In this blog post I evaluate the Shure SM58 dynamic microphone for use as a measurement microphone for loudspeaker testing. Why would you want to use this type of a microphone for a speakermeasurement? Well, recently I’ve been suspicious that my current microphone has been limiting my distortion measurements to about -65dB. In other words, I’ve never been to better this amount regardless of the dozens of drivers I’ve evaluated. It’s too coincidental that I don’t see figures better than this. This is specifically with the multitone IMD test. This leads me to believe that the microphone is the limiting factor in my test set up. For years I’ve used the Dayton UMM-6 reference measurement microphone and it served me well. However as I delve into the topic of intermodulation distortion I find that there is certainly a limit. As I investigated other types of microphones I realize that the condenser microphone has Self Noise which is mainly a function of the internal preamp. Additionally, the small microphone capsule is only a quarter inch in diameter. This means that for a given input the diaphragm has to move significantly more than, for example a half inch diaphragm, to produce the same signal to the microphone preamp. This is identical to the concept of a larger loudspeaker having lower distortion by virtue of its large radiating area. I’ve done live sound mixing for many years, and the Shure SM58 is ubiquitous. It’s known for its excellent sound and reasonable cost. I would also like to select affordable components for my test setup so that others can easily replicate what I am doing. This helps the DIY community.
I recently did a quick video that evaluates the Dayton UMM-6 on its own using a special microphone distortion test. The video can be found here. The conclusion from this video is that the microphone indeed has self noise of around -65dB. I followed a standard test procedure which can be found here.
This blog post follows from that video. I directly compare the Shure SM58 against the Dayton UMM6.
Below is multitone intermodulation distortion results of a Dayton UMM6. The mic is placed at the mouth of a 600Hz horn which uses a high quality compression driver. The test level at the mic is 95dB. As we can see there is a flat noise floor -63dB below the fundamental tones. 
I then swap out the microphone for the Shure SM57 which is connected to my Tascam US-1641 Preamp which is then connected to my laptop via USB. The same IMD test is shown below.
As you can see, we are now seeing distortion products as low as -80dB at 1.5kHz. This reveals much more resolution into the driver’s performance, assuming that this revealed data is attributable to the driver and not the test setup.
I will continue to investigate my test setup to see how more precision can be utilized in evaluating loudspeakers.