# REW repeatability?



## robertcharles123 (Jan 12, 2008)

elee532 said:


> I gave this a try on another computer... 0 to 200Hz sweep using 128, 256k, 512k, and 1M sweep lengths (graph 1, seperated traces is turned on). Graph 2 shows 0 to 20,000Hz sweeps at 128k and 1M.
> 
> Computer 1 is my HTPC... 2.5ghz dual core, Gigabyte GA-E7AUM-DS2H motherboard using the onboard Realtek Digitial Input and Output.
> 
> ...


This is a perfect example of what I have been saying for quite some time. I just do not trust all of the measurements I see from some of the subs being tested. I just cannot beleive that at 75 dB we can get so much room gain from some of the systems being tested. I would think that most systems would somewhat resemble the results of the modeling software- within reason. Some room gain should be expected, but for the graphs to roll up below tuning is hard for me to beleive. That is why I keep asking about repeatability with different computers and equipment. I am super happy you got some good data. Repeatability is my biggest problem with this setup. I am getting close with my REW setup, but I want to be able to judge my results against another independent tester. That was the purpose of my GTG, but little interest has been given to the idea. Maybe at a different time of the year.

Good job finding out about the measuring problem.

Thanks,

Robert


----------



## Wayne A. Pflughaupt (Apr 13, 2006)

*Re: Comments on my first measurements?*



> I just cannot beleive that at 75 dB we can get so much room gain from some of the systems being tested.


SPL level has no correlation with room gain, but I'm intrigued by your issues with repeatability. Could you elaborate?

Regards,
Wayne


----------



## robertcharles123 (Jan 12, 2008)

*Re: Comments on my first measurements?*

Well, I feel that if you measure the driver and the spl of the signal- at the driver is 75dB, you are going to get a different reading concerning room gain than if you measure listening position of 75dB. It has to make a difference. My room is 4800 cf. When I put the mic at 1/2" from the cone and point the mic directly at the cone, I get completely different graphs. I am, at that point measuring the driver itself. As I back the mic up so it won't hit the cone, my graphs change shape and then I start getting room interactions, but nothing like a slope going up below the tuning. For instance, At the GTG, a few months back, we used 2 different microphones at 2 different locations and got pretty much the same looking measurements. The room was causing some differences, but a roll off at 20- 25 Hz measure at 3 feet for one particular sub did not result in a graph at the opposite wall measure showing a rise beyond the tuning. The graphs just showed less rolloff per octave. In the graph from the previous post, I would almost certainly think that the measures were in error due to the fact that they look so different and are from the same position. 

I need to go, but I need to get a handle on this, so please bare with me. 

Thanks, 

Robert


----------



## robertcharles123 (Jan 12, 2008)

*Re: Comments on my first measurements?*










This was my first measure at listening position. I know it is not really scaled correctly, but disregarding that, if I accepted the response measure as "good", I would be seeing extension down to 2Hz. I did not believe this, so I kept measuring and checking the adjustments and calibrations.

Now my graphs are different again, but very closely resemble what was modeled when designing the system. 










Here are one driver and top and bottom passives measured 1/2 inch from the cones. My newest graphs at listening position are resembling this general shape-give or take, but no upslope below the cabinet tuning. In order for me to believe that that can occur, I would have to put my sub in the room where this occurs and use it against the sub that is tested there now and run multipleREW setups on different computers to see if it is a computing error or measuring error or actual reality. I have yet to experience it myself and I wat to do just that.

Any better? I want to understand How this works and do it correctly. I want accurate and repeatable results that can be done by anyone and the result remains the same.

Thanks,
'
Robert











This is a measure of my subs at listening position-basically in the center of the room.

I could and can't repeat this measure now, although I was getting this measure or something like it for quite some time.


----------



## Wayne A. Pflughaupt (Apr 13, 2006)

robertcharles123 said:


> Well, I feel that if you measure the driver and the spl of the signal- at the driver is 75dB, you are going to get a different reading concerning room gain than if you measure listening position of 75dB. It has to make a difference.


Well sure, close-mice measurements remove most of the influence of room gain.



> For instance, At the GTG, a few months back, we used 2 different microphones at 2 different locations and got pretty much the same looking measurements. The room was causing some differences, but a roll off at 20- 25 Hz measure at 3 feet for one particular sub did not result in a graph at the opposite wall measure showing a rise beyond the tuning. The graphs just showed less rolloff per octave. In the graph from the previous post, I would almost certainly think that the measures were in error due to the fact that they look so different and are from the same position.


