A HHR performes both simple reflection and self oscillation. At the
beginning of the triggering phase the sound runs into it and is
reflected at the inner walls. A little part of the sound comes out
through the hole while a larger part is reflected creating steady waves
as known in all rooms as modes. These partly again come out. So far this
is just reflection.
But the little hole causes losses and local over pressure so it is also
an obstacle for the air inside which will start to pump. This way a
continous wave in beneath the HH-frequency is so to speak "loading" the
HHR which will become louder and louder. This causes a shift of the
phase difference between the incoming and outgoing waves coming to zero
finally when the feeding signal reaches the losses.
Therefore the HH might emphasize but also eliminate certain frequencies
in the room depending on the phase and volume difference and the length
of the signal. It might happen that a triggering sound is thus limited
and does not overload the room with modes but after the sound is over,
the HHR resonator will continue to feed the room with sound when
unloading his energy, unless damping material is used inside to limit this:
http://96khz.org/files/2003/helmholtz-resonator-damped-silenced.jpg
A classical HHR built as a heavy iron sphere will have only one dominant
frequency and low losses, while a wooden case might have up to three. A
light wooden case might even pick up energy from the moving air inside
starting to emit sound on it's own resonance freqs.
So does the guitar: it picks up energy from the stings and performs both
energy storage in the wood and direct reflections inside the corpus. In
theory a guitar even has little HHR capabilites :-)
To distinguish both effects in the meaning of signal processing, one
could describe the reflections as a FIR behaviour while the energy
storing is an IIR. Real time simulation of loudspeaker cases can be done
this way, e.g. bass reflection tubes. In a very simple model the
reflections can also be replaced by IIR since the reflection of a sine
wave with a smoothed volume curve will also add to something like a II
response.
gtx
Dipl.-Ing. Jürgen Schuhmacher
Am 22.07.2018 um 23:20 schrieb Stefan Sullivan:
Yes. The term helmholz resonator should be a hint ;) Basically when a
sounds gets added to itself after a delay you end up adding energy to
the frequency that corresponds to that delay amount. For very long echos
we don't hear it as a resonance, but for shorter delays it will boost
higher and higher frequencies into the audible range.
Stefan
On Sun, Jul 22, 2018, 08:10 <rolfsassin...@web.de
<mailto:rolfsassin...@web.de>> wrote:
Hello all
Is "feedback with delay" really resonance? I recognize many people
describe the effects of "room resonanes this way", but to my
understanding these are no resonances in the basic meaning but
reflections. A resonance is a self standing oscillating system like
a guitar string or an air mass in a Helmholtz resonator.
Rolf
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