VIAS

VIAS - Opto-Acoustical Input-Admittance Measurement of String Instruments
supported by Österreichischen National Bank, Projekt 6352/3

Measurement of the sound characteristics of string instruments is not a simple undertaking. The musician playing the instrument, the room where it is played, the bow, the strings and many other factors contribute essentially to the sound of the instrument in question. To determine the quality of an instrument - and often its worth - independently of variable factors, a repeatable measurement system must be developed to draw objective conclusions regardless of who performs the measurements or where they are performed. The results should however still agree with the subjective judgements of experienced musicians.

The objective determination of the acoustical spectrum of an instrument played by an artist during a concert in the concert hall, with comparable and repeatable results is imaginable but is extremely difficult to achieve. Above all, the attribution of measured sound characteristics to variable factors such as the musician, the composition and its interpretation, bow, strings, instrument, hall acoustics, and recording set-up is beyond the present technical possibilities. This renders conclusions of such a test unsatisfactory in terms of quality and the character of the specific instrument.

The first step toward standardization of such tests is normally to withdraw into a room with known acoustical characteristics. To separate acoustical characteristics from subjective factors, the player is usually replaced by the artificial means of an actuator, which is much easier to standardize. A method of excitation directly at the bridge with muted strings finally frees measurements from factors such as strings, bow and bow hair.

The measurement method normally used with string instruments is to record its frequency curve in an anechoic chamber. The instrument is made to vibrate at the bridge, either using artificial means (an actuator), usually in the form of an electro-mechanical transducer (loudspeaker principle), or through a so-called impulse hammer with which the entire transmission spectrum is concentrated in a short needle impulse (more or less close to the ideal Dirac-impulse). Sound waves emitted from the violin are recorded through microphones, processed by computer and evaluated. This technique delivers details about the transmission qualities of the instrument. The exactness and repeatability of results depends directly on the characteristics of the room (the quality of the anechoic chamber) and the absolute and relative position of the measurement microphones and the violin. The essential criteria for the repeatability of such measurements, the anechoic chamber, is available only at great cost. Also, the most interesting and valuable historical instruments often cannot be measured in such a chamber because they are bound to their place of safe- keeping by conservation or insurance limitations.

Fig. 1.: Measurement device construction is shown, without the electronic amplifier. In the lower center, two parallel tracks are visible which allow free positioning of the exciter element. The violin is suported by a clamp at pegbox, and the strings are muted with foam rubber.

The measurement method developed by the Acoustical Research Team in Vienna is notable because it eliminates dependence on the room acoustics where the test takes place. The anechoic chamber is therefore no longer a requirement for repeatable and comparable results. This is possible because this measurement method no longer relies on the instrument's sound in a room recorded through microphones, but on optical data collected directly at the bridge. Technically speaking, it is not the transmission function of the instrument that is measured, but instead its closely related input admittance. Input admittance is the function of frequency that shows how the sound pressure (the force with which acts on the bridge) is translated into sound speed (the speed with which the bridge, and therefor the table, vibrates). Resonance frequencies of an instrument are expressed in such an admittance curve as peaks- the bridge vibrates with higher amplitude without higher input force. Frequencies with little or no vibration are expressed as troughs. It is possible to make conclusions about the transmission function of the instrument because not only the absolute values, but by using applicable assumptions, also the phase relationship between sound pressure and sound speed is measurable. This method for optically measuring input admittance of string instruments therefore allows the construction of a portable measuring device with which historical instruments can be tested where they are kept. The Acoustical Research Team's method liberates the test process from considerations to change or improve the test environment. The name VIAS stands for "Violin Analysis System" and identifies this method and its device. It consists of an adjustable holder for the violin, the so-called actuator which contains a built-in electro-mechanical transducer for the excitation of the bridge and a laser-optic sensor, an electronic module containing the controls of the transducer and sensor, and the interface to a standard sound card. A computer (notebook) contains the sound card and operates software for evaluation and representation of the measured admittance curves.

Satisfactory transducers have already been introduced by H. Duennwald. The following describes this type of electro-mechanical transducer.