Certainly, and elee532 told us he had found that his computer was the source of the problem.

Regards,
Wayne


----------



## Wayne A. Pflughaupt (Apr 13, 2006)

robertcharles123 said:


> This was my first measure at listening position. I know it is not really scaled correctly, but disregarding that, if I accepted the response measure as "good", I would be seeing extension down to 2Hz. I did not believe this, so I kept measuring and checking the adjustments and calibrations.


Due to the extremely low level (30-40 dB??) this graph would be essentially useless – mostly noise from the system and room background noise. Plus, I don’t think anyone would say the Radio Shack meter (if that was what you were using) is going to be accurate much below 20 Hz.



> Now my graphs are different again, but very closely resemble what was modeled when designing the system.
> 
> 
> 
> ...


I’m afraid you lost me as to exactly what the various measurements in that graph were…




> This is a measure of my subs at listening position-basically in the center of the room.
> 
> I could and can't repeat this measure now, although I was getting this measure or something like it for quite some time.


What do your current graphs look like?

Regards,
Wayne


----------



## robertcharles123 (Jan 12, 2008)

Wayne A. Pflughaupt said:


> Due to the extremely low level (30-40 dB??) this graph would be essentially useless – mostly noise from the system and room background noise. Plus, I don’t think anyone would say the Radio Shack meter (if that was what you were using) is going to be accurate much below 20 Hz.
> 
> I’m afraid you lost me as to exactly what the various measurements in that graph were…
> 
> ...


The graph with the three measures are the driver and the upper and lower passive 1/2" from the cone. 

I will get connected tonight and run some stuff and see if I can gleen some knowledge from a few guys here to get some descent results.


----------



## robertcharles123 (Jan 12, 2008)

I am using a cm 140 not a radioshack meter


----------



## DrWho (Sep 27, 2006)

robertcharles123 said:


> This is a perfect example of what I have been saying for quite some time. I just do not trust all of the measurements I see from some of the subs being tested


Dynamic range (the volume difference between the loudest signal and the noise floor) of the measuring system can play a huge role when measuring low frequencies. If you look at all the calibration files, there's a lot of internal digital gain to compensate for the natural roll off of electronics and of the measurement microphones being used. While this gain is trying to offset what the system is doing, it is also going to increase the low frequency content of the noise floor of the system. So if your measurement's output is near the noise floor of the measurement rig, then you're going to see energy at the low frequencies that didn't come from the speaker itself.

This is of course assuming that all of the calibration was done correctly. I personally prefer to run without calibration files (although I'll post my results with the calibration files in place) because I can learn to recognize the behavior of the microphone and distinguish it from the behavior of the system I'm trying to measure.

Also, the purpose of the frequency response graph is not to tell you that your system sounds good. There are all sorts of ways to have a system that measures flat, but that doesn't mean all those ways are going to sound good. It's also interesting to observe just how much low frequency extension is overstated in its effects on perceived sound quality.

And lastly, the room gain of every room is very different and is a function of the room volume, how rigid the walls are and how well sealed the room is.


----------



## robertcharles123 (Jan 12, 2008)

Wayne A. Pflughaupt said:


> Due to the extremely low level (30-40 dB??) this graph would be essentially useless – mostly noise from the system and room background noise. Plus, I don’t think anyone would say the Radio Shack meter (if that was what you were using) is going to be accurate much below 20 Hz.
> 
> I’m afraid you lost me as to exactly what the various measurements in that graph were…
> 
> ...












Red line is listening position with no mic cal file.

Blue line is listening position with mic cal file

I am running my cabs through a crown ITech 8000. It has a built in HPF at 8 Hz. It is not defeatable. I am getting a couple of FACE amps soon, and I will measure with them and see if any changes occur.

Any thoughts?

Here are my LFE cabs.


----------



## Ricci (May 23, 2007)

DrWho said:


> Dynamic range (the volume difference between the loudest signal and the noise floor) of the measuring system can play a huge role when measuring low frequencies. If you look at all the calibration files, there's a lot of internal digital gain to compensate for the natural roll off of electronics and of the measurement microphones being used. While this gain is trying to offset what the system is doing, it is also going to increase the low frequency content of the noise floor of the system. So if your measurement's output is near the noise floor of the measurement rig, then you're going to see energy at the low frequencies that didn't come from the speaker itself..


