Final answer:
On a xylophone, higher pitches are produced by shorter and thinner bars, due to faster vibration which results in higher-frequency sound waves.
Step-by-step explanation:
The qualities on a xylophone that increase the pitch of a key include the size and the material of the bars. Higher pitches are produced by shorter and often thinner bars, while lower pitches come from longer and thicker bars. This is because when you strike a xylophone bar, it vibrates to produce sound. Smaller bars vibrate more quickly, which results in higher-frequency sound waves and thus, a higher pitch.
The pitch of the xylophone keys is not adjustable; they are tuned to specific notes when manufactured. Some xylophones also have resonating tubes or cavities below the bars, which amplify the sound much like the pots or gourds amplify the tones of a marimba.