In terms of the frequency spectrum, brass instruments all tend to be rich in harmonics. For example, although the fundamental of a trumpet is centred around 1200Hz, it generates strong harmonic components beyond 8kHz for the upper notes, with the implication that the harmonic series continues well into the ultrasonic region. The harmonic balance also varies enormously with volume, the fundamental and first overtone remaining relatively constant but the upper partials exhibiting a much greater dynamic range. Thus the trumpet (see Figure 1 below) can sound quite mellow when played softly, but brilliant when blown hard.
The trombone, on the other hand, has its fundamental centered around 550Hz (see Figure 2), but can produce frequency components above 10kHz when played hard. When played more softly, however, the balance of harmonics doesn't change quite as much compared to the trumpet, so it retains much of its rich and bright quality even during quieter passages. The lowest brass instruments are the bass and contrabass tubas (and their relatives) which are capable of generating notes as low as 29 Hz (a low Bb), although the first overtones are generally much stronger than the fundamentals.
