Keystone Sub Using 18, 15, & 12 Inch Speakers

[weltersys]

Originally Posted by Brian Steele

1)In the case of the KS, wouldn't that result in the mouth facing upwards?
2)How different though is the loading on the driver between the KS and the TH18?
3) My POC3 uses the same fold at the TH18. I may be imagining things, but I've noticed a difference when it's placed on its side compared to when it's upright. I thought that might have something to do with the bottom panel not being against the ground when the sub is used on its side (of all the panels, the side panels flex / vibrate the least) . I've never tried to quantify it by measurements though. I'll try to do that this weekend.

Brian,

1) Yes.
2) The Keystone has a more rapid drop in frequency response below F3 than the TH18 or DSL TH118, indicating the loading is a bit different.
3) The differences you notice could be due to flex or the effect of different dispersion due to proximity of the exit to the ground plane. There was a fairly significant difference in the smoothness of the upper response of the Keystone when measured on it's side or upside down compared to the "normal" apex up configuration.

[Brian Steele]

Originally Posted by weltersys

Brian,

1) Yes.
2) The Keystone has a more rapid drop in frequency response below F3 than the TH18 or DSL TH118, indicating the loading is a bit different.
3) The differences you notice could be due to flex or the effect of different dispersion due to proximity of the exit to the ground plane. There was a fairly significant difference in the smoothness of the upper response of the Keystone when measured on it's side or upside down compared to the "normal" apex up configuration.

I'd expect the results you saw with the KS in (3), as the mouth does not have rotational symmetry. The square(ish) mouths of the TH* subs do have this symmetry, so any changes in FR when rotated near to a planar surface (e.g. the ground) should not produce any significant changes in FR due to a change in loading at the mouth.

Panel flex however will introduce some changes, which should show up in the distortion plot as well as FR.

Here's a quick and dirty FR I did of my POC3 (which shares a similar layout to the TH* designs) in upright and sideways position. FR seems to be basically identical up to 150 Hz, but the distortion curve is different. Above 65 Hz, the distortion is actually HIGHER for the "sideways" position, but that may be due to the fact that I've done some changes to my PC to ensure that the mouth's bottom panel only touches the ground on one edge, which should minimize any additional distortion caused by the panel slapping against the ground.

Or who knows - maybe the change in distortion is due to the change in orientation of the driver . I'll have to think up another experiment to eliminate that as a possibility...

[weltersys]

Originally Posted by Brian Steele

Panel flex however will introduce some changes, which should show up in the distortion plot as well as FR.

Here's a quick and dirty FR I did of my POC3. Above 65 Hz, the distortion is actually HIGHER for the "sideways" position, but that may be due to the fact that I've done some changes to my PC to ensure that the mouth's bottom panel only touches the ground on one edge, which should minimize any additional distortion caused by the panel slapping against the ground.

Brian,

All the subs I have tested have progressively less distortion in the upper passband, in the range of 1-2% above 100 Hz.
Distortion exceeding 100% above 130Hz indicates something is definitely wrong with the POC3 or the test gear.


Art

[Brian Steele]

Originally Posted by weltersys

Brian,

All the subs I have tested have progressively less distortion in the upper passband, in the range of 1-2% above 100 Hz.
Distortion exceeding 100% above 130Hz indicates something is definitely wrong with the POC3 or the test gear.


Art

The only thing I changed during the testing was the orientation of the subwoofer.

[weltersys]

Originally Posted by Brian Steele

The only thing I changed during the testing was the orientation of the subwoofer.

With as much upper distortion as your subwoofer indicates, unless there is a problem with the test gear, I'd suspect a loose panel or dragging voice coil to be at fault, either of which could change with the orientation.

A slow sine wave sweep makes either of the above mentioned problems easy to identify.

[Brian Steele]

Originally Posted by weltersys

With as much upper distortion as your subwoofer indicates, unless there is a problem with the test gear, I'd suspect a loose panel or dragging voice coil to be at fault, either of which could change with the orientation.

A slow sine wave sweep makes either of the above mentioned problems easy to identify.

I believe HolmImpulse's distortion graph shows distortion relative to the main signal. So I don't think you're seeing "100% distortion" there. The DSP in the amp is also providing something like a 48dB/oct LP signal @ 100 Hz or thereabouts. I'll have to check.

[weltersys]

Originally Posted by Brian Steele

1)I believe HolmImpulse's distortion graph shows distortion relative to the main signal. So I don't think you're seeing "100% distortion" there.
2)The DSP in the amp is also providing something like a 48dB/oct LP signal @ 100 Hz or thereabouts. I'll have to check.

Brian,

1) Distortion relative to the fundamental signal means when they are equal level, distortion is 100%. Your graph displays THD crossing the fundamental signal both in the low range (expected) and in the upper range (unexpected).
-10 dB is 31.62% distortion, -20 is 10% and -40 =1% etc.
The Keystone (or bass reflex) loaded with the B&C or Lab 12s has distortion around -40 dB in the upper range.

2)Your graph does not appear anything like "48dB/oct LP signal @ 100 Hz or thereabouts." That said, panel flap or rubbing voice coils don't really care what the electrical crossover does.

Art

[Brian Steele]

Originally Posted by weltersys

1) Distortion relative to the fundamental signal means when they are equal level, distortion is 100%. Your graph displays THD crossing the fundamental signal both in the low range (expected) and in the upper range (unexpected).
-10 dB is 31.62% distortion, -20 is 10% and -40 =1% etc.
The Keystone (or bass reflex) loaded with the B&C or Lab 12s has distortion around -40 dB in the upper range.

I don't think that when the curves cross that distortion is 100%. That level of distortion would be VERY audible and my ears aren't that bad yet . Here's another measurement I took of the same sub, in upright position with all DSP turned off. Check the distortion curve..

Originally Posted by weltersys

2)Your graph does not appear anything like "48dB/oct LP signal @ 100 Hz or thereabouts." That said, panel flap or rubbing voice coils don't really care what the electrical crossover does.

The DSP section of the amp was applying a 100 Hz 24dB/oct LP filter to the signal in my previous measurements.

Note, both tests were done indoors, so there may be some in-room resonances that are adding their character to both the FR and distortion graphs.

[Brian Steele]

Originally Posted by Brian Steele

I don't think that when the curves cross that distortion is 100%. That level of distortion would be VERY audible and my ears aren't that bad yet .

As a sanity check, here's a TrueRTA measurement at 138 Hz, just above where the FR and distortion curves cross. Notice the level of the distortion components next to the fundamental. I moved the mic a bit closer to the subwoofer to get the level up a bit. Ain't no way that could be "100% distortion".

[CRESCENDO]

Originally Posted by Brian Steele

As a sanity check

What was the orientation of the subwoofer during this test?