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Re: Introduction...

Subject: Re: Introduction...
From: "Tim" pteropus_tim
Date: Wed Jun 15, 2011 8:17 am ((PDT))

Thanks for the comments and feedback.

In reply, I can only say (while pointing out that I'm still very new to thi=
game, am still almost desperately trying to learn, and really appreciate an=
feedback!) that I did try a number of different setups, including an x-y
array at various angles, and decided that for my purposes the ORTF array
seemed to work best. Remember I'm not trying to get the most aurally
'impressive' recording; what I'm trying to do with the stereo setup is: a)
get a general soundscape that played back over (typically) computer stereo
speakers gives a comparative impression of the noise in that particular
flying-fox camp at that particular time; b) allows me to get an averaged
sound level and frequency distribution spectrum for the 10 minute duration
of the recording (using Raven 1.4); and c) if I'm lucky enough to get a goo=
distinct call on one channel; isolate, extract and analyse that call

The ORTF array seemed to me to give these requirements, while also providin=
a (relatively) dead spot immediately behind the array - where I am usually
sitting and taking notes.

In many cases I am in the middle of a flying-fox camp; these can be a 1-2
hectare area containing up to 30-40,000 individual bats, all talking
simultaneously (these animals haven't yet learned that it's polite to put
your wing up and wait for a break in the conversation before speaking!) so
need to record on a wide field to get an overall impression.

This is also, of course, why I am using a parabola for my specific targeted
recordings - in spite of its being a little more painful to handle, the
parabola (providing it's pointed directly at the target) allows the specifi=
animal's vocalisations to be so much louder than the surrounding general
noise that they are (relatively) easily abstracted and analysed.

The shotgun mics I have access too (Sennheiser ME66 and ME67) do not seem t=
be able to abstract to this extent. Maybe a better mic (SennheiserMKH8070??=
may achieve this but sadly at the moment it's out of my league.

I must admit to being a little worried about the frequent comments I come
across in various areas of the scientific literature regarding supposed
"colouration" of the sound by the parabola? Although both reviewing posting=
on this list, and doing a little research of my own into the published
literature, this does not seem to be a huge problem providing you are
concerned with on-axis recordings of above the critical frequency for the
dish diameter you are using. Or have I missed the point entirely somewhere?=

As I stated above, effectively only a parabola seems to be able to abstract
the individual calls from the surrounding verbiage successfully.

If I understand things correctly, there's no necessarily perfectly correct
answer for anything in this game; like in many fields, the answer is usuall=
"it depends."

Reading this list there's so many techniques and bits of equipment I'd like
to try! Now, where's that lottery win??



 On Behalf Of Avocet
Sent: Sunday, 12 June 2011 12:05 AM
Subject: Re: [Nature Recordists] Introduction...


Welcome to the list. It's full of ideas and the odd controversy. :-)

It sounds like a good kit and the only comment I would make is that
the ORTF array is related to the "dummy head" types of array and gives
its best effect on headphones. If you are analysing the recordings, a
mono front signal mix of the two tracks will give a comb filter effect
at some angles due to the time shift which may or may not show up. The
first extinction frequency will be about 1.5KHx at 45deg. Your ears
will not hear this on headphones. You can test this with compresssed
air hiss - which gives "blue noise" - and a power spectrum analysis.

For a directional recording for measurment rather than for listening I
would compare the ORTF with a coincident cardioid capsule array with
one mic over the other with horizontal directional angles from 90deg
to 120deg. The time differences that the ear responds to is in
fractions of a millisecond, not easy to measure.

If you want to go to town on direction recording consider four
coincident or near coincident cardioids or two double cardioids like
two AT 8022's. I've just been recording a jackdaw nest in a chimney
using two stereo recorderas for inside and outside and synchronising
them easily using Audacity so you can try out the idea without a

I'm using gun mics for a lot of my recordings and I would not advise
taking off-axis measurements from these as their frequency rsdponse
off axis is very wonky.


David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce

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