I purchased a colorful five-pack of these whistles (Figure 1) for both myself and my dog-themed Little Free Library. Besides books, I like to make sure the neighbors have access to other items as well—stickers, dog bags, tags, stamps, bookmarks, and more.

This is an unconventional entry in my collection of musical instruments. The whistle produces a single, piercing tone and is marketed as ultrasonic, intended for an audience other than humans. I can hear a high-pitched sound when I blow it, which made me curious about how far that frequency extends into the ultrasonic range—and whether I’m only hearing its lower edge.

The best way to find out is to measure it. Audacity is a free, cross-platform audio program that can display a spectrogram. I started recording with my computer’s microphone and blew the whistle (Figure 2).

The results were clear. The primary tone appears as a bright, thick band at approximately 9.6 kHz, well within the upper range of human hearing. A second, fainter band appears at 19 kHz, which corresponds to the second harmonic (2 × 9.6 kHz). Dogs would hear both components easily, while most humans would only perceive the lower tone.

This whistle is better described as a high-frequency audible tone with ultrasonic harmonics, rather than a purely ultrasonic instrument. That combination makes it especially noticeable to dogs—and, unfortunately, more annoying to humans (Video 1).