The cyclical motion of sound pressure waves or force pressure waves moving through the air can be represented by a waveform that repeats once every ‘T’ seconds (left side graphs). The number and relative height of the peaks and valleys of these waveforms directly influences the loudness of individual harmonics in the corresponding sound (right side graphs). Instruments like the flute (A) and oboe (B) playing the same note will produce sound pressure waves that repeat at the same rate (T) but have differing waveforms. This makes the harmonic content, the timbre, of the notes different. Hightower et al. showed that hovering animals – such as hummingbirds (C), parrotlets (D), mosquitos (E), and compact flies (F) – also produce unique pressure waves which repeat each time they flap their wing. Here, the waveforms represent vertical, lift forces instead of sound, but the link between the pressure waves and their harmonic content is the same. For complex waveforms (B, E, and F) with many peaks and valleys, the corresponding harmonics tend to be dominated by harmonics that are louder than the fundamental frequency (f). Whereas, in simpler waveforms (A, C, and D), the fundamental frequency is usually the loudest harmonic.