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Scietific research

 

ARVEDI LABORATORY of NON-INVASIVE DIAGNOSTICS University of Pavia, based at the Museo del Violino

There has been growing interest in recent years in research into cultural heritage, for both documentary and conservation purposes. Describing an object’s composition as accurately as possible allows the data acquired to be shared, and, most importantly, enables the best strategies to protect it to be identified.

In parallel with the growing interest in research, the constant development of new technology has enabled diagnosis involving ever more sophisticated equipment to be introduced to the protection of cultural heritage.
There are two major branches of research in this sector: one founded on micro-destructive diagnostics, and the other on non-invasive methods.
The first type involves taking various samples from the item being analysed. The research provides highly detailed and very interesting information, but involves the destruction of the samples, which cannot be returned to their original place or restored to working condition at the end of the process. This method is carried out in the Arvedi Laboratory at the University of Pavia, based in Pavia itself.
Non-invasive diagnostics, meanwhile, involves analysis carried out without taking any samples from the items; it is already in use in the scientific world, but has never before been applied to historic musical instruments. When the Museo del Violino was founded in 2013, a team from the University of Pavia built a lab space there for this kind of research, designed especially for musical instruments.
The lab’s location within the museum immediately gives it an enormous, hugely important advantage: its vicinity to the staff that work at the Museo del Violino and the items undergoing the research, meaning that priceless objects can be handled with the necessary care, and simplifying various logistical and conservation issues relating to moving the items, transportation, insurance, maintaining environmental conditions, etc.

Major innovation

Unlike cultural heritage in general, conservation and restoration in the musical instrument sector has not, until now, been a major research and development topic, leaving significant gaps in the theory underpinning instrument restoration, in the application of cleaning processes and in the knowledge of historic materials. The Arvedi Laboratory today is an example of the highly innovative work being conducted in Italy in the study of and research into antique musical instruments. Historic musical instruments: doubly precious objects Deciding on the most suitable conditions for conserving an antique musical instrument
involves considering how the object is viewed, and whether it is seen statically or dynamically. In the case of the former, the instrument is considered as a material object with historic value, an item that bears witness to an ancient culture of craft-smanship and art, and which should be preserved in both its form and materials; for the latter, meanwhile, the musical instrument’s value lies in the fact it was built to produce a sound, thus focusing attention on its ‘immaterial heritage’ and prioritising its function.
This duality today raises several questions on how best to assess the conservation and function of a musical instrument, often generating differences of opinion between cultures, particularly at a national and international level. The aim of the Arvedi  Laboratory’s research, in close contact with the curator of the Museo del Violino’s collection, is to overcome the dichotomy between the two models by combining different profiles and scientific skills. Indeed, scientific research can be used to define the best conditions for using a musical instrument, and at the same time set the parameters for the proper conservation of the instrument, indicating the times when the item should not be played.
Studying the ethics of the conservation of musical instruments is an extremely complex job that ultimately must involve creating scientific research procedures that cover all existing antique musical instruments and that include ongoing checks over time.
Gathering as much information as possible on the materials used for antique instruments, as the Arvedi Laboratory is currently doing, is laying the foundations for more precise instrument conservation procedures, and creating a significant corpus of information and analytical results unparalleled across the entire world.

The Arvedi Laboratory predominantly focuses on studying the materials used to make historic musical instruments, and particularly bowed and plucked string instruments, analysing the varnishes, pigments and wood treatments to rediscover the ancient recipes and construction techniques used in classic Cremonese violin making, and sharing this knowledge with the violin-making community of the city of Cremona.
Research into musical instruments can be divided into three types:

  1. Imaging
  2. Non-invasive analysis
  3. 3D modelling and printing
1 – Imaging

This covers the activities carried out to establish the condition of a musical instrument, using photographs acquired in standard, repeatable conditions. Standard
acquisition techniques are paired with highly sophisticated tools, such as UV-induced fluorescence, ultra-detailed analysis, UV-induced fluorescence and stereomicroscopy, endoscopy (visible light and UV) and digital radiography using x-rays.

UV-induced fluorescence

This type of study involves a Wood’s lamp – a light source that emits electromagnetic radiation, mostly in the ultra-violet range. It is used to illuminate the surfaces of an object, causing a fluorescent effect and producing characteristic colours that are invisible in normal white light. Using this technique, the staff at the Arvedi Laboratory can map the surface of an object and assess the distribution of materials such as the original varnishes, any retouching, restored areas and worn patches.

Ultra-detailed analysis: gigapixel images

Gigapixel images are digital photographs created using ‘stitching’, i.e. using software to combine multiple images of the same object. This produces results with an incredibly high definition. The ultra-high-definition images reveal extremely small details – a particular craquelure in the varnish, spots of colour in the varnish caused by particular pigments, and so on. This technique has previously been used on other items of cultural heritage, and is now being applied for the first time to musical instruments thanks to the work of the Arvedi Laboratory.

