How is the sound of a musical instrument radiated into the room?
This is determined by the directional characteristic of individual instruments. It describes how the sound is radiated as a function of position and frequency. To determine this property, an automatic measuring system was developed at the Institute of Musical Acoustics in Vienna, which makes it possible to stimulate a musical instrument and to measure the radiated sound pressure at different angles. This was realised by an automated turntable system. It also is remotely controllable by a supervising PC.
Turntable for acquisition of the directivity (Cerveny B-Tuba)
The measurements are done in an "anechoic chamber" of the IWK. Therefore acoustic influence of the room (such as reflections from the walls) is mostly avoided. Only in this way it is possible to determine the directional characteristics of the instrument clearly.
The System mainly consists of following components:
In following figure the control unit of the turntable drive is shown. The elements of this board are controlled by an Arduino Mini micro-controller. One part of this board are two stepping motor drivers which supply the two motors with energy and the appropriate clock rate. Communication with the outside of the anechoic chamber is realised with a serial interface (RS-232). The 7-segment display allows simple status monitoring of the electronic control system. Buttons offer the user a manual control of the turntable system. Additionally, the Hall sensor inputs trigger important switching pulses. These include reaching the original position or an error detection Locked turntable for instance.
Step Motor Controller(SMC): Automated control of the turntable
High Air Pressure Artificial Mouth (HAPAM): Exciting mechanism for brass instruments providing high output power
The HAPAM excitation system consists of a wooden box wherein a 5,25" mid range loudspeaker was mounted. It has a rated power of 50 Watt. There, a plastic cone is placed directly above the loudspeaker membrane. This focuses the sound energy into the cup of the mouthpiece. To widely avoid parasitic radiation in other directions, the box is almost built sound-proof. A reference microphone is placed near the mouthpiece edge's plane. It serves to acquire the actual sound pressure inside the cup.
The flow chart below describes how the measurement process is conducted by the SMC and the PC including the LabVIEW analysing program. Both programmes work simultaneous:
Flowchart of the directional characteristic's measuring process
|Radiation of a Tuba calculated from measurement data gathered:|
|Graphical description with different frequency bands||Dynamic representation|
|Radiation of a "Cigarbox Guitar" calculated from measurement data gathered:|