4.2 Measuring Input Impedance
123
software package has been adopted by many brass instrument manufacturers and
has also been widely used in brass instrument research.
4.2.2 Complementary Cavity Methods
A number of input impedance measurement systems have been developed using
a rigid piston as a driver (Benade and Ibisi 1987). An attractively simple idea is to
couple the back of the driving piston to a closed cavity containing a microphone. We
use the term ‘complementary cavity’ to describe this approach because a volume
flow into the test system is complemented by an equal and opposite volume flow
into the cavity. From a knowledge of the acoustical properties of the cavity, it is
possible to deduce the volume flow from the measured pressure. This is particularly
straightforward at low frequencies, since the cavity behaves as a lumped impedance
(Dalmont 2001a).
A complementary cavity system using a loudspeaker driver described by Singh
and Schary (1978) suffered from problems associated with the lack of rigidity of the
driver and imperfect acoustical isolation between the cavity and the measurement
system. A successful complementary cavity device using a piezoelectric disc as
a driving piston has been developed and marketed by the Centre de Transfer de
Technologie du Mans (CTTM) in association with the Laboratoire d’Acoustique de
l’Université du Mans (LAUM) (Dalmont et al. 2012). The principle of the system
is illustrated in Fig. 4.17, and Fig. 4.18 shows Jean-Pierre Dalmont using the CTTM
system in a study of trumpet bore optimisation (Macaluso and Dalmont 2011).
4.2.3 Wave Separation Methods
A disadvantage of using a capillary tube of very high acoustic resistance to drive
the flow in an impedance measuring device is that the flow amplitude is typically
very much smaller than the values generated in human performance. This is not in
Fig. 4.17 Schematic diagram
of the CTTM impedance
measuring system. Courtesy
of Jean-Pierre Dalmont
123
software package has been adopted by many brass instrument manufacturers and
has also been widely used in brass instrument research.
4.2.2 Complementary Cavity Methods
A number of input impedance measurement systems have been developed using
a rigid piston as a driver (Benade and Ibisi 1987). An attractively simple idea is to
couple the back of the driving piston to a closed cavity containing a microphone. We
use the term ‘complementary cavity’ to describe this approach because a volume
flow into the test system is complemented by an equal and opposite volume flow
into the cavity. From a knowledge of the acoustical properties of the cavity, it is
possible to deduce the volume flow from the measured pressure. This is particularly
straightforward at low frequencies, since the cavity behaves as a lumped impedance
(Dalmont 2001a).
A complementary cavity system using a loudspeaker driver described by Singh
and Schary (1978) suffered from problems associated with the lack of rigidity of the
driver and imperfect acoustical isolation between the cavity and the measurement
system. A successful complementary cavity device using a piezoelectric disc as
a driving piston has been developed and marketed by the Centre de Transfer de
Technologie du Mans (CTTM) in association with the Laboratoire d’Acoustique de
l’Université du Mans (LAUM) (Dalmont et al. 2012). The principle of the system
is illustrated in Fig. 4.17, and Fig. 4.18 shows Jean-Pierre Dalmont using the CTTM
system in a study of trumpet bore optimisation (Macaluso and Dalmont 2011).
4.2.3 Wave Separation Methods
A disadvantage of using a capillary tube of very high acoustic resistance to drive
the flow in an impedance measuring device is that the flow amplitude is typically
very much smaller than the values generated in human performance. This is not in
Fig. 4.17 Schematic diagram
of the CTTM impedance
measuring system. Courtesy
of Jean-Pierre Dalmont