Which change to a sound wave lowers its pitch?

• A. Increasing the amplitude
• B. Decreasing the frequency
• C. Increasing the frequency
• D. Increasing the speed

Answer: (B)

Pitch is how high or low a sound seems, and it mainly comes from the frequency of the sound wave—the number of vibrations per second. Fewer vibrations per second mean a slower oscillation, which we perceive as a lower pitch. So decreasing the frequency lowers the pitch. Changing how big the wave is affects loudness, not pitch. Increasing frequency would raise the pitch, and increasing the speed of the wave doesn’t change the pitch for a given source frequency (pitch tracks the source’s frequency, not how fast the wave travels). Therefore, the change that lowers pitch is decreasing the frequency.