Microscopy

This research is carried out using tools that are much more powerful than the human eye. The musical instrument is placed securely below the microscope’s lens, allowing details on the surface to be observed in up to 100-times magnification. This technique is used by researchers at the Arvedi Laboratory to investigate microscopic cracks in the varnish and to assess the composition of the decoration and inlay.

Endoscopy

Just like the technique used in the medical sphere, endoscopes are inserted into the instrument. This allows an assessment to be made of the internal state of repair of the instrument without having to dismantle it, using a system of visible light and UV photographs that can identify restored areas and any structural reinforcements and potentially allow inscriptions, signatures or labels inside the instrument to be read.

Radioscopy

The laboratory also has a digital radiography room for conducting x-ray imaging. The radiography equipment allows the instruments to be x-rayed in real time, meaning the plate can be viewed immediately. This allows researchers to obtain information on the materials found within the instrument that cannot be seen from the outside (nails, screws, reinforcements, etc.) and to assess the scale of any woodworm attacks.

2 – Non-invasive analysis

This analysis investigates the inorganic materials and organic compounds that make up instruments, and basically involves two different forms of spectroscopy: x-ray fluorescence spectroscopy and attenuated total reflection IR spectroscopy (FTIR).

X-ray fluorescence spectroscopy (XRF)

This process analyses the chemical elements found in materials and provides information on any inorganic substances (salts, pigments, heavy metals, etc.) Knowing which chemical elements were used in the construction of antique instruments offers vital pointers towards uncovering the working methods of some of the most important violin makers in Cremona’s history, for example helping to establish which materials they used to dye the varnish or to treat the wood before applying the finishes to the surface.

Attenuated total reflection infrared spectroscopy (FTIR)

This technique analyses molecules by studying the bonds that form between atoms. It is a diagnostic technique, best suited for analysing organic substances like oils, proteins or natural and synthetic resins. The data obtained with this technique, together with the results of the x-ray fluorescence, provide a clearer and more complete overview of the instrument, highlighting, for example, whether an oil-based varnish, associated with an iron-based pigment, has been used on an historic violin. This allows the researchers at the Arvedi Laboratory to hypothesise which blends of materials were used, and to state their composition.

3 – 3D modelling and printing

This innovative laser-scanning technique enables the staff at the Arvedi Laboratory to obtain completely identical three-dimensional digital models of violins. The 3D model can then be used to obtain information on the violin’s size, measurements and volumes without having to physically measure the original violin, instead working directly on the 3D model. The Arvedi Laboratory is currently scanning the historic violins held at the Museo del Violino, and so building a ‘digital collection’ based on the real one. In recent months the laboratory has also been involved in printing original violins using 3D models, producing faithful, reproducible copies of the original historic instruments.

 

MUSICAL ACOUSTICS LAB Politecnico di Milano, based at the Museo del Violino

Modern bowed instruments evolved over time due to practical experiments by generations of violin makers, all with the aim of improving the projection and quality of the sound. However, the way in which the instrument (as an ‘object’) influences the structural acoustics and the timbre produced is still far from being understood, since the complexity of the physical and perceptual phenomena being analysed requires a contribution from many different disciplines.

This is the concept on which the Politecnico di Milano’s Musical Acoustics Lab, based at the Museo del Violino in Cremona, is founded – all aspects of the sound production of bowed string instruments are investigated, both collecting and recording an enormous and varied quantity of information and conducting a comprehensive analysis of this data. The twentieth century saw various pieces of research into bowed string instruments with a scientific approach.

Many aspects of antique instruments made by the most renowned violin makers were analysed: the material, the shape and the characteristics of the sound they produce, using methods adopted from acoustics, material science, structural mechanics and computational science. In addition, significant research was conducted into the vibrations of the sound box, into how the strings and the bow interact and into radiance patterns, i.e. how the sound of the instrument travels through space. Today the work of the Musical Acoustics Lab represents a major step forward both in terms of technical and scientific development and the evolution of the city’s cultural life: it has succeeded in bringing together the various parts of society, the economy, research, education and culture linked to the field of music, bowed string instruments and the craft of instrument making.

Analysis has been conducted in four areas in the first few years of the laboratory’s existence:

  1. vibrational analysis
  2. acoustic radiance analysis
  3. timbral analysis
  4. 3D geometric modelling
Vibrational analysis

When one picks up an instrument, the first thing to do is work out how the instrument vibrates.

This is regularly conducted by violin makers by listening to the acoustic response of the vibrating structure when hit with their knuckle (tap tones). Nowadays a more detailed and precise overview of the phenomenon can be achieved thanks to sophisticated equipment that produces and records vibrations, such as impulse hammers, accelerometers and laser Doppler vibrometers, as well as the software developed to process this data.
This method allowed the lab to analyse, for example, the timbral behaviour of a modern instrument during the major stages of its production, from a piece of wood through to the end product. It emerged that, due to the unique geometric structure of the instrument, certain aspects of the timbre were highlighted, while others were suppressed.
Most importantly, the researchers could verify which parts of the structure contribute to making certain vibrational behaviours more or less relevant, and what impact they have on the perceived quality of the instrument.

