Room EQ Wizard Doubles as Crossover Design/Measurement Tool

[Chrisbee]

terry j wrote:

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!

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!

[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