Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Ivan Beaver]

Alright, call it a 2ohm box and give the amp an easier life. I don't think I'd ever put a pair of 3ohm loads across an amp unless it was an absolute emergency.

Chris

But the issue is not so much the number of the lowest point on the impedance curve.

But rather the overall impedance.

THAT is why a graph of the curve is important, NOT just a simple number.

What if it only drops down to the 3.3 ohm for a small range of freq-but yet the other freq in its operating range are much higher?

If those couple of notes are not hit, then it does not present a load (at least enough to be considered) on the amp.

Once again-this is where the "simple single number" gets people into trouble.

And it is not just a curve-but also the RESOLUTION of the curve.  You could put a bunch of smoothing on my example and raise the "apparent" 3.3 ohm to a higher impedance.

But if you look at unsmoothed data (depending on the number of actual data points-that is another way to "cheat", use unsmoothed but few data points and it will average out) you can see more where the peaks and dips are in the impedance curve.

Then you can get a better understanding of the actual load on the amp across the operating range.

[Mark Wilkinson]

Ivan brings up a valid point... "What is the bandwidth"? It's not really possible to cover every sub or speaker with a single generic bandwidth. Some subs go below 20Hz and others offer nothing much below 40Hz. The same could be said above 100Hz. It's not going to be fair to rate sensitivity by using the same bandwidth for every cab.

Another thing I agree with Ivan on is NOT using a noise signal for sensitivity, impedance or SPL specs, for the reasons he has brought up. I prefer a swept sine measurement signal.

I completely agree with all your testing methodology, and have really learned a lot from your excellent site..which BTW, thanks for it!
But I still think the SPL-LEQ methodology has a lot of merit....for a sensitivity spec, and  for the +/- response specs put into words.

One question first pls: 
Do you get better magnitude and phase measurements with swept sine, than with noise (ala smaart or systune etc)
I've compared mag and phase many times with smaart's transfer pink noise vs REW's swept sine, and consider the measurements equal.
Especially so when any smoothing is added.

I understand that swept sine is needed for harmonic distortion, and maybe offers more gating alternatives than pink noise.
But for mag and phase only, does swept sine offer any real advantage?


Going back to LEQ using noise...I'll give another try at why I like it...

OK, say you've built a new sub and it's time to write up the specs.

You run a swept sine measurement to get raw mag and phase response (with some chosen reference voltage)

You then eye ball what the rough bandwidth looks like, and what SPL level kinda runs through the middle of the bandwidth to get a sensitivity SPL, say 102dB for example.

If you're honest, you use that eyeballed sensitivity spec to define the -3dB, -6dB, etc, response corners, which then becomes the implied if not outright claimed bandwidth.  So you've ended up with a bandwidth spec after all...

Now, how well does your sensitivity spec match through that bandwidth?
Well, perfectly, everywhere the response curve hits right on 102dB. 
But how about the + and - variations, how well does the sensitivity spec represent the average sensitivity through the bandwidth?

Put HP and LP filters in place that preserve the corners you listed, run a noise based LEQ-SPL at the same chosen reference voltage,
and you'll see how good you did at eyeballing the sensitivity spec, that is how close to 102dB LEQ measures.

[Josh Ricci]

But I still think the SPL-LEQ methodology has a lot of merit....for a sensitivity spec, and  for the +/- response specs put into words.

One question first pls: 
Do you get better magnitude and phase measurements with swept sine, than with noise (ala smaart or systune etc)
I've compared mag and phase many times with smaart's transfer pink noise vs REW's swept sine, and consider the measurements equal.
Especially so when any smoothing is added.

I understand that swept sine is needed for harmonic distortion, and maybe offers more gating alternatives than pink noise.
But for mag and phase only, does swept sine offer any real advantage?


Going back to LEQ using noise...I'll give another try at why I like it...

OK, say you've built a new sub and it's time to write up the specs.

You run a swept sine measurement to get raw mag and phase response (with some chosen reference voltage)

You then eye ball what the rough bandwidth looks like, and what SPL level kinda runs through the middle of the bandwidth to get a sensitivity SPL, say 102dB for example.

If you're honest, you use that eyeballed sensitivity spec to define the -3dB, -6dB, etc, response corners, which then becomes the implied if not outright claimed bandwidth.  So you've ended up with a bandwidth spec after all...

Now, how well does your sensitivity spec match through that bandwidth?
Well, perfectly everywhere the response curve hits right on 102dB. 
But how about the + and - variations, how well does the sensitivity spec represent the average sensitivity through the bandwidth?

