room responses etc.

[Martin Czech]

Yes, the ground reflections...  I observed quite long impulse responses
so far, it would mean quite a large construction...  A tower would be
nice, but the construction elements would reflect, too!  Perhaps a dive
tower in a swimming pool?

A dear, getting arrested again. My first time beeing arrested was
because of list members suggesting the use of a cap pistol in the
Cologne Cathedral.

Now I fear getting arrested when climbing the fence at my local public
swimming pool...

;_>



Perhaps I should not try to eliminate the reflections at all?  What if I
LAY the speaker on ground, so that the chassis main axis points towards
heaven? Say on a great agricultural area, no buildings around. If I
build a wooden "collar" for the speaker, basically a 2x2m area with a
hole inside to fit the speaker just on the surface, I'd have a controlled
reflection from that collar but certainly not from heaven.

And OTOH , would it be possible to remove the reflections by comparing
different elevations above ground in case of a "tower" ?
Basically, if the speaker is above a concrete plain (road),
i can say that only one reflection comes up, with a predictable
delay, that should be equal for all frequencies (I hope).

Can't we apply a mirror image speaker below the road, this kind of
model usually does the trick for field problems...
Should give characteristic peaks and notches...

Only if the concrete would give an ideal reflection.
I know nothing about that.

??

m.c.


:::I have done zillions of free air recordings of speakers. Now, this is why the
:::correlation technique is used! Yes, you will get a ground reflection and
:::possibly a zillion others. However, the first trick is to cut the responce 
just
:::before the first reflection (which should be the ground reflection if you 
have
:::done things right). Now, the time length from the begining of the impulse
:::(as seen by the mike) to the truncated end of the responce will set the 
lowest
:::frequency for which the responce says anything. If you have very, very little
:::energy after the truncation then the error of extending with zeros is not 
very
:::large, but otherwise you don't look beyond that point. Also, an FFT should 
only
:::be done for the valid area. The second trick there is to lift your speaker 
and
:::mike assembly up in the air, at least 3-4 meters. Now, naturally you have a
:::tower around that allows you to do that. When you do this you effectively
:::moves the ground reflection away in time (and amplitude!).
:::
:::The ground reflection is approximatly twice the higth away, so a lift of 4 m
:::is worth 8 m to the first reflection, so that gives 8 / 343 = 23.3 ms of time
:::or 42.875 Hz of frequency.
:::
:::Also, the longer away the first reflection is, the more has the speaker
:::silenced, so a truncation and then concatenation of zeros provides less 
error.