This is discussed in every audio forum around the globe. But most often it is called BBC dip. A search here in the forum also immediately provides a few hits
There are arguments about the name, the origin, ... whatever. For "modern"/recent loudspeakers this does not really matter.
What is relevant is that there are situations where a dip in certain frequency ranges makes perfect sense and the approach to establish a frequency response (FR) and listening window (LW) as flat as possible does not make sense.
This is the case when the loudspeaker shows a strong, especially in the horizontal, widening of the radiation in certain frequency ranges and loudspeaker designers who do not design a crossover based on axial frequency response alone (almost all should be), consider the resulting change in sound power (and ER and PIR). This then leads to the dip in "modern" speakers.
This is very often the case due to edge diffraction. This strongly depends on the width of the loudspeaker and the crossover frequency. Edge diffraction results in a dip in the axial frequency response (more precisely in on-axis dips and humps, but mostly only one dip is really relevant) and a more or less strong widening in the radiation outside the axial frequency response.
Since most speakers have a width of 0.15-0.4m, the effect occurs in the range 1700 - 4500 Hz.
This phenomenon is particularly severe when a "normal" tweeter is placed in the center of the baffle and a low crossover frequency is selected.
Here is an example with
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View attachment 128508
A simple crossover without additional parametric EQ or notch filter gives us a wide dip in the frequency range 2.3-4kHz.
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What you should not do in any case is to get the frequency response as smooth as possible, because then this happens (FR normalized to the axis frequency response):
View attachment 128509
The sound power in the 2-4 kHz range is much too dominant.
In fact, it is very likely that you will have to deepen the dip in the frequency response to avoid a too harsh sound at higher sound pressure levels, since the vertical FRs also show a widening in the radiation in the same frequency range:
View attachment 128514
Therefore, it is not always a bad sign when an attempt is made to compensate for design weaknesses via the axis frequency response (and thus usually also the LW) - and a linear on-axis FR/LW is dispensed with for this purpose.