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post #11 of 45 Old 10-31-06, 04:25 PM
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Re: Finnish Subwoofer Tests Explained

Hello Ilkka,

Thanks very much for the excellent explanation of your measuring techniques.

I have been using ETF5 for several years and recently added TrueRTA. I use stereo ACI Maestros to help out the L/R mains and a Servodrive ContraBass for the dedicated LFE channel. I have a few questions. All my measurements are done indoors.

Quote:
10 second sine sweep from 200 Hz to 10 Hz was used.
How do you do a reverse sweep with TrueRTA? For a 10 second sweep do you just set the sampling frequency to 24k and then use QuickSweep? Did you ever compare the ETF low frquency sweep (short or long) against TrueRTA? Curious as to why you picked TrueRTA over ETF to do the frequency measurements.

Quote:
Mathematically group delay is the negative derivative of the phase response. Phase response was measured using the ETF-5 program, which calculates it from the measured impulse response. Since frequency response and phase response correlate with each other, also FR and GD correlate with each other. Meaning flat FR results in low GD.
My experience with ETF5 and its Minimum Phase graph has been very inconsistent. I can do five measurements in row and each one will have a different phase graph but the frequency responses will be identical. I asked Doug Plumb (ETF designer) about how the phase graph is calculated and he told me not to bother with it (his new program does not have a phase graph). I had hoped to use the phase graph to help phase align the subs and mains but it has not proved very useful.

I wonder if it has to do with measuring indoors with its complex reflections. Have you every compared the phase graphs outdoors vs. indoors? Do you have the same problem indoors that I have?

Thanks for your help.
George
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post #12 of 45 Old 11-02-06, 05:37 PM Thread Starter
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Re: Finnish Subwoofer Tests Explained

Quote:
GGA wrote: View Post
Hello Ilkka,

Thanks very much for the excellent explanation of your measuring techniques.

I have been using ETF5 for several years and recently added TrueRTA. I use stereo ACI Maestros to help out the L/R mains and a Servodrive ContraBass for the dedicated LFE channel. I have a few questions. All my measurements are done indoors.
Hi George,

I think I recall you from AVS. You know there aren't that many people who own a dual Maestro & ContraBass system.

Quote:
How do you do a reverse sweep with TrueRTA? For a 10 second sweep do you just set the sampling frequency to 24k and then use QuickSweep?
I use an external signal generator to produce both 10 second and 30 second reversed sweeps. So TrueRTA acts only as a "receiver". TrueRTA can not do anything else than the (forward) quick sweep.

Quote:
Did you ever compare the ETF low frquency sweep (short or long) against TrueRTA? Curious as to why you picked TrueRTA over ETF to do the frequency measurements.
I have compared them and they produce pretty much identical results. I choose the TrueRTA because it's much better with this kind of measurements. The ETF can only show one measurement at a time, it doesn't show the absolute SPL, it has a linear frequency range instead of log...etc. ETF is good for some things but not for all.

Quote:
My experience with ETF5 and its Minimum Phase graph has been very inconsistent. I can do five measurements in row and each one will have a different phase graph but the frequency responses will be identical. I asked Doug Plumb (ETF designer) about how the phase graph is calculated and he told me not to bother with it (his new program does not have a phase graph). I had hoped to use the phase graph to help phase align the subs and mains but it has not proved very useful.

I wonder if it has to do with measuring indoors with its complex reflections. Have you every compared the phase graphs outdoors vs. indoors? Do you have the same problem indoors that I have?
The phase measurement is pretty accurate and consistent/repeatable, but you need to know a few things. Since the ETF uses the impulse response as a "base" for its every graph, you need to make sure the impulse itself is aligned correctly. Copy/paste from the help file:

Quote:
The impulse response from the system must be aligned properly in time for the loudspeaker/mic distance and the phase response. Move the Impulse Time Alignment slider one step back or forward and press "Update Graph". Repeated clicking on "Update Graph" will cause the impulse response to move in animation.

This is used to align the measured impulse so that the first part of it aligns with the sound card impulse response. In most cases ETF will have these aligned perfectly, but is some rare cases an adjustment is necessary.
Although I would say that the last sentence doesn't hold water. In some rare cases an adjustment isn't necessary.

