One of the major differences between great microphones and others is the smoothness of the off-axis frequency response.
Do you consider the off-axis response when you are buying a new mic? There are good reasons why you should.
The Science of Sound Recording
First of all – what is it? Off-axis response (in directional microphones) concerns what happens when sound waves hit the microphone from an angle - as opposed to on-axis when the sound source is straight in front of the microphone. Sounds coming from an angle (i.e. the sides) will hit different parts of the membrane at different times – resulting in phase cancellations and other issues - and the larger the membrane the larger the time difference. Therefore small membrane microphones usually exhibit a better off-axis performance than microphones with a large membrane.
The normal frequency chart (that comes with most microphones) shows the frequency response for sounds arriving from the front of the mic. Most serious manufacturers will include a chart that shows how the polar pattern appears at different frequencies – or a frequency chart with curves measured at different angles - but these charts may be difficult to interpret and different manufacturers use different charts and methods. The diagram to the right shows the cardioid polar pattern of the Milab DC-196 at six different frequencies from 100 Hz to 10 kHz.
"Real spatial sensitivity patterns deviate from the ideal ones and often vary dramatically with frequency. Off-axis response can seriously alter the sound of a microphone in real-world use, since in a room there are reflections of the direct sound picked up off axis and combined with the on-axis signal. The combination can radically alter the transduced sound quality through frequency-dependent reinforcements and cancellations." – Jay Kadis, The Science of Sound Recording
Although the off-axis response may not be that important when you are recording studio vocals (depending on room reflections), there are literally thousands of recording situations when the sound enters the microphone from the side and/or from a wide angle – drum overheads, ambience, choir, orchestra, piano and so on. When you are working with a stereo configuration such as X/Y or ORTF, sounds originating from the front of your setup are entering both mics at an angle - 45° if you are working with an X/Y 90° setup (see illustration to the left) - and therefore a microphone with a poor off-axis response will not perform well in these situations. (Using a wider X/Y setup makes the off-axis performance even more critical). As a result, many large membrane microphones are simply not suited for these kinds of stereo setups or other situations that demand good off-axis performance.
Simply put: It doesn’t matter how expensive your microphone is or how wonderful it sounds on-axis. If the off-axis performance is poor and you are using it for anything other than an isolated on-axis sound source, you are not getting the sound you paid for.
However, the Milab rectangular capsule (found in the DC-series, VIP-50, SRND 360 and more) is a large membrane capsule with an off-axis response equal to that of small capsules. This gives the user a unique possibility to combine the benefits of a large membrane microphone (low noise, high sensitivity etc) with the off-axis response of a small membrane microphone. As a result, you get a microphone that can be used for a wider range of applications and with better results than traditional large membrane microphones.
Don’t take our word for it. Here are a couple of quotes from magazine reviews:
"Off-axis response on the cardioid setting is extremely smooth and consistent, with minimal colouration." - Resolution Magazine on the Milab DC-196
"One if its most impressive traits is the distinct lack of tonal colouration as sound sources move off-axis to the directional patterns." - Resolution Magazine on the Milab DM-1001
"What I want to examine with the pink noise test is the off-axis performance of the microphones. And all I can say is that Milab’s theory regarding the rectangular capsule seems to be very accurate. When I turn the DC-96C 45 degrees from the speaker, the very highest top-end drops a bit, but not much, and the drop feels very natural, smooth and pleasant. The other two (AKG C414EB & Microtech Gefell M930) are definitely not bad, but they get noticeably darker when turned the same way. There is no doubt that the DC-96C would perform very well when used for XY and AB stereo recording – probably just as good as a small membrane microphone." - Monitor, Sweden – Milab DC-96C review
There are several ways to test the off-axis response of a microphone. You can start by recording your voice and moving sideways in front of the mic. The level should change (-6 dB at 90 degrees for a cardioid mic), but the sound should change as little as possible. How far to the side can you move without noticing a change in the sound?
Quotes from: https://ccrma.stanford.edu/courses/192a/SSR/Microphones.pdf