Great post.:T I've been trying to educate people on this, but I don't think I've been able to word it so succinctly.

20db of calibration for your mic/meter plus another couple of db from your soundcard added to 45 or 50db of low frequency background noise = Voila! 75-80db of apparent <15hz response and..."Wow my ported 150w 12" White van sub is really digging deep!"


----------



## DrWho (Sep 27, 2006)

robertcharles123 said:


> I am running my cabs through a crown ITech 8000. It has a built in HPF at 8 Hz. It is not defeatable. I am getting a couple of FACE amps soon, and I will measure with them and see if any changes occur.
> 
> Any thoughts?


You are 10dB down by 34Hz...I wouldn't worry about the 8Hz highpass right now.


----------



## Wayne A. Pflughaupt (Apr 13, 2006)

Ricci said:


> Great post.:T I've been trying to educate people on this, but I don't think I've been able to word it so succinctly.


Yes indeed, that was an excellent post. Filed away for future reference. :T



robertcharles123 said:


> Any thoughts?


Only that I’m confused about the repeatability question. Are you telling us that these readings were taken in the same location, with the same sub? I would have a hard time believing that...




















Regards,
Wayne


----------



## Ricci (May 23, 2007)

Robert,

How about a little more background on these measurements? Just the LMS subs, or with mains, or other JBL subs? What crossover freq? Any EQ enabled in those Itech's or in your processor? Audyssey? What microphone do you have? I'd highly suggest getting one of the calibrated ones available here from Cross Spectrum labs if you haven't already for about $90. What sound card?

The 35hz and over response is not too bad with just the 90hz dip. The action in the 16-32hz octave is bad though and this is where those big subs should be at their best. You drop some 20db from 38hz to 28hz and then staying at 10db or more lower overall level than the 80-35hz range. This is a strange variation in response to me. Almost like you have some cancellation, or EQ going on.


----------



## robertcharles123 (Jan 12, 2008)

Those measures are taken at listening position with the soundcard plugged directly into the amplifier. Nothin else in the mix at that point- just the Itech8000 to the cabs.

What do you think could be the problem?


----------



## robertcharles123 (Jan 12, 2008)

No crossover- just the 0-200 sweep into the amp.


----------



## robertcharles123 (Jan 12, 2008)

I honestly do not think my computer is powerfl enough to handle the setup. It seems like it glitches at times when it is sweeping. That is the reason why I want to use another person's computer who has good repeatable measurements in order to compare graphs. I will say, regardless what these graphs say at this point, I have never heard or felt better bass in a home environment or theater for that matter, so I am satisfied either way. I just want to make sure I am getting the most out of the setup and learn to use these programs correctly.

Any ideas?


----------



## robertcharles123 (Jan 12, 2008)

Wayne A. Pflughaupt said:


> Yes indeed, that was an excellent post. Filed away for future reference. :T
> 
> Only that I’m confused about the repeatability question. Are you telling us that these readings were taken in the same location, with the same sub? I would have a hard time believing that...
> 
> ...



That is what I have been trying to tell you. I got some really good measures that were relartively flat at one time, only to lose the use of the program to a virus and now you see what I get. I am saying that all of those graphs are from the same spot where my head would be. the mic is on a stand and held in the same spot throughout.

Any thoughts as what to do to get the measurements to be reasonable and believable?


----------



## robertcharles123 (Jan 12, 2008)

This is a graph of listening position the night I got REW working again. This was about 2 weeks ago.


----------



## Wayne A. Pflughaupt (Apr 13, 2006)

The only time I've seen response differences that drastic has been when people moved the sub to a different location. If everything is sounding good to you, I'd sure be questioning what your getting from REW. You might download some sine waves from the BFD Guide for 20 Hz and 40 Hz. If you register a 10-12 dB difference between them that would confirm whether or not REW is in the ball park. 

Regards,
Wayne


----------



## robertcharles123 (Jan 12, 2008)

Cool. There really is no moving my cabinets. They require lots of planning in order to get from one point to another. I was thinking of moving them a little, but other things are going to be in the way, so there they stay. I will try what you said. I might need a little coaching. Thanks.

Thanks,

Robert


----------



## DrWho (Sep 27, 2006)

Try turning off your wi-fi when taking measurements...or anything else that does automatic stuff in the background.


