PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

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Arisa Kuramoto1; Yoshinori Takahashi1; Akisato Mizuno2
1

Tokyo Metropolitan College of Industrial Technology, Japan
2

Kogakuin University, Japan

ABSTRACTA Japanese traditional wind instrument, shakuhachi, is made of bamboo, and it has a different timbre for each
instrument. There are some invaluable shakuhachi crafted over 500 years ago and it is desirable for those tobe protected appropriately. The purpose of this study is to restore the shakuhachi from the shape information
recorded in advance just in case the instrument is lost due to unforeseen circumstances. Reproducing
shakuhachi by additive manufacturing (3D printing) has been attempted based on the X-ray images takenfrom two directions. Then, it has been confirmed that the shakuhachi were sufficiently restored with thefeeling of playing the original shakuhachi. However, since the irregularities on the surface of bamboo cannot
be reproduced with images taken from only two directions, the appearance of the shakuhachi was quite
different from the original one. In this paper, a 3D model was created by introducing the adjustment
processing to the contour of the original shakuhachi from the X-ray CT image, and then the reproduction wasattempted so that the shakuhachi has a shape close to the original one using 3D printer.
Keywords: Shakuhachi, Wind instrument, Additive manufacturing (3D printing), X-ray CT, Reproduction ofcultural properties

1. INTRODUCTION
Shakuhachi is a wind instrument made of the root of natural bamboo. It is a kind of air-lead
instrument that produces sound by the sound generation phenomenon called edge tone that occurs
when the airflow from player’s mouth hits the knife edge. The inner surface of the instrument issmoothly polished with a mixture of Japanese lacquer "urushi", polishing powder "tonoko", plasteretc. [1-2]. Due to the large individual differences of instruments, it is difficult to reproduce theshakuhachi as the completely same shape when it was damaged or lost by a disaster. In addition, there
are shakuhachi that are not allowed to test the sound because the value is too high as cultural asset.
On the other hand, in air-lead instruments that acoustic energy generated by air column resonanceis dominant, it is known that no change in timbre can be seen when the material is rigid to a certain
extent. [1]. In this study, the reproduction method of the instrument using an additive manufacturing(3D printing) is investigated based on the shape information of the instrument recorded before
damaged or lost due to unforeseen circumstances.
First, a 3D model of shakuhachi was created based on the X-ray photographs, and then theinstrument was reproduced by using a 3D printer. As a result, it was confirmed that the reproduced
shakuhachi was performable and partially kept the feeling of playing the original one [3]. Next, Xray computed tomography (X-ray CT) images of the shakuhachi were taken, and then the 3D modeland the reproduction of the instrument were conducted [4]. However, when extracting the shape witha constant threshold value for the luminance of the X-ray CT image, it is difficult to extract the contourof the shape accurately due to the large gap of the luminance between the bamboo part and the innersurface part coated by "tonoko". Because of the effect, it was confirmed that the inner diameter of the6497

reproduced shakuhachi became smaller than the original one. Therefore, an adjustment process of thecontour on the X-ray CT images was introduced for the 3D modeling, and the reproduction wasconducted to make the appearance and the inner diameter be as close as that of the original one. [5].

2. Reproduction based on X-ray photographs
In this paper, 3D model was created based on the X-ray photograph of a real shakuhachi, and it wasreproduced by using a 3D printer [3]. The original shakuhachi used in this work is an old instrument craftedby Shiro Yamaguchi (1885-1963). This shakuhachi is called “Shiro-kan” and well known as a valuable
instrument [6]. The shakuhachi used in this work was found around 1969 with a crack on the top of the pipe,
and it was consequently repaired and has been used.
Figure 1 shows X-ray photographs of the original shakuhachi taken from the front direction and the sidedirection. Since shakuhachi generally has an intermediate junction, the shakuhachi was photographed bybeing divided at the junction into the upper and the lower part. The upper two images in Fig. 1 were takenfrom the side direction. The lower two images were taken from the front direction. The inner diameters of
the tube were measured every 30 pixels from the mouthpiece to the end of tube on the X-ray photograph, andthe shape of the shakuhachi was estimated by elliptic interpolation. As the inner diameter of the shakuhachi
was adjusted by "tonoko" etc., it was necessary to measure the adjusted tube not the substrate bamboo whenmeasuring the inner diameter from the X-ray photograph. In this paper, the boundaries of the instrument werevisually observed from the diameters on the photographs to build the 3D model. In addition, the magnificationof the image caused by the positional relationship between the X-ray source and the subject was compensated.
The original shakuhachi and the reproduced shakuhachi by the 3D printer are shown in Figure 2(a) and(b), respectively. It is possible to produce sound by the reproduced shakuhachi, and the owner of the originalshakuhachi stated in the introspection report that the feeling of playing is similar to that by the original
instrument. However, it is impossible to reproduce the details from the X-ray photographs taken from only
two directions.

