New user with a few questions about measurements

5325 Views 32 Replies 7 Participants Last post by DrWho, Mar 30, 2009

burzel2

Discussion Starter · Mar 11, 2009

2

First let me say that REW is fantastic. Since I discovered it last weekend I can't stop using it! I have a DIY sub with a 12" Dayton driver and a 300 watt amp in a 2 cubic foot sealed enclosure. F3 using WinISD is about 38 Hz. I'm using the older analog RS sound level meter. The first measurement below was taken with a 90 dB target with the preamp crossover set at 60 Hz. The second measurement was taken with a 95 dB target. There's always a spike at 45 Hz which is exactly the first order resonance of the width of the room. The second spike is around 90 which is perhaps the second order resonance. Here are my questions:

Why does the spike at 45 Hz get less pronounced at higher volumes? At 75 dB target the spike is huge.
Why do the measurements not look anything like WinISD? I don't see a distinct F3. Is that room gain?
How the do you minimize a first order resonance without putting the sub in the middle of the room or adding a second subwoofer?

Any help is greatly appreciated.

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DrWho

· #2 · Mar 11, 2009

If you want your graph to look like WinISD, then you'll need to take your sub outside to do the measurements since that's the environment that REW is predicting in. You might also see if your sub's frequency response doesn't change with output level outdoors as well....that will allow you to see what is room gain and what is the driver.

Btw, are these plots smoothed, or EQ'd? Can you post the raw responses?

burzel2

Discussion Starter · #3 · Mar 11, 2009

3

Great suggestion. Is testing outside similar to an anechoic chamber? I'm not sure my laptop has a good enough sound card, but I'll look into it. To answer your question the plots have the 1/3 octave smoothing applied. The plots below do not have any smoothing. I've also included a third plot showing a 75 dB target from a previous measurement. I think the crossover is disabled in that plot (ignore the target curve). It shows the pronounced spike at 45 Hz and again at 90. I have no EQ in my system.

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DrWho

· #4 · Mar 14, 2009

Yea, outdoor ground planes are how all the speaker companies achieve "anechoic" measurements at lower frequencies.

For what it's worth, I'm not seeing any of your spikes changing with output level. Instead, what I think you're seeing is non-linear room gain. Basically, as the lower frequencies get louder and mask the fact that you've still got a ~45Hz spike.

Could you provide more information about your setup? Like room size and the subwoofer you're measuring and where the microphone is located?

The 45Hz stuff looks like it might be two different frequencies and/or some of the polar response effects of having a subwoofer surrounded by walls. Looks like your sub is about 2ft from the walls? (or your microphone is). I would expect to see about a 6dB rise in response starting at around 60-70Hz....kinda like a shelf-filter until the driver response starts to naturally roll-off.

Are you opposed to plotting your WinISD predictions too? (preferably with the same scale).

burzel2

Discussion Starter · #5 · Mar 15, 2009

My sub is a DIY 2.0 cu ft sealed enclosure using a Dayton RSS315HF-4 and Bash 300W amp. The room is 20'W X 12.5'L X 7.5'H. The couch is against the longer wall. The right side of the room has stairs and two open door ways. Behind the couch is a bay window. The sub is currently to the left of the front left floor standing speaker (Paradigm Studio 60s) and is near the back wall and about 5' from the left wall. I can reduce the 45 Hz spike by moving it to the center of the room, but there's a fireplace along the side wall. Experiments against the back wall were worse. The mic was located at the listening position in the middle of the couch.

I very much appreciate your help. I enjoy learning the technical aspects of this hobby.

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DrWho

· #6 · Mar 15, 2009

Are you setting your "microphone" on the couch when taking the measurement? And do you get the same results if you feed your signal directly to the sub amp instead of through your receiver?

If you use the all-measured tab, you could provide 10 measurements at 10 different SPL's...maybe start at 70dB and move up in 3dB steps? I think this will show the trend with SPL....I'm wondering if it's not voice coil temp rise / room gain / power compression? There are examples of this performed by Ilkka in the subwoofer measurement forum (it's down on the list somewhere).

Btw, are you more curious about the source of the behavior you're seeing, or are you trying to tweak things to sound better? or both?

Also, you can model VC temp rise in WinISD....basically, the resistance of the VC rises and causes a voltage-divider against the non-linear impedance response of the speaker.

burzel2

Discussion Starter · #7 · Mar 15, 2009

4

My cheap mic is on a tripod pointing horizontally right behind where your head would be. I did a little experimenting with having it vertical and in a few other locations, and the results were marginally different.

Btw, are you more curious about the source of the behavior you're seeing, or are you trying to tweak things to sound better? or both?

Definitely both.

First plot below is through my preamp (Rotel RSP-1066):

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The next plot is directly to the sub amp. I would say the results are very similar to the previous plot:

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This sweep has the source fixed and I'm changing the sub amp volume knob:

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This sweep is through the preamp and includes the main speakers:

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Wayne A. Pflughaupt

· #8 · Mar 15, 2009

burzel2 said:
My cheap mic is on a tripod pointing horizontally right behind where your head would be.

What "cheap mic" are you using? Do you have a calibration file for it? If not, none of your measurements are valid.

Regards,
Wayne

burzel2

Discussion Starter · #9 · Mar 15, 2009

I'm using the Radio Shack 33-2050 with the proper calibration file. Sorry for the confusion.