Acoustic radiance analysis

Acoustic radiance describes how the instrument distributes its sound energy in different directions. The Musical Acoustics Lab has developed an innovative technique that can measure this radiance at an extremely high directional resolution, in a completely non-invasive way. This technique is based on an acoustic field analysis

tool known as a ‘plenacoustic camera’, which is revolutionising the applied acoustics sector. Making use of two plenacoustic cameras, movement reconstruction techniques adapted from the computer vision sector and computational geometry, the researchers at the Musical Acoustics Lab developed a method that can reconstruct ultra-high-resolution radiance patterns during a violinist’s performance. This method has been widely discussed at international scientific conferences, and has become one of the laboratory’s flagship results. Using this method, the acoustics and projection of the great instruments of the Cremonese violin-making tradition could be studied in detail (including the 1715 Cremonese Stradivari).

The Musical Acoustics Lab has also developed methods for predicting the acoustic  radiance of an instrument, based on the structure obtained with a 3D laser scanner and the acoustic properties of the materials used in its construction.
This geometric information forms the starting point for a system that predicts the vibro-acoustic behaviour of the violin, and thus how it radiates its sound.

Timbral analysis

As well as research into vibrations and acoustics, the Musical Acoustics Lab has also investigated the timbre of instruments, using methods drawn from artificial intelligence (machine learning). Timbral analysis refers to a combination of techniques used to describe the quality of a musical instrument’s sound, and it is one of the most innovative aspects of the Politecnico di Milano’s Musical Acoustics Lab. Although the qualities of a violin’s sound have been studied for decades, the physical phenomena involved in describing its timbral qualities are not yet fully understood. For this type of research, the classic approach involves compiling a list of objective descriptors (for example describing the harmonic content of the signal).

The problem is that these descriptors are less semantically rich than the descriptions commonly used by violin makers and musicians, and do not always coincide.
For example, what is a bright sound? Or a warm sound? What makes it different from a cold sound?

Describing sounds is subjective. And the choice of terms used to describe the sounds themselves is equally subjective. A strong correlation has been shown between timbral description, sound perception and physics, but working out which aspects of the sound have most influence on our perception is no easy task.

The following procedure was carried out on the instruments in the lab. First a corpus of sounds was recorded using both antique and modern violins – 13 historic violins (three by Amati, two by Guarneri del Gesù and seven by Stradivari) and 15 modern violins from the collection of the Museo del Violino and the Antonio Stradivari International School of Violin Making – played by a professional musician and following a specific procedure. A number of descriptors were extracted from each of these recordings: the centre of mass of the spectrum, its distribution, the relationship between the energy of the harmonic and non-harmonic components of the sound, and so on. Next a group of violin makers and violinists were asked to rate the timbral qualities of each violin using a standard questionnaire. Analysing the correlations between the two reveals which descriptors are most relevant in determining the timbre. In parallel with this analysis, an ‘ontology’ of the violin was put together.

An integral part of this work was a glossary of terms commonly used to describe the properties and qualities of a musical instrument in common speech. This glossary includes a ‘metric’, i.e. a description of the ‘semantic proximity’ of the various terms it comprises, and rules that define the conversational context in which they are used.

This semantic structuring of the language, achieved through numerous questionnaires filled in by violin makers and musicians, was needed to understand, formalise, model and increase the quantitative significance of the subjective descriptions offered by the violin makers and musicians. All of this is dealt with and relayed using natural language, which is easier to understand and communicate.

Research is also currently underway into the use of artificial intelligence to predict the timbral characteristics of an instrument directly from the vibro-acoustic data acquired.

The aim is to predict the sound features of bowed string instruments from the mechanical behaviour of their sound boxes, to help in the design of new instruments.

3D modelling of instrument structure

Since its foundation, the laboratory has been equipped with a laser scanner, a device that can measure the three-dimensional layout of any surface, entirely non-invasively, to a very high level of accuracy

(20μm). The laser scanner is used within the Musical Acoustics Lab for two different purposes:

  1. Predicting the impact of changes in structure on vibrational and acoustic behaviour: once the geometric structure of the instrument has been scanned, one can make controlled changes to the geometric model, and so analyse the impact of these changes on the violin’s vibrational behaviour and sound projection.
  2. Analysis of deformation when the string tension is varied: when the strings are put under tension, they change the curvature of the instrument slightly. Although this change is in the order of tenths of a millimetre, it is nevertheless very important when assessing the state of repair of an historic instrument. The Musical Acoustics Lab, with help from the School of Violin Making, has perfected a very simple measurement procedure, which allows an assessment to be made of which parts are most subject to deformation in just a few minutes.