When measuring outdoors and only subwoofers, getting the peaks aligned is pretty simple. But when measuring indoors with lots of reflections and output coming from both speakers and subwoofer(s) (usually sub's peak lags a bit), it isn't that easy. So it doesn't surprise me at all that you got different results each time.

I haven't used ETF that much indoors. I usually tweak the phase by basic FR measurements and by ear. I can try some indoor phase measurements and post the results here if you like.
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post #13 of 45 Old 08-12-07, 02:41 PM
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Re: Subwoofer Tests Explained

"Maximum long term output level was measured using a 30 second linear sine sweep from 100 Hz to 10 Hz. First sweep was level matched at 90 dB at 50 Hz. Drive level was raised by 5 dB after each sweep."

How long was the time between sweeps, and was it consistent? It would affect voice coil temp and thus the amount of compression.

"But we do sence the second derivative, acceleration, without any help."

Actually, it takes one more derivative, called "jerk".

The best example is the constant acceleration of gravity.

Also notice that when taking off on a plane, the noticeable part is when the thrust is being increased, but when it reaches its maximum, it's less perceptible because it's constant.
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post #14 of 45 Old 08-12-07, 04:33 PM Thread Starter
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Re: Subwoofer Tests Explained

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noah katz wrote: View Post
How long was the time between sweeps, and was it consistent? It would affect voice coil temp and thus the amount of compression.
The time between the sweeps was around 60 seconds. I didn't use a stopwatch to measure it, but it was fairly consistent. And actually it doesn't matter that much because at high output levels voice coil heats up really fast. I remember Dan Wiggins quoting that the VC in AA Tumult takes around 2 seconds to go above 200 deg. C with 1000W input power. So it doesn't matter that much whether the starting temperature is 30 C or 60 C.

Actually the heated up air inside the enclosure is a bigger problem. It would be interesting to measure Bossobass' "cooling system" to find out how much it actually helps.

Quote:
Actually, it takes one more derivative, called "jerk".

The best example is the constant acceleration of gravity.

Also notice that when taking off on a plane, the noticeable part is when the thrust is being increased, but when it reaches its maximum, it's less perceptible because it's constant.
Yes, we do sense a changing acceleration better than a constant one. What I wrote is based on Mr. Geddes' research. IMO group delay is one of the least important variables in subwoofer's performance.
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post #15 of 45 Old 08-13-07, 12:25 AM
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Re: Subwoofer Tests Explained

"And actually it doesn't matter that much because at high output levels voice coil heats up really fast. I remember Dan Wiggins quoting that the VC in AA Tumult takes around 2 seconds to go above 200 deg. C with 1000W input power. So it doesn't matter that much whether the starting temperature is 30 C or 60 C."

Good point.

I've quoted that same thing from Dan several times when people start talking about the continuous power capacity of some subwoofer.

"Actually the heated up air inside the enclosure is a bigger problem. ♠"

Yes. I wonder why more car audio subs don't burst into flames with a kW or two being fed into a small, sealed, well-insulated box.

Maybe the crest factor is high enough that the average power isn't nearly so high.

Or maybe they do; I don't follow car audio, I've just read references to what they're up to.
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post #16 of 45 Old 08-13-07, 08:11 AM
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Re: Subwoofer Tests Explained

Quote:
noah katz wrote: View Post
"And actually it doesn't matter that much because at high output levels voice coil heats up really fast. I remember Dan Wiggins quoting that the VC in AA Tumult takes around 2 seconds to go above 200 deg. C with 1000W input power. So it doesn't matter that much whether the starting temperature is 30 C or 60 C."

Good point.

I've quoted that same thing from Dan several times when people start talking about the continuous power capacity of some subwoofer.

"Actually the heated up air inside the enclosure is a bigger problem. ♠"

Yes. I wonder why more car audio subs don't burst into flames with a kW or two being fed into a small, sealed, well-insulated box.

Maybe the crest factor is high enough that the average power isn't nearly so high.

Or maybe they do; I don't follow car audio, I've just read references to what they're up to.
Hi Noah,

The high power boxes don't burst into flames for the same reason the drivers heat up so fast. Most drivers are very in-efficient at radiating heat away from the voice coil. You can't heat up the enclosure unless there is a good means of transferring the heat out of the VC. Over time you can heat things up in pro use where averages are high, but with typical crest factors, a dark colored subwoofer sitting in the shade vs sun for extended time probably has a greater impact.

PS - It looks like I might make it out your way again late September.

