Some pipe organs can produce frequencies lower than the threshold of human hearing which is around 20 cycles per second. The Bösendorfer Imperial which goes all the way down to the C below the standard low A on other pianos produces notes lower than 20 cycles per second. Part of this phenomenon can be explained by the how you can feel rather than hear those extremely low frequencies. Certainly a massive pipe organ in a cathedral can energize an entire room with sonic energy with low, rumbling frequencies. But there is much more to it than that and it has to do with the overtone series.
I have a video about atonality which touches on the overtone series:
The overtone series is a characteristic of all pitched sounds in nature as well as musical instruments. Anything that makes pitched sounds contains color tones above the fundamental tone. It’s a series of tones that goes up by an octave, then a fifth, then two octaves above the fundamental pitch and on and on. All these color tones affect the quality of the tone. That’s why in its simplest form a trumpet sounds distinctly different from a violin playing the same pitch. It has to do with the overtone series and how these overtones interact. So when you’re hearing an extremely low note, you are actually hearing more overtones than fundamental pitch! Your mind constructs the fundamental pitch particularly in descending lines that go lower than your hearing.
As a young child I performed an experiment utilizing my father’s tape recorder and my tape recorder. I recorded the lowest note of the piano at one speed, then played it back 4 times faster which raised the pitch 2 octaves. To my shock, instead of hearing a single note I head a chord! This is because on smaller grand pianos, the fundamental tone is so weak, that the overtones are actually as loud or louder than the fundamental tone! So, this is how you’re able to hear notes that are below 20 cycles per second such as the Bösendorfer Imperial which goes lower than your hearing as do some pipe organs with immense pipes that produce frequencies in the low double digits of frequencies. It’s not only that you feel the room shaking, but you hear the overtones and you surmise the fundamental pitch that you can’t actually hear. So the question is answered for you very simply, you’re not hearing something you can’t hear but your mind makes an image of that low tone in a convincing way.
Again, this is Robert Estrin at LivingPianos.com. info@LivingPianos.com