Course:PHYS341/Archive/2016wTerm2/Erhu Acoustics

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Erhu Acoustics

The erhu is a bowed instrument with two strings that originated in China. It is the most common instrument belonging to the huqin family[1], with a playing range from pitches D4 to A7. In contrast to its Western counterpart, the violin, its acoustics have received little attention.


In the traditional Chinese organology system, the erhu is classified as a silk instrument[1]; in the Western Hornbostel system, the erhu is classified as a spike lute, namely a ‘spike fiddle’[2]. The er (二) character of the erhu’s name means ‘two’ and is thought to refer to the two strings of the instrument; the hu (胡) character refers to the Hu people, or the ‘Barbarians of the North and West’, suggesting it originated from nomadic tribes[1]. It is believed to have been derived from an older name, huqin (胡琴), from as far back as A.D. 618-907 (the Tang Dynasty). Found in the Song Dynasty (A.D. 960-1279), a possible ancestor of the erhu was the xiqin, an instrument of the Xi tribe also made of two strings with a ‘bamboo slip’ pressed in between the strings that could make plucked and continuous sounds[1]. The bow with horse-hair was not mentioned in historical Chinese documents until the description of the mawei huqin (马尾胡琴) or ‘horse-tail huqin’ by Shen Kuo in his poem, Song of Triumph, written in the 11th century[1]. Historian Jonathan Stock suggests that the “conceptual ancestor of today’s erhu, was imported intact into China, by, at the latest, the late eleventh century”[1].

Physical Structure of an Erhu

Fig. 1. Erhu with labeled parts.

The standard erhu has a unique structure and consists of two strings wound around tuning pegs, connected to a wooden neck that pierces the wooden soundbox vertically. The front of the soundbox is covered with python skin; the rear of the soundbox is open and usually decorated with wooden lattice-work[1]. The strings pass over the python skin onto a bridge, and are then fixed on the underside of the soundbox (usually under a wooden rest or base). To keep the loose bridge in place, a loop of string or qian jin (千斤) acts like a nut and pulls the strings toward the python skin, slightly increasing the string tension. Additional features may include decorative carvings on the top of the neck and fine-tuning devices on the strings. A small piece of soft material may also sit below the bridge, between the strings and the python skin, with the purpose of damping unwanted vibrations.

The erhu consists of many important parts that contribute to its characteristic sound or timbre. These include its two steel strings, bamboo bridge, soundbox, and use of python skin. The bamboo bow is also connected to the instrument with the horsehair lying in between the two strings. Fig. 1 is a labelled picture of the many different parts of the erhu with its bow detached for clarity. In contrast to the violin, another unusual feature of the erhu is its lack of a fingerboard. In order to produce different notes, players must learn the appropriate amount of pressure required of the fingers to press onto the strings.

Geometric Measurements

Without the rest or base below the soundbox, a standard erhu is around 80 cm in length. The faces of the soundbox are about 10cm in diameter and span 13 cm from its python-skin front to open rear. The horsehair of the bow lies between the two strings – a distance of 0.5 cm. The distance between the strings and the neck are determined by the tightness and height of the qian jin. The erhu in Fig. 1 has a distance of 2 cm between the strings and the neck at the qian jin.

String and Bridge Properties

The two erhu strings are generally tuned to D4 and A4; the lower pitch string is closer to the player’s body when held in playing position and thus called the inner string or nei xian (内弦). Erhu strings were traditionally made of silk, and were completely replaced using steel by 1958[1]. The length of the strings is fixed by the “qian jin”, analogous to the nut of the violin. Vibration of these strings due to bowing is the basis of how the erhu produces sound. The bridge on the erhu is small, round, and typically made of bamboo[1].The bridge transitions the vibration of the string to the soundbox and python-skin “soundboard” for amplification.


Fig. 2. The leftmost diagram shows the structure of the first three erhu soundbox modes. The centre plot shows the changing phases between the membrane and the contained air. Contrastingly, the right plot reveals little significant differences in left-right or up-down membrane motion suggesting that the entire surface of the membrane moves in unison.

