SPL Meter question (1 Viewer)

[Phaseshift]

I do not know the specifics of the setup where I sit; I am 8200 + miles from our lab in a hotel room, but I can assure you that my staff gave this program every bit the same care as Listen, MLSSA or Clio… Being the owner of a service company that is known for expertise in difficult test and measurement solutions, particularly related to repeatability and accuracy, I am 100% confident that our results are on the spot. If there is any question about the accuracy or validity of what I have posted, please let me know.

As I said earlier, a client paid for the results and we are not going to breach that by disclosing specifics. Their marketing department does monitor this and other forums and asked us to include REW as a “sideline” comparison. We actually did not originally agree as this is not a competitive suite for a couple of reasons-

1.It is freeware- written by enthusiasts and I bet, done on a “beer and burgers budget”. Frankly speaking, I commend the folks that wrote this program and think that they are doing the right thing. I am not a programmer, but I can see where the REW shortcomings show up are in areas that the hardware poses limits in one way or the other- not the fault of the developers who are doing this for the love of the game or have some long term plan. To the developers, drinks on me, I think you did a great job, particularly for what I bet you are paid for it.
2.Adjustability - As far as we know, there is not a MLS time window adjustment which is going to have a big negative impact on the repeatability and absolute measurement on the lower end of the scale. You can gate the measurements, but in the league where we are playing, the gated measurement is not allowed. Not fair to the REW camp.
3.Ultimate variability- The measurement is at the mercy of the sound card and whatever voodoo is going on there. And if that were not enough, your sound card, my sound card, our friend’s sound card and his brother's sound card are not likely to be the same and are damn sure not built with precision level components in the audio and processing chain. At least the others supply hardware. Again, not fair to the REW camp.
4.Tracking Filter - I am not sure about this, but it does not appear to have tracking filters. This is a big hit on repeatability and again, not fair when comparing free programs to those costing big $$$. If it does and I am wrong, I stand corected.
5.Commercial venture - The other suites are far more advanced in terms of additional measurements, adjustments, macros etc…. For a freeware program, it is not a reasonable comparison.

Other stuff, I can generalize-

•Mic- I believe that we used a cheapo mic to start with and tossed the results in favor of using a B&K or GRAS (typical for our measurement lab). I do know that our tech did run a electrical stimulus test through the REW to “calibrate” it as best as we could. This was mainly an exercise of seeing how much calibration curve would have to be applied electrically to the system, but the result is the same.
•Computer – I would have to guess on that. One of our lab computers that does not have a built in sound card- we hate those things. Running Win XP SP2 is the likely OS.
•Calibration files- Do not know and do not think that is the issue. Actually REW is relatively good above 500Hz or so. Below that, it is weak and I do not believe that due to calibration. With cal errors, you will usually see specifics in a curve signature that are there with every measurement- every stimulus and every transducer or system. With REW, we saw a good static response measurement and consistent curve signatures. The problem that we observed is that the measurements are not repeatable on the low end and and the absolute measurements were only loosely associated with calibrated measurements, including the pistonphone.

Maybe it seems I am being unfair to REW- I am not. I can put it in context as saying that I am accustomed to getting spot on results on audio and electrical measurements. We pay the bills by this stuff and it is up to me in my company whether to accept measurements or not. If someone gave me a curve from REW, I would question the accuracy below 500Hz based on what I have observed in our lab and listening room.

Is REW a cool program? Absolutely.

Is REW done by some cool folks? No question about it.

Could I write a better program? Not a chance; forget it.
Could the program be better? Yep, but keep in mind the cost.
Is it a lab quality program or suite? Unfortunately not, but I do not think it is intended to be that at all- My hat is off to the folks that are dedicated enough to write it for free and give it away for free. Great job; we need more folks like you in the industry.

If soneone wants to nit pick this to death, please check all my spelling, the facts and so on... I am doing this for free as are the REW people. So if you have some nit pic commentary...... My response will be something like- 'Yo momma.

Phase

 

[Jason Sweet]

My digital RS model displays the "0.1 to 0.9" range of a decibel as little squares under the number, so 75 with 3 squares on the display would be "75.3".

 

[John_Mulcahy]

Hello Phil,

I'm the author of REW. In the initial development of the app I used both MLSSA and AP Systems 1 and 2 as references (I was Technical Director of an audio company at the time) and didn't observe any measurement discrepancies, above or below 500Hz. It would be interesting if you or your staff could provide some more information on how the app was tested, as I'd like to understand how your results could occur - my email address is [email protected].

REW uses logarithmically swept sine signals as excitation for the greater resistance to nonlinearities they offer over MLS, so neither MLS time window adjustments nor tracking filters are relevant. The sweep length, number of sweeps to average and of course the impulse response windowing are all configurable by the user.

Regards,

 

[clubfoot]

Don't jump off the deep end, raise your shields or load your photon torpedoes just yet Phaseshift. It was a straight forward inquiry not intended to start a flame war.

