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So today, we're at Boston Symphony Hall, and we're
going to be recording a string quartet using ambisonic techniques.
In order to do so, we're going to run through the steps of what's
necessary to set up and calibrate your system.
An ambisonic microphone, such as this Sennheiser AMBEO,
is putting out four channels known as first order ambisonics.
Depending on the brand of microphone, they
vary what is coming out of each channel.
Or in other words, how it's interacting with the four capsules
that the microphone has.
In this case, we have channel 1 putting out left front up.
Channel 2 is putting out left right down.
Channel 3 is putting out back left up.
And channel 4 is putting out back right down.
One very important aspect of recording ambisonic information
is making sure that all four channels coming out of your microphone
are going into a system that can be properly calibrated.
Ideally, you're going to want that calibration
to be within a 10th of a DB.
The easiest way of recording ambisonics is using something
like this Tascam 4-channel field recorder that I have,
which allows me to digitally control each channel on the recorder,
and more so, link all the channels on the recorder
so that with the turn of one gain stage, I can actually turn all four channel
preamps up or down as needed.
Next, I'm going to make sure that I have some way to play back signal tones.
In case you don't have access to something
like this or one of the many other forms of signal generators out there,
you can also opt to use a signal generator app on your iPhone.
The first thing I'm going to want to do is calibrate the input to my recorder.
So I'm going to play pink noise out of my signal generator
and plug it into the first channel of my field recorder.
As I calibrate my signal, I'm going to want
to make sure that I allow plenty of head room
so that once this is actually the microphone inputting into the recorder,
it allows the performers to have plenty of head room to interact with the hall.
Because they're all linked, that's going to be calibrated
through the rest of the channels.
So here, I'm taking my signal generator, connecting it to input 1,
running test tone, and I'm going to make sure
that this hits roughly negative 12 DB.
Once the four channels of my recorder are calibrated,
I'm going to go ahead and plug in, carefully in order,
channels 1 through 4 to channels 1 through 4 on the recorder.
After you do that, you're probably going to notice
that the levels on each of the channels are no longer going
to be absolutely identical, and that's to make up for the fact
that the four tracks on the microphone are actually
in 360 degrees listening to the environment that we're currently in.
Therefore, you're going to have some that
are getting on-axis access information and some that are
getting fully reflective information.
We found that first order ambisonics don't give us
quite enough detail of the information that we're actually capturing
in a setting like music recording.
In order to make up for this, we will set up
spot microphones on every single instrument
that we intend to record in order to give us
really close and accurate detail, so that when the viewer turns
his or her head, there is still enough detail
of the sound sources in the room.
Because of that, we're going to want to have some form of reference to time
delay the spots to our ambisonic ambience recording later
in post-production.
Once I'm sure that my microphones are placed correctly in the spots
that I want, I'm going to give myself a click that
will allow me to then have an accurate delay reference later on.
In a situation capturing a musical performance,
we're going to want to be mindful of things like proximity effect, on-axis
or direct sound, and off-axis or reflective sound.
That will obviously be largely affected by the space that you're recording in.
In a hall like Boston Symphony Hall, which has a huge reverberant character,
we want to make sure that our primary microphone is
close enough to the ensemble so that the direct sound arriving
at the front of the microphone or both of the capsules in the front
is greater than the reflective sound coming from the back.
I'm going to want to position my microphone from a vantage point
as though I were a conductor sitting in front of them.
Finally, let's discuss how to monitor an A-format ambisonic while you're
recording.
Some field recorders, like the Zoom FM8 or the Sound Devices
MixPre Series will give you the option to monitor A-format
to B-format conversion in real time, both an AMBIC or FuMa formats.
Today, because we're actually recording 16 channels of ambisonics in the four
mics that we have, we're actually recording
to a DAW, where we can either use a plug-in to monitor through
and do the conversion from A-format to B-format or a pseudo left
and right using two channels out of any of our ambisonic microphones.
So in summary, in order to capture an ambisonic recording,
we need to calibrate our gain stages at our mic pre's, measure the delay
time between our spot microphones and ambisonic sound sources,
be mindful of the mic placement and the acoustic space that we're recording in,
and finally, monitor the recording accurately.
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