This graph is what started me with folds and harmonics. As you can see a single fold gives the best suppression of harmonics.  Tyrland had both driver and pipe opening on the top surface. In my case I have the driver on the forward face and the opening at the top rear. Measurements are at the center of the pipe opening and 1-2 cm in front of the dustcap of the driver (NF, near field)

I start with the IMF folded one.  Drivers are two PP cone 5” woofers with Fr of about 60 Hz. The Peerless one has a Q of 0.5 and the Taiwan one a Q of 1.0. Pipe:22 cm wide and taper from 20 to 5 cm.

Pretty much as I expected. 3-7^th harmonic is clearly seen. Note that the peak output is at 40 Hz while the dip in driver response is above 45 Hz.

Here there are peaks all over 500-1500 Hz! Note that the peak output is below 40 Hz while the dip in driver response is above 45 Hz.

Now it is time to investigate that pipe folded once that is supposed to intrinsically tame that third harmonic. . Pipe:22 cm wide and taper from 20 to 5 cm.

Lets start with the Peerless again

  Note that the fundamental has more of a peak and it also matches the dip in driver response. The  3^rd and 5^th harmonic is the same and the 7^th is worse. Not what I had hoped for.

Very broad fundamental and harmonics that is no improvement.

Then I have the Daline fold. That is a top chamber and then 3 equally wide pipe section of 5.5cm  with the opening at the very bottom of the rear panel.

Here the Q of the fundamental is lower, harmonics similar to the IMF fold.

Very low Q fundamental and some peak suppression in the 500-800 Hz range.

I do have impedance measurements as well but then I will overload the post.

It does seem that the harmonics is not only due to pipe geometry but that features of the driver comes into play.

There is obviously a lot to learn. More to come…