----------



## robertcharles123 (Jan 12, 2008)

I turned off the internet and the connection last night for the last measures. Funny you say that about the noise. I had a lot of noticeable noise when the rca cable was plugged in, I took it out and used another, I am going to have to use a good plug and see what the story turns out to be. I will try that later on.


----------



## robertcharles123 (Jan 12, 2008)

So I should play a 20 Hz sine wave and get a SPL.

Then, measure a 40 Hz sine wave and get a SPL.

No changes should be made in the setup from measure to measure-whatsoever?

I will do this.

What about the tone generator in the REW?


----------



## robertcharles123 (Jan 12, 2008)

DrWho said:


> Try turning off your wi-fi when taking measurements...or anything else that does automatic stuff in the background.



You know, I bought the crowns as b stock. They might have a filter turned on somewhere, I just do not know how to tell. I am going to run another amp to the cabs and see what happens. Sooooon!


----------



## robertcharles123 (Jan 12, 2008)

I tried another amp and no change in graphs, so amp seems to be ok.

Next, I ran the 10 Hz, 20 Hz and 40 Hz tones- I got 89.5 dB for 10 Hz
93.4 dB for 20 Hz
110.2 dB for 40 Hz

When that 40 Hz tone started, it was very intense.

Also, I ran a 100HZ to 1HZ sweep and it started at about 86 dB and stayed pretty much between 85 and 93 and got no lower than 86 on the meter before it stopped and the meter dropped off instantly. If correction values were able to be added, The numbers would be very high at the lower Hz readings. The 100Hz to 1Hz sweep seemed to be more level and much more intense at the end of the sweep than the 0-200 sweep is at the beginning. 

When I do the 0-200 sweep, the sound seems to build and has a few nulls in the sweep, but the 100Hz to 1Hz sweep did not have that type of reaction. It actually felt like it built up as the Hz decreased and it showed it on the Galaxy CM140. If I were to be able to do a reverse sweep on REW, I would definitely see better results. Any way to do reverse sweep on REW?


----------



## brucek (Apr 11, 2006)

Use the "Frequency Tracks Cursor" checkbox with the signal generator and you can drag the cursor around the REW graph plot and the frequency will follow. Turn on the signal generator and drag the cursor around to create any manual sweep you like.

You could also do a standard measurement first so you have a plot on the graph to compare the sound against while you use the 'Frequency tracks cursor' feature. If the plot shows a dip, drag the cursor there and move it around the dip and see if the sound agrees.......

brucek


----------



## robertcharles123 (Jan 12, 2008)

I just found this little bit.


[PDF]The Legend Lives On - The Dolphin Group - MK Sound ...
Vented subwoofers and subwoofers with passive radiators roll off too sharply (24 . dB/octave) to take advantage of Room Gain. ...

www.thedolphingroup.org/pdfs/MK_Sound_march_09.pdf - Similar pages 


Would this have anything to do with the funny looking graphs I am getting?

I did not have a chance to do more measures, but I did see the generator and curser option.

Thanks,

Robert

By the way, I am going to test a sealed sub in my HT and see how that goes.


----------



## robertcharles123 (Jan 12, 2008)

brucek said:


> Use the "Frequency Tracks Cursor" checkbox with the signal generator and you can drag the cursor around the REW graph plot and the frequency will follow. Turn on the signal generator and drag the cursor around to create any manual sweep you like.
> 
> You could also do a standard measurement first so you have a plot on the graph to compare the sound against while you use the 'Frequency tracks cursor' feature. If the plot shows a dip, drag the cursor there and move it around the dip and see if the sound agrees.......
> 
> brucek


OK. I have a quick question I hope could make a difference. I used the cursor feature and just played around to get the feel for it. Before I get to involved with it, I need a little bit more info if possible. When I drag the cursor onto the frequency and let it play, I seem to be getting the SPL reading at the Galaxy at the listening position that corresponds to the number on the Galaxy. If there is a calibration file being added to the graphs, shouldn't the readings be of by the cal file number corresponding to the frequency being tested. For instance, when I got a reading of 64 dB on the meter, it read 64 on the graph for 10 Hz. If a cal file was used to create the graph, shouldn't the reading be 64+13.45 dB?

