Last night we had a thunderstorm roll through with some halfway decent thunder production, and I grabbed the two wildlife microphones to do some recording and comparisons. This turned out to be a great test subject, because it demonstrated just how different the methods captured sound.
The first microphone is the newest addition, though I’ve had the main part for a while: a Sony ECM-44B lavalier mic, but fitted to a 3D-printed parabolic dish to focus and magnify sound (I’ve reprinted it since that post, so it’s slightly improved now.) Initial tests with this showed that it was excellent at being able to pinpoint a sound source by the changes in volume as it reached the focal direction. But the smaller the parabolic dish is, the more low-frequency sounds it fails to capture; ideally, it should be in the realm of a meter across, but that’s unwieldy in just about any application, and commercial designs seem to fall a little less than a half-meter. Mine, however, is 26cm across the widest section, limited by the size of the printer bed.
The second microphone has been in use for a while now, an Azden SGM-2X unidirectional ‘shotgun’ mic intended for exactly these kind of uses, fitted with only a ‘dead cat’ wind guard. Shotgun mics are really slick in how they reduce off-axis sounds, and I don’t want to go into it here but encourage you to look it up if you’re unfamiliar with it, because it’s great.
Both mics have their own power source, a single AA battery, and both were feeding into an Olympus VN-8100PC digital audio recorder. Both were also pointed in the same general direction from the same location on the front step, about 30° upwards over the tops of the nearby trees where I could see the flashes and occasional lightning bolt, and by this time it was raining steadily, as can be heard. Unfortunately, I couldn’t do a direct side-by-side comparison since I only had the one recorder, so I could only pick thunder examples that seemed roughly the same, so precise it’s not. That said, the audio recordings showed a drastic difference in tone.
First, the parabolic dish mic:
Parabolic dish mic test
I had amplified this one slightly to get the ambient rain sound in the same general realm for both recordings, so the thunder could be more directly compared, and I let the recording run until I could no longer make out the echoes of the thunder.
Followed by the shotgun mic test:
Shotgun mic test
That’s a world of difference – the parabolic dish really doesn’t capture the lower registers well at all, though I think the shotgun mic is both optimized for them and has more of a built-in pre-amplifier. It also has a ‘low pass filter,’ which can reduce much of the very low frequencies/longer wavelengths, since a lot of ambient noise falls into this category. However, I was not using this setting since I wanted to see how low it captured.
Another example from the shotgun mic, this time of the loudest thunder that I heard while out there. Listen carefully, because you’ll hear the rain vanish for a few moments:
Thunder through shotgun mic
I actually heard the rain seem to stop through the monitoring headphones, amusing because I was standing right there and knew that it hadn’t, and I was a little concerned that the audio had issues. But what I suspect now is the Olympus recorder has an auto-levels setting and it quashed the volume as it began to creep too high, resulting in the quieter rain dropping off.
I also have to mention that I stepped out into the rain with the parabolic dish, and this is not recommended: the thin plastic of the dish picks up the raindrops hitting it all too well, producing a tinny but distinct tapping sound, close to the sound of rain on tight sheet plastic (which I guess it is.) Meanwhile, the dead cat on the shotgun mic prevents nearly all noise from rain hitting the mic, though you can still here faint taps when it hits the mic body outside of the wind guard. And both, of course, need to be handled gently with as few hand movements as possible, since these transmit through the bodies and come out quite distinctly in the recordings.
But I think the answer is clear, and I’ll be staying with the shotgun mic for audio and video recording, though I intend to do some tests with the wood ducks, notably much higher in pitch, to see if the parabolic mic amplifies those better. I think it’s likely that the parabolic will be used mostly to locate subjects with precision, because it is good at that, while the useful recording will be with the shotgun mic.
[I just have to add that I have a lightning tracking app on my smutphone, and I purposefully changed the alert tone to an actual recording of thunder that I’d made some years ago – which went off while I was listening to the audio of this very post in draft form. Nice little surround-sound effect…]