DEPARTMENT OF MUSIC ACOUSTICS
WIENER KLANGSTIL (IWK)
Institute         Projects         Research         Teaching         Publications         Service

Turntable for an Automatic Acquisition System for Measuring the Directional Characteristic of Musical Instruments

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.

Turntablesystem with tuba in b

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.

Principle of the Measurement System

The System mainly consists of following components:

  • Stable turntable with a fixing construction which suits the individual test instrument
  • Motor (here: 2x stepper motors), which drives and rotates the turntable
  • Motor controller, which stimulates the motor/s and communicates with the PC outside the chamber
  • Measuring equipment like:
    • Exciting mechanism for the instrument (stringed instruments with shaker or brass wind instruments with artificial mouth)
    • Probe microphone placed in far field
    • Pre-amplifiers, conductors, power amplifiers
    • Analysing computer unit

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.

SMC

Step Motor Controller(SMC): Automated control of the turntable

High Air Pressure Artificial Mouth - HAPAM

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.

Measuring process of the directivity

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:

  1. Initialisation:the rotary table is driven to its initial position besides the separate settings for the analysis can be made.
  2. Calibration: here, the position of the turntable can be adjusted manually. If this state is confirmed with OK (Push button or command over serial interface), the first measurement will be conducted from this position.
  3. First Measurement: then, the first acoustical measurement begins. This analyses the transfer response over a defined frequency range by dividing output sound pressure at the measurement microphone by input sound pressure in the mouthpiece (reference microphone). Die excitation of the systems (HAPAM mounted on the instrument) occurs as logarithmic stepped sine.
  4. Rotation of the instrument: the instrument is turned by a rotation of the turntable. This changes the angle of the instruments to the recording microphone relatively. The rotation is scaled by the predefined amount of degrees. The system has an accuracy of about 1 degree!
  5. Waiting: the analysing computer waits until the desired position of the turntable is reached. For signalisation, a RDY command will be sent via the serial interface.
  6. Further Measurements: then, another measurement will be start. This procedure (incl. Point 4 and 5) will be repeated as long as the desired end position of the turntable is reached (commonly 180° or 360°).
  7. Data processing: finally, the gathered data of the acquisition system will be analysed and post processed. Then the gained information will be prepared for several output file formats. These files are: WAVE-ffiles (= raw data of the measurements), VIAS-files (= include frequency response with magnitude, phase, real- und imaginary part) und a graphical illustration as a polar diagram as SVG-file.

Flowchart of the directional characteristic's measuring process

Results

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:
  Dynamic representation
© IWK 2012–2015