# Room EQ Wizard Doubles as Crossover Design/Measurement Tool



## aktiondan

Is anyone else using REW to design crossovers? I've been working on a new set of MTM speakers for a friend's home theater and discovered that REW works great for measuring crossover networks. To some degree it will even do impedance plots and measure Zobel networks. See the example below. Though I don't believe the absolute impedance value is correct (i.e., the graph shows the resonant frequency of the woofer [red] to be at 39Hz and a dB level that might correspond with an impedance of only 8.7 ohms, when in fact it's more like twice or three times that (you'd need to convert the log function to a linear function), however the profile of the curve is accurate). Notice the classic impedance rise which is easily combated with a simple Zobel network. Using just 15uF cap and 8 ohm resistor I was able to measure the transfer function [purple] of the Zobel (into an 8 ohm resistive load) and then see what the resultant impedance plot of the woofer [green] looks like - much better! (The black line is my reference, which is an 8 ohm resistive load). From here I can easily adjust the capacitor and resistor to tweak the profile and achieve something that looks even better.

Anyway, this also works for crossover networks, (I've got a bunch of plots of those, too) as well as measuring the near-field response of woofers and tweeters. So far I haven't seen any inherent limitation in REW that would indicate it isn't capable of doing these kinds of measurements (other than the proper scaling for impedance). But there is one thing I would love to see - the ability to export the graph to a .csv file. That way impedance plots and measured FR plots of drivers could be imported into Passive Crossover Designer 5.00 and then used for actually designing crossover networks (instead of relying on mfg's data) and then once the crossovers are designed, REW can be used to measure them and tweak them. And of course once the speaker is built, you can measure the resultant 20-20kHz response of whole speaker.

Just thought I'd share, REW has proved to be invaluable to me, and I am glad I ran across this site. Sorry this is my first post and kinda long, but this forum is great.


----------



## brucek

Welcome Dan..... yeah, we use REW for all kinds of in/out measurements. I don't remember anyone using it for crossovers, so that's a new one for sure. I love the tuning of the Zobel - very cool.

We definitely measure receiver/ processor, line driver and BFD equalizer responses. One of the things you can do when you use an ECM microphone and preamp for audio measurements is to put the preamp in the cable loop when you are creating the soundcard.cal file and it will increase the accuracy of your measurements by compensating for any response HPF effects from the preamp electronics when the file is created....

I used REW to find the correct spot on the poorly marked dial of my X-30 crossover box that corresponded to the crossover frequency I needed.

brucek


----------



## Chrisbee

Dan

How exactly did you do the impedance plot? 

No great accuracy required just a bump in roughly the right place to check Fs.

Thanks


----------



## aktiondan

Put a 100 ohm, 10W resistor in series with your driver. Then connect the input of your sound card across terminals of the driver (in place of your mic/SPL meter). Make sure you turn off any mic/SPL calibrations curves, but be sure to use the sound card cal. Then you set up REW just like you would for a regular measurement and hit go. The graph that results has just measured impedance instead of SPL. Hope that was descriptive enough, but it's not too tough and works great for determining fs of a driver.


----------



## Chrisbee

Perfect! Thanks. :T


----------



## terry j

not sure I followed it exactly, will probably make sense when I do it for real.

Would that work once the driver is in the box??? ie would it tell you the tuning of the box be it vented or sealed or whatever?


----------



## aktiondan

It should make sense once you do it, but I'll try and take a picture or do a quick drawing or something. Yes, it works for a complete speaker. Here's a plot I just ran of a pair of two-way monitor speakers I built a year ago. Fb of the box lies between the first two dips, in this case it would be about 35Hz. I believe I was shooting for a tuning of 36Hz or so. If this were a sealed box, there would be a single peak at the low end, and Fb would be at the top of that peak.


----------



## Chrisbee

Dan

Just for clarification: The driver is put in series with the resistor between the input and output of the soundcard? 

I'm getting a tiny peak at 50Hz which doesn't make any sense at all.

I'm also getting a constant buzz on the driver.