The soundbox of the erhu is extremely different compared to its Western counterpart, the violin. The erhu soundbox is entirely symmetrical, contrasting the asymmetry found in a violin soundbox. This difference is due to the different bowing position depicted in Fig. 3 resulting in different forces placed on the bridge. The angle at which the bow is drawn on the erhu avoids the rocking motion seen on the violin bridge, thus avoiding the asymmetrical use of the sound post. Aside from the difference in symmetry, the shape of the erhu soundbox vastly differs from that of a violin; an erhu soundbox is approximately cylindrical – it can be hexagonal, octagonal, or round, and is made of padouk, ebony, or sandalwood [1]. Acting similar to a pipe that is approximately closed at one end with python skin, it produces resonant frequencies in odd harmonic ratios (1:3:5:7… modes)[3]. Sound is radiated more from the open rear than the python-skin front of the soundbox. The length of the soundbox (13 cm from its python-skin front to open rear) is also similar to the length of the human vocal tract. Researchers have found that the lowest erhu membrane mode frequency is around 2 kHz[4].

Python-Skin "Soundboard" and Soundbox Modes

The erhu “soundboard” is a membrane made of python skin and its vibrational 2D surface helps amplify the loudness of the vibrating strings[5]. Due to the python-skin membrane covering one end of the soundbox, each mode of the erhu soundbox is split as the membrane can move with (in phase) or against (antiphase) the air contained inside the soundbox[3]. Fig. 2 is a sketch of the structure of the erhu soundbox modes by physicist Christopher Waltham. In the leftmost diagram, the arrow at the far left represents the python-skin membrane motion, and contrasting colors represent opposing air movement[3]. In 2016, acoustic engineer Florian Pfeifle compared violin and erhu properties and discovered that even when string properties were reproduced, the body of the erhu (soundbox and soundboard) has important contribution to its timbre and sound quality[6].

Playing Technique and Mechanisms

Fig. 3. Erhu Playing Position.

The erhu is traditionally played in a seated position like in Fig. 3: the soundbox is placed on the performer’s left thigh, the left hand is placed on the neck (its location on the strings changes the pitch), and the right hand holds the bow with an underhand grip seen in Fig. 4.


Fig. 4. Leftmost picture: Right hand bowing position for playing the inner string. Rightmost picture: Right hand bowing position for playing the outer string.

Contrast to the violin, the erhu makes use of both sides of the horsehair because of its unique bow position between the two strings. Thus, the strings are vibrated perpendicular to the surface of the python-skin membrane allowing the erhu soundbox to be symmetrical. Fig. 4 shows the different right hand bowing positions necessary to switch between playing different strings. Despite the different bowing position, the bowed mechanism in which the erhu produces sound is the same as the violin – the stick-slip mechanism, a phenomenon where the horsehair of the bow and the string alternate between sticking (displacing string) and sliding against each other (releasing the string) resulting in sound produced by string vibration[5]. Also similar to the violin, the erhu can be plucked to produce pizzicato. However, because of the bow lying in between the strings, the bow must be positioned away from the plucked string to prevent it from muting the plucked sound.

See Also


  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 Stock, J. (1993). A historical account of the Chinese two-stringed fiddle erhu. The Galpin Society Journal, 83-113.
  2. E. M. von Hornbostel and Curt Sachs, 'Classification of Musical Instruments: translated from the original German by A. Baines & K. P. Wachsmann', GSJ XIV (1961), pp. 3-29.
  3. 3.0 3.1 3.2 Waltham, C.E. (2017). Private Communication.
  4. Xiulan, C. T. C., & Minhua, Z. H. E. N. G. (1991). Acoustical properties of Erhu [J]. Acta Acustica, 1, 010.
  5. 5.0 5.1 Garrett, S. L. (2017). Understanding Acoustics: An Experimentalist’s View of Acoustics and Vibration. Springer.
  6. Pfeifle, F. (2016). Organologic and acoustic similarities of the European violin and the Chinese erhu. The Journal of the Acoustical Society of America, 140(4), 3143-3143.