Also, I ran some sine 10 Hz waves and put the SPL where the mic was in a corner for some of the measures. I get a 15 dB higher reading than when I went right up to the cabinets. But when I run the sweeps, I do not get that higher reading on the graphs. It still shows a serious roll off. Any thoughts? I can actually feel the pressure increase tremendously when I walk from the cabinet to the corner of the room. So there is room gain down low, but I quess I won't be able to benefit from it because of the position of the cabinets.

Thanks,

Robert


----------



## brucek (Apr 11, 2006)

> If a cal file was used to create the graph, shouldn't the reading be 64+13.45 dB?


The soundcard and meter calibration files are only added after a response sweep measurement is taken and applied to the graphs.
The SPL Calibrate routine is a rough calibration using a band limited pink noise to ensure the real SPL meter and REW SPL meters are within a ball park of each other.



> But when I run the sweeps, I do not get that higher reading on the graphs.


I would think you should.

brucek


----------



## robertcharles123 (Jan 12, 2008)

brucek said:


> The soundcard and meter calibration files are only added after a response sweep measurement is taken and applied to the graphs.
> The SPL Calibrate routine is a rough calibration using a band limited pink noise to ensure the real SPL meter and REW SPL meters are within a ball park of each other.
> 
> 
> ...


So, would it be possible to 1st take a graph and use that graph to see the effects of a cal file? I would like to look at one graph and edit that graph. Is that possible?


Or, do I just have to take a graph with the cal file and then take another without?


Thanks,

Robert


----------



## brucek (Apr 11, 2006)

> So, would it be possible to 1st take a graph and use that graph to see the effects of a cal file? I would like to look at one graph and edit that graph. Is that possible?


Sure, although I can't imagine why since the mic being used requires some offset from the cal file to render it flat.

Either way, simply clear the mic cal file and take your measure. Then if you want to add the cal file to that measure, load the cal file and click Apply Windows in the IR Windows popup. This will apply the file.

You can in fact, take any measurement you've already taken and remove the cal file effect by clearing the file and clicking Apply Windows. You see, all your measurements with REW are taken without any soundcard or mic calibration files - they are simply added later. REW takes a raw measurement and creates the impulse response. When the frequency response plot is generated from that IR response, the cal files are then added. So, you can add or remove them from the response graphs whenever you want.

brucek


----------



## robcart65 (Oct 10, 2009)

Thanks a lot


----------



## robertcharles123 (Jan 12, 2008)

brucek said:


> Sure, although I can't imagine why since the mic being used requires some offset from the cal file to render it flat.
> 
> Either way, simply clear the mic cal file and take your measure. Then if you want to add the cal file to that measure, load the cal file and click Apply Windows in the IR Windows popup. This will apply the file.
> 
> ...


I see how that process is performed and I like thank you for the info.

Another ?

I have an older computer and way less than 1 GB RAM. I seem to be leaning toward that being the reason why I am getting some really oddball measures. Maybe not the complete reason, but maybe something to it???

Any thoughts?


Thanks,

Robert


----------



## brucek (Apr 11, 2006)

> I have an older computer and way less than 1 GB RAM. I seem to be leaning toward that being the reason why I am getting some really oddball measures. Maybe not the complete reason, but maybe something to it???


I can't say for sure, but a computer with "way less than 1 GB" may be a bit challenged to run REW. If the operating system gets busy with some other duties and you get some hitching during the sweep, it can offer some poor results. You do require a clean sweep - if you hear any hitch, redo it.

brucek


----------



## robertcharles123 (Jan 12, 2008)

brucek said:


> I can't say for sure, but a computer with "way less than 1 GB" may be a bit challenged to run REW. If the operating system gets busy with some other duties and you get some hitching during the sweep, it can offer some poor results. You do require a clean sweep - if you hear any hitch, redo it.
> 
> brucek


I have definitely experienced "hitching". 

A new computer should do it. They all have multi GB of RAM.

We will see. Either way, I will know the measures are close once another computer is used.

Thanks,

Robert


----------



## glaufman (Nov 25, 2007)

robertcharles123 said:


> I have definitely experienced "hitching".
> 
> A new computer should do it. They all have multi GB of RAM.
> 
> ...


If it really is just a memory issue, you usually can add memory to an old computer for a fraction of what it would've cost when the computer was purchased, and for a fraction of the price of the new computer... I just refurbed my wife's machine, maxing out the memory for $35 delivered where it would've been hundreds when we bought it. Not quite as fast as my machine with the modern processor, but almost...


----------

