RealTraps - Measuring Microphones

Ten omni microphones compared

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Ten small-diaphragm condenser microphones from $40 to $1,800 are compared.

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By Ethan Winer

People often ask which affordable microphones I recommend for measuring their rooms. Best is a condenser microphone having a small diaphragm for an extended high frequency response. If the diaphragm is small enough its self-resonance will be higher than 20 KHz. Many "tiny diapragm" microphones such as those from Earthworks and DPA have a usable response to past 20 KHz, and they are ideal. But most of these microphones are very expensive.

With the help of my partner Doug Ferrara and some friends I gathered up ten popular microphones ranging from an inexpensive DIY model to a Josephson with Microtech Gefell capsule that costs $1,800. Five of us got together in my large one-room home studio to compare these microphones, and following is my report.

Note that this test did not attempt to evaluate anything other than raw frequency response. We didn't measure distortion, off-axis response, maximum SPL capability, build quality, or residual noise. Nor did we record any musical instruments. Our only goal was to determine the suitability of these microphones for measuring in-room loudspeaker response.

I created a sort of "anechoic chamber" using six RealTraps MiniGobos in a semicircle, plus two more MiniGobos to absorb and deflect potential reflections from the front of the room. One more HF style bass trap reduced any floor bounce reflections. The test setup is shown in the photos and drawing at left.

Since this is not a true anechoic chamber you will see some influence from the room, especially at low frequencies. So even if a particular microphone is perfectly flat, its response will not appear flat in the graphs due to the room. However, the relative responses are valid, letting you compare how closely the inexpensive microphones match the expensive calibrated models that are known to be highly accurate.

For these tests the microphones were suspended 25 inches in front of a Mackie HR824 loudspeaker whose published response (-3 dB points) extends from 37 Hz to 21 KHz. The microphone capsule tips were on-axis directly opposite the front of the tweeter. To ensure that all the microphones were in exactly the same place, within 1/8 inch in all directions, a piece of string was stretched to set the height and distance from the loudspeaker. An ink mark on the string identified the horizontal location. Each microphone was lowered so its tip was exactly one inch below the string, to avoid potential minor reflections from the string itself.

Added August 11, 2013: The Dayton EMM-6 microphone available from Parts Express is similar in cost to the Behringer and Nady models we tested, but it comes with a custom calibration curve for the exact response of the individual microphone you receive.

 
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Rear, left to right: dbx, DPA, Behringer, Nady, Josephson, Radio Shack, AKG, Earthworks. Foreground: Neutrik (left) and Mitey-Mike.

 

 

"When measuring rooms and loudspeakers, the convention is to 'aim' the microphone upward."

THE MICROPHONE LINEUP

Below are the microphones we tested, though some are no longer available so links are not provided:

Mitey-Mike
Nady CM 100
Behringer ECM8000
Radio Shack Analog SPL Meter
Radio Shack Digital SPL Meter
dbx RTA-M
Neutrik 3382
DPA 4090
AKG C 451 EB with CK 22 capsule
Earthworks QTC-1
Josephson C617 Set

There were actually 11 microphones because we tested two different Radio Shack SPL meters - an analog model about 20 years old, and a more recent digital version. Both SPL meters were set for C weighting. For these tests the microphones were hanging from their audio cable pointing straight down, though we also tested one of the SPL meters pointing toward the speaker.

Although omnidirectional microphones supposedly receive sound equally from all directions, when measuring rooms and loudspeakers the convention is to aim the microphone straight up. No omni microphone has exactly the same frequency response from all directions, though microphones with tiny diaphragms and slim bodies are often more uniform than larger models. So when balancing loudspeaker volume levels on a surround system, pointing the microphone toward the ceiling favors all of the loudspeakers equally. Top

 

 

 

"What separates the men from the boys is how high their response extends at high frequencies."

 

 

 

 

 

 

"It would seem that the $40 Nady and $50 Behringer are within a few dB of the most expensive options."

THE RESULTS

Even inexpensive omni condenser microphones are quite flat at bass frequencies - what separates the men from the boys is how high their response extends, and how flat they are above 1 KHz. We tested to only 20 KHz, though I imagine some of these microphones are useful to even higher frequencies.

The Nady and Behringer microphones are visually identical except for the name printed on the side. However, I didn't take them apart to examine the capsule or electronics, so it's possible only the cases are the same. I also noticed that the Behringer's output was about 3 dB higher than the Nady.

All of the data is shown in the graphs below. It's difficult to show 11 microphones all on one graph, so they are divided into groups. The first two graphs show the four "high-end" microphones - both the raw response, and averaged into third octaves. The raw data is more accurate at low frequencies, but shows too much detail above 1 KHz. Therefore, the remaining graphs are all smoothed, using the Earthworks as an arbitrary reference for each graph, plus four other microphones. Top

All of the tests were done using the fabulous freeware program Room EQ Wizard, and the graph line heights were adjusted vertically to compensate for varying microphone output levels. As you can see, most of the microphones track extremely well at low frequencies, deviating mainly above 1 to 2 KHz. In the last graph you can see how the response of the Radio Shack digital SPL meter is much more extended when aimed at the loudspeaker source.

From this data it appears that for measuring loudspeaker setup and room treatments, the $40 Nady or $50 Behringer are quite adequate - within a few dB of the most expensive options - and far better than the Radio Shack SPL meter which is often recommended.

Thanks to Grekim Jennings, and also to Mike Barney and Pete Basel of the Connecticut Audio Society for their participation. Special thanks to Fletcher at Mercenary Audio for supplying the Josephson / Gefell microphone. Top

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