Geddes on Waveguides

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#1,941
gedlee said:


I'm saying that I achieve a nearly ideal impulse response - which means that the end result of the crossover response and the driver response does have to have low group delay and linear phase. I use the unavoidable group delay in the crossovers to "time-align" the woofer and the tweeter. There are other ways to achieve good results than with complex digital EQ.

Actually my crossovers are quite complex - they are not simple at all.
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Earl,

Thanks for the explanation.

Mike
 
#1,942
soongsc said:
An externally hosted image should be here but it was not working when we last tested it.

On axis, the sound is slightly different. I am also starting to think more in relation with what kind of directivity pattern might be best assuming toeing in to cross in front of the listener,
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I thought I'd post some measurements even though they don't look good.

Below are driver response, horizontal response in 7.5 deg increments, and vertical response in 7.5 deg increments.

An externally hosted image should be here but it was not working when we last tested it.


need to step away for now, but will be back for more explanation. Any comments are welcome.
 
#1,943
The center of rotation was about 5~7 cm in front of AC, horizontal and vertical were not exactly at the same distance. The vertical off-axis span is actually more to my liking although I'm not sure whether it is actually good or not. The horizontal off-axis span seems more CD orientated.

Currently it seems that the vertical lip might benefit from an improved design. The dip at around 8KHz was unexpected, I expect measuring inside the wave guide probably will show some useful data.

A better driver would definetly be beneficial.
 
#1,944
measurements

Any thoughts on the details of these measurements? As in, units/division (5dB, 2dB, ???) for the vertical axis, etc.

And why run the graph out to 50Khz? especially with all the garbage represented out there. Presentation can make or break an argument. Are these gated FFTs of impulse response, or some other methodolgy?

Maybe try to show some sort of reference driver and describe the experimental setup so one can correctly interpret the data presented.



John L.
 
#1,945
Sorry, missed the scale. 5db/div for both horizontal and vertical.
This is not a sales pitch, it's just data that I normally look at. Measurements are done with SoundEasy using MLS signal gated with a 2.5ms rectangular window.
This is really a first cut at doing horn/wave guides. Obviously the driver has breakup modes that are not good. Other thoughts I had mentioned previously. Impedance measurements were shown a while back. Not exactly sure what else you are asking for, but I'll try to answer if you can be more specific.
 
#1,953
soongsc said:
My initial guess about the dip at around 8KHz is some mismatch between driver and throat. But I think running some more tests will show what the problem is. It could also just be the driver.
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Its almost certainly the driver. Thats because it is a global problem, i.e. it occurs equally at all spatial points. This means that the problem itself cannot be spatial in nature, such as a throat mismatch, it must be localized, which eliminates everything from the cone forward. The problem is in the voice coil, spider, the magnet structure or the mounting.
 
#1,954
gedlee said:



Its almost certainly the driver. Thats because it is a global problem, i.e. it occurs equally at all spatial points. This means that the problem itself cannot be spatial in nature, such as a throat mismatch, it must be localized, which eliminates everything from the cone forward. The problem is in the voice coil, spider, the magnet structure or the mounting.
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Thank you Dr. Geddess, the mounting interface between the driver and the throat is an important issue since this is a direct radiating driver. This is also what I refer to as matching of the driver and the throat.
 
#1,955
Before conducting further measurements with the horn/wave guide, I'll be trying to tweak the driver a bit to obtain better performance. Here is a first attempt. There will be two goals, one is to reduce the breakup modes, second is to maintain the bandwidth.

In the first attempt, the low frequency is reduced due to stiffening and damping of the surround. This did however reveal some tweaking of the diaphragm is necessary.
 

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#1,956
And you can clearly see that something is going on at 8 kHz. in the driver itself. The waveguide amplifies this problem. You need a better driver (Like a compression driver!). I'm guessing that the 8 kHz problem will be in the voice coil - one that you cannot correct (been there, done that).
 
#1,957
I'm guessing the 8Khz part is related with the cavity behind the diaphragm. Lots of driver manufacturers do not do enough analysis on this aspect of the driver. That's why we see many DIYers removing the dust cap and replacing them with phase plugs.

There is still a chance to tweek it. Been there, done that, significant improvements but nto perfect unless basic design is modified.
 
#1,959
gedlee said:


I'm betting otherwise. Its too high in frequency and Q, and too deep to be an acoustic issue.
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Did not think you were the betting type. 😉 I know this driver has a shorting ring (which I normally don't like in drivers of this range); but since the notch sort of focuses at the 8KHz point when loaded, it seems that you have a good point. But also note that the dip at 13K or so originally in the driver disappears when loaded, and gradually appears off-axis.
 
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