What are the acoustics of brass instrument pedal tones?

Question

The natural range of a standard Bb trumpet, for example, extends down to F# (i.e., concert E), but by adjusting one's embouchure and air, it's possible to produce lower pitches. Those lower pitches -- pedal tones -- have a distinctly different timbre than the natural tones of the instrument -- duller and less brassy, less stable. Through practice they can be made useable and their timbre brought closer to that of other notes, but....

What is happening acoustically that makes pedal tones different? (Or, maybe, why can't brass instruments naturally produce those pitches in the first place?)

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Related information

• Physics behind why a bugle can play several notes, while a whistle only plays one note
• Playing 440 Hz, what are the harmonics for a trumpet? For a flute?
• Trumpet Peculiar Frequency spectrum

Answer 1

Check here for the basics:

Trumpet Peculiar Frequency spectrum

In the above answer, it’s stated that the trumpet’s “natural” overtone series of only odd harmonics is altered by the flare of the bell and the shape of the mouthpiece so that instead of being only the odd harmonics, it has essentially a normal harmonic series, but the fundamental is missing.

A pedal tone is a way to play the “missing” fundamental. Any resonant system has a lowest resonant frequency and for bell and mouthpiece brass it’s there, it’s just not as easy to sound as the first harmonic and much of the series above it.

It’s also a different embouchure from the rest so pedal tones aren’t as frequently taught because many believe learning to play then might create bad embouchure habits in the student.

One thing about the above linked answer I’d be careful of is the reference to the clarinet, which does only have its odd overtones but is also a conical bore instrument so I think there are some subtleties beyond the half closed cylinder model at work in the clarinet.

There’s a Wikipedia article on the topic also.

Everything I’ve found suggests the concept of pedal tones applies to all members of the brass family, namely cylindrical bore metal instruments with a flaring bell and hemispherical mouthpiece that are primarily played by sounding an overtone series along with valves that change the sounding length of the primary cylinder. The fact that the horn uses a different mouthpiece shape doesn’t exclude it from the category.

Answer 2

On any wind instrument, the basic idea behind the physics is that there's something vibrating at the front of the tube (the player's lips for brass instruments, the reed for reeded instruments, and air turbulence for flutes), which powers a vibration of air inside the tube, and the physics of constructive and destructive interference only allow a resonant frequency to ring out. The tone quality produced is determined by the spectrum of frequencies produced by the mouthpiece (and the skill of the player), refined by the characteristics of the resonator tube.

With trumpet pedal tones, you buzz a low frequency below the fundamental frequency of the tube, and "insist" on it hard enough that you overcome the physics of the resonator and force that pitch through. When this happens, the tube is no longer refining the tone as normal, and so the sound is just the mouthpiece buzz being saxamaphoned through the horn.

Answer 3

Much the same as the acoustics of any other note. Just somewhat harder to produce due to the proportions of the instrument and mouthpiece.

More interesting are the 'false' notes between pedal (written) C and F♯, bottom of the 'normal' range. There is a definite F fingered 1-2, E fingered 2-3 etc. (i.e. one fingering 'lower' than might be expected). You have to force them, but they definitely exist and with practice can almost match the normal notes.

On trombone, the pedal notes are easier (wider tube, larger mouthpiece) and are routinely asked for. The 'false' notes are also easier, but unlike the pedals are not normally asked for except as an 'extended technique'.

Modern trombones (especially bass trombones) have switchable extension tubing that enable the lower range without trickery. But the characteristic rasp of a smaller instrument's pedal tones is still useful.