Sound of 64 feet pipes

Quite interesting, I presume, for the sound technician in each of us! I stumbled on this recording of 64 feet opened pipes from the oversized organ in Atlantic City. More precisely, we hear the two lower scales (only the white notes), and the 15 notes are played descending, one at a time.

I insist on opened pipes because it is quite usual, in organs, to close the top of pipes as a way to force the belly of the sound wave at the end of the pipe, while in an opened pipe, the belly is located at the middle of the pipe. So, a closed pipe has its effective wave length multiplied by two, compared to an opened pipe of the same length, and consequently, the sound frequency is divided by two. For the lowest notes, which require the biggest pipes, closing them is a way to be economical in space, weight and money.

However, odd harmonics have more amplitude in a closed pipe than even harmonics, and this gives the sound a particular colour. The overall sound is also a bit weaker. But in the recorded sound as above, the longest pipes really use 64 feet, there is no trickery on 32 feet pipes.

Could we really speak about sounds here? Frequencies are below the sonic threshold, we can even hear the separate wave peeks one by one for the lowest notes. These pipes are like enormous whistles, where sound happens in very slow motion. Yet, imagine the blow it would take for sounding such gigantic whistles!

It is especially amusing, for me at least, that we can recognize the notes even if the pipes play subsonic waves. This is a consequence that many harmonics of the fundamental are in the sonic range, and these are what we hear briefly beyond each separate vibration of the fundamental.