----------



## aktiondan

The driver is connected to your amp/stereo with the 100 ohm resistor in series with the (+) terminal on the driver. The line out from the sound card goes into the CD/AUX in on the stereo. This is the conventional method for using REW I believe. Then you connect a pair of wires to the terminals on the speaker and run that to the line in on your sound card. Most sound cards are only line-level out (about 1Vrms) which isn't enough to drive the speaker straight - you need the external amp to do that. Let me know if that doesn't get you going.


----------



## Chrisbee

aktiondan said:


> The driver is connected to your amp/stereo with the 100 ohm resistor in series with the (+) terminal on the driver. The line out from the sound card goes into the CD/AUX in on the stereo. This is the conventional method for using REW I believe. Then you connect a pair of wires to the terminals on the speaker and run that to the line in on your sound card. Most sound cards are only line-level out (about 1Vrms) which isn't enough to drive the speaker straight - you need the external amp to do that. Let me know if that doesn't get you going.


I wish you'd mentioned the amplifier in your first post. 
I have just spent 5 hours trying to get a peak straight off the soundcard.


----------



## aktiondan

Sorry about that, I thought it was understood that an external amp was used when using REW. Have you tried it again with the amp?


----------



## aktiondan

Chrisbee, well I felt bad for steering you astray and causing you to waste your time on this test, so I thought I'd try it out your way, and connect my speaker straight to the sound card (with the series resistor still), and it actually still works. So maybe there's something else not right with how you're hooking it up? Without using the amp, I've got the volume control full blast on the WAV as well as the main output, and the line in is set at about 1/3 the way up. When I go into to check levels, it comes up around -14dB. Then I just hit measure. I've got it set to measure a 1M log sweep to 20kHz. It runs a sweep, I can hear the speaker go through the sweep, and the resultant plot comes looking like an impedance plot.

Most of the other settings in REW are at their default value, so I'm not sure if there's something not set up right there? Anything else I can clarify to get this working for you?

Here's a quick hook-up diagram (w/o the amp):

Line Out (+) -------------------/\/\/\/\------------------Speaker (+)-------------------Line In (+)
Line Out (-)--------------------------------------------Speaker (-)--------------------Line In (-)


----------



## terry j

whilst this procedure can't give absolute values to the measurements (unless s/one with the suitable mathematical skills can work out a correct formula-is that possible?)I'm also guessing that it may help us determine the correct amount of stuffing in a sealed box for example?

IIRC the resonance (and therefore the impedance) of the system drops with stuffing and will cease falling and start rising when the correct amount has been added. I would have to hunt around to get the exact data, but I think that's right??

If that is kinda correct then it might take the guesswork out of how much stuffing to put in. Maybe it would help with the vented boxes and LLT's too, one for Steve Callas perhaps, tho he's probably way ahead of such crude techniques.

Am I on the right track here?? cause I'm soon to start building my subs and so it would be a timely discovery.


----------



## aktiondan

It was years ago that I built this little impedance analyzer, and the method for measuring impedance was much more time consuming. It involved using a signal generator and a DMM and you had to go through and manually test and measure each frequency. REW just replaces the signal generator and DMM, and it does it quite nicely too. Since all it's really doing is comparing an output voltage to an input voltage.

I've got a little 1/8" mono plug that runs to an RCA jack with a couple of wires soldered to it, and that hooks up right across the woofer/speaker. The 8 ohm resistor can be used to calibrate the measurement, so you have a reference to 8 ohms on the graph in REW. Though it shows up as dB, you can convert back to ohms with the formula Z=CALz*10^(A/10) where CALz is your reference load (8 ohms in this case) and A is the delta dB from the 8 ohm reference. For example, if you set it up so that 80dB is your 8 ohm reference, then if the plot rises 3 dB to 83 dB, that would mean the impedance actually doubles from 8 ohms to 16 ohms. A 6dB rise would be 32 ohms. The trick would getting the plot into Excel so you can manipulate the data.

I hadn't thought about using it to evaluate the ideal amount of stuffing, but it certainly can help you "see" what you're doing by adding/removing stuffing. For measuring the tuning frequency of a newly built sub this method is ideal. A more accurate way to measure the Fb of a ported box is to plot the impedance and measure the frequency at which each of the two lower peaks occur and label them Fl and Fh. Then seal the ports, and measure the frequency at which the single peak occurs and label it Fc. Run the numbers through the formula Fb = SQRT(FL^2 + FH^2 - FC^2) and you get Fb right on the nose.