As a user of REW, my questions were for educational and informational purposes and to gain insight into were or what was contributing to the errors or lack of precision your team observed. And I do appreciate you posting your findings.

As you pointed out there are many variables. As far as sound cards, REW creates a compensation file for "any" card that is configurable in the software by looping the output to input, so they are removed from affecting its measurements. Same goes for loading .cal flies for an SPL meter or measurement mic of your choosing.

 

[clubfoot]

To the OP, if you're just setting equal levels then the accuracy between the digital an analog meter is not that important, once all speaker outputs match. But if you need to know accurate numbers any meter you use will have to be calibrated.

 

[Sonnie Parker]

We've tested the Radio Shack meters to death and there is really nothing new been revealed here in this thread.

I would consider the Galaxy CM-140 SPL Meter before I would buy the RS Meter. It still needs correction, but not near as much as the RS meters and they have proven to be fairly consistent, contrary to the RS meters.

 

[Phaseshift]

John- We calibrated per the instructions and I am sure that it was tested using 2 different sound cards since the results were not what we anticipated. I am not certain of the specifics other than once setup and running, a basic R&R test using REW was poor at the lower end of the scale. Compared to MLS with the time windowing properly set, the results are not nearly as repeatable. That is my point and that is the reason I said that I would question data from 500 and down with this program. This was clearly a bigger issue in a listening room environment where the source and mic positions were not changed in any way. The way you can do that with different mics is to have them all mounted and simply switch at the cable connector end- no movement of the mic, no movement of the source. All units were tested with all microphones BTW.

My comment about tracking filters is in reference to susceptibility of noise during the test cycle. We found REW to not have the noise immunity that the other do. Again, the others are charging big $$$ and REW is free. This is a great program and I commend you on the execution of it.

For the record- This is a good program for the intended use and I would strongly recommend it to individuals who are looking for a T&M system for home use. There is nothing wrong with REW for that application and it certainly beats the pants off a plain Jane SPL meter for setup work.

 

[John_Mulcahy]

Phil, I can only speculate that the impulse response windowing was incorrectly set, particularly in view of your comment of poor resolution above 500Hz - REW does not have much in the way of automated setting for the IR windowing, so perhaps the problems lay there. Out of interest, do you have a record of which REW software version was tested? Or failing that, the date on which the test was done?

 

[Phaseshift]

The issues I have observed are from 500Hz and down. I think that is what you meant, but want to clarify that.

I do not know what the version details are, but I am sure it is current as this test was done only 3 weeks back and the program was newly downloaded .

At some point, we will have an opportunity to compare in the listening room and I will ask the techs to keep an eye out for such a opportunity. When that happens, perhaps we can share more info, curve overlay etc...

 

[Wayne A. Pflughaupt]

Phil,

Why did you use REW? It wasn’t designed as a platform for testing microphones.

Regards,
Wayne A. Pflughaupt

 

[Phaseshift]

?? This is not about testing mics.

It was included in a test of meters / handhelds vs. known solid test platforms (AP / Listen / Clio / MLSSA). The microphone tests and comparisons are there for a couple of reasons, one being to demonstrate the similarity between the 2 upper end mics, the 2 mid level mics and the el-cheapo mics.

I really regret even posting this stuff in the first place. I do not think anyone actually got the point of the comparison and my comments about REW seem to be taken as blaspheme. Lesson learned.

 

[Wayne A. Pflughaupt]

Hee hee – I do know some people who are grousing pretty bitterly about it, but I fully understood the context of what you were talking about – i.e., big-buck lab-grade vs. not.

As to “testing mics,” the thread was opened as a discussion on SPL meters, and you had recently tested a few, along with some reference mics. That I got, and what you presented was excellent info. I just didn’t see what the platforms had to do with it – i.e., why that info was included at all, and especially a software program that wasn’t even designed for testing mics. Hope that makes sense...

Hey, you did note that it would be painful for some, so they were warned!

Myself, I want one of those Ivies!





Regards,
Wayne A. Pflughaupt

 

[Phaseshift]

Actually, we were using the 35 which is the Dell Axim based unit. I do not have enough "seat time" with the 35 to say how well the other features work, but I am betting that they are good. Even though we own a few of the NTI handheld analyzers, I can say for sure that the Ivie 35 is a better value even at higher cost. To be blunt about it, I wish we had known about the 35 before we spent so much on the NTI stuff. It is cool.

How does it compare with the 33 series (Wayne pictured it in his post)?? I do not know but would guess that the measurement performance is similar. The difference may be in futures more than anything.

The one that looks really cool to me is the 45- That is a neat machine and you have plenty of computing power to support your application. May have to ask Santa Clause for one 'o those....

 

[Wayne A. Pflughaupt]

I guess that would be this one? Not sure why it says “Samsung” on the front...





Here’s the 35, as long as we’re doing show-and-tell. Even cooler than the first one, looks like a 1/3-octave display. $1700 - ouch!





Regards,
Wayne A. Pflughaupt