Trumpet Mouthpiece Backbores: An Investigation of Interior Volume and Timbre

Abstract

In the trumpet playing community, the total interior volume of the trumpet mouthpiece backbore is generally thought to be a major component in the timbre a trumpet produces. Generally, a backbore with a larger interior volume is thought to sound darker and a backbore with a smaller interior volume is thought to sound brighter. As defined in this research, the total interior volume of the trumpet mouthpiece backbore does not have a strong correlation to the timbre a trumpet produces. However, the shape of the backbore does have a strong correlation to the timbre of the trumpet. Exploring the relationship of trumpet mouthpiece backbores and the timbre of the trumpet is the basis of this research. Many mouthpiece manufacturers describe backbores with larger interior volumes as sounding darker than backbores with smaller interior volumes. Twelve different backbores from trumpet mouthpieces currently in production and available from various manufacturers were copied for testing and discussion in this study. Five different trumpet players with varying areas of specialization and degrees of development were used for the testing. Three frequencies were tested: 233 Hertz, 466 Hertz, and 932 Hertz. Each frequency was tested at three dynamics as perceived by the players: piano, mezzo forte, and forte. Other than changing backbores, all the testing was done with the same equipment and method. The harmonic components, or spectrum, of the trumpet sound for each test was analyzed to describe timbre. An oscilloscope was used to convert the sound waves into harmonic spectrums. In this study, the method of describing timbre uses the harmonic spectrums in three ways. The first description of timbre uses the ratio of the peak harmonic to the total harmonics in the sound. The second description of timbre uses the ratio of the odd harmonics to even harmonics in the sound. The third description of timbre presents the harmonics in a line graph. Each method of describing timbre was compared to total interior volume of the backbores and analyzed for any correlations. There were moderate correlations of the total interior volume and the odd to even harmonic ratio on the 233 Hertz frequencies at mezzo forte and forte and on the 466 Hertz frequencies at piano and mezzo forte. These correlations were not strong correlations. The interior shape of the backbores was also compared to the timbre. The shape of the backbores was described by dividing the backbores into five segments of the same length and finding the interior volume of each segment. Comparing the interior volumes of each segment generally describes the rate of taper, or shape, of the backbore. The shape of the first three segments (small end) had a strong correlation to timbre. The middle of the backbore, segment three, had a strong correlation to the timbre as described as a ratio of peak harmonic to total harmonics at the 233 Hertz and 932 Hertz frequency at mezzo forte. The middle-small end of the backbore, segment two, has strong correlations at the 466Hz frequency at piano and mezzo forte. The small end of the backbore, segment one, has strong correlations at the 466 Hertz frequency at piano and mezzo forte. In general, as the rate of taper in the middle section of a backbore increases the timbre gets darker; conversely, as the taper decreases the timbre gets brighter, in the lower and upper registers. When the rate of taper of the small-middle end of the backbore is increased, the timbre of the trumpet gets brighter in the middle register. When the rate of taper of the small-middle end of the backbore is decreased, the timbre of the trumpet gets darker in the middle register. When the rate of taper of the small end of the backbore is increased, the timbre gets brighter in the middle register. When the rate of taper of the small end of the backbore is decreased, the timbre gets darker in the middle register. The shape of the backbore has a strong correlation to timbre. The information in this study differs from the general belief in the trumpet community that larger trumpet mouthpiece backbores have a darker timbre, and smaller backbores a brighter timbre. The information begins to specify what aspects of backbore design are responsible for timbre. Also, the methodology is a departure from only using audio descriptors as a means to communicate about and describe timbre, and trial and error as a means to select and design equipment.