Because variable humidity affects the acoustic properties of wood, manufacturers of wood instruments must minimize dimensional changes caused by the absorption of water. Acetylation reduces the moisture content of the cell wall, thereby increasing the stability of the acoustic and dimensional properties of wood under conditions of changing humidity. The acetylation of wood slightly reduces sound velocity (by about 5%) and also reduces sound absorption when compared to unreacted wood. Hence, acetylation does not change the acoustic converting efficiency

Norimoto, Misato

Rowell, Roger M.

Yano, Hiroyuki

English

Wood and Fiber Science (E-Journal)

STABILIZATION OF ACOUSTICAL PROPERTIES OF WOODEN MUSICAL INSTRUMENTS BY ACETYLATION Hiroyuki Yano Lecturer Department of Forestry Kyoto Prefectural University Sakyo-ku, Kyoto 606, Japan Misato Norimoto Professor Wood Research Institute Kyoto University Uji, Kyoto 6 1 1, Japan and Roger M. Rowel1 Team Leader USDA Forest Service Forest Products Laboratory1 One Gifford Pinchot Drive Madison, WI 53705-2398 (Received August 1992) ABSTRACT Because variable humidity affects the acoustic properties of wood, manufacturers of wood instru- ments must minimize dimensional changes caused by the absorption of water. Acetylation reduces the moisture content of the cell wall, thereby increasing the stability of the acoustic and dimensional properties of wood under conditions of changing humidity. The acetylation of wood slightly reduces sound velocity (by about 5%) and also reduces sound absorption when compared to unreacted wood. Hence, acetylation does not change the acoustic converting efficiency. Keywords: Dimensional stabiliration, acoustical properties, acetylation, specific dynamic Young's modulus, internal friction. INTRODUCTION Wood is a hygroscopic material, so when the ambient humidity changes, the dimensions of wood products change too. This aff'ects the tone quality of wooden musical instruments be- cause as moisture content increases, the acous- I The Forest Products Laboratory is maintained in co- operation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright. tic properties of wood, such as the specific dy- namic Young's modulus (E'ly) and internal friction (Q--I), are reduced or dulled (James 1964; Sasaki et al. 1988). The absorption of water molecules between the wood cell-wall polymers acts as a plasticizer to loosen the cell- wall microstructure. The decrease in cohesive forces in the cell wall also enhances the defor- mation of wooden parts under stress. Stress may be due to forces in the strings of guitars and violins and the pin block in a piano. Varnish applied to the wooden parts helps Wood and Frhcr Scrcnce. 25(4), 1993, pp 395-403 396 WOOD AND FIBER SCIENCE, OCTOBER 1993, V. 25(4) TABLE 1 .  Description of test specimens. Test Species Specific gravity Specimen Dimensions (mm)' Free-free vibrationb Sitka spruce 0.42 Longitudinal strips 180 by 20 by 3 Radial strips 20 by 180 by 3 Clamped plate Sitka spruce 0.43 Quarter-sawn plate 150 by 150 by 3.5 Creep Maple 0.68 Longitudinal strips 120 by 14 by 1.8 Pin block Maple 0.68 Longitudinal strips 125 by 125 by 14 .' Longitudinal by radial by tangential. Tree-free vibrat~onal beam test. stabilize the dimensional and acoustic prop- erties of wooden musical instruments by de- laying the absorption of moisture. However, excessive varnishing causes suppression of the sound level radiated from the top plate of the violin (Schelleng 1968). Another method, chemical modification of wood, can effectively reduce the hygroscopicity of wood (Rowel1 1983). It has been known for almost 50 years that acetylation of wood reduces hygroscopic- ity and increases dimensional stability (Stamm and Tarkow 1947). Acetylation, when applied to the wooden di- aphragm of speakers, was shown to stabilize tone quality under conditions of changing hu- midity (Ono and Norimoto 1983). Acetylation also affects the vibrational properties of wood (Yano et al. 1986; Akitsu et al. in press). Dynamic vibrational analysis can help de- termine how the acoustic properties of wood change when ambient humidity changes (Ono and Norimoto 1983, 1984). Dynamic Young's modulus (E') divided by specific gravity (y) measures the square of sound velocity, while Q ' measures sound absorption or damping within the wood. FIG. 1 .  