Re-tweaking my Center Channel speaker

[Patrick Sun]

Now that I've gotten my SunOnes and SunTwos squared away (in my mind, at least) this past Friday and Saturday (until I get another opinion on their sound later), I figured I'd take another stab at my Center channel speaker beast I created about 4-6 months ago.

I asked one of my friends what he thought of the sound quality of it about 3 months ago, and he thought it sounded a little funny, but once you got used to it, it started to sound okay. Well, that just wasn't music to my ears, so I ordered some capacitors a few weeks ago so that I could be flexible in my CC tweaking/tune-up.

I had nothing to do Sunday, so I decided to give it a shot using the lessons I've learned since, plus the perspective I've gained by tweaking the other 2 sets of speakers. I was sort of amazed at the speed of my tweaking this time around. Then again, I didn't "document" every tweak/change in an orderly fashion, but I knew where I was, and where I wanted to be. I did make notes of each change, but didn't produce graphs upon graphs with each change
this time around.

I was bummed that my plans to replace the tweeter with the Morel MDT30s that I used in my other speakers were thwarted by the existing tweeter hole being too big! Doh! So I decided to press on with the original tweeter (from my old DIY speaker project). (after tweaking, I can't really find any reason to feel too bummed about not being able to use the Morel tweeter - I'm using an old Phillips AD11600/T8 fabric tweeter).

I used a more systematic approach this time (the previous efforts had me changing something and then taking measurements of the whole frequency response and then deciding if the change was beneficial - I
had no idea if the individual filter slope were actually working as designed). I broke the tuning down into 3 phases:
measuring the current individual drivers with their current filters in place, graphing their response, and then making
changes to get the filter slopes to cross at the desired frequencies.

So once I was happy with the woofer low pass filter, I moved on to the midrange high pass filter, got the integration of it with the woofer filter where I wanted, and then moved onto the integration of the midrange low pass filter and the tweeter high pass filter. Sounds simple, eh?

I can't believe I didn't do this the 1st time around, and I was tweaking pretty darn blindly.

I can't show the "before" situation yet (left it at home - will post it later tonight), but I can show the "after" frequency response down below. I started with the woofer response. I knew I wanted to slope off the parallelled
woofers' response around 500-700Hz (to try and get that baffle step loss minimized), and then use my midrange from around 500-3000Hz, and then let the tweeter take care of the rest.

Let just be nice and say that my initial attempt at the crossover was pretty marginal.

Once I plotted the individual response of the driver/filters, I knew why my friend said the CC sounded weird: the tweeter was padded too much, and the midrange was carrying too much of the load on the high end, and IIRC, the woofers were playing too high up the road (around 1200Hz).

After about 5 hours of tinkering with the filters, seeing if the textbook filters were doing their things (or if I noticed that the filters were "delayed" in when the slope started to kick in, I would calculate a new filter with the break frequency that was lower by 1/3 or 2/3 of an octave to get the filter to cross over as desired), I got something that started to sound "normal" to me, with slightly more tweeter output, and was able to shape the midrange output to covered the region I wanted it to cover.

I think it took 2 changes to the woofer filter, 5 changes for the midrange (for the high pass and low pass filtering), and another 3 for the tweeter filter.

What also helped was when I got my other speakers measured on the LAud setup, I was able to produce a table of RS SPL meter measurement correction values for my particular meter that gets me in the ballpark. I put these corrections values (what I needed to add or subtract from any given frequency's SPL reading - since I use 1/3 octave test tones, it's a simple matter to standardize on the corrections) in an Excel spreadsheet, and it gave me a
"better" idea of the frequency response, especially in the high frequencies after 3KHz. Believe me, it's a lot closer
than the correction values I was using before (which led me to pad my tweeters too much - usually 3dB too much).

Just keep in mind that for the low end (60-150Hz) I have this weird room mode that always accentuates the bass response, it's not as bad as it sounds/looks once the speaker is put in its place above the TV. The extended ridge in the 1KHz range is actually beneficial once the speaker is in place as well, so that's why I did not take steps to pad down the midrange. It's not all that "flat-line" pretty, but without serious measuring gear and filter design software, it gets most of the job done. So without further ado...

SunCC frequency response plot.




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[Patrick Sun]

Here's the "before" frequency response when I started yesterday. As you can see it was a bit messy (too much of the woofers were working past 1KHz, the midrange kicked in too late, and the tweeter was padded too much).




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[Patrick Sun]

Okay, here's one more attempt at getting my DIY center channel speaker to sound better.

When we last visited, the response looked like this:

Last FR plot

It sounded okay, but there was too much growl in male vocals, still a little boxy, and some yucky glare that I'm sensitive from the midrange in the 3KH-5KHz region (more apparent when music is played through it).

So just for more kicks, I revamped the filters once more (all 2nd order), the tweeter filter (originally crossed over around 3.2KHz) is now crossed even lower using a LR - 1.9KHz, against good judgment given the old tweeter I'm using (with a high resonance frequency around 1.6KHz). I factored in the cross-over for some of the benefits in crossing it over so early. The current L-pad on the tweeter is -5.5dB.

Then I changed the filters on the midrange. The highpass is a LR crossed over around 575Hz. I used the LR for a more damped response in the crossover region with the woofers (crossed over around 500Hz) to smooth out the minor summation hump in the 500Hz region (originally it was a Butterworth filter, so it was a little strong at the XO region). The lowpass is rolled off around 1.9KHz as well using a LR. This is done to take out as much midrange glare as possible. I also popped in a -2dB L-pad to equal out the output level. This appeared to work well enough for me and my ears. Originally it was crossed higher, around 3KHz, I think, and was probably a Butterworth as well.

The woofer filter was left alone as it since it appeared to be working as planned.

I was a little lazy because I wasn't really sure if I would like what I heard with this crossover melting pot (using textbook filters, but adjusted based on observed behavior - i.e. to get the roll-off I wanted on the woofer, when I wanted to roll it off around 500Hz, I had to ask for a filter that broke around 250Hz). Strange but true.

Now, the tweeter is carrying more of the upper frequency load. The midrange is carrying less, but it an important portion that gives depth to mainly male vocals without over-powering the rest of the audio content coming through the center channel.

I had entertained bypassing the midrange all-together, and just running a MTM configuration, but due to a dip in the woofer response around 1.5KHz, I decided to give it one more try at the current W(T/M)W configuration.

I re-listened to DPL material with strong center channel content (like TV programming on tape, and DVDs) that showed me the growling male vocal problems from the last crossover design, and now it's gone, plus the voices sound more natural now.

So while I was sitting on my couch I decided to at least take a set of measurements with the center channel installed in its normal position, right on top of the TV, about 3 meters away.

Here's just the overall frequency response of the current crossover incarnation:

Current FR plot

Now things look pretty good, and actually sound good too. There is a little dip around 500Hz, but nothing too critical. I'll try to do the individual driver responses when I get some time/energy/motivation. Heh.




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[Patrick Sun]

Okay, finally measured the individual drivers at 1 meter away (couldn't do the 3m measurement too easily), so here it is:

Final? CC frequency response graph

It looks like the midrange doesn't do much, but it does fill in what I need in the 700-2000Hz region, so I can't say it's a total waste given the layout of the center channel front baffle. The hump down under 100Hz isn't too problematic, and nothing to get concerned about (especially if I run the speaker in small mode).




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