In this article, we consider the timbral and ergonomics aspect of the dizi. In discussing the timbre, we inadvertently have to foreshadow a bit of the set of articles talking about music styles and the dizi. It may make sense to quickly read through this article, look at the later ones, and then return here for more insights.
This is the most controversial aspect of a dizi and is what separates the amateur players from the professionals.
Scientifically, timbre is the quality of sound that comes from the [higher] frequencies arising from the resonance that is not the pitch frequency. The dizi, being an open pipe-type instrument, shares similar timbre as the concert flute, but sounds more ``buzzy'' and ``bright'' due to a larger number of high frequency overtones that are imparted by the dimo.
But here, we are assuming that ``buzzing'' aspect of timbre and consider instead the feel of the tone as it is played.
Early dizi repertoire is split roughly into two big schools of style, namely the northern style (北派 or beipai) and the southern style (南派 or nanpai). Later and more modern repertoire (1980s and beyond) introduce the ``new'' style (新派 or xinpai) which treats the dizi as a total instrument, eschewing the distinction of bangdi and qudi techniques.
Northern Style (北派 or beipai)
Northern (beipai) style emphasises strength and brilliance of tone. The notes tend to be piercing with liberal use of staccato. High notes are the norm, and notes are heavily ornamented at times---expect lots of glissando, flutter-tonguing, double-tonguing and ``crushing'' acciaccatura (剁音 or duoyin, a type of acciaccatura that starts with a strong jet-stream followed by the acciaccatura itself). As such, a dizi with quick response and a hearty timbre that can hit the high notes well will play beipai style pieces well. Such dizi for the beipai are usually called 梆笛 or bangdi.
In terms of the base key, bangdi will be F, G or A. The bangdi will also have 6-holes, and appear ``short'' (around half the length of a concert flute).
Southern Style (南派 or nanpai)
Southern style emphasises a more rounded tone. The notes do not climb as high as in the beipai style and tend to be played in legato. While long notes are often heard, it is more likely to hear various runs of sixteenth or shorter notes, often with the various forms of grace notes in the form of mordents, turns, appoggiatura, and ``sent notes'' (贈音 or zengyin). As such, a dizi with good response and warm resonance will play nanpai style pieces well. Such dizi for the nanpai are usually called 曲笛 or qudi.
In terms of the base key, qudi will be C or D, and will have 6-holes. The length of the qudi is roughly three-quarters that of the concert flute.
New Style (新派 or xinpai)
The north/south style divide is the most traditional means of specifying timbre requirements for the dizi, but in the modern age, is considered relatively obsolete unless one were playing really classical folk music. As dizi playing became more professional, such divides were considered to be artificially limiting. This spawned the so-called new-style (新派 or xinpai). The new style treats the dizi as a holistic instrument and inter-marries all the available techniques in both the northern and southern styles into the composition, while incorporating even more radical techniques that hitherto were not used in either style.
In terms of the base key, the traditional xinpai dizi will likely to be in E and will have a middling length between the bangdi and qudi.
With the concept of xinpai in mind, it is clear then that a neutral timbre is what is sought.
A timbre is considered neutral if it doesn't specifically accentuate the high pitches while neglecting the lower pitches, and vice versa.
In comparison, a bangdi specifically built to play beipai pieces will ensure that the brightness is highly accentuated, while a qudi specifically made to play nanpai pieces will be overly mellow.
A neutral timbre dizi will provide the same amount of a characteristic dizi timbre for every tone played in the range of the dizi.
From the perspective of selecting a decent dizi, seek out one where the response is nimble, the highs are brilliant while the lows are round---a blend of both bangdi and qudi characteristics, even for a dizi that traditionally falls in either of their specific range.
Technically, a neutral timbre has a very high signal-to-noise ratio, or equivalently, the amplitudes of the non-harmonic [high] frequencies are minimised as much as possible.
A neutral timbre comes with a distinct disadvantage: the player will need to put in more effort to deliberately emphasise the specific timbre requirements when playing pieces specifically of the northern or southern style. This is, however, par for the course for modern dizi pedagogy and performance.
Once the intonation, material, jointedness and timbre requirements are met, the last thing one needs to evaluate the dizi on its ergonomics.
Since everyone's physiology is a bit different, ergonomics alone provides the largest amount of variation with regard to how well the dizi plays under one's hands.
To simplify explanations and exposition, we will consider ergonomics from two perspectives: breathing and weight distribution.
A decent dizi should have good response---when one blows across the embouchure hole lightly (to gain say the piano dynamics), the tone should appear immediately and at the right pitch.
The dizi should also be efficient, i.e. when a note is sustained at moderate loudness (say mezzo-forte), it doesn't require a tremendous amount of effort to do so.
An efficient dizi affects sound production positively because the excess effort/air that was used before to control the tone at the loudness can be used to colour the tone more.
An inefficient dizi in this regard is considered to be 费气 or feiqi, literally a wastage of breath.
On another note, it is very hard for a note to crack on the dizi because the resonator is not just the air column---the dimo plays a very large role that provides that distinctively ``buzzing'' timbre of the dizi. And that dimo is really resistant to note cracking.
The weight distribution is also an important factor for the dizi. It is not an issue that usually crops up, but when it does, it can be a serious problem.
Generally, the dimensions of the tone producing parts (spacing of cork, internal/external diameter, position of the various holes) are well regulated from acoustic theory---deviations from them will make the dizi fail on intonation from the get-go.
What isn't regulated is the bit of bamboo to the left of the cork (or embouchure hole for an easier landmark).
Ideally, when holding the dizi in playing position, the weight should be evenly distributed between the left and right thumbs, the two points that support the weight of the dizi. A dizi that is top-heavy will be harder to play since it has a tendency to slide off the lower lips/chin.
Sometimes the top-heavy effect can be ameliorated by adjusting the way the left hand holds the dizi, but this is by no means always possible. We will spend more time talking about how to hold the dizi at a later entry.
What these boil down to is that the dizi must be comfortable to play. And comfort is individually defined due to physiology.