Fig. 2: Principle of the electro-mechanical excitation transducer (sizes not shown in proportion)

A hollow steel tube of 1.2mm diameter runs between the two poles of a strong permanent magnet whose magnetic field is arranged perpendicular to the wire. If a sine-formed current flows through the tube, force F, the so-called Lorenz force, is created on the wire perpendicular to current and field direction. When the edge of the bridge touches the wire, the wire exerts a force proportional to the sine-form current upon the bridge. The [eigenmode] of the wire must be chosen that the force within measurement parameters is practically independent of the frequency. Since vibrating violin strings could falsify results, they are muted. Normally the excitation occurs on the sound post side of the bridge.

Using VIAS, vibrations are measured through optical means without touching the instrument, which minimizes falsification of its acoustical characteristics. A simultaneous comparative measurement of audible qualities is also possible.

Basically, optical sensors are divided into two groups: those which measure distance[measurement object], and those that make use of an optical effect. The majority of optical sensors are based on the effect that a mechanical displacement of a fiber, aperture, mirror, or a pressure plate causes a variation of the intensity of a light ray. Intensity modulation (IM) was the technique used at the beginning of optical sensor development because it offers many advantages. For example, the simpler construction of such sensors allows for mechanical and thermal stability and reliability for long periods of time. The sensitivity of IM sensors is primarily determined by the effectiveness of the modulator, and secondly through the optical power of the transmitter and optical sensitivity of the receiver. A disadvantage of IM sensors is that they are sensitive to light energy losses within the system. This disadvantage can be avoided, however, by the use of stable sender and receiver units and high-quality transmission channels. Further, a reference channel may be omitted if the [measurement object] is a periodic alternating signal whose frequency does not lie within the band of variable losses.

This type of sensor has an aperture situated between the connected laser and detector. The position of the aperture depends on the measured variable. The amount of light transmitted modulates the detector. The aperture for this measurement system is in the form of the current-carrying steel tube, which is the sole contact between the violin and any other mass.

The illustration below shows the structural principles and modulation curve of the aperture sensor. The modulation slope is determined exclusively by the diameter of light beam diameter, through which an additional linear area on the modulation curve results.

The instrument is excited by a constantly increasing frequency and the amplitude is recorded simultaneously. Normally a frequency generator and an [analog-writer *Analogschreiber) are required. The function of both of these implements is assumed by a computer equipped with a sound card. Through digital signal processing, the VIAS Program computes and renders an admittance curve from digitalized sensor signals. The curve contains an abundance of information about the vibration characteristics of a string instrument and can be considered a "fingerprint" of the violin. Because all resonance frequencies of an instrument can be taken from this curve, and because the qualities of these various resonances is visible, the input-admittance curve offers valuable references to violin makers to improve a given instrument, or rather, to more closely realize a certain acoustical ideal.

An admittance measurement contains a lot of information which can be described by 4 different curves: magnitude, argument, real part and imaginary part. Figure 6 gives an example for such curves for a Guarneri violin.

The software of VIAS can calculate the radiated sound of any measured instrument. Using a soundcard the "virtual" sound created by synthesis can be heard. This feature gives the opportunity to compare measured instruments by hearing their sound. Another way to illustrate the sound characteristic of measured instruments is a kind of sonogram (Figure 7). X-axis: playable notes, Y-axis: partials related to the notes. The intensity is color-encoded: high intensity=bright, low intensity=dark. The grey line is the RMS, the white line indicates the spectral center of the note. Beside the information on the sound characteristic this methode of graphical representation is useful to get information on the playability of various notes, for instance detecting wolf notes. The figure 7 shows such an example.

	similar to the bowed string, the transmission of a frequency-independent, but perfect sine wave force to the bridge
	no burden to the instrument though an external mass and no other changes to the characteristics due to the transducer
	exact repeatability of all excitation characteristics (static thrust for example)
	no damage whatsoever to the instrument or bridge
	no sound emitted from the transducer
	no disturbance to the signals from the violin
	development of sufficient stimulation for satisfactory signal/ noise relationship

	higher sensitivity
	less initiation time
	highly adaptable
	simple mechanical structure

	displacement sensors
	reflex sensors
	aperture sensors
	fiber loss sensors