The Truth About Human Hearing of Low Frequencies

As a child, I experimented with a pair of hard-surface crystal headphones that produced sinusoidal waves all the way down to DC. At about 20 Hz, the vocalic structures in my middle ear started generating harmonics, while below 10 Hz, I became essentially deaf. This experience taught me an important lesson about how our ears handle low-frequency sounds.

Hearing Mechanism and Experiment

Most loudspeakers, especially those with a port, only produce audible frequencies up to around 60 Hz due to their Helmholtz resonance. This resonance causes the speaker to generate harmonics rather than the fundamental frequency. When you play a 30 Hz sound, the harmonics at 60, 90, and 120 Hz supply the essential pitch, making it sound coherent, even though the fundamental is not directly provided. Similarly, at 20 Hz, the harmonics at 60, 80, and 100 Hz create the pitch, while the speaker is not actually producing the low-frequency fundamental below 50 Hz.

Old AR speakers were designed with solid enclosures without ports, heavy magnets, and long voice coils. These features allowed them to produce authentic fundamentals at 20 Hz, a feat that few contemporary speakers still manage. One particular model, the AR1, was noted for its ability to rattle keys on a table before enough volume was produced to be heard or felt.

The Role of Subsonic Waves

Have you ever experienced the 16 Hz from a live pipe organ's 32-foot stop? You likely felt a pressure in your ears and a pulsation in your chest. This sensation is not due to the direct perception of the 16 Hz frequency but rather to the harmonics that our ears process. The fundamental frequency is not strong enough to make the tiny bones in the ear vibrate significantly; instead, our internal sensations help us detect the presence of these bass notes.

Conclusion

Loudspeakers and our ears work together in a manner that often masks the true nature of low-frequency sounds. Our ears are adept at creating the perception of low-frequency fundamentals by processing higher harmonics. Understanding this mechanism can deepen our appreciation of the nuances in audio and help in designing more effective audio systems. Whether it's a 30 Hz hum or the 16 Hz rumble of a pipe organ, the human hearing experience is far more complex than meets the ear.

Note: Personal anecdotes have been shared to illustrate the points discussed, and the information provided is based on personal experience and general understanding of acoustics.