Schematic diagram of simplified model of string- sustaining part used in the experiment. (1) wood screw, (2) tuning pin, (3) pin block, (4) iron plate, (5) string. The purpose of this research was to deter- mine the effects of acetylation on: (1) changes in E'ly and Q - ' ;  (2) stabilization of acoustic properties of wood using a free-free vibrational beam test and an all-sides-clamped plate test; and (3) reduction of drops in tuned frequency caused by deformation of wooden parts under stress, using a full-scale model of the string- sustaining part of a piano pin block. EXPERIMENTAL METHODS Test specimens The specimens for the free-free vibrational beam test were cut from a Sitka spruce (Picea sitchensis Carr.) wood block that had been se- lected for piano sound boards. These speci- mens and the specimens for the clamped plate, creep, and pin block tests are described in Ta- ble 1. The quarter-sawn Sitka spruce plates for the clamped plate test were prepared from a plate selected for a guitar sound board. Three specimens for the pin block test were cross- laminated with the longitudinal direction of the face specimens parallel to the strings, using a resorcinol resin to construct the pin block as shown in Fig. 1 (Yano et al. 1988). Acetylation The acetylation of the wood specimens (Rowel1 et al. 1986) involved chemical reac- tions in 120 C neat acetic anhydride for time spans ranging from 30 min to 4 h. Specimens were dried at 70 C in a vacuum for 10 days. Weight percentage gains (WPG) were deter- mined from measurements of each specimen's dry weight before and after acetylation. Mea- surements for the longitudinal and radial strips ranged from 0.7 to 24.9, with 22.0 for the plates, Yano et al. -STABILIZATION OF ACOUSTICAL PROPERTIES BY ACETYLATION 19.1 for the creep test specimens, and 18.7 for the pin blocks. Acoustic properties Flexural vibration with a free-free end con- dition [20 C, 65% relative humidity (RH)] was used to determine the specific dynamic Young's modulus and the internal friction of strips (Ono and Norimoto 1983, 1984). The acoustic prop- erties at the first mode of vibration were mea- sured before and after acetylation. E' ly  and Q-'  were measured from longitudinal speci- mens. To vary the resonance frequency of the specimens, we gradually cut their lengths (1 8 to 6 cm) and increased their vibrational mode (from the 1 st to the 4th). Resonant frequencies ranged from 0.5 to 7 kHz. Ten replicate spec- imens were used and the results averaged. The free-free flexural vibration technique used also determined how the acoustic prop- erties of the strips changed when placed in a methyl methacrylate box with variable RH. For the equilibrium moisture content (EMC) experiment, sulfuric acid aqueous solutions of various concentrations controlled the RH level from 30% to 85%. For the nonequilibrium moisture content experiment, the dried spec- imens were placed in a vessel at 20 C, 95% RH, and acoustic properties were measured when the moisture content reached equilibri- um. Finally, the specimens were moved back to the dry box containing silica gel, and acous- tic properties were measured again until the moisture content reached equilibrium. Five replicate specimens were used and the results averaged. Acetylated and unreacted plates, condi- tioned at 20 C, 65% RH for 10 days, were attached to a plexiglass frame using a cyano acrylate adhesive with a 5-mm wood frame overlap. The transverse sides of the frame were attached to a speaker having an 80-mm-di- ameter flat aluminum diaphragm. This assem- bly was placed in a glass vessel conditioned at 20 C, 95% RH, for 36 h. Before and after ad- sorption of moisture, the plate was excited by the speaker. A microphone placed 150 mm away from the center of the plate detected t 0 :longitudinal direction :radial direction Weight gain ( % I  FIG. 2. Ratio of acoustic properties (20 <', 65% RH) after acetylation to before acetylation