Mark Seaton
"Make no little plans; they have no magic to stir men's blood..." - Daniel H. Burnham
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post #17 of 45 Old 07-28-08, 05:32 AM
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Multi-Frequency Testing

Hi Ilkka,

Don't all of your tests including the CEA test use a spectrally pure, single frequency input at a time?

The reason I ask, it seems it would be useful to also have a different test that measures the ability of a sub (or any speaker for that matter) to accurately reproduce multiple frequencies at the same time. One of the things that floored me when I got my DD-15 several years ago was that I could hear musical (intended) harmonics and textures that I could not hear with my previous subs.

This relates to something I've always wondered about ported subs. At port tuning, the acoustic load on the driver increases dramatically thereby reducing cone excursion. This is great if the input only has this one frequency while the port creates most of its output, but what happens if there are other frequencies present in the input signal? Won't they be affected (attenuated?) by the non-linear acoustic loading over frequency? Is the lack of any acoustic loading in IB/OB subs the reason why many claim they have superior SQ?

As a proposed test method, it will be necessary to create a test signal that contains a constant frequency tone in the range of 50-80 Hz (referred to as the "dut" tone) combined with an equal amplitude swept tone that varies from 1/2 octave below the dut tone to lower than box tuning. The spectral content/amplitude of the dut tone can then be compared against several different frequency points of the swept tone. As a reference data point, the spectral plot of the dut tone by itself should also be included. In order to minimize other distortion sources that might obscure the spectral plots, the test should be restricted to a moderate starting SPL of say 90 db with the input amplitude of both tones held constant during the sweep.

Any feedback is appreciated as I am half trying to propose something, and half trying to learn.

Thanks,
Darrell
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post #18 of 45 Old 07-30-08, 08:06 PM
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Re: Multi-Frequency Testing

darrell, he used to include just such a measurements in his tests. The results can be found here. Ultimately I believe he stopped because the results showed a very strong correlation to regular THD and spectral contamination measurements and they didn't offer enough unique information on their own. You will notice that ported subs have nothing inherently holding them back from having very low IMD.

As for your theory about behavior around tuning, why wouldn't you assume the same type of thing would happen in a small sealed where pressures can get very high at low frequencies?


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post #19 of 45 Old 07-31-08, 06:28 PM
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Re: Subwoofer Tests Explained

Hi Steve,

Thanks for the response and link. I missed the IMD measurement sticky before, but after reading it, it still doesn't exactly address my concerns. Instead of measuring IM content for 30 Hz and 72 Hz fundamentals, I wanted to measure the affects on the upper fundamental as the lower fundamental is varied down to box (port) resonance and lower. When I say the affects on the upper fundamental, I mean how is its magnitude and any spectral contamination affected as the lower fundamental is varied.

The idea for this multi-frequency test actually came about as I was reading recent posts on the AVS "Lord of the Bass" thread. Instead of questioning whether or not a linear air spring minimizes THD in smaller sealed subs, I was wondering about the opposite affect in ported subs. Since a ported sub presents a non-linear and much higher magnitude acoustic load to its driver (at port resonance) compared to a sealed sub, I wanted to see if there was a test that could isolate and measure this affect (if any). Logically since a ported sub's driver is contributing virtually no output at port resonance (if that's the only frequency present), a second "victim" frequency must be introduced to determine if this peak acoustic load is having any detrimental affects on the driver. I proposed one method, but there may be others that are better.

It may be that a well implemented ported sub closely approximates a linear system in spite of its non-linear acoustic load. If true, the principle of frequency superposition says that individual frequencies will not be affected by one another, and everything is good. I just wanted to see if there is a direct way to test this.

Thanks,
Darrell
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post #20 of 45 Old 07-31-08, 06:55 PM
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Re: Subwoofer Tests Explained

"At port tuning, the acoustic load on the driver increases dramatically thereby reducing cone excursion. "

An interesting question.

The loading at Fb ought not restrict excursion at higher freq, and would have certainly been noticed in ported systems.

The loading at Fb isn't acoustic, it's mechanical pressure loading (hydraulic, if you will) from the air in the port compressing the air in the box.

If it were increased acoustic loading, the efficiency would be higher. Peaked response from underdamping doesn't count, the same can happen in a poorly designed sealed box.

Mark,

Thanks for the response, I missed it til just now!
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