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post #21 of 114 Old 10-13-08, 12:21 AM
 
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Re: Sealed test enclosure

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A quarter-wave dip, if it's there, would be measured back at the driver, however the energy reflecting off the far wall should still register as a slight rise in SPL for that frequency. This is not seen in the results.
Understood, I just want to cover all the bases. There are a variety of factors that would prohibit this from registering.

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I'm going to do another set of measurements to look at how the intensity of these peaks is reduced outside the box. This involves taking a measurement as before (just at one spacing, probably 500mm), and then removing the meter without moving the wall. A second sweep will then be done and the SPL measured outside the box at 1m from the driver.
When you do this could you also meausre 1" from the driver? This would satisfy the conditions I previously outlined.

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This will show just how much importance should be attributed to these resonances. As an adjunct, any quarter wave dip should be visible in the second measurement. How does that sound..?
The audibility of these peaks and dips will be related to Q, source material and frequency (there has been large amounts of research on the audibility of such issues). In the end it doesn't really matter since simple steps can be taken to fully remove these anomalies via proper damping . Then again, my goal is always perfection...
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post #22 of 114 Old 10-13-08, 02:29 AM
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Re: Sealed test enclosure

1 inch it is!
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post #23 of 114 Old 10-13-08, 10:13 AM Thread Starter
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Re: Sealed test enclosure

Great tests Collo. I look forward to your further results.

Andrew,
I saw your anouncement and checked out the site. It looks good and I'll be waiting to see what you have in store in the coming months. Once you get things rolling I could possibly help provide some drivers for testing.
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post #24 of 114 Old 10-13-08, 10:21 AM
 
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Re: Sealed test enclosure

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Andrew,
I saw your anouncement and checked out the site. It looks good and I'll be waiting to see what you have in store in the coming months. Once you get things rolling I could possibly help provide some drivers for testing.
Thanks! It has been a lot of work!

PM sent.
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post #25 of 114 Old 10-13-08, 10:29 AM Thread Starter
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Re: Sealed test enclosure

Back to the sealed test box and the graphs I posted...The focus here so far has been about resonances and their effects, which is great. I also have a post in my big thread at AVS and the discussion there has been about the FR itself. The main one being that the roll off shown is greater than the expected 12db octave that a sealed box would normally show and is closer to 16db octave. Now this box & driver combo (XXX18 10ft) has a Q of about .875 or thereabouts because these model like a overbuilt IB driver and you can forget about getting a normal .7q. This can contribute to a bit faster roll-off and so could a bit of inductance, but not to this extent I would think. The other effects in play are that the CE4000 amp has a 3rd order butterworth HPF at 8hz and the Rane PE-17 has a 12db octave at 10hz. Together these affect everything from about 12-13hz down.

Now I also have a pair of SDX15's in 4 cu ft each, so I took a close mic FR of each of them and they matched each other very well. They exhibited a faster than 12db roll off too. Again it's more like 16db octave. These are in a different room, with a different system and I'm using a different CE4000 with them. Here is that graph. It does have a 24db octave filter at 200hz.







At this point I'm like at the steep roll off which has been exhibited by everything so far, so I switched out the CE4000 for an AETechron 7560 which is a dc coupled amplifier that has been tested by Chuck over at AVS in the measuring amplifiers thread as being -0.2db at 10hz and took another measurement.








As you can see this did flatten things out quite a bit. Here are the 2 graphs together. Blue is the 7560 trace. Notice the 30hz area. I have no clue what's up there. Better control of back EMF?









So at this point Illka chimed in with his measurements of the SDX which are much flatter and extended in the HF's and in a similar sized (smaller) enclosure, his show what appear to be a 40hz peak while mine are at 30hz. Also he has 2 CE4000 also and both of his measure nearly dead flat to 10hz. I find it hard to believe that both of mine are exhibiting the same weird response irregularity.

Here is Illka's 2m groundplane outdoor FR of an SDX15 in 100L.



