Course:PHYS341/Archive/2016wTerm2/BrassVsPlastic
pTrumpet
The brass wind instrument family or labrosones have several things in common but, surprisingly, being made of brass isn’t one of them. [2] The labrosone family which includes the trumpet, as well as the didgeridoo have been constructed of more than just brass or wood. Recently the world’s first all plastic brass wind instrument the pBone was invented in 2011, by the Warwick Music Group in the United Kingdom. In 2014, the pTrumpet was invented and is the second completely plastic instrument. [3] The pBone and pTrumpet are made of Acrylonitrile butadiene styrene or ADS, a commonly used thermoplastic polymer that is durable. [3]
Specifications
The pTrumet plays is in the key of Bb which is the most common trumpet played. The length of tubing is equal to that of a standard trumpet, of about 1.48m and the bell’s circumference is 11cms. The main difference between the pTrumpet and a standard brass instrument is the size and shape of the valves. Each valve has a 3cm diameter, an extra cm larger than a standard model, Vincent Bach Tr300. The braces holding the pTrumpet add extra plastic to the exterior of the leadpipe. The diameter of the of the pTrumpet’s leadpipe is 8cm, and double the size of the Bach Tr 300. A major downfall to the design of the pTumpet is that the player musician cannot control valve slides. Brass instruments are have imperfect intonation wise and frequencies in the lower register that involve multiple depressed valves are sharp. Lengthening the valve slides lowers the frequency to allow trumpet players to match intonation and avoid beating. The pTrumpet weighs only 500 grams, slightly less than half of the weight of a Tr300 1084 grams. Brass musicians generally prefer lighter instruments because they take less effort to play.
Acoustics
Trumpets and all brass wind instruments are played by a stream of air causing the player’s lips or embouchure to vibrate, creating a “buzz” sound. The vibrations from the player’s lips cause a standing wave vibration in the air column of the instrument. Trumpets have been made of wood, shell, animal horn, ceramics, tree bark, various metals, and most recently plastic. Since the sound is created from the air column inside the instrument, theoretically the material shouldn't impact the timbre. Acoustically, the trumpet’s sound comes from primarily cylindrical tubing except for the bell.[2] In a cylindrical tube that is open at one end and closed at the other, the fundamental frequency’s wavelength is four times the length of the pipe. It is common knowledge that a closed-open pipe only resonates odd harmonics but the shape of the bell alters the resonance of the frequencies enough to include the whole harmonic spectrum.[2] It is not easy to obtain a meaningful measurement of the external sound spectrum of a trumpet. [1] The bell of brass wind instruments concentrates the high frequencies into a narrow beam, while the low frequencies spread more or less equally in all directions. [1] This is common amongst string instruments and similar to how sound boxes project sound. When comparing the sound quality of a standard brass instrument to the pTrumpet, the pTrumpet is has a far less resonant and brilliant sound. A good instrument is one that performs well in the low register, as it is harder to produce low frequencies rather than high ones. Figure 1. is a spectrogram of the pTrumpet and Bach Tr 300 at 163Hz or E3 the lowest frequency of a trumpet. When analyzing the two spectrograms it becomes apparent that the pTrumpet is missing the overtones of the harmonic series above 1.6kHz, which are present in the Bach Tr300. Without the harmonic series present the tone and timbre of the pTrumpet in the low register is less than optimal.
Figure 2. shows spectrograms of the two trumpets in the middle register sustaining a F4 or ~349 Hz. Comparing these two spectrograms the overtone series of the pTrumpet is more complete but above 4kHz the overtone series is notably fainter than that of the Tr300.
This trend continues into the upper register of the pTrumpet as Ffigure 3. highlights ~937Hz or “High C” on a trumpet (Bb trumpet is a transposing instrument A# is enharmonically equivalent to Bb). The harmonics between 6.3-10kHz are less important due to the condition of ability human hearing. Comparing the spectograms of the pTrumpet and Tr300 across the range of the instruments, it is clear that the the plastic one doesn’t resonate as much of the harmonic series as the brass trumpet would. To non beginner or experienced brass wind performers the tone quality and intonation of the pTrumpet is less than satisfactory.
Advantages
Although the pTrumpet will not replace a standard trumpet in professional performance settings there are advantages to having an instrument made of plastic. The cost of the horn is a third of your standard student model Bach Tr300. The plastic is incredibly durable and light, and it is suitable for children. Without as brilliant or resonant of a sound the pTrumpet is great for practicing quietly with neighbours or a mute. Plastic can also perform in cold temperatures where the metal trumpet will fall flat or lower it’s pitch in the cold. Plastic is a good insulator while metal is a thermal conductor, the environment greatly affects the temperature of a metal instrument.
References
[1] Benade, Arthur H. Fundamentals of musical acoustics. New York: Dover Publications, 2012. Print.
[2] Moore, Thomas R. "The Acoustics of Brass Musical Instruments." Acoustics Today 20 Dec. 2016: 30-37.
[3] www.pTrumpet.com