Rolf Bader,
Institute of Musicology,
University of Hamburg, Germany
R_Bader@t-online.de
 
Lay-language version of paper 3aMU7
"Acoustic Holography of Musical Instruments using Radiation Directivity Method"
Presented on November 12, 2008
156th Meeting of the Acoustical Society of America
 
 
Musical instruments have a very complicated way to radiate their sounds which then approache our ears. When singers change their singing style from barock to romantic, they change the amount of energy being radiated also from the body and not only from the mouth. The character and quality of a guitar is very much dependent upon the distribution of radiation areas over its surface. If a grand piano is on stage the complicated geometry leads to a complex radiation pattern which is much differentiated and does a lot to the fascination of the instrument.

Now measuring these radiation patterns is very complicated as radiation is coming from different sources and different material. To perform such a measurement, one of the largest microphone arrays is used especially developed for this purpose at the Institute of Musicology. Here 128 microphones record the sound field simultaneously with a sampling frequency of 48 kHz differentiating also the complicated initial transient phases which music is full of. Using an algorithm developed at the Institute, the radiation pattern at the singers body or at the instrument surface is calculated using the recorded sound field data. Here also the directivity of radiation is calculated. From these calculation results it will also be possible to perform a virtual radiation of the musical instrument back into the room and so one could listen to the same recording while ‘sitting’ in another place in the concert hall.

 

 

So for example is the classical guitar. Here it could be shown that the character of the guitar can clearly be associated with the radiation pattern of the guitar top plate. A high priced guitar being known for its very stable and clear sound also showed very accurate and precise radiation patterns. A low priced guitar on the other hand did not have such a clear pattern and so not such an accurate sound as the guitar of higher quality. Also a flamenco guitar and a guitar with is constructed like a bass reflex box were investigated. Here also the characteristics expected showed up. So this method could lead to an answer of the question still very much under dabate: how can a character and a quality of a guitar be associated to its physical parameters? Further investigations with the violin and other instruments will be performed in the near future.

 

 

Another example is an opera singer uses different techniques when singing in different musical styles. When singing in a romantic opera where the orchestra is quite loud and the space is large, the singing sound need to be loud, too and so the singer tries to ‘shoot’ with the voice into the space. This is different with barock singing. As the orchestra is much smaller and the pieces were written mostly for chambers rather than big concert houses the singer is able to differentiate here much more. In an experiment a professional singer was asked to sing the same musical phrase once in a romantic and once in a barock style (see Figure 2 below). When the sound radiation fields were compared, the different singing techniques clearly appeared in the measurements. The barock phrase was radiated from different points of the singer, her mouth (and never discussed but of course also true, the nose), but also from the upper part of her body. The patterns also depend on frequency. This is not the case with the romantic style which could also clearly be heard during the experiment and was verified by the singer describing her technique. The romantic radiation pattern nearly only concentrated on the singers’ mouth and was stable for all frequencies investigated.

So this technique not only helps to understand the playing styles instrumentalists perform. It also could help instrument builders getting new ideas about instrument designs. Additionally it may also be used as an educational tool for singers or players improving their techniques or choosing an instrument while comparing the radiation pattern results with their hearing experience.