----------



## Chrisbee

I have copied your soundcard-only circuit exactly but I'm still getting only the slightest peak at around 31.5 Hz.

I've double checked and my resistor still measures 105 Ohms. 

Only if I mess about with the graph limits can I make it look as if that peak actually means something. 

These AE IB15 drivers are supposed to have a 16Hz Fs!


----------



## Chrisbee

Okay, at last. Impedance curve using an old 30WPC stereo amp I dug out of storage. 

I didn't want to risk my 450WPC IB amp on this test with all those bare wires dangling all over the place..

I think this is absolute confirmation that my AE IB15 drivers don't remotely match the claimed 16Hz spec. The green line is my soundcard-only plot.

I suppose I ought to test all four just to double check because I get a very different response between the two pairs in the same manifold. 
They don't even look the same in some minor details.


----------



## Chrisbee

Confirmation all 4 drivers have much the same impedance curve all peaking at ~32Hz.










I switched off the amplifier and readjusted REW SPL levels between each sweep. 
This accounts for the slight variation in level between each driver.


----------



## brucek

> The trick would getting the plot into Excel so you can manipulate the data.


Dan, REW has an Export Measurement as Text feature under the File pulldown. Copy and paste the results of this feature's text file into Excel and use the Text Data to Columns feature in Excel to separate it using the space as the delimiter. This yields a very resolute (frequency and SPL level) set of columns................ Here's a quick export and paste into excel showing how resolute the results of a measurement are between 10Hz and 15Hz. You get the idea anyway.











> I have copied your soundcard-only circuit exactly but I'm still getting only the slightest peak


Chrisbee, the line driver circuitry in the soundcard line-out is meant to operate as a voltage bridge sourcing high impedance loads. A speaker and 100 ohm resistor don't really qualify. Especially (as it is in your case I believe), if you're driving and measuring with long lines, which would add their own reactance to the mix. Best stick with using a power amp to do your test ... 

brucek


----------



## aktiondan

brucek, thanks for posting how to export the data, not sure why I didn't notice that option before, guess I just wasn't clicking around enough. So I took the plot I had measured, pulled the data into Excel, converted to ohms, and plotted the results and compared them against the manufacturer's data. With the exception of the absolute impedance values seeming a bit exaggerated, the resonant frequency is right on - considering the driver is brand new out of the box, it reads a bit higher than one that's been broken in.

Chrisbee, that's weird about your drivers reading 32Hz when they're suppose to be 16Hz. Are the drivers mounted in an enclosure? I'm guessing they are in some sort of infinite baffle configuration, just based on the name of the driver, but if you're measuring the speaker in their enclosure, even if it's an IB enclosure, you end up measuring the Fb of the box more so than the Fs of just the driver. If the box is sufficiently large enough, the Fb of the enclosure should approach the Fs of the driver, so it should effectively be the same thing as measuring the driver in free air, but maybe not. And that's how the driver should be measured, no baffle, suspended in free air at least 3 feet away from other objects. Otherwise the enclosure comes into play and drives the impedance peak (Fs) upward.


----------



## Chrisbee

The drivers were tested in their open-sided 4' high IB manifold with the door right next to the box wide open. There should be no real effect on Fs with the front and backs of the drivers exposed to the same volume of air. Worst-case path length between the front and back of the cones was only about 4 feet. The impedance peaks look far too close to each other in frequency for there to have been much variation in air loading despite different path lengths.


----------



## brucek

> think this is absolute confirmation that my AE IB15 drivers don't remotely match the claimed 16Hz spec.


I'm confused about why your results show this.

I'd like to see the results with the voltage across the driver measured with a simple AC voltmeter, rather than the cable you're using to REW line-in soundcard.

Use a single driver with a series dumping resistor with REW feeding a power amp using 'frequency follows cursor'.

Clip the AC voltmeter across the driver terminals.

Vary the frequency and observe when the AC voltmeter shows its maximum voltage. This is when the driver is at resonance. Is it still 32Hz?

brucek


----------



## Chrisbee

Hi brucek

As requested:

Smack on the nose 32.2 Hz showing 0.369 Volts AC. 

