Applied Acoustics Systems Chromaphone 3 Upgrade Acoustic Object Synthesizer Plug-In (Download) - Manual (Page 40)

40 The Editor View
The coupling of objects results in a bidirectional transfer of energy between the objects. In
physical terms, the amount of exchange is determined by the relative value of the mechanical
impedance of the different objects. The impedance is a notion which measures how much an
object opposes motion when subjected to a force. It is a frequency domain function as the response
of an object can vary greatly with frequency. For example the amplitude of the motion of an object
will be much greater when excited at a resonance frequency.
In simpler terms, the effect of coupling can be understood by considering how rigid one object
is compared to the other which determines how much energy can be transferred from the first object
to the second one. Let’s imagine a string attached to a very stiff sound board. While some energy
will be transmitted to the sound board through the bridge, it will not greatly affect the motion
of the string; most of the energy will be reflected back into the string at the bridge resulting in
a standing wave in the string and a long decay. Now let’s imagine that the soundboard becomes
much less rigid. The string can now set it into motion more easily at the bridge. This implies that
more energy will be able to flow from the string to the soundboard resulting in a shorter decay as
less energy is reflected back into the string. But the soundboard also moves according to its own
vibration modes which are different from that of the string. This motion interacts with that of the
string which modifies the tone that we hear. One could say that we now hear more the soundboard
in the resulting sound. The amount of coupling between the resonators therefore affects both the
resulting tone and its decay time.
The material of the objects is not the only thing to consider. Their respective tuning, which
can be related to their geometry, also greatly influences the response of the combined objects. For
example if the objects are tuned at the same fundamental frequency, their respective motion will
be synchronized and result in a sound having a large amplitude. For example, in a vibraphone,
the tubes are tuned to the fundamental of the bar above them in order to amplify the fundamental.
But there is also another effect which might seem contradictory at first. The fact that energy is
well transmitted from the bar to the tube also implies a faster decay of the oscillations. Hence,
the overall effect of the combination of the bar and the tube is to amplify the fundamental while
decreasing the decay time of the note.
As we can see, the overall effect of coupling can be quite complex as many factors must be
taken into account. As a rule of thumb, in traditional musical instruments, a first resonating object
with a long decay is usually coupled to a second resonator having a very short decay time (try
knocking on the sound board of a guitar) in order to avoid unpleasant resonance effects.