in longitudinal and radial directions versus acetyl weight percent gain. sound radiation from the plate. A spectrum analyzer recorded the sound spectrum while the speaker received a relative amplitude of sound pressure level to white noise level. The Chladni's figure of the 1st mode of acetylated and unreacted wooden plates was photo- graphed simultaneously by the use of fine si- licic oxide powder. Triplicate samples were used and the results averaged. Creep test Ten specimens of each type were subjected to a bending moment at a distance of 60 mm between load points and a span of 100 mm. A linear variable differential transducer mea- sured the deflection at the midpoint between supports. The load was 18.35 N, which cor- WOOD AND FIBER SCIENCE, OCTOBER 1993, V. 25(4) 2 2  - - d - a - 0  0 0 0  0  0  0 0 %  0 O O O  0 :unreacted 20 - :acetylated, 24.9 WPG I 0 - 4 0 1 8 -  \ - d w w 00 8.8 8- 1 6 -  Frequency (kHz) Frequency (kHz) FIG. 3. Effect of acetylation on frequency dependence of E'ly and Q ' (20 C, 65% RH). responds to stress levels of 16% and 14% of the bending strength (at 20 C, 65% RH) for unreacted and acetylated strips, respectively. Resonance frequency of string-sustaining model To measure the resonance frequencies of the strings, we used an acoustical pickup attached to the frame to detect their plucked vibration, feeding the signal (Fig. 1) into the spectrum analyzer. The resonance frequency was read to 0.32 Hz accuracy. Triplicate measurements were taken and the results averaged. RESULTS AND DISCUSSION Figure 2 shows the effect of treatment on E' ly  and Q-' at 20 C ,  65% RH, resulting from acetylation. The ratio of (E1/y),/(E'/y), re- mained constant up to 8 WPG. As the degree of acetylation increased, the ratio decreased up to 15% and 10% in the longitudinal and radial directions, respectively. This was probably be- cause very little cell-wall bulking took place up to 10 WPG; above 10 WPG, bulking be- came a significant factor. The ratio of Q-',/ Q- '. decreased up to 15% in both longitudinal and radial directions as the degree of acety- P: 0.8 longitudinal direction ': " t 0.0; A.12.0 ;  0 , 2 4 . 5  WPG': redial direction b 0 . 0 ;  A,12.0; H,l8.9 WPG Relative humidity (%) Relative humidity (%)  FIG. 4. Ratio of acoustic properties (20 C) at various humidities to those at 30% RH. Yano et a1.-STABILIZATION OF ACOUSTICAL PROPERTIES BY ACETYLATION 399 01 AEMC = EMC at 85% RH - EMC at 30% RH 0 0 4 8 12 16 20 24 Weight gain  (%) FIG. 5.  Changes in equilibrium moisture content (EMC) at 20 C with acetylation. lation increased. So, as weight gain resulting from acetylation increased, both sound veloc- ity and sound absorption decreased. Figure 3 shows the frequency dependence of E'ly and Q-' for acetylated and unreacted 20 WPG Adsorption 10 20 3 0 strips. The value of E'ly decreased slightly while Q-I increased greatly as the frequency in- creased. However, no differences in the fre- quency dependence were observed between acetylated and unreacted strips. The acoustic converting efficiency, which is related to the ratio of acoustic energy radiated from the mu- sical instrument to the energy given by the string, is proportional to (Ta- naka et al. 1987; Yano and Matsuhisa 199 1). The increase of air-dry specific gravity caused by acetylation of 23.9 WPG was 9.2%. This suggests that acetylation has a minor effect on the acoustic converting efficiency of wood for musical instruments in the audible frequency range. Figure 4 shows the ratio of E' ly  and Q-' with elevation of RH above 30% compared to the value at 30°/o RH. The ratio of E' ly  de- creased while Q-I increased with increasing RH for both control and acetylated specimens. However, the degree of the distortion of the acetylated specimens was about one-third that of the unreacted specimens in both longitu- @ ( % I  @ ( % I  FIG. 6 .  Relationship between acoustical properties in longitudinal direction and average moisture content for unreacted and acetylated specimens. 