Side view of lower pipes

Side view of upper pipes

Front view of lower pipes

Front view of upper pipes

Figure 1 - X-ray photographs of shakuhachi

㻌㻌6498(a) Original shakuhachi(b) Reproduced shakuhachi based on X-ray photographs

Figure 2 - Comparison between the original shakuhachi and the reproduced shakuhachi based on X-ray
photograph

3. Reproduction Based on X-ray CT
The polygon data (STL file) are required to reproduce the shakuhachi using a 3D printer. STL fileis a format of polygon data which represents the surface of the object by means of many triangles,
and generally used in most 3D printers. Figure 3 shows a part of the polygon in a STL file, and thatthe surface is composed of many triangles. In the reproduction of shakuhachi shown in the previouschapter, the loft function in CAD software (whose function is to connect multiple contours smoothly)
was sufficient to make 3D data of the shakuhachi since few values are obtained to represent the shapeof the instrument. However, this method is insufficient to analyze more than two thousand seats of Xray CT images. This chapter describes a method to create STL file from X-ray CT images and shows
the reproduced result.

Figure 3 - Triangular polygon in a part of 3D model
3.1 3D Modeling and Making Polygon
Figure 4 shows one of 2048 seats of shakuhachi tomographic images taken every 150 μm by X-ray
CT. These photographs are loaded in MATLAB to create 3D model. First, eight voxels are grouped
into a cube for one analysis unit as shown in Figure 5. If the luminance value of each voxel is higher
than a certain threshold level, the voxel is judged inside the solid. If the value is lower than thethreshold level, the voxel is judged outside of the solid (red circle means inside, blue circle means
outside in Fig. 5). Next, when the labels of the apex are different one another, the middle point among
the apexes of the cube is marked as shown in Figure 6 (dark blue circle in Fig. 6). If all marks in eachanalysis unit are finished, a centroid (yellow circle in Fig. 6) is calculated. After that, the boundary
surface is expressed by multi-triangle polygons using the centroid and the marked points.
However, some errors were observed in the represented 3D model. This seems to be a defect inpolygon generating algorithm. Therefore, the polygon errors are repaired by the use of an application
called Meshmixer, which can process polygons. Figure 7 shows the polygon errors, which needs to berepaired, indicated by Meshmixer. In Fig. 7, the blue marks indicate a hole of the mesh, the red marks
indicate a non-manifold area and the magenta marks indicate a subregion (too small area). In order tocreate STL data that allows a 3D printer to read, the automatic repair by the application and the manual
repair with the brush tool were repeated.

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lower pipes

upper pipes


Figure 4 – X-ray CT images

Voxel center

Voxel for whether it
is judged solid or not

Figure 5 - Analysis unit (eight voxels) with voxel information in the center of voxel.

Center of gravity

Figure 6 - Polygon creation from cube with voxel’s inside and outside information at the vertex.

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Figure 7 - Locations of triangular polygons displayed by Meshmixer

3.2 Reproduction of shakuhachi by 3D printing
In this paper, a 3D printer “DaVinci Pro” produced by XYZprinting was used to output the 3Dmodels, which is a fused deposition modeling (FDM) type 3D printer. Since the maximum output sizeof the printer was limited to 200 mm × 200 mm × 200 mm according to the specifications of the printer,the shakuhachi was divided into four parts as shown in Figure 8. The printer was set to the lamination
pitch as 0.3 mm and the internal density as 60%. As a result, it took about two and a half days tooutput the 3D models. After that, the joints except for the intermediate joints were bonded with glue.

Upper side ofUpper side ofupper pipe

lower pipe

Lower side ofLower side oflower pipe

upper pipeFigure 8 – The output from 3D printer6501

3.3 Reproduction result and evaluation
The original shakuhachi and the reproduced shakuhachi are shown in Figure 9 (a) and (b), respectively.
On the outside surface of the reproduced shakuhachi, several granular protrusions were observed although
they were not on the 3D model. It is considered that the filament was dripping due to injecting the excessive
filament in narrow area when the 3D printer output the fine and complexed shape.
In this paper, the inner diameter at the upper end of mouthpiece, at the intermediate joint and at the endof tube were compared between the original one and the reproduced one in order to evaluate the reproduction
accuracy of shakuhachi. Table 1 shows the comparison results of inner diameters. Any errors over 1 mm were
not found between the 3D model and the reproduced shakuhachi. However, some errors over 1mm wereobserved at the upper end of mouthpiece and the intermediate joint between the 3D model and the original
shakuhachi. Therefore, it can be concluded that there is a problem in creating 3D data rather than the problem
in the reproduction accuracy of the 3D printer.