Ricci

· #10 · Mar 16, 2009

Looks like you are compressing on your highest level measurements. Either the sw itself or otherwise.

burzel2

Discussion Starter · #11 · Mar 17, 2009

I admit I had to Google subwoofer compression. I read about thermal compression and power compression. They seem to have the similar definitions. I'm not familiar with this so bear with me. My understanding is power compression occurs when the voice coil heats up, the impedance goes up. So as the power goes up the coil heats up, impedance rises, and less power is delivered to the driver. But my plots show compression at lower levels. Is my understanding of this correct?

Wayne A. Pflughaupt

· #12 · Mar 17, 2009

Ricci said:
Looks like you are compressing on your highest level measurements. Either the sw itself or otherwise.

That was the first thing that crossed my mind. But then it came to me that the peak is acoustic, not electronic. IOW, the amp does not "know" that there is a peak at 45 Hz and therefore would not be trying to flatten it at higher volumes.

burzel2 said:
I admit I had to Google subwoofer compression. I read about thermal compression and power compression. They seem to have the similar definitions. I'm not familiar with this so bear with me. My understanding is power compression occurs when the voice coil heats up, the impedance goes up. So as the power goes up the coil heats up, impedance rises, and less power is delivered to the driver. But my plots show compression at lower levels. Is my understanding of this correct?

That's a different issue. What Ricci is talking about, some subs have a limiting feature that restricts their output at a pre-determined level in order to prevent damage. It's most noticeable in that at a certain point, the sub just won't put out any more volume, irrespective on how high you crank the receiver's volume control.

Regards,
Wayne

DrWho

· #13 · Mar 17, 2009

Well at first I was going to mention that the Bl isn't linear with excursion and that is a common source of compression...basically, as the voice coil starts to leave the magnetic gap, the amount of force it can generate goes down and thus the cone doesn't move as far. The suspension can also get stiffer as the excursion increases too.

However, if this was the problem, then you would see everything compressing below 40Hz especially in a sealed box because the excursion goes up with lower frequencies.

Instead, I think you might be clipping your SPL meter....or something in the measurement chain (and REW is probably not noticing). It looks like the compression starts happening near the same SPL at other frequencies once they get that high.

robbo266317

· #14 · Mar 17, 2009

If its the SPL meter then moving it closer and running the same sweep at the same level again will tell you. (ie don't change any settings, just move the meter closer to increase the level it "hears")

JohnM

· #15 · Mar 17, 2009

You can look at the scope tab to see whether the input signal is externally clipping, if you are leaving your meter on the 80dB range for all those tests then the meter will be the limiting factor. If the meter needle reaches the upper end stop you need to increase the meter range. You can either use a higher meter range for all the tests and live with reduced signal/noise at the lower levels, or switch the meter setting as the level increases and offset the measured curves accordingly (though the steps on the meter will not be exactly those indicated depending on component tolerances).

burzel2

Discussion Starter · #16 · Mar 19, 2009

2

It appears to be a measurement issue (fantastic advice from everyone by the way!). The following plot has the mic right next to the sub. I set the meter to 80 and ran the sweep.

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For the next test I adjusted the meter as it got louder. I re calibrated each time and verified by testing at the same volume as the previous run. The meter appears to be accurate even when the needle becomes pegged. However it becomes inaccurate beyond a certain point. For example when it's pegged during almost the entire sweep. Here's the plot with it adjusted and calibrated. I also set the crossover to 60 Hz on this run.

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I'll re run the sweeps at the listening position this weekend. It's too loud to test with the family around. Some quick tests show the traces look identical at different volumes. Looks like there is no compression happening. My 45 Hz spike just goes up and up.

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brucek

· #17 · Mar 19, 2009

The meter appears to be accurate even when the needle becomes pegged.

As John suggested above, you only need to examine the Scope to see if there is distortion from the meter.

Are you familiar with interpreting the Scope information?

brucek

DrWho

· #18 · Mar 19, 2009

Ah good, I'm glad to see things are starting to make sense :T

Btw, I was looking at your plots earlier....I bet you can get that 70Hz dip to move around by changing the height of your meter.

burzel2

Discussion Starter · #19 · Mar 19, 2009

Are you familiar with interpreting the Scope information?

Negative. I looked at it, but didn't dive into the help section. Any quick pointers?

I bet you can get that 70Hz dip to move around by changing the height of your meter

Unfortunately it's at the same height as my head in the main listening position. But I'll try some tests to see the effects of height. I did a few tests with the sub on it's side and at various heights. Some things looked better and some looked worse. I'm thinking of building another sealed enclosure that's a bit smaller with a front facing driver.

The 70 Hz dip is smoothed out a bit when driven with the mains, but both the sub and mains have that spike at 45 Hz. I can't get rid of it and have the sub in a wife approved location! I might have to bite the bullet and build two smaller subs.

brucek

· #20 · Mar 19, 2009

2

interpreting the Scope information

The scope is available after each measure and is lost when you do the next measure, so you do have to look directly after a measure.

It simply gives a picture of the output and input signal.

See a typical measure below where the output is the purple signal from the soundcard and the red is the input signal once it's received by the soundcard. See I have one peak there received that is about 80% of full scale (FS). That's a good sweep signal as long as the meter itself wasn't clipping.

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Hard to see anything when you look at the entire horizontal axis of the sweep, so........

Below is an expanded horizontal scale version and you can see the output and input are not clipped. If I increased the input level control so that the VU meter on the Settings page showed more than 0dBFS, then the red signal would be as large as the output signal and you would see the red signal clipping (flat top). If on the other hand, the signal from the meter was clipping, you would see a flat top or other distortion of the input signal without it being 100%FS. This is likely the case when you leave an SPL meter on a lower scale and measure high SPL readings (your case). This is one of the other advantages of a microphone over an SPL meter for measuring audio.

Text Line Plot Pink Magenta

brucek

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