THE PHYSICS OF A TROMBONE

The trombone is an instrument that can make the most interesting characterizing sounds: from the triumphant victory of war, to prestigeous royalty, to the sensual moves of a go-go dancer, to the uncomprehendible but comic adult voice on the "Peanuts" cartoon.

All this is possible thanks to the slide of the trombone, the only instrument of the brass family with this special feature (which makes it the best, especially because I've played the trombone!)

The goal of this web page is to explain the instrument's function using PHYSICS--how the trombone can make such a wide range os sounds, articulations, pitches, intensities. Soon you'll love the trombone as much as I do! So let's get started...

The trombone, like all wind and stringed instruments, produces its sound using standing waves, waves that overlap. Trombones have longitudinal sound waves.To get a sound out of a trombone, the player buzzes his/her lips on the mouthpiece and blows air through the horn. The vibrating lip of the player helps to set up vibrations of the air column. Because of the disturbance, the air within the tube vibrates with a variety of frequencies. Only certain frequencies persist, however, which correspond to standing waves.

Waves can be described objectively, through physical wave properties, or subjectively, through sensory effect:

Objective:
• intensity
• frequency
• waveform (harmonics)
  Subjective:
  • loudness
  • pitch
  • tone quality

    The objective and subjective descriptions are related. The loudness of a sound varies from very soft to very loud and depends on the amplitude of a sound wave-- the greater the amplitude, the louder the sound produced.

    The pitch is the perception of highness or lowness and depends on the frequency or the sound wave--the greater the frequency, the higher the pitch.

    Tone quality is used to distinguish two different sounds even though they have the same pitch and loudness and depends on the waveform of the soundwave.

    If the air column or standing wave has no movement, that is, the player is not blowing into the trombone, then this activity is described to be a node. If, however, there is a large amplitude of vibration, caused by the player blowing into the instrument, then this activity is described as an antinode. Nodes separate successive antinodes. That is why we can hear separate musical notes played, instead of a jumble of sound. An illustration of this concept:

    Node-Antinode-Node-Antinode-Node

    The natural frequncies of the trombone (an open tube) are described in the equation:

    fn=v/wavelength of n= nf1, where n = 1,2,3,...

    When the player plays a musical note or tone on the trombone, it is composed of a number of overlapping frequencies. The lowest frequency standing wave is called the fundamental. Higher frequency standing waves called overtones or harmonics overlap the fundamental. The 1st harmonic corresponds to a single antinode. The 2nd harmonic corresponds to two antinodes.

    The tone quality of a sound coming from the horn depends on waveform, the number of harmonics or overtones present and their relative amplitudes. When a note is played, the fundamental and the overtones are present at the same time:

    Fundamental

    1st Overtone

    2nd Overtone

    The sum of all three forms the composite waveform.

    There are seven positions on the trombone, depending on how far the slide is extended. The 1st position is the highest pitch of the seven and the 7th position is the lowest of the seven. The player can also adjust his/her jaw opening and airflow to obtain musical notes 7 steps above or below the note in the same position.

    So, the next time you're listening to music and you hear the distinguishing sound of a trombone, you'll know the physics behind how it works!

    References:

    Giancoli, D.C. Physics, Third Edition. Prentice Hall: Englewood Cliffs, 1991.

    Wilson, J.D., Buffa, A.J. College Physics. Prentice Hall:Upper River, 1997.

    Ostdick, V.J. and Bird, D.J. Inquiry into Physics. West Publishing Company: St Paul, 1987.

    This page hosted by
    Get your own Free Home Page