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ttruman 28th August 2007 12:52 AM

Stuck in a dip
 
2 Attachment(s)
Hello everyone,

Been away for a while reading, learning. I think I feel a bit like Alice falling down the rabbit hole sometimes. As part of course I believe I am now getting reliable measurement data from Speaker Workshop as to utilize the cross over designer features and I have begun to carefully measure all the drivers to obtain a good modeling.

In that regard the tweeter measurement has exposed some kind of destructive cancellation when the driver rests flush mounted in the cabinet. I have tried extending the baffle using cardboard both vertically and/or horizontally. I also tried covering the upper portion of the baffle around the tweeter with felt containing a small hole (30mm) exposing just the dome. The location and the severity of the dip remained unchanged! However pulling the tweeter out about 100 mm from the baffle changes the response dramatically. The original dip disappears but is replaced with two sharp dips located on opposite sides of where the dip previously occurred. If I remove the tweeter from the baffle and measure its response positioned resting on top of the cabinet, the dip disappears. I have read much about diffraction and although it appears to explain the location it does not explain the severity of almost 8 db.

Below is a chart showing the anomaly. The red line is the tweeter flush mounted on the baffle. The black line is the tweeter removed from the baffle and measured sitting on top of the cab.

My only theory at this point is that the design of the tweeter faceplate is creating a waveform that when encountering the baffle creates a very destructive cancellation that so happens to be centered around a baffle step diffraction dip.

I am hoping the experience on this board can help me make some sense of this. I don't know what else to try short of removing the grill and applying felt directly to the tweeter faceplate. The process will damage the grill; there is risk of damaging the dome and the felt test I tried above doesn't strongly suggest it would help anyway.

Measurements farfield 1M gated.
The box is 220 x 1080 X 310 (WxHxD)mm
The Accuton C23 is horizontally centered on the baffle 110mm from the top.

Svante 28th August 2007 06:13 AM

Re: Stuck in a dip
 
Quote:

Originally posted by ttruman
Hello everyone,

Been away for a while reading, learning. I think I feel a bit like Alice falling down the rabbit hole sometimes. As part of course I believe I am now getting reliable measurement data from Speaker Workshop as to utilize the cross over designer features and I have begun to carefully measure all the drivers to obtain a good modeling.

In that regard the tweeter measurement has exposed some kind of destructive cancellation when the driver rests flush mounted in the cabinet. I have tried extending the baffle using cardboard both vertically and/or horizontally. I also tried covering the upper portion of the baffle around the tweeter with felt containing a small hole (30mm) exposing just the dome. The location and the severity of the dip remained unchanged! However pulling the tweeter out about 100 mm from the baffle changes the response dramatically. The original dip disappears but is replaced with two sharp dips located on opposite sides of where the dip previously occurred. If I remove the tweeter from the baffle and measure its response positioned resting on top of the cabinet, the dip disappears. I have read much about diffraction and although it appears to explain the location it does not explain the severity of almost 8 db.

Below is a chart showing the anomaly. The red line is the tweeter flush mounted on the baffle. The black line is the tweeter removed from the baffle and measured sitting on top of the cab.

My only theory at this point is that the design of the tweeter faceplate is creating a waveform that when encountering the baffle creates a very destructive cancellation that so happens to be centered around a baffle step diffraction dip.

I am hoping the experience on this board can help me make some sense of this. I don't know what else to try short of removing the grill and applying felt directly to the tweeter faceplate. The process will damage the grill; there is risk of damaging the dome and the felt test I tried above doesn't strongly suggest it would help anyway.

Measurements farfield 1M gated.
The box is 220 x 1080 X 310 (WxHxD)mm
The Accuton C23 is horizontally centered on the baffle 110mm from the top.

Hmm, my bet on this is still that it is diffraction that causes it. A simulation of the baffle step shows that the placement on the baffle is far from optimal. I suppose it has to do with that the tweeter is mounted 110 mm from three edges, and that produces a dip of some 4-5 dB at ~2600 Hz.

http://www.tolvan.com/ttruman_01.gif

Another thing, which is a longshot; as far as I can see from the measurement, the resolution is ~100 Hz. That would mean that the gating window is ~10 ms (?), which in turn means that the pathway of the first reflection must be no less than 0,01*345=3,45 m. Is that so? Could it be that you also have a reflection from one of the room surfaces inside the gating window?

Svante 28th August 2007 06:53 AM

Re: Re: Stuck in a dip
 
Quote:

Originally posted by Svante
Another thing, which is a longshot; as far as I can see from the measurement, the resolution is ~100 Hz. That would mean that the gating window is ~10 ms (?), which in turn means that the pathway of the first reflection must be no less than 0,01*345=3,45 m. Is that so? Could it be that you also have a reflection from one of the room surfaces inside the gating window?
Should be :

...at least 0,01*345=3,45 m longer than the direct pathway...