400 W O O D  AND FlBER SCIENCE, OCTOBER 1993, V. 25(4) Unreacted Acetylated I Radial FIG. 7. Chladni's figures of all-sides-clamped plates at 20 C, 65% R H  (solid line) and 20 C, 95% R H  (dashed line). dinal and radial directions. Changes in hu- midity had a greater effect on radial properties than longitudinal, and acetylation stabilized these changes. Figure 5 shows the effect of acet- ylation in reducing EMC changes between 30% and 85% RH with a WPG of 124:  EMC changes were reduced by > 50%. The displace- ment of the hydrophilic hydroxyl group with a more hydrophobic bulky acetyl group de- creased the amount of moisture being ad- sorbed and/or desorbed under the same hu- midity changes. This reduced changes in acoustic properties. The effect of acetylation on acoustical prop- erties was more pronounced in the nonequi- librium moisture condition. Figure 6 shows the changes in acoustic properties against av- erage moisture content. Although acoustic properties changed more in the nonequilibri- um moisture condition than in the equilibrium moisture condition, acetylation greatly sup- pressed these changes, especially in the process of adsorption, even though changes in average moisture content were constant. Reducing cell-wall moisture increased the cohesive forces among wood constituents, causing them to resist dimensional changes as the humidity increases. Stabilizing both the physical dimensions and the internal cell-wall polymer conformation in wood used for mu- sical instruments greatly affected their acoustic properties. As the RH increased from 30% to 85% at 20 C, the unreacted specimens swelled Frequency (kHz) FIG. 8. Sound spectra of all-sides-clamped plates at 20 C ,  65% RH (solid line) and 20 C, 95% R H  (dashed line) for (a) unreacted and (b) acetylated specimens. 1.28% in the radial direction, whereas the acet- ylated specimens swelled only 0.59% at a WPG of 18.5. When an instrument containing a wooden plate clamped at all sides is exposed to humidity changes, the mechanical restraint of the dimensional changes causes internal forces that can deform and actually destroy the instrument. Internal forces due to changing from 65% RH to 95% RH caused changes in the Chlad- ni's figure as shown in Fig. 7. The RH change only minimally changed the Chladni's figure for the acetylated plate. Acetylation also sta- bilized the sound spectra for the all-sides- clamped wood (Fig. 8). Although mutual in- terference between vibration modes in both plates complicated the sound spectra over 2 kHz, the change in the resonance frequency of the first peak from 65% to 95% RH was 14.5% Yano et al. -STABILIZATION OF ACOUSTICAL PROPERTIES BY ACETYLATION 0 .rl Unreacted c, 0 2 -  8 4 w 8 - n Acetylated Time (hr) FIG. 9. Effect of acetylation on creep deflection compared to humidity cycling. in the unreacted plate, while it was 6.2% in acetylated plate. Acetylation reduced the changes in resonance frequency to the same degree as in the free-free beam test, in spite of the restraints. The reduction in the peak level in the lower frequency range was smaller for the acetylated plate than for the unreacted plate as the RH increased. It is well known that the adsorption and/or desorption of moisture enhances the defor- mation of wood under stress (mechano-sorp- tive behavior). Although the humidity change magnified the creep deflections (Fig. 9), acet- ylation greatly reduced them. This can be at- tributed to the decrease in hygroscopicity of the cell wall as well as to the reduction in the plasticizing effect of water on the cell-wall polymers. To stabilize piano tones under humidity changes, a pin block must have dimensional stability. Since the string is under great tension, any movement of the tuning pin causes a re- duction in the pitch of the vibrating string (Fig. 10). For this purpose, we investigated changes of the string's resonant frequency on a full- scale model of the string-sustaining part at the 72 key (G sharp) of the piano using acetylated and control cross-laminated wood pin blocks, FIG. 10. Cross section of the string-sustaining part of a piano. (1) pin block, (2) iron plate, (3) tuning pin, (4) pressure bar, (5) wood brace, (6) string, (7) sound board, (8) bridge. WOOD AND FIBER SCIENCE, OCTOBER 1993, V. 