My browsing suggests that the AE IB15 went right off spec long before John disappeared off the radar. Denials and heated discussions changed nothing.


----------



## brucek

> Smack on the nose 32.2 Hz


Huh, fascinating. I guess this kinda tells you why you've had trouble all along with your IB response....

brucek


----------



## Chrisbee

Thanks goodness for the BFD!


----------



## terry j

yeah, but how do you now fix the IB response??

good thread, and couple it with the distortion measurements Chrisbee showed me (all of you guys knew tho!!) recently has expanded the horizons so to speak.


----------



## Chrisbee

terry j said:


> yeah, but how do you now fix the IB response??
> 
> good thread, and couple it with the distortion measurements Chrisbee showed me (all of you guys knew tho!!) recently has expanded the horizons so to speak.


Hi terry

Being able to measure things so easily is all new to me. The only previous way I could get a signal generator was to borrow one from the local technical college. There was never enough time to do anything serious except to put a quick curve on graph paper by hand before it had to go back an hour of two later before anybody noticed except the lab tech. 

Now I can play for hours with REW until sweep fatigue sets in. It is so flexible and offers so many options I'm still finding new things to try after many months. JohnM, the REW software author, is a genius! :hail:

With a Behringer feedback Destroyer (a popular parametric equaliser) I can change the response curve using boost and cut filters of various widths and depths. The BFD is only suitable for modifying subwoofer response curves but is not good enough for speakers.


----------



## terry j

yeah you're right chrisbee, it is a fantastic resource. I'm sure there is so much more to learn yet, like the impulse graphs and all that they can tell us, so get onto it NOW will you??!!

Ok, so plenty of boost will do the trick eh? excellent, but couldn't you know you needed boost simply from the response curves you have been generating?? Could you explain what new things you have learnt from this recent bout of discovery?? 

For example, how does it help you to know the impedence, wouldn't you simply boost or cut as required? I know it is a simple(ton) question, but hey I'm a (very) simple guy ha ha.


----------



## Chrisbee

I didn't really want to know the true impedance as I had already checked the DC resistance with a meter. I just wanted to check the free air resonance of my drivers. I have had doubts about their true Fs since the heated online discussions about these driver's actual parameters. The manufacturer claimed 16Hz but others have measured 32 Hz. 

Dan's timely post about using REW to show impedance curves gave me a chance to check for myself. I am no electronics expert so needed an easy way to check Fs without breaking anything.

By looking at the impedance curve I was able to check exactly where the impedance peak showed up on the Hz scale. It was just as I feared; at 32Hz rather than 16Hz. 

Very low frequency boost is only possible where there is sufficient headroom and stiff suspension. I was very wary about boosting at first and worried about exceeding the excursion limit on LFE. It seems I can push them very hard indeed without running into excessive cone travel.

It is not generally a good idea to boost the bottom end of ordinary subs as they quickly run out of excursion and will produce masses of distortion anyway. With 4 large (15") drivers I had a lot of headroom to play with. 

The BFD should really be used to cut higher frequencies to leave the VLF more exposed. I found that wasn't enough to overcome the poor lower bass response of my IB. So I'm boosting by +16dB at 20Hz with maximum bandwidth of two octaves. With some small cut filters at 25, 30, 40 and 50 Hz just to keep the upper bass response curve flat. 

As an organ music enthusiast I probably feel more need for a smooth low frequency roll-off than most. The infrasonics are vital to the reproduction of and the modulation of the largest organ pipes by each other.

It is never enough to be able to play these very low frequencies at suitable levels to make them audible. It must be done with very low distortion to make the exercise worthwhile. The true IB is probably the only practical and easily affordable subwoofer that can manage this really well. The IB is an absolute revelation after using a large 16Hz tune, ported cylinder for a couple of years. 

REW rocks! :T


----------



## Anthony

I don't know if he checks in this forum a lot, but BoomMCT uses the Behr 8k and REW to measure all his speakers.

I'm currently building some open baffle HT speakers, and plan on using REW to help me tweak placement, crossover design, and baffle design.

At its heart, it's a sweep and measure program -- and that's all you need to do a lot of speaker tweakin'!