CeramicMan 28th August 2007 10:43 AM

Considering the relatively long wavelengths in question (circa: 110mm), you may need a layer of damping material that's in the vicinity of 3~4cm thick.

How about covering the corners and box edges with some pieces of sheep-skin rug? A 100mm x 660mm strip (with two cutouts for the overlapping corners) may be sufficient.

Cheers,

Lech

ttruman 28th August 2007 03:48 PM

not convinced it's all baffle step
 
Hi Svante,

Quote:

Originally posted by Svante
Hmm, my bet on this is still that it is diffraction that causes it. A simulation of the baffle step shows that the placement on the baffle is far from optimal. I suppose it has to do with that the tweeter is mounted 110 mm from three edges, and that produces a dip of some 4-5 dB at ~2600 Hz.

I am little confused by your interpretation of the simulation. Based on the graph I agree with the frequency but the dip amplitude appears to be about -2.5 db given a mean of the 6db on the chart. Apparently I am not interpreting that correctly. I also donít see the adjacent peak in my measurement which is the same amplitude as the dip. The tweeter exhibits the classic smooth 12db roll off of a closed box system when not measured in the baffle so I would expect to see the effects of the peak as well. Yes I am confused. Additionally I mentioned extending the baffle with cardboard in both horizontal and vertical directions. The dip did not move or change amplitude.

Quote:

Originally posted by Svante
Another thing, which is a longshot; as far as I can see from the measurement, the resolution is ~100 Hz. That would mean that the gating window is ~10 ms (?), which in turn means that the pathway of the first reflection must be no less than 0,01*345=3,45 m. Is that so?

I am working on memory here but if I recall correctly the pulse response shows the initial pulse at ~6ms (the latency of the SC is about 4.5ms). The first reflection is at ~10 ms from the floor. The start gate is set ~4ms and the end gate about 9ms. In order to get consistently useable measurements I had to have a gate width of at least 5ms. The sample rate is 96 KHz but the sample size is low.

Quote:

Originally posted by Svante
Could it be that you also have a reflection from one of the room surfaces inside the gating window?
If I do its not apparent in the pulse response. But itís worth double checking everything again except this time I will use the formula to calculate the arrival of the surface reflections D/R=T and see if they match the pulse response indicators. Maybe I am doing something wrong. I know the floor arrives at ~10ms. I can clearly see it and reduce it by acoustically padding the floor. The ceiling is approximately the same distance away and is hung acoustical tile.

Inductor 28th August 2007 04:57 PM

2 Attachment(s)
Hi, -2,5dB, -2,5dB = ... (a lot)

ttruman 28th August 2007 05:46 PM

Hi Lech,

Quote:

Originally posted by CeramicMan
Considering the relatively long wavelengths in question (circa: 110mm), you may need a layer of damping material that's in the vicinity of 3~4cm thick.

I assume you mean around the tweeter area. I think that much material would severely affect the off axis dispersion characteristics. But it may be worth trying to in narrow down the cause.
Quote:

Originally posted by CeramicMan
How about covering the corners and box edges with some pieces of sheep-skin rug? A 100mm x 660mm strip (with two cutouts for the overlapping corners) may be sufficient.

Did you experience a similar issue with your two way Accuton configuration? What doesnít make sense to me is that if I change the baffle dimensions using cardboard, the anomaly does not change. The measurements taken with the extended baffle combinations do show a slight influence in the frequency response like those in BSD simulations.

ttruman 28th August 2007 05:59 PM

Quote:

Originally posted by Inductor
Hi, -2,5dB, -2,5dB = ... (a lot)
Hi Inductor,
I think in order for that to be usefull, the particulars of the circumstances under which that measurement was taken must be known. Assuming that the measurement was taken without the unit being flush mounted in baffle as you suggest
is it should resonable to expect that I would see the same dip in my my out of baffle measurement, which I dont. In all fairness it may very well be, as Svante suggested, my measurements may not be accurate. I will check my setup this evening and confirm my original results.

Inductor 28th August 2007 06:42 PM

Quote:

Originally posted by ttruman

Assuming that the measurement was taken without the unit being flush mounted in baffle as you suggest is it should resonable to expect that I would see the same dip in my my out of baffle measurement, which I dont.

Yes. I know. Something fishy (for a $200,00 driver).

ttruman 28th August 2007 06:47 PM

Quote:

Originally posted by Inductor
Yes. I know. Something fishy (for a $200,00 driver).

I don't believe that's been established yet.


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