A sine wave carries a single frequency: a pure tone. A square wave also contains the odd harmonics of its fundamental. A 100 Hz square wave simultaneously carries components at 300 Hz, 500 Hz and 700 Hz, at amplitudes falling as 1/3, 1/5 and 1/7. Fourier analysis shows this mathematically; a square wave is the sum of infinitely many sines.

The practical consequence: when you say “I am playing 100 Hz”, 100 Hz is not the only thing in the room. That is not a flaw of the square wave — it is its definition. But it needs to be known.

Why square waves?

The circulating frequency lists were historically prepared for devices generating square waves. Rezzonix uses the same waveform so the catalog value matches the signal you hear. With a sine wave you would get an entirely different spectrum at the same Hz value, and the relationship between catalog and output would break.

There is a second practical advantage: because it carries harmonics, a square wave is more audible on small speakers. A phone speaker can barely produce a pure 100 Hz sine; but the 300 and 500 Hz harmonics of a 100 Hz square wave fall in the region where the speaker works comfortably, so the ear hears something. Note carefully: what you hear is not the fundamental. The ear tends to “fill in” a missing fundamental, so you think you hear 100 Hz while what leaves the speaker is 300 Hz and above.

Duty cycle

A square wave also has a duty cycle: the percentage of its period spent in the “high” state. A 50% duty cycle is the classic square wave and contains only odd harmonics. As the duty cycle departs from 50%, even harmonics enter the spectrum and the tone changes noticeably. The same Hz value sounds different at a different duty cycle — which is why saying “I played this frequency” does not fully describe the signal.

Sampling and aliasing

Digital audio consists of discrete samples. At a 44.1 kHz sample rate, 44,100 samples are taken per second and, by the Nyquist theorem, the highest reproducible frequency is half of that: 22.05 kHz. Force a frequency above it and the signal “folds back” as a lower, entirely spurious frequency. This is called aliasing.

It matters especially for square waves, because a square wave contains infinite harmonics and some of them inevitably exceed the Nyquist limit. A naively generated square wave leaks false frequencies into the audible band. The way around it is band-limited synthesis — leaving out the harmonics above the limit.

Your device’s real limits

The catalog spans 20 Hz to 43 kHz. But every link in the chain has its own ceiling:

Human hearing covers roughly 20 Hz – 20 kHz, and the upper bound falls with age. Most adults in their thirties already cannot hear above 16 kHz.
Phone speakers are drivers a few millimetres across. They typically cannot produce meaningful output below 300 Hz or above 15 kHz, and they boost unexpectedly at their own resonance. The number on screen and the energy leaving the device are not the same thing.
Sample rate sets the mathematical ceiling: 22.05 kHz at 44.1 kHz. Some devices support 48 kHz, raising it to 24 kHz. Reproducing 43 kHz correctly would require a sample rate above 86 kHz, which consumer devices do not offer in practice.
Bluetooth codecs narrow the band further and apply compression. If you are listening on wireless headphones, the signal has already been processed.

So seeing 43 kHz in the catalog does not mean your device plays it. The app flags items that cannot be played. We write this down rather than hiding it — because something being inaudible does not mean “it works but you cannot hear it”; most often it means it was never produced at all.

If you want to measure

If you are curious about what your device actually produces, a spectrum analyser app running on a second device will show you. Place your phone at a fixed distance, play a tone and watch the spectrum: you will see whether the fundamental is really there, where the harmonics sit and how much energy comes out. It is a far more instructive ten minutes than guessing.

Safety

Avoid high volume levels. Prolonged loud headphone use — at any frequency — carries hearing risk. High frequencies you cannot hear still deliver energy to the ear; “I cannot hear it, therefore it is harmless” is not a sound inference. Keep the level comfortable and take breaks.