2/19/2021 - Senior Project Update 5

This week, I sent my microphone off to one of my good friends to have some outside opinion and information about the performance of my microphone. He tracked some vocals for a big project he's been working on, and he gave me some notes on his findings.

First off, it's important to mention that he put my S-87 alongside his Rode NT1 A, with the capsules near each other and recorded through both simultaneously to get his results. He mentioned that the S-87 has a higher base sensitivity in relation to the NT1 A, since he had to turn the gain up a bit higher on the Rode. Here's a picture he took, showing the NT1 A's gain on the left, and the S-87 on the right:

The S-87 turned out darker than the NT1 A, with a little more midrange but overall fuller and probably closer to the true sound source. High end on the S-87 is a bit more tame and less harsh than the NT1 A, which could allow for more flexible processing when mixing. Lastly, the S-87's frequency response is overall more balanced. When comparing his findings to my A/B testing on the U87, it seems that we both had pretty similar experiences. In the future, I plan to figure out if there's a way I could tame the midrange a bit and make the sound less muddy.

Next week, I'm planning to do some simple homemade tests to find out some of the finer characteristics of my S-87. I've done some research beforehand to get a feel of what I could do and what to look for while I'm testing my microphone.

Fox Audio research has an informative webpage about the process they use to test and improve microphones they receive for service. They perform two tests on the microphone you send in. First off, they test the base electrical and acoustic performance of the microphone. I interpret this as determining the amount of voltage output for a certain amount of volume from a sound source, and graphing the frequency response. After this test is finished, they analyze the data in order to devise a procedure to improve the microphone's overall performance. Following these modifications, the microphone is then tested for the second time, determining what differences the modifications have created, and the new performance statistics for the mic. Lastly, Fox will return your microphone to you along with some documents describing the changes made and its improved performance.

At the bottom of the page, they explain some of the tests that are performed a bit more in depth. Up first, we have the electronic tests, beginning with the preamp frequency response test. In this test, the engineer will use electrical testing equipment to determine the electrical frequency response of the preamp inside your microphone. This is a different test than the acoustic frequency response test, which I will explain in a minute. Another electrical test is the preamp harmonic distortion test, in which the distortion of the 2nd and 3rd harmonics are measured. Modifications Fox makes in response to this test tends to be somewhat of a dramatic difference on a before-and-after response chart:

The final electronic test noted on this page is the preamp noise floor test. Fox will plot the raw noise output of the microphone with its input grounded while it's inside a device called a Faraday cage. Using this test, you can determine the total amount and the frequencies of the raw noise floor of the microphone's preamp.

Here's an example of a type of Faraday cage:

The second group of tests are the acoustic tests, however Fox only lists one in this webpage, and that would be the Frequency Response test. If you've ever bought a microphone new before, it most likely came with a booklet or manual of some sort with a frequency response graph on it. That graph is determined by doing the test described on this page. From what I understand, Fox uses a specialty precision measurement microphone called the Brüel & Kjær Type 4135. A quick google search of this microphone gave me this eBay listing where a used module is going for about $800. I couldn't figure out how much this product costed new, but this mic is no joke. By comparing your microphone to this one which is as close to a perfectly flat response as possible, the graph of your device's frequency response can be made.

Just as an extra note, Fox mentioned some facts about the Brüel & Kjær Type 4135 that I found quite interesting:

"This is a laboratory grade measurement microphone which has a flat response (+/- 2 dB) from 5 Herz (vibrations per second) to 100,000 Herz. For audio range testing it has a flat response from 10Hz to 20KHz +0 and -.5dB."

The important thing to take away from this information is that it has a flat response from 5Hz to 100kHz! To give you a frame of reference, the average human hearing frequency spectrum ranges from 20Hz to 20kHz. Also, the "(+/- 2dB)" means the flatness of the mic's response is not perfect, and does vary a bit. However, keeping in mind that 1dB of volume change is just barely imperceivable to a human, 2dB is a remarkably low number. This microphone really goes above-and-beyond.

DPA microphones has a neat page here that describes some more ways to test your microphone, one of which is off-axis coloration testing. They recommend placing your microphone 1 foot away from the sound source, and 45º off-axis to the side. It's mentioned in this article that off-axis coloration is very important if your goal is to have a clear and transparent recording from your microphone every single time. For asymmetrical microphones (I assume mine is one of these considering the grate around the capsule is tapered at the top), they suggest moving the microphone at several different 45º angles, such as where the sound source is point upwards in relation to the microphone. Performing this extra step of the test can be very revealing about the qualities and characteristics of your microphone.

Tune in next week to see what else I can find out about my mic's performance!