closing the back of the mid does indeed raise the Q, which is another way of saying it causes a peak in the response, which would be audible - but only until you eek 🙂 it flat
I only have experience with two small mid drivers, the 4fe35 and the 4ndf34.
Maybe I've never had a small enough back chamber to get the peak you describe. Response has been about the same whether open back or enclosed .
I'm guessing the back chamber was a little under a liter per driver....I threw away the chambers or I'd go measure.
One thing I've come to think about peaks is that they are essentially of no value, unless all you use the driver for is it's region of peak response.
I like a driver section to be maximally flat between its xover roll-offs, so for me peaks are just an unwanted EQ task.
On the graph below, bottom panel is raw 4ndf34's on syn10, next up is raw impulse.
Top two panels are the processed 250-750Hz bandwidth currently being used.
That would be interesting way to get rid of internal reflections...
I've been finding that steep (lin-phase) xovers do a great job making out-of-band box reflections, like the 4kHz spike on the raw response above , a non-issue.
Processed response is down by -60dB before 1150Hz....4kHz is bound to be off the radar.
well Danley's original Synergies used closed back mids. They work fine so long as their limited Xmax is allowed for. I wouldn't hesitate to 3D print a tight fitting cap for a 4NDF34, if I were doing a Synergy. It doesn't have to be all that tight fitting given a low pass filter of around 1 khz ...There will be reflections from the cap but they will be so close in time/phase within the passband they will reinforce the front wave.
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Yep, Danley had to enclose the mids, as they share a common box volume with the large low drivers.
A nice 3D cap would be great...if i had a printer!
For my builds, with the large low drivers mounted on the horn's top and bottom in separate sealed enclosures, there's no spillover pressure from the large cones to modulate the mids. And why I think I get away nicely, with open back mids..
Nope. I haven't built anything yet. Long shorty short, I moved to a house where OB doesn't quite fit.
I did morph the design into this slightly smaller version, which still didn't fit.
The 15OB350 has been scaled down to 12OB150 high in a shallow Uframe, high passed at 100 Hz to a 12" slot loaded down firing sub at floor level.
That also doesn't fit my rooms, so I'm back working on a cardioid. My current favorite is this, which can be placed right up against the front wall with its center as close as 500 mm to a side wall.
It uses the same 5208C Radian 8" coax passive cardioid above an active cardioid consisting of an 8" front woofer and 2 8" side subwoofers. It will be less than 6 db down at 25 Hz in room and cardioid-like from 80 Hz up. A quad class D amp is in the stand and EQ and XO using Camilla DSP in a Rpi4 with Motu UltraLite Mk5 for a DAC.
I did morph the design into this slightly smaller version, which still didn't fit.
The 15OB350 has been scaled down to 12OB150 high in a shallow Uframe, high passed at 100 Hz to a 12" slot loaded down firing sub at floor level.
That also doesn't fit my rooms, so I'm back working on a cardioid. My current favorite is this, which can be placed right up against the front wall with its center as close as 500 mm to a side wall.
It uses the same 5208C Radian 8" coax passive cardioid above an active cardioid consisting of an 8" front woofer and 2 8" side subwoofers. It will be less than 6 db down at 25 Hz in room and cardioid-like from 80 Hz up. A quad class D amp is in the stand and EQ and XO using Camilla DSP in a Rpi4 with Motu UltraLite Mk5 for a DAC.