ERK'2022, Portorož, 313-316 313
Tonewood Treatment Ideas for Musical Instruments 
Soundboards 
Mehran Roohnia
1
, Franz Policardi
2
 
1
Karaj Branch, Islamic Azad University, Moazen BLVD. Karaj - Iran 
2
 University of Ljubljana, Ljubljana - Slovenia 
E-mail: mehran.roohnia@iau.ac.ir 
 
 
 
Abstract. A suitable piece of wood for a 
musical instrument soundboard must show the 
proper and within the acceptable ranges of 
acoustic radiation and damping capacity to 
form its acoustic conversion efficiency and 
must have a mechanical impedance match with 
those of the vibrating string. It has shown 
mathematically that each acoustic parameter 
is somehow related to elastic stiffness and 
density.  
 So, manipulating the density separately 
but beside the elastic stiffness, through a 
proper chemical or non-chemical treatment 
procedure would lead to either a positive or 
negative change into any of these parameters.  
           An idea for tonewood treatment was 
proposed to tune the density versus the elastic 
stiffness, simultaneously, to reach to a suitable 
set of the acoustic radiation, damping capacity 
and mechanical impedance.  
 
1 Introduction 
Wood is a critical component for string 
musical instruments. For such a chordophone, 
the sound produced by the vibrating string, 
attacks the soundboard and regarding to 
frequency and mechanical impedance match 
between the wood and the string, the 
soundboard begins the resonance.  
Human ears hear the musical sound through 
the soundboard acoustic radiation and its 
damping or sustain capacity.  
There are some sets of the acoustic qualities of 
wood that must stand within the acceptable 
ranges, introduced earlier by Wegst (2006) and 
Roohnia (2019) [1, 2]. But if a piece of wood 
didn't match these criteria, a scheduling 
treatment would become necessary to tune the 
tonewood acoustic parameters, accurately as 
they are already precisely defined. 
First we start to define the acoustic properties, 
and then the treatments ideas are shared.  
  
2 Acoustic properties  
Definitions: 
A suitable piece of wood for a musical 
instrument soundboard must show the proper 
and within the acceptable ranges of acoustic 
radiation and damping capacity to form its 
acoustic conversion efficiency (Wegst 2006; 
Roohnia 2019) [1, 2]. 
 tan
K
ACE  
In which, K is acoustic radiation and tan  is 
the damping capacity.  
Acoustic radiation depends on Elastic Stiffness 
and apparent density of wood.  
3

E
K  
E corresponds to the dynamic Elastic stiffness 
of the body, evaluated through one of the 
dynamic vibration methodologies and  is the 
apparent density of the specimen at servicing 
condition moisture content calculated directly 
from mass and the dimensions of the 
specimen.  
Meanwhile, Sound wave resistance or the 
mechanical impedance of wood (z) in 
(N.s/m^3) is calculated through the elastic 
stiffness and density, too. 
 
 . E z  
 
Low impedance facilitates resonance. 
Damping capacity is an indicator for internal 
friction of the material that causes the 
dissipation of vibration while guaranteeing 

314
 
enough sustains for each playing note. There 
are several methodologies to evaluate damping 
capacity of wood but a direct measurement of 
the sound wave attenuation is the most 
common one. Considering n times of full 
oscillations, damping capacity is calculated 
from the logarithmic decrement, as: 
 
n i
i
X
X
n

 ln
1
tan

 
 
 
 
In 1983, Ono and Norimoto [3] showed that 
damping capacity is also affected by and 
depends on the elastic stiffness and density. 
This truth was revised again later by Bremaud 
et al. (2012) [4].  
 
68 . 0
23 . 1
10 tan










 


E
 
 
Ono and Norimoto's proposed regression 
model is not as strong as the logarithmic 
decrement model, but is just enough to show 
that the vibration and acoustic behavior of a 
piece of wood, even its damping capacity is 
always under the effect of the ratio of elastic 
stiffness divided by apparent density. So, 
adjusting the vibration and acoustic properties 
of a piece of wood, indeed, means the tuning 
of its elastic stiffness beside its apparent 
density, through the possible chemical or 
physical treatments.   
 
Acceptable ranges: 
Wegst (2006), characterized the acoustic 
parameters of wood in different components of 
the various musical instruments in their 
experimental acceptable ranges. Table 1 
summarizes these minimum and maximum 
borders in soundboards of the string 
instruments. 
 