(a) Original shakuhachi(b) Reproduced shakuhachi based on X-ray CT images
Figure 9 - Comparison between the original shakuhachi and the reproduced shakuhachi

Table 1 - Accuracy of inner diameter in the reproduced shakuhachi

Upper end ofmouthpiece
Joint area
(upper pipe)
Joint area
(lower pipe)
End of tube

Original

Shakuhachi 3D Model

Shakuhachi output by 3D

shakuhachi/ Error from theprinter / Error from the[mm]original one [mm]original one [mm]22.35

21.05 / 1.44

20.91 / 1.30

23.56

22.46 / 0.83

22.73 / 1.10

18.12

17.97 / 0.94

17.18 / 0.1517.52

17.80 / -0.12

17.64 / -0.28

4. Inner diameter adjustment of 3D model
When creating a 3D model from CT images, the inner diameter was narrower than that of the original one.
The reason may be that the contour is not extracted correctly as the luminance value of the part coated with“tonoko” etc. is extremely high comparing with the part of bamboo. Therefore, in this paper, an adjustment
method is investigated for the inner diameter in the X-ray CT image. This chapter describes the adjustment
method of the inner diameter and its effect.

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4.1 Inner diameter adjustment method
In the adjustment of the inner diameter, the edges are detected from the brightness gradient after the Xray CT images are binarized. Although 3D models are generally obtained from a binarized image, it ispossible to make the solid area narrower by subtracting one pixel of the detected edge from the binarized
image. Based on the result in the previous chapter, this operation was repeated six times, and the adjustments
were performed to extend the inner diameter by six pixels.
4.2

Result and evaluation of inner diameter adjustment
Table 2 shows the reproduction accuracy for the inner diameter of the reproduced shakuhachi created by
the 3D printer. By the inner diameter adjustment method for the X-ray CT image, the inner diameter wasreproduced as precisely as that of the original shakuhachi.
In addition to adjusting the inner diameter, the weight was also considered by changing the output densityof the 3D printer. The results are shown in Table 3. Reducing the density, the weight became closer to that of
the original shakuhachi. As the density decreased, however, many faint gaps were recognized and the breathleaked from these faint gaps.
Table 2 - Accuracy after inner diameter adjustment
Original

Inner diameter without

Inner diameter after

shakuhachi

adjustment / Error fromadjustment / Error from[mm]the original one [mm]the original one [mm]22.35

20.91 / 1.44

21.17 / 1.18

23.56

22.73 / 1.83

23.54 / 0.02

18.12

16.18 / 1.94

18.10 / 0.02

17.52

17.64 / -0.12

17.53 / -0.01

Hole ofmouthpiece
Joint area
(upper pipe)
Joint area
(lower pipe)
End of tube

Table 3 - Changes in the output density of 3D printerReproduced, print density 90 %Reproduced, print density 60 %/ Error from original one/ Error from original one

168

200 / -32

152 / 16211

244 / -33

181 / 30

Original
Upper pipeweight [g]Lower pipeweight [g]5. CONCLUSIONSIn this paper, the preservation of the shape information for an ancient Japanese instrument “shakuhachi”
and the reproduction of the instrument from the preserved shape information using a 3D printer wereinvestigated for the purpose of protecting cultural asset. The reproduction method based on X-ray images
and X-ray CT images were carried out in this work. Although the reproduced shakuhachi based on the X-ray
photographs was playable, its outside shape was different from the original one. Since more details ofshakuhachi shape were observed by means of the X-ray CT images, a 3D model of shakuhachi was created6503

again. Due to the high luminance value of the X-ray CT image in the inner part of shakuhachi coated by“tonoko”, the inner diameter adjustment method was employed so that the reproduced shakuhachi has more
precise shape compared to the original one. Moreover, the weight of the reproduced shakuhachi was reduced
as much as that of the original one by changing the output density of the 3D printer.
This paper presented that there was a possibility to reproduce a shakuhachi by a 3D printer if the shapeinformation based on the X-ray CT images is preserved. The results strongly suggested that X-ray CT images
should be taken for any invaluable shakuhachi designated as an important cultural property before it isdamaged or lost due to unforeseen circumstances.

REFERENCES[1] Hayasaka, T., Science of musical instruments (Gakki no kagaku), CORONA PUBLISHING CO.,LTD, 1992, pp. 20--41, (in Japanese).
[2] Ando, Y., Acoustics of musical instruments (Gakki no onkyogaku), ONGAKU NO TOMO SHACORP., 1996, pp. 79--80, (in Japanese).
[3] Takahashi, Y.; Mizuno, A., Shakuhachi replication utilizing 3D printer for enhancement ofJapanese musical instrument guidance at junior high school, Spring meeting of the Acoustic
Society of Japan, 2018, pp. 1519--1520 (in Japanese).
[4] Kuramoto, A.; Takahashi, Y.; Mizuno, A., Reproduction of shakuhachi utilizing 3D printer forprotection of cultural heritage, Autumn meeting of the Acoustic Society of Japan, 2018, pp. 1271-1272 (in Japanese).
[5] Kuramoto, A.; Takahashi, Y.; Mizuno, A., Restoration of shakuhachi from X-ray CT images
utilizing additive manufacturing, Spring meeting of the Acoustic Society of Japan, 2019, pp. 1335-1336 (in Japanese).
[6] Mizuno, K., (pseudonym of Mizuno, A. for a shakuhachi master), Yamaguchi Goro: the teacher who
didn't teach, European Shakuhachi Society Newsletter, 2, 2014, pp. 13--15.

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