At this point I'm trying to track down what if anything is skewing my measurements. Maybe my ECM8000 does not correlate well with the mic cal file? My testing is somehow flawed? Don't know. Next stop is FR testing all of my amplifiers, outdoor measurements and possibly getting a 2nd ECM and having one professionally calibrated.

Last edited by Ricci; 10-13-08 at 10:40 AM.
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post #26 of 114 Old 10-14-08, 02:27 AM
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Re: Sealed test enclosure

Inside SPL vs Outside SPL for a spacing of 500mm - (unsmoothed graphs)



Red is the original measurement with the SPL meter through the baffle measuring the resonances inside the sub.

Blue is measured outside the sub, 100mm (4 inches) from the driver - couldn't get the meter any closer due to the way the driver was mounted.

Gold is measured outside the sub, 1 metre from driver.

The 1m measurement is basically the same as the close miked result apart from picking up some extra environmental reflections.


To get a clearer view, here's just the first two, with 1/3 octave smoothing applied.....



- No sign of a quarter-wave resonance around 170hz.
The first peak is the 1/2 wave resonance around 340hz
The second peak is the ( 1/2 * 2 ) wave resonance around 680hz
The third peak is the ( 1/2 * 3 ) wave resonance around 1020hz

- The "dip followed by peak" is presumably due to phase differences as the sweep moves through each of the resonances. The resonance initially interferes with the driver, and as the frequency is raised, comes into reinforcement.

- The intensity has dropped from a 20dB peak inside the box, to a +/- 7dB ripple for the fundamental, and each successive harmonic diminishing by half.
Naturally the details would depend on the implimentation, but the principle is there....
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post #27 of 114 Old 10-14-08, 08:25 AM
 
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Re: Sealed test enclosure

Interesting results. I still wonder about something to do with perhaps pipe harmonics preventing any quarter-wave anomaly from showing. This anomaly can clearly be seen on Ricci's test box as well as some other measurements I have seen on undamped systems.

Sadly, my understanding of these physics is not strong enough. Due to this I have invited an expert on the subject to the discussion, Martin J. King. Hopefully he joins us!
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post #28 of 114 Old 10-14-08, 01:33 PM Thread Starter
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Re: Sealed test enclosure

Interesting. I'm not very strong on physics and wave propagation either, but I am surprised that there is no evidence of a 1/4 wave effect at all. It would make sense that the 170hz anomoly in my box was a half wave resonance from the longest dimension from baffle to back wall but it should be slightly higher in frequency. To correspond with the 880mm distance. I suspect that I may have something higher up around 300-320hz like was suggested also due to the side to side resonance which would have 4 walls instead of 2 interacting, but I don't remember seeing anything. I'll have to look again.
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post #29 of 114 Old 10-14-08, 04:54 PM
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Re: Sealed test enclosure

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...I still wonder about something to do with perhaps pipe harmonics preventing any quarter-wave anomaly from showing. This anomaly can clearly be seen on Ricci's test box ....

.....I have invited an expert on the subject to the discussion, Martin J. King...
The 170hz on Ricci's box is certainly something. I'm just not convinced that it's quarter-wave. The front-to-back half-wave is the most likely culprit, but the round trip path length is too short by around 160mm.

With the magnet occupying most of the central hole in the brace, I wonder if there's a path that goes through one of the smaller holes in the brace and returns via another.

I'm just fishing here. Truly, an expert pair of eyes on this would be great!
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post #30 of 114 Old 10-14-08, 05:13 PM
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Re: Sealed test enclosure

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Interesting results. I still wonder about something to do with perhaps pipe harmonics preventing any quarter-wave anomaly from showing. This anomaly can clearly be seen on Ricci's test box as well as some other measurements I have seen on undamped systems.
I find this interesting as well. I wonder if the measurement technique only picks up harmonics as if it were in a closed-open pipe. But I'm not sure why this would happen. Have a look at this article I found.


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