Table 1. Minimum and maximum acceptable values of each 
acoustic parameter for wooden soundboards 
Sound board min max 
 (Kg/m^3)
E (GPa)
tan 
K (m^4/Kg.s)
z (kN.s/m^3)
ACE
300 
6 
0.003 
9 
1342 
1000 
550 
20 
0.009 
16 
3300 
5000 
 
3 Ideas for acoustic manipulating of wood  
Density 
First of all it is necessary to predict that, what 
would happen to the introduced acoustical 
parameters, as the density of wood decreases. 
Decreasing the density while keeping the 
elastic stiffness unchanged, will increase the 
acoustic radiation, decrease damping capacity, 
increase the acoustic efficiency and decrease 
the mechanical impedance.  
In most of the engineering materials but the 
wood it is not possible to decrease the density 
without keeping the stiffness constant. Solvent 
extraction using distilled water or an ethanol 
acetone solution will decrease the wood mass 
without doing any important destruction to the 
cell walls and the elastic stiffness will be 
nominally remained unchanged.  
This idea has already tested in several 
researches using several wood species. Some 
of them were successful to decrease damping 
capacities while increasing the acoustic 
radiation coefficient. Padauk, Maple and 
Spruce are taking into account in this group 
(Roohnia et al. 2014; Miao et al. 2017; Traore 
et al. 2010) [5, 6, 7]. In Some species the 
extractive content found strangely useful for 
acoustic vibration and decreasing the 
extractives content increased the damping 
Figure 1. Logarithmic decrement of vibration to represent 
damping capacity 

315
capacities and decreased the radiation 
coefficients.   For an example, Pink silk wood 
and Pernambuco are the members from this 
group of the species (Farvardin 2015; 
Matsunaga 1999; Alves 2008) [8, 9, 10]. 
Previous experiences showed that the 
extractives from Pernambuco were also useful 
to improve the acoustic quality of spruce, if 
impregnated with.  
 
Elastic stiffness 
Manipulating the stiffness is not easily 
possible without doing the changes on density, 
but it is possible to increase the apparent 
stiffness, virtually. One of the oldest 
traditional treatments was bowing the 
structural material to produce arches and etc. 
But for wood, an orthotropic material, it has 
some important aspects that are not concerned 
in other isotropic ones. Wood has directional 
properties i.e. bowing by grinding would be 
highly different from bowing by elastic 
bending in longitudinal direction. While the 
elastic bending is done, the growth direction 
would not be destroyed (Figure 2).  
 
 
 
 
So, the grinding would decrease the 
longitudinal elastic stiffness. 
Though, this idea has not been published yet, 
but has been tested successfully by two Iranian 
skillful luthiers, i.e. Mr. Saeed Peymani and 
Mr. Naeem Khatooni in Persian Setar 
instrument.  
Increasing the longitudinal stiffness of wood, 
even apparently, by elastic bowing the 
soundboard would increase the radiation 
coefficient dramatically. But it is important to 
note that this manipulation would increase the 
mechanical impedance as well. So, to keep the 
resonator soundboard, easy to vibrate, it is 
recommended to combine the two proposed 
ideas for increasing the stiffness and 
decreasing the density, together. We 
recommend doing these both together, unless 
making changes to the impedance is also of 
interest.  
Noteworthy is that the elastic bowing of the 
soundboard is traditionally done by bridge's 
tensile or compression force in string 
instruments and also the Sound-post in the 
violin family instruments do an elastic bowing 
on the top plate, too. This additional elastic 
bending approach must be carefully tuned in 
terms of the radiation coefficients and the 
mechanical impedances of the wooden 
soundboards, unless the instrument would lose 
some notes in its frequency range. 
 
4 Further comments 
There are lots of cases that a selected wood 
specimen falls naturally within the acceptable 
ranges, same as table 1. So, such a wood 
would not need any treatment at all. If there 
would be any of the out of range acoustic 
parameters, it is recommended to be tuned in 
terms of the density and the elastic stiffness. 
One must be concerned about the simultaneous 
unwanted changes in some other parameters 
beside the scheduled program.  
For example, in a hypothetical specimen, 
programming for increasing the acoustic 
radiation might be done by increasing elastic 
stiffness or by decreasing density of the wood. 
It must be noted that while increasing the 
elastic stiffness, the mechanical impedance 
will increase as well and causes an additional 
unwanted resistance against the resonance. On 
the other side, decreasing the density to hit the 
higher acoustic radiation would decrease the 
mechanical impedance, too. It could be good 
for facilitating the resonance but some times 
the impedance matching might be disturbed. 
So, a sufficiently good idea of tonewood 
treatment deals with an accurate tuning of 
elastic stiffness and density to keep all the 
Wood in longitudinal direction 
Elastic bowing 
Bowing by grinding 
Figure 2. Elastic bending vs grinding the wood to reproduce 
bowing  

316
 
acoustic parameters still remained within the 
finest acceptable ranges. 
 
References 
[1]   U. Wegst: Wood for sound, American Journal of Botany, 
93(10): 1439–1448. 2006. 
[2]   M. Roohnia: Wood: Vibration and Acoustic Properties, 
Book Chapter #01996 in: Reference module in materials 
science and materials engineering, ELSEVIER, 2019. 
https://doi.org/10.1016/B978-0-12-803581-8.01996-2  
[3]   T. Ono and M. Norimoto: Study on Young's Modulus and 
Internal Friction of Wood in Relation to the Evaluation of 
Wood for Musical Instruments, Japanies Journal of 
Applies  Physics, 22(4): 611–614, 1983. 
[4]   I. Bremaud: Acoustical properties of wood in string 
instruments soundboards and tuned idiophones: 
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