[Paul Bissell]

I was looking closely at some waveforms of percussion instruments I have
recorded recently when I saw that the initial 'compression' of the wave
is showing up as a TROUGH. Shouldn't the waveform show an compression
and not a rarefaction at the beginning? If so, I have checked my set up
and there are no phase switches turned on. This is the case with various
microphones, cables and mic pres - that leaves the MOTU828 mkII. It is
possible the MOTU is reversing the polarity of all incoming signals?

Thanks
--
Paul Bissell
Go Fish Music

[Scott Dorsey]

Paul Bissell wrote:
I was looking closely at some waveforms of percussion instruments I have
recorded recently when I saw that the initial 'compression' of the wave
is showing up as a TROUGH. Shouldn't the waveform show an compression
and not a rarefaction at the beginning? If so, I have checked my set up
and there are no phase switches turned on. This is the case with various
microphones, cables and mic pres - that leaves the MOTU828 mkII. It is
possible the MOTU is reversing the polarity of all incoming signals?

Maybe. To be honest, most designers are not very careful about absolute
phase.

On the other hand, maybe you recorded from the other side of the drum.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Richard Crowley]

"Paul Bissell" wrote ...
I was looking closely at some waveforms of percussion instruments I have
recorded recently when I saw that the initial 'compression' of the wave
is showing up as a TROUGH. Shouldn't the waveform show an compression
and not a rarefaction at the beginning?

Why? Are you striking the drumheads from below?
Are you micing them from below?

When I hit a drum, the head does down and the waveform
from the perspective of my ears (or from a microphone on the
drummer's side) is an initial rarefication of the air pressure, not
a compression.

I actually hadn't thought of this before, but your question
prompted a quick "dry-lab". :-)

Of course, micing a kick-drum from the "backside"
would result in an initial compression.

I am more intrigued by the waveforms of speech I frequently
see which appear significantly asymmetrical with much more
"energy" in the positive side than below zero. It is not DC
offset, but very asymmetric waveforms.

[Fred Johnson]

In my 'tests' I used a 12inch Tom as well as a Marimba. The tom was
miced from above about 3 inches right above the rim. The marimba
(interestingly enough) was actually a M/S stereo configuration. In that
case, both the R and L channels showed the first movement from the 'zero
pressure' point as being a negative movement. When I say 'first
movement' I mean the absolute start - in all the cases, the first large,
dare I say fundamental waveform is positive
In both cases however. I uploaded a small .png file so we can all make
sure we are talking about the same thing. This is an image of the strat
of me saying "Dum.." into a standard dynamic (going straight into the
MOTU fyi) Note the first real motion is downward, but the fundamental is
positive.

http://www.gofishmusic.com/waveform.png

Thanks for all the responses...

Paul

In article ,
"Richard Crowley" wrote:

"Paul Bissell" wrote ...
I was looking closely at some waveforms of percussion instruments I have
recorded recently when I saw that the initial 'compression' of the wave
is showing up as a TROUGH. Shouldn't the waveform show an compression
and not a rarefaction at the beginning?

Why? Are you striking the drumheads from below?
Are you micing them from below?

When I hit a drum, the head does down and the waveform
from the perspective of my ears (or from a microphone on the
drummer's side) is an initial rarefication of the air pressure, not
a compression.

I actually hadn't thought of this before, but your question
prompted a quick "dry-lab". :-)

Of course, micing a kick-drum from the "backside"
would result in an initial compression.

I am more intrigued by the waveforms of speech I frequently
see which appear significantly asymmetrical with much more
"energy" in the positive side than below zero. It is not DC
offset, but very asymmetric waveforms.

[Richard Crowley]

"Fred Johnson" wrote ...
In my 'tests' I used a 12inch Tom as well as a Marimba. The tom was
miced from above about 3 inches right above the rim. The marimba
(interestingly enough) was actually a M/S stereo configuration. In that
case, both the R and L channels showed the first movement from the 'zero
pressure' point as being a negative movement.

But isn't that exactly what we would expect from "pushing"
(the slow-motion equivalent of striking) a drum head or a
marimba key? The air following it would rarefy first before
the drumhead (or marimba key) oscillated the other way
(towards us and the microphone diaphragm). I would be
surprised if it DIDN'T work this way.

A much slower and macro illustration would be what happens
to the rope around the ring when one of those "wrestlers"
pushes their opponent into the ropes. The rope first distends
away from the "wrestlers" before "oscilating" back towards them.

[Paul Bissell]

Well the marimba was recorded about 4 feet away from the bars - AND out
front where the bar would be seen going 'up and down' in relation to the
microphone's diaphragm and not 'towards and away'.
This will also be the case when the tom is 6 feet away and miced from a
neutral angle.


In article ,
"Richard Crowley" wrote:

"Fred Johnson" wrote ...
In my 'tests' I used a 12inch Tom as well as a Marimba. The tom was
miced from above about 3 inches right above the rim. The marimba
(interestingly enough) was actually a M/S stereo configuration. In that
case, both the R and L channels showed the first movement from the 'zero
pressure' point as being a negative movement.

But isn't that exactly what we would expect from "pushing"
(the slow-motion equivalent of striking) a drum head or a
marimba key? The air following it would rarefy first before
the drumhead (or marimba key) oscillated the other way
(towards us and the microphone diaphragm). I would be
surprised if it DIDN'T work this way.

A much slower and macro illustration would be what happens
to the rope around the ring when one of those "wrestlers"
pushes their opponent into the ropes. The rope first distends
away from the "wrestlers" before "oscilating" back towards them.


--
Paul Bissell
Go Fish Music

[Richard Crowley]

"Paul Bissell" wrote ...
Well the marimba was recorded about 4 feet away
from the bars - AND out front where the bar would be
seen going 'up and down' in relation to the microphone's
diaphragm and not 'towards and away'. This will also
be the case when the tom is 6 feet away and miced from
a neutral angle.

Seems to me that both the marimba keys and the conga
head have their top sides open to the air, but their bottom
sides more constrained so that the effect of what happens
to the top surface would predominate unless the mic is
sampling the wavefront at the bottom end of the conga
shell or the marimba resonator tube.