----------



## aktiondan

Yes, REW definitely rocks. And so does this forum, there's a lot of bright people kicking around here. After years of building speakers and subs and never being able to actually measure them, I've decided to purchase an ECM8000 and 802 Mixer/Preamp and start measuring all my speakers, past and future. I've always wanted to know what some of my older speakers look like. And I've got a couple of plans in the works for some new speakers and so far REW is going to help out considerably in the design of both the box and the crossovers. I can't wait.

I started this thread after finishing a set of MTM speakers for a friend's basement theater when I realized how well it measured the crossovers I'd through together, and last week we installed the speakers and watched bits and pieces of a few movies for the first time. I was going to get some in-room responses, but I left my meter at home so I don't have any real response plots of the entire setup. But they sounded really good. Here's some pics of how they turned out in the nearly complete room. It was a lot of fun putting together, but I'm afraid the bug has just kicked in again and now the only thing on my mind is speaker designs....


----------



## clubfoot

Nice


----------



## Anthony

Okay, after spending a month or so playing around with Speaker Workshop and building a test jig for that, I now understand this thread a lot better.

SW uses MLS signals and FFTs to measure impedance by the same method: Line Out--> Amp --> Speaker --> Line In.

Seems this is the same, except using the burst sweep tones. Very cool.

Some things to remember if you all are going to try this: check your output voltages carefully! BEFORE YOU PLUG INTO LINE-IN!!! I always make sure the levels are set at less than 1V before plugging in. My jig has a 16:1 voltage divider built in, so that's not a problem, usually.

I'm using an M-Audio MobilePre, which is nice because I can easily go from impedance measurements (where the input signal is a reading across the speaker terminals) to sound measurements (ECM8000 mic swapped in for one of the channels directly to the MobilePre).

Next time I set the rig up for testing, I'll fire off some impedance tests with REW and compare them to the SW and manuf. specs. Always nice to have another tool in the toolbox, especially one that's already loaded on the computer!


----------



## brucek

> Some things to remember if you all are going to try this: check your output voltages carefully! BEFORE YOU PLUG INTO LINE-IN!!! I always make sure the levels are set at less than 1V before plugging in. My jig has a 16:1 voltage divider built in, so that's not a problem, usually.


Yeah, it would definitely be smart to estimate the drop you're expecting first and create the voltage divider to be in the ball park depending on the power output you expect at the speaker. The higher the power, the larger the voltage drop required (to ensure you don't feed the line-in too much).

So, if you expected 120 watts across a 4 ohm speaker and your dividers output impedance was 600 ohms, then the drop required to match a typical +2dbV (1.26 volts) maximum input to the line-in would be about a factor of 17:1 (~25dB in attenuation). So around 10k ohms would work fine for that situation.

--------------------------------------------
simple calculation: 
120 watts RMS across 4 ohms

e = sqrt (120 x 4)
= ~ 22 volts RMS

so 22v / 1.26v = 17 factor of attenuation.
20 log 17 = 25dB attenuation in voltage divider.

so if we use a 600 ohm divider with a factor of 17 = 10200ohms
-------------------------------------------

That's a quick way to be sure you're in the ball park. Some of these subwoofer amps produce a lot of power, so you have to be careful if you're testing with line-level converters....

brucek


----------



## Anthony

the way I did my divider was:

Feed an undistorted sine wave to the amp with plenty of overhead. Put another way, I set wave out to full then adjusted the master volume so that there was no clipping (and that clipping was a ways away so that I had room to get more output if needed). That turned out to be around 50% on the master volume with clipping starting at 80% or so (hard to translate, because the M-Audio MobilePre shows 0db in the middle and +/- dB readings for all the levels as opposed to Windows' mixer that has no markings at all  )

After that was done, I fed one channel of my power amp, a 60W car audio amp powered by a 12V, 10A power supply (all I had laying around). At minimum gain setting, it was around 16V RMS output. So I went with a 16:1 voltage divider. I then picked values for the resistances to impedance match with the line in of the sound card.

It worked exactly as planned and I get great readings. Since all this was sized for sound card and amp (and was software non-specific), I see no reason why this jig wouldn't work with REW.

I have a thread in DIY Speakers about this, so I didn't repeat a lot of the calculations here. Check out that sub-forum if you want more info on what I did.

Anthony


----------