Put HP and LP filters in place that preserve the corners you listed, run a noise based LEQ-SPL at the same chosen reference voltage,
and you'll see how good a job you did at eyeballing the sensitivity spec.

Thanks...

Sine measurements can provide more detail and higher resolution. I always use very long / slow settings for better background noise rejection. Noise based measurements are mostly the same but gloss over some edges here and there. You are correct if you will be throwing on 1/6th octave smoothing it's mostly irrelevant. An easy way to see this difference is to do an impedance measurement on a driver or cabinet. A very slow sine wave capture will result in slightly higher peaks and a bit more detail vs PN. I've found that the resulting calculated driver parameters are closer to factory specs using sines. A few of the manufacturer's I've talked to have said it is what they also use to capture the impedance curve so that makes sense that the match is better that way. See attachment.

I don't see anything inherently wrong with Leq averaging but I see potential issues in a few areas.
What weighting do you use? A, C, Z?
What type of noise? PN, WN, periodic?
What bandwidth is the noise?
2 volts of sine wave and 2 volts average noise. Crest factor of the noise signal?
The calculated SPL is an average. I mentioned in an earlier post that this requires converting to Pa and then back to dB SPL (done inside the program or meter typically) . This type of averaging is always going to anchor itself primarily to the loudest peak in response. The lions share of recorded SPL will be from the peak area. This favors speakers with a less linear response when compared to another that has ruler flat response over a greater bandwidth. This alone is why I consider a measured voltage response graph as necessary whether the single spec is Leq derived or by other means.

You don't happen to have a comparison of a sine sweep based measurement at a known voltage and the Leq SPL of the same cab at the same average voltage do you? I'd like to see a comparison. I might mess around with it the next time I have the equipment out.

[Ivan Beaver]

Thanks...

Sine measurements can provide more detail and higher resolution. I always use very long / slow settings for better background noise rejection.

With the TEF, you MUST use long slow sweeps in order to get accurate low freq information.

As a general rule you need to have the resolution to be 1/10th of the lowest expected (typically -3dB point) freq.

If you use a fast sweep the display will "look" like the cabinet rolls of much higher than it actually does.

Once again-just because it shows up on a screen DOES NOT mean that it is accurate-just accurate for the data points in the measurement parameters.

This goes for EVERY measurement program.  There are all sorts of places in various programs that can easily give you a erroneous reading

[Mark Wilkinson]

I don't see anything inherently wrong with Leq averaging but I see potential issues in a few areas.
What weighting do you use? A, C, Z?
What type of noise? PN, WN, periodic?
What bandwidth is the noise?
2 volts of sine wave and 2 volts average noise. Crest factor of the noise signal?
The calculated SPL is an average. I mentioned in an earlier post that this requires converting to Pa and then back to dB SPL (done inside the program or meter typically) . This type of averaging is always going to anchor itself primarily to the loudest peak in response. The lions share of recorded SPL will be from the peak area. This favors speakers with a less linear response when compared to another that has ruler flat response over a greater bandwidth. This alone is why I consider a measured voltage response graph as necessary whether the single spec is Leq derived or by other means.

You don't happen to have a comparison of a sine sweep based measurement at a known voltage and the Leq SPL of the same cab at the same average voltage do you? I'd like to see a comparison. I might mess around with it the next time I have the equipment out.

Thanks Josh,

I use flat full-spectrum PN (from smaart, 12 dB crest factor).
That signal gets band limited by speaker processor.  Typically, I use linear phase 48dB BW HP and LP filters at chosen corners.
Hopefully, crest factors aren't an issue.  I set the bandpassed PN RMS voltage to the voltage used in sine sweeps, using a fluke 189 voltmeter.  I guess alot depends on how well the fluke makes RMS time-averaged measurements of pink noise.
I average the simultaneous RMS voltage and LEQ-SPL, for about two minutes or so....after not seeing any average variance past about a minute....

I'll try to dig up some old sub comparisons of REW sweeps vs smaart LEQ at the same RMS voltage.  I got convinced of SPL equivalence some time ago, and haven't made any recent comparisons. If I can't find any, I'll run a new comparison since I've ballyhooed this so much.
 

[Josh Ricci]

Thanks Josh,

I use flat full-spectrum PN (from smaart, 12 dB crest factor).
That signal gets band limited by speaker processor.  Typically, I use linear phase 48dB BW HP and LP filters at chosen corners.
Hopefully, crest factors aren't an issue.  I set the bandpassed PN RMS voltage to the voltage used in sine sweeps, using a fluke 189 voltmeter.  I guess alot depends on how well the fluke makes RMS time-averaged measurements of pink noise.
I average the simultaneous RMS voltage and LEQ-SPL, for about two minutes or so....after not seeing any average variance past about a minute....

