the different pickup patterns of the two mics made my calibration procedure wrong. I was calibrating for direct, on-axis sound, but I was measuring diffuse off-axis sound. I needed to do the calibration using only the diffuse sound field, which meant moving the speaker some distance from the mics during the calibration.

A Better Test Design

At this point I also figured out that I didn’t have to move the noise source to reduce the level of the test signal, I could create test tones that got lower and lower in level and play them back from the same spot as the calibration (well, duhhhh).

Here’s the result. For the best fidelity, here’s 45db-66db-xt.wav. Or if your connection is a bit slow, the compressed version is 45db-66db-xt.mp3.

You’ll need to download one of the files and pull it into a player that can select only the right or left channel. The mic in the left channel is the condenser, a Shure KSM141. The mic in the right channel is a dynamic, the Shure SM57. This is a little excerpt from the demonstration recording. I selected the area from -45 to -66 dBFS, which is where the test tone slipped into inaudibility. I raised the level of these files substantially and there is plenty of broadband noise, so be careful not to play them too loudly. The condenser mic is in the left channel, the dynamic in the right. Listen to first one side, then the other, and see if you can hear tones at lower levels from one mic or the other.

Conduct Your Own Demonstration

If you’d like to conduct this demonstration with your own mics and room, all you need is a calibration tone and a test tone series. You can generate the calibration tone in most audio editors, and you can download my test tone series.

If you can’t figure out how to create a tone in your favorite audio workstation software, download Audacity and install it (I recommend the Beta 1.3.xx or later version). Start Audacity and choose

Generate | Tone

then fill out the Tone Generator form:

Waveform: Sine
Frequency (Hz): 1000 (a 1 Khz test tone is the normal industry standard for basic testing)
Amplitude: .6 (a very loud long 1 Khz tone can damage your speakers and possibly your ears)
Duration: 600 seconds (10 minutes should be enough)

Click OK and you’ll see a strange solid waveform. That’s your calibration tone. Just export it from Audacity:

File | Export
Save as type: (either MP3 or WAV Microsoft signed 16 bit PCM)
(Choose a directory and file name)

You can download my test tone file here. The test tone file contains volume level announcements and 1000 Hz tones starting at -9 dBFS and going down to -90 dbFS in 3 dB increments.

Here’s a screen shot of the test tone file:

Mic sensitivity test tone in Adobe Audition 3

and a sample of the tones, starting at -9 dBFS and going to some of the lower level tones:

To conduct your own demonstration, connect two mics to your recording system. Place the mics at least 6 feet from the speaker, then play the calibration tone. Adjust the preamp gain so the two mics show the same input levels. Do this very very carefully, this is the most critical step in the process.

Next, simply play the test tone file while recording the two microphones. You’ll want to wait until a quiet part of the day, and be prepared to sit very quietly while the test file plays. When you’ve completed recording the test tone sequence, listen to one of the tracks you just recorded. When you can no longer hear the test tone, switch to the other track (other mic) and listen again. If your experience is like mine, the test tones will fall into inaudibility at the same level for both mics.

Better Recording By Knowing Our Tools

Mics are fascinating devices, but they’re engineered objects in the physical world. We can make better use of them if we have a better understanding of the way they really work instead of relying on incorrect assumptions and erroneous analogies. In the past audio testing required lots of expensive dedicated equipment, but now with our computer audio systems we can easily perform simple but fairly sophisticated evaluations of our audio gear, and learn to make better recordings in the process.