A musician playing a woodwind instrument can influence the characteristic of a sound by changing the form of the vocal tract. This can cause variations of the tone with regard to frequencies and amplitude. The vocal tract works similarly to a resonator. It adds a personal “flavor” to a musical tune during the performance. Using the vocal tract to allow the airflow to move into the instrument is an important part of performing music as it generates a characteristic received by the audience. The vocal tract's geometry itself can influence the tone of the instrument. By doing this, each musical player contributes a significant individuality in terms of their personal musical performance. Another personal influence of performing a tune is taken by the variation of the tempo, and the slow changes of sound volume (e.g. crescendo). A musician will also have a noticeable influence on the frequency and harmonic tone of a woodwind instrument. The musician will control the frequency and the harmonic tone by the movement of their tongue, mouth, and vocal tract while playing a woodwind instrument. Most studies agree that the vocal tract has an influence on the tone and the pitch of woodwind instruments. It is debatable how much the structure of the tract affects the performance, as there are difficulties in measurement. In the special case of using the voice track for influencing the airflow, a woodwind artist has to take care of health related consequences.
Basics of Physics and Music
In general, music is a special kind of “sound” or “noise” caused by the vibration of a sound source; vibrating matter in the form of strings, air columns, in a combination of resonance bodies from different materials, and their shape, including holes in the resonance bodies. This sound is organized in frequency and amplitude over time. This sound energy is transported by longitudinal waves through matter (gas, liquid, and solid matter). The wave energy obtained stimulates the vibrations inside the eardrum. The brain translates this vibration into a sound impression. Usually, the combination of tones can be interpreted as pleasant, unpleasant, or an inconvenience. Basic parameters of a single sound wave are frequency (realized as pitch: high frequency=high pitch, and low frequency=low pitch). The amplitude indicates the amount of energy that is transferred by the sound wave and is realized as volume. Music is composed of different tones, generated by various instruments such as woodwind instruments, string instruments, or human voices. Musical instruments and human voices emit a combination of waves which interfere with each other and form complex waveforms with different frequencies and varying amplitudes.
Influence of the Player and the Vocal Tract on Woodwind Instruments
Players of such instruments use their vocal tract to influence the harmonic tone of their instrument. The vocal tract can be divided into three portions; the oral and nasal tract, and the resonating cavities. The mouth, both parts of the resonating cavities as well as the oral tract, play an interesting role in the tone of several woodwinds such as the clarinet, soprano saxophone, and recorder. According to x-ray fluoroscopic examinations on the vocal tract, the shape changes while playing these instruments. It is worth noting that tongue movements appear parallel to those used in the formation of speech vocals.
Furthermore, an examination of the sound spectra of the clarinet demonstrates that the vocal tract shape had a strong influence on the quality of the note. 
The sound of woodwind instruments is initiated by the airflow of the player of such instruments, which is already modulated by the vocal tract. The varying pressure of the airflow results in the modulation of amplitude. The vocal cord can be passed without any modulation of the airflow. The instrument player may, however, intentionally change the shape of the vocal tract. This usually occurs when an individual is a singer. The vocal tract forms a resonator, which causes variations in the airflow.
Playing a woodwind instrument can be described as a chain of creating, transmitting, receiving, and perceiving sound as music: human airflow through the vocal tract ➔ vibration of an air column in the instrument ➔ transfer of vibrating energy to the surrounding air ➔ transfer of vibration energy to the human body by the ears ➔ analysis of the “sound signal” of ears to the brain by the brain ➔ stimulation of an emotional impression of “music.”
Each of the sound “stations” will transform their intensity and their waveform combination of tones of the sound by absorption, reflection and sound velocity change of the matter (e.g. the inside area of a music hall), and therefore change the characteristics of the sound. The relation of the frequencies of harmonics in a tone defines the characteristics of a special sound of an instrument. The influence of a player's harmonic tone is prevalent in the clarinet. Clarinet players "bend" the pitches of their instruments with vocal tract configurations. By changing the vocal tract resonances, clarinet players can alternate the impedance and the influence of the frequency while playing their instrument
Physics Behind Playing Woodwind Instruments
An extensive study on various wind instruments and the effects a player has on the sound was conducted. The researchers concluded that the geometry of the player's vocal tract does affect different woodwinds instruments, all in different ways. For example, in instruments, such as the Didjeridu, the geometry of the player's vocal tract will have an effect on the instrument's timbre. However, the effect will be smaller if it took place in a trombone. The geometry of the player’s vocal tract will have an effect on the instrument’s timbre whereas a smaller effect will take place in a trombone. The geometry in that instrument would influence the pitch significantly in both reed and other instrument-dominated areas, such as woodwinds. According to the same study, the tongue can increase the vocal tract impedance and aid the player in finding instrument resonances and adjusting the pitch. Therefore, even before the air enters the instrument, it can be given a special “characteristic” of flow in the strength for its “crescendo”, or a low-frequency change similar to the and “vibrato” of a violin caused by the motion of the fingers on a violin string. This characteristic is carried forward through the other “stations” of the transmission chain, which will further interfere with the following changes of the tones of the sound while passing other “stations” of the sound transmission, until it reaches the ear of the listener. This intentional human airflow influence is a forming part of the artist’s special characteristics that allows a musician to make music in a concert.
Difficulties in Measurement
While it is obvious that there are ways that the vocal tract can adjust and influence the frequency of a sound produced by an instrument, it is difficult to measure exactly how much the influence is contributing to the sound. This difficulty exists due to the limited amount of methods to access the vocal tract while an individual is playing a woodwind instrument. Real-time feedback of the behavior or the resonance may aid musicians in the use of their vocal tract in order to fine tune the power of sound. The measurement of the spectrum may aid too since they produce sound while playing instruments. In the study a correlation was found between the intensity of variation of the vocal cord while playing a woodwind instrument and health consequences for the player’s voice.
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