25(4) Time (hr) n - 4  - 8 - V -uA* A A A  - 8  - 7 - AA A A A A  A ,h A, u r  cw '4, 'G Y - 12 - 4. -16 - - A f  = ((fT-fo)/fo) x l o o ( % )  FIG. 1 1. Changes in resonant frequency and moisture content of acetylated and unreacted pin blocks, where f, and f, represent average resonant frequencies of 12 strings at start of measurement and that after time T, respectively. -20 n 8 V +' 20 d a, +' 16-  F: 0 L) 12 with humidity cycling. The decrement of the resonance frequency for the acetylated model CONCLUSIONS was much less than that for the control blocks The acetylation ofwood slightly reduced both as the moisture content increased (Fig. 1 I). It sound velocity (due to the decrease in E1/y) is expected that a laminated acetylated pin and Q-' (sound absorption) when compared block could be extremely effective in reducing to unreacted wood. Acetylation greatly re- the changes of resonance frequency in a piano. duced variability in the moisture content of .. .. me L I - :unreacted - A : acetylated - - =.,. -*.. 0- a w a, 8 -.-."t k - 8 -  A - - I' .rl * *UAUA -U-..YA+ A A* 1 1  n 8 0 V A A 4. AA A A A ~  1 1  1 1  1 1  h 90 c, a, '* .5,2 4 5 ,  - - +' ~ J J  a o ~ I I ~ I I I I I I I I I ~ ~ " I ~ ~ I I I I I  ,-I A 2 0 500 1000 1500 2000 1 1 1 1 1 1 1 1  1 1  1 1  1 1  1 1  Yano et 01.-STABILIZATION OF ACOUSTICAL PROPERTIES BY ACETYLATION 403 the cell-wall polymer, thereby stabilizing the physical dimensions of wood and its acoustic properties. These tests did not determine whether acetylation enhanced sound quality. A violin and guitar in Japan and a violin in Sweden have been made from acetylated spruce, and actual changes in sound quality are presently being investigated. ACKNOWLEDGMENT The authors wish to thank MI-. Takayuki Sasaki for assisting with the experimental por- tion of this investigation. REFERENCES AKITSU, H., M. NORIMOTO, T. MOROOKA. AND R. M. ROWELL. 1993. Effects of humidity on vibrational properties of chemically modified wood. Wood Fiber Sci. 25(3):250-260. JAMES, W. L. 1964. Vibration, static strength, and elastic properties of clear Douglas fir at various levels of mois- ture content. Forest Prod. J. 14(9):409-4 13. ONO, T., AND M. NORIMOTO. 1983. Study on Young's modulus and internal friction of wood in relation to the evaluation of wood for musical instruments. Japan. J. Appl. Physics 22(4):6 1 1-6 14. -, AND - . 1984. On physical criteria for the selection of wood for soundboards of musical instru- ments. Rheol. Acta 23:652-656. -, Y. KATOH, AND M. NORIMOTO. 1988. Humidity- proof, quasi-isotropic wood diaphragm for loud speak- ers. J. Acoust. Soc. Japan 9(1):25-33. ROWELL, R. M. 1983. Chemical modification of wood: A review, vol. 6(12). Commonwealth Forestry Bureau, Oxford, England. Pp. 363-382. , A. M. TILLMAN, AND R. SIMONSON. 1986. A simplified procedure for the acetylation of hardwood and softwood flake for flakeboard production. J. Wood Chem. Tech. 6(3):427448. SASAKI, T., M. NORIMOTO, T. YAMADA, AND R. M. ROWELL. 1988. Effect of moisture on the acoustical properties of wood. J. Japan Wood Res. Soc. 34(10):794-803. SCHELLENG, J. C. 1968. Acoustical effects of violin var- nish. J. Acoust. Soc. Am. 44:1175-1183. STAMM, A. J., AND H. TARKOW. 1947. Acetylation of wood and boards. U.S. Patent 2417995. TANAKA, C., T. NAKAO, AND A. TAKAHASHI. 1987. Acoustic property of wood. J. Japan Wood Res. Soc. 33(10):811-817. YANO, H., AND H. MATSUHISA. 199 1. Study on the tim- bre of wood, 11. Analysis of the sound spectrum of wood using viscoelastic Timoshenko equation. The Scientific Reports of the Kyoto Prefectural Univ. No. 43:24-3 1. -, J. MUKUDAI, AND M. NORIMOTO. 1988. Im- provements in the piano pin-block. J. Japan. Wood Res. SOC. 34(2):94-99. -, M. NORIMOTO, AND T. YAMADA. 1986. Changes in acoustical properties of Sitka spruce due to acetyla- tion. J. Japan. Wood Res. Soc. 32(12):990-995. 

Stabilization of Acoustical Properties of Wooden Musical Instruments By Acetylation

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Stabilization of Acoustical Properties of Wooden Musical Instruments By Acetylation

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