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Re: Digest Number 1235

Subject: Re: Digest Number 1235
From: Walter Knapp <>
Date: Wed, 21 Jan 2004 11:23:48 -0500
From: Matteo Trisoglio <>
> Subject: omni-directional mic
> 
> Dear all,
> I just started with the world of sound recording, I have a Sharp Minidisk and 
> a little stereo mic. At the moment I'm having some good fun recording city 
> birds and sound like people passing, wind, sea...
> I read also some e-mails of the group but I'm rather lost with all these 
> surrounding records, parabolic mic,ecc.
> My question is simple: when should I use a directional mic rather than an 
> omni-directional one? Mind that my budget is limited, at the moment I'm quite 
> dummy and I don't dare to buy expensive stuff before having tried the basic 
> ones.

I think of a omni mic as a "local" mic. It's ability to pick up sound 
well at any kind of distance is limited. And it picks up from 
everywhere, the bird in front of you, the freeway behind you etc. If you 
try to amplify a signal from a omni mic you get all this surrounding 
stuff amplified too. This makes it good for recording ambiance, the 
general sound of a area, but not the best choice if you want to 
concentrate on some subject. Unless you can get the mic very close to 
the subject.

BTW, one can think of nature recording as dividing into ambiance 
recording or subject recording. Though with most recordings it's a 
mixture. Those two divisions take different mics if you concentrate on 
one or the other. And some form of compromise if doing both.

Directional mics up to and including shotgun mics don't have any extra 
ability to pick up at a distance, but they do limit the directions they 
record. The sequence in increasing directionality is cardioid, 
super-cardioid, short shotgun, long shotgun (there are other pickup 
patterns too). You still have to amplify to pick up at a distance, but 
the signal is not carrying as much surrounding sound, giving you the 
ability  to cut out unwanted sound. So, I think of these mics as capable 
of short to medium distance. Since they still pick up a fairly wide 
angle, they all can be used for both ambiance or picking out a single 
subject. The more directional they are, the more they become specialized 
at picking out a single subject.

Now, in all of the above, to get distance you electronically amplify the 
signal. The mics themselves have some noise they produce, regardless of 
the sound they are recording. This is their self noise. And, in 
amplifying the signal after the mic, you amplify this as well. That 
means you can only increase the gain so much before the noise of the mic 
(or preamp) becomes too intrusive, in quieter sites this will be the 
ultimate limit of how far you can pick up. Thus the self noise spec of a 
mic becomes very important in nature recording. In general you want mics 
with a self noise below 20dBA (or if given S/N greater than 74 or so). 
And the lower the self noise the better. Unfortunately the lower the 
self noise the greater the cost of a mic, in general terms. Starting out 
it hard to afford the best in this characteristic. Realize that even a 
noisy mic can be used, just it limits you to noisier sites.

Even a long shotgun mic is still picking up a angle of more or less 60 
degrees. You can get even more precise directionality by going to a 
parabolic mic. The actual mic in a parabolic mic may be a omni or 
another pattern, the reflector provides the directionality. And the 
reflector provides something else, it provides a considerable increase 
in sound volume from the subject, like being close to the subject. And 
this is free amplification that does not amplify the self noise of the 
mic or preamp, so for the same quality mic and listening volume, you 
will get cleaner recordings with a parabolic. Or, in another view, be 
able to record at much greater distance.

There is another consideration you may or may not have realized. You say 
you are recording with a small stereo mic. If you went to a single mic, 
say a omni, then the character of your recordings will change. The easy 
way to describe it is the soundfield you now hear as a volume would 
collapse to where it was sounding like it was located all at a point 
between your ears, inside your head if listened to with headphones. 
That's what mono does, you loose all directional cues, only have sound 
volume to help with distance.

Back to thinking about noise. If you record mono and everything ends up 
sounding as the same location, not only the bird, but all the unwanted 
noise is there, making listening to the bird harder. If instead, you 
record in stereo, even if there is noise, you should hear the bird in 
one location the noise in others. And our brains are very good at 
sorting that out to hear the bird better.

So, you need to think about this aspect as well. From a money standpoint 
stereo has the disadvantage that two mics (or two capsules in a single 
housing) are required, increasing the cost. And good reproduction of the 
soundfield you are hearing requires some organization of the mics. Two 
mics placed randomly may produce a "soundfield" but it is unlikely to be 
much like the original one you listened to. The easy way out is to buy a 
all in one stereo mic that's well designed, the more elaborate way is to 
buy two evenly matched mics and use those in various standard ways. More 
to learn in any case. Mono is simpler and less costly, but with the 
disadvantages I gave above. There are ways to do stereo with any mic 
type, including parabolics.

I'd suggest that, since you already have the basics, a recorder and a 
mic, that you start from that, and don't get in too much of a rush. 
Think about what you like and dislike about the sound you get out of 
your current mic. Think about things it won't do well enough for you 
that you want to do. And, most important, think about what you like to 
record. Knowing that you will have a set of criteria to judge potential 
mic choices that will be personalized to you. Don't be in a rush to get 
new equipment, build your equipment set as money and need dictates. It's 
well to think through early where you are going with mics and work 
toward a system of mics for what you want to do. No one mic will do it 
all, and it's really nice to have a balanced system that gives sound 
consistent with what you want. Quality mics are very durable if taken 
care of, it's a long term investment. And, more than anything else you 
buy, will determine what you get. Though gaining recording experience is 
just as critical, record as much as you can and pay attention to what 
you get and sort out why it came out the way it did. As you gain 
experience judging mics will get easier.

It also helps to realize some of the discussions going on in the group 
are wishful thinking, not actual stuff people are using. At all levels 
you can still think of something more to do.

Walt






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