Presentation:

This appendix shows why it is preferable to use a speaker whose resonant frequency is equal to the horn cut-off frequency with measurements made on other pages of this site.

Originaly, this document shows the frequency response of a compression driver with a tube of 80cm long, it also shows a marked decrease in response of comb filter at 2KHz (surrounded by black); this frequency is the resonant frequency of the compression driver.

The question is: why the frequency response is smooth naturally to the resonant frequency of the transducer?

The answer lies in the following two documents:

This document shows in purple and orange the first echoes of compression driver with a 1 meter tube long with or without horn to its other end.

We note that the maximum energy is captured in both cases to 1600hz; resonant frequency of the compression driver used in this experiment ...

We note also, the horn reduces the amount of energy which returns in the approach of its acoustic cut-off frequency (Fa) as explained on its page of origin.

In view of this two documents, we can conclude that the acoustic energy returned by the horn can be absorbed by the compression driver at its resonant frequency (Fs).

Schematic explanation in three steps:

Conclusion:

Following the explanations provided in this page you will understand why it is likely to have Fa = Fs for better results in electroacoustic response.

From a practical point of view, this can all be filtered at lowest frequency with a lower increase of the distortion on linear domain in the case of Fa≠Fs.

First step: The sound is emitted from the compression driver and is transmitted to the external environment through the horn.

Second step: In closer of Fa, the horn will return more acoustic energy to the compression driver.

Third step: At Fs and its vicinity, the compression driver will absorb more sound energy that is returned by the horn.

The compression driver will stop back and forth of sound between the horn and transducer at Fs.

The result of this association should be a strong reduction of the group delay as shown in the diagram below:

Red, an association whose horn acoustic cutoff "Fa" and the resonant frequency "Fs" are not equal.

Green, an association of Fa and Fs are equal.

Explications:

Thus, on the "Appendix1" we have this document:

Above, surrounded by black, we saw an hole in the frequency response on the first 4 echoes, but not in what is called "the real response of frequency compression driver", it shows that the sound energy is absorbed somewhere when its made a return trip in the tube.

In another document from the "Horn's reflection measurement":