Improving acoustic properties of walnut wood (Juglans regia) by artificial, speeded up mechanical vibrations

Background and objectives: Wood has been used for making musical instruments for a long time around the world. Manufacturers of these instruments are always trying to improve the quality of wood to make better sound by using special treatments. Santour (Iranian traditional music instrument) is usually made from walnut wood and the quality of its sound is highly dependent on the quality of wood and fabrication methods. Besides choosing the proper wood and manufacturing technics, all performers agree that if an instrument is kept in proper situation, by playing it, the sound quality will become better over the time. As the wood in instruments is under mechanical vibrations during playing, this hypothesis has been investigated that if the wood is imposed to artificial vibrations similar to real music playing, before being used to make the instrument, what changes will occur in its sound characteristics.
Materials and methods: To do this, 10 specimens of walnut wood Juglans regia (5 tangential and 5 radial) with dimensions of 200×40×8 mm3 has been prepared. In order to be sure about the similarities of specimens, the acoustic properties of them has been measured. Then all the woods were imposed to light vibrations for 1 month and then heavier vibration for next 2 months. Two vibrating machines were designed and built which produced light vibrations by magnetic force and heavy vibrating by eccentric mass. The acoustic and mechanical properties of wood like Young's modulus (E), sound speed, sound radiation coefficient, Tanδ, were measured before and after vibrating using nondestructive Bing method. Then the woods remained without vibration for another month to find out the durability of changes and the acoustical properties of them were measured again.
Results: The results of this research show that these vibrations have improved the acoustic properties of wood. The light vibration had almost no effect on the specimens but the heavier vibration after one month improved the sound quality of wood up to 3% including Young's modulus, sound speed, sound radiation coefficient, and Tanδ. But in the second month of imposing heavy vibrations no significant change was observed. However statistical analysis shows that this changes are not meaningful. One month after finishing the vibrations, Tanδ returned to the previous amount but improvement in E and other acoustic properties looked permanent.
Conclusion: The results were not statistically significant and the 3% improvement was very low but meanwhile it can be concluded that if artificial vibrating is imposed for a long time there is a possibility to improve acoustical propertied of wood.