I'll try to dig up some old sub comparisons of REW sweeps vs smaart LEQ at the same RMS voltage.  I got convinced of SPL equivalence some time ago, and haven't made any recent comparisons. If I can't find any, I'll run a new comparison since I've ballyhooed this so much.
 

Good deal. I'd appreciate seeing the comparison. I'll go ahead and put this on my list of to do items also. I need to look into REW's distortion limited burst testing function as well and compare it with what I get in IgorPro with Don Keele's app, so I'll probably look into both at the same time. I don't have Smaart so I'll probably just use REW for both PN and sine signals and see what I get. Might take me a few weeks though.

[Chris Grimshaw]

I need to look into REW's distortion limited burst testing function as well

Didn't know it'll do that!
Is that a built-in routine, or would it need setting for the CEA burst, open RTA and manually alter drive voltage until distortion passes the limit?
The Help file doesn't mention much about it.

I've just finished some 15" subs here that I'd love to put through their paces.

Chris

[Mark Wilkinson]

Didn't know it'll do that!
Is that a built-in routine, or would it need setting for the CEA burst, open RTA and manually alter drive voltage until distortion passes the limit?
The Help file doesn't mention much about it.

I've just finished some 15" subs here that I'd love to put through their paces.

Chris

Chris, it's recently added functionality.   
You really have to hand it to John M.  He is forever improving an already super program.
  https://www.avnirvana.com/resources/v5-19-beta-5-windows-installer-with-jre-allowing-multiple-instances.16/

@Josh, hope to make some new measurements today...maybe even get to try REW's stepped sine

[Josh Ricci]

Didn't know it'll do that!
Is that a built-in routine, or would it need setting for the CEA burst, open RTA and manually alter drive voltage until distortion passes the limit?
The Help file doesn't mention much about it.

I've just finished some 15" subs here that I'd love to put through their paces.

Chris

I'm not entirely sure. I very briefly messed with it when it was first added to the generator function. it still had some problems at that time but that's been well over a year and a number of updates ago. I don't think it is a full routine. I don't think it will auto check the distortion levels and give a pass or fail flag for example. I'll have to fire it up and play around to see what it is doing.

The other current systems that have this built in are IgorPro / Wavemetrics with the routine setup years back by Don K. ($600 or so for a license of IgorPro. Don's routine plug in is freeware or was many years ago.). CLIO also has this functionality. I've not tried it myself. REW would be by far the cheapest way to do the burst tests ( Good for testing subs, speakers, amps!).

Chris, it's recently added functionality.   
You really have to hand it to John M.  He is forever improving an already super program.
  https://www.avnirvana.com/resources/v5-19-beta-5-windows-installer-with-jre-allowing-multiple-instances.16/

@Josh, hope to make some new measurements today...maybe even get to try REW's stepped sine

Awesome.

Yes this is one of the reasons I use REW. Jon is hands on and easy to communicate with. If there is a problem or suggestion with the program he will work on it and next thing you know it's fixed or implemented. The high dollar, "pro", measurement systems do not behave with this type of direct feedback, implementation, or speed in my experience. Most of them have a user interface that looks and functions like something 20 years old comparatively. Some have less functionality. Simplicity and ease of use is high on my list of requirements.

[Mark Wilkinson]

Ok, here's an attempt to post a simultaneous sine sweep and a SPL-LEQ, using REW.
Sine sweep is a ported sub's raw response with linear phase HP and LP filters in place. 48dB BW at 25 and 100Hz.
SPL signal is periodic pink noise, band limited 30-100Hz.  I chose 30 and 100Hz because they are the f3 points.
The sub has a tuned f3 at 30Hz, the HPF is hopefully just there for protection.
Both signals were at -12dBFS. 
REW is really cool in that it holds the same correct sine equivalent RMS voltage for whatever specified bandwith pink you set.
Josh, the sub btw, was inspired by your BMS box...only it's a little larger @175L with a bit more port area.

Anyway, measured LEQ was 91.7dB, at 14.3vrms.  Measurements were at 8 meters.
That gives a 1 meter LEQ derived sensitivity of 95.6 dB @ 2.83v....which sounds right for this 8 ohm driver.
So on the graph below, LEQ of 91.8 (off a tenth) is plotted on the sine sweep....
Hope this all made sense...

[ProSoundWeb Community]