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Bending wave, low order crossovers and tradeoffs

Stereophile just reviewed an interesting bit of kit, the Manger P1. The mid-tweeter is a bending wave transducer, while the woofer is conventional. Crossover point is around 400 Hz.

The DIY community has a similar type of design called Woofer Assisted Wide Band.  It is 90% wide-band, plus a woofer.

https://www.stereophile.com/content/manger-p1-loudspeaker-measurements

Of course, you have to listen to make any sort of real assessment as to the value of the speakers and their suitability to your own home, I just wanted to share with the measurement readers how I might look at a crossover and the paths not taken so others can gain some insight into just how much is happening in the crossover design of a speaker.  Let's take a looksie ...


There are many who feel the best crossovers are 1st order (6 dB / octave) as high order rob dynamics or something. This is not an effect I have heard. Lots of 1st order, time aligned speakers I have not like at all, and one with active woofer system, was spectacular. So for me this is not a compelling sales pitch.


But still, lets say low-order or no-order filters is a very desirable characteristic, so lets talk about the negative consequences of having a very simple crossover, as apparent here.


Comb Filtering
Those armchair speaker designers who get frothy mad at driver arrays, claiming "comb filtering" when there's no evidence in the measurements are oddly silent when it's right in front of them. This is a good example. Look at figure 5. Plenty of comb filtering visible here.  What's going on? The low-order filters used is letting the woofer interfere with the mid-tweeter. Lots of great speakers do much better off-axis than here.


Next, lets look at the impedance chart, Figure 1. See that 3 Ohm dip around 200 Hz? With a low rise above 8 Ohms around 1,500 Hz? This is evidence of a minimalist crossover. It's quite possible that the woofer is run full-range with no low pass filter at all, and the mid-tweeter may have only a cap.


I will say that I do not like speakers with a dip in this region, as I find them quite demanding of amplifiers. My usual reaction is "WHY WOULD YOU DO THIS?" and then I am reminded that audiophiles LIKE demanding speakers. We are rather masochistic when it comes to the care and feeding of speakers. Some manufacturers deliberately drop the impedance in this range for exactly this reason.


Lastly, lets look at the overall shape of the output, Figure 4, showing a subdued mid-range. Not exactly recording studio attributes here, but possibly a good speaker at lower volumes. The shape here is a function of the cabinet, drivers and crossover design. No one thing contributed to the speaker's tonal balance, which we can lay it the foot of the designer, but if you choose to use a minimalist crossover as evidenced here there's only so much you can do to change things.


So, to recap, the designer picked a minimalist crossover and accepted poor lateral response and a low minimum impedance, and a lumpy frequency response curve as trade-offs. Or we could say he/she wanted all three.




Best,

E
erik_squires

Showing 2 responses by csmgolf

csmgolf
902 posts
 
It's quite possible that the woofer is run full-range with no low pass filter at all, and the mid-tweeter may have only a cap.
I have never designed or built speakers, but am trying to understand this statement. In figure 3, it appears that the woofer is rolling out at or near 12 db per octave above 300 hertz. It is also showing no output greater than -40 db above about 3.5khz. How could it be running full range if this is the case? There would almost certainly be a lot of output above that with an unfiltered 8 inch driver. 

This is a good example. Look at figure 5. Plenty of comb filtering visible here. What's going on? The low-order filters used is letting the woofer interfere with the mid-tweeter. Lots of great speakers do much better off-axis than here.
How is the woofer contributing to this, if it's contribution is more than 40 db down at 3.5 khz? All of the comb filtering I see in figure 5 is well above that frequency. Are you sure that is not something associated with the way the bending wave driver operates? I have never seen measurements on one of these types of drivers before and am wondering if it is something due to it's design. Could the poor off axis performance be laid at the feet of the driver instead of the cross over design? I agree that the off axis performance looks bad. So does the spectral decay plot. 

Could this excerpt from the body of the review explain the comb filtering seen in the measurements?

Unlike most loudspeaker drivers, the diaphragm of the Manger Sound Transducer (MST) does not operate pistonically. Instead, voice-coil excitations generate transverse waves along its flat surface, like ripples in a pond.

Read more at https://www.stereophile.com/content/manger-p1-loudspeaker#Wb5r3k9kVdizr0LV.99


Lastly, lets look at the overall shape of the output, Figure 4, showing a subdued mid-range. Not exactly recording studio attributes here, but possibly a good speaker at lower volumes. The shape here is a function of the cabinet, drivers and crossover design
Again looking at figure 3, this would appear to be all due to the bending wave driver with likely some cabinet contribution. I am not trying to be argumentative here, rather trying to learn how to interpret provided measurements and understand how you reached the conclusions that you did.
csmgolf
902 posts
 
Could that comb filtering be caused by the bending wave transducer? Possibly, but then why is it different horizontally and vertically?
Look at the scales on the graphs. The horizontal goes fully to 90 degrees off axis. The vertical only goes to 15 degrees off axis. If you look at the plot line on the horizontal that would correlate with 15 degrees, it looks suspiciously like the vertical at 15 degrees. 

Right, and if you have a very simple crossover, you don’t have the ability to flatten any such shape, so you are left with whatever the driver starts with.

My point here was to reinforce something I said elsewhere, most crossovers incorporate some amount of equalization, but very simple circuits have to avoid them.
After my first post in the thread, I went back and read the reader comments at the end of the review. There is a lot of good discussion about this very thing and some of the problems with equalizing this particular type of transducer. 
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