ï~~Analysis of "Touch Effect" on the Transient of Pipe Organs with Mechanical
Transmission.
Laura Bazzanella, Giovanni B. Debiasi
C.S.C.- D.E.I., Universita' di Padova, via Gradenigo 6A - 35131 Padova (Italy)
tel. +39+49-8287500, fax +39+49-8287699, Email debiasi@paola.dei.unipd.it
Abstract
In pipe organs with mechanical transmission, the organists notice a response from the instrument depending on
the type of solicitation, i.e. the so called "touch". In order to explain this intuitive observation, we have
considered the sounds of several stops of organs with mechanical transmission, using for all notes two different
ways of "touch": slow and fast, employing for this aim a mechanical finger. With different analysis methods,
we got the evolutions of the timbre of the considered sounds, obtaining very different behaviours. Our results
have proved a real difference between the same note played with a different "touch". From a general point of
view, we can notice a remarkable sensitivity of the second spectral component. The results of the present
research can be useful for the synthesis of pipe organ sounds in high-level electonic organs.
1. Introduction
Sounds of pipe organs are generated from pipes
boosted with an air jet driven by the keys of one or
more keyboards. Most organists judge that the
pipe organs with mechanical transmission are
preferable in order to act with the touch on the
transient of each note. However there are
discordant opinions about the effectivness of the
touch influence and so we decided to give a new
contribution for the solution of the question.
2. Methods of sound recording and analysis
We considered the sounds of flue pipes of two high
quality modem organs with mechanical
transmission, namely the organ of Vincenzo
Mascioni of the Conservatory "C. Pollini" in Padua
and the organ of Franz Zanin of the S. Giorgio
Church in Vicenza. For each organ we recorded
all stops considering for each note two modalities
of touch: slow and fast. The definition of these two
categories has been done by expert organists and
they adjusted a mechanical finger to obtain two
very constant key velocities, slow and fast,
according with their subjective judgement. Each
note has been recorded from the starting transient
up to five seconds with an AKG condenser
microphone mod. C414B-ULS and with a D.A.T.
Denon mod. DTR 2000, at the sampling frequency
of 48 kHz. We also used a B&K phonometer with
a condenser microphone for monitoring the
intensity level (dB). We adopted a system of
acoustic shields in order to reduce the incidence of
the reverberating sounds on the microphone. The
sampled sounds have been processed with different
analysis methods: in particular with the STFT (see
Limg & Oppenheim [1988]), with a proprietary
algorithm for discrimination between harmonic
and inharmonic components of the sound and with
a method for automatic evaluation of timbre and
fluctuations of pipe organ sounds [Dal Sasso et al.,
1991].
3. Experimental results
For both organs, the starting transients of the
sounds analized with STFT showed a remarkable
difference between the time-evolution of the
amplitude of the first six harmonics in
correspondence with slow and fast touch.
Particularly, we find that the steady-state of the
amplitude of the second harmonic is reached with
an overshoot in the case of fast touch, while in the
case of slow touch we can observe no overshoot
(Figure 1).
The automatic assessment of the timbre on a polar
chart demonstrates a real difference in the timbre
evolution on the starting transient of sounds with a
different touch (Figure 2).
Finally, the extraction of the inharmonic
component has shown that its starting transient
also presents a different evolution, according to the
type of touch (Figure 3).
These two last methods of analysis, to our
knowledge, have not be used before for this kind of
analysis.
The differences of the second harmonic evolution
are in accordance with the theoretical prevision of
Fletcher [1976], Finch & Nolle [1986], Caddy &
Pollard [1957], and all other measurement we did
confirm the effective dependence of the starting
transient on the touch.
Nevertheless, we also noticed that sometimes the
notes of the stops with long-distance mechanical
transmission (having very long torque bars) can
present no differences between slow and fast touch.
We try to explain this fact thinking that, with slow
touch, there is torsional energy accumulating in the
bars and, as this energy reaches the needed value,
it causes the abrupt opening of the air valve.
ICMC Proceedings 1994
485
Acoustics
0
