Another SongTower FR Question

elee532 · « **on:** 12 Jul 2009, 08:45 pm »

So, I was reading audiocrazy's thread and noticed that my FR graph taken with REW was dramatically different than his. As you can see, the lower end is fairly smooth but the upper end is all over the place. My SongTowers are crossed over at 60Hz to a Rythmik custom built sub. Does something look wrong here?

Thanks!

DMurphy · « **Reply #1 on:** 12 Jul 2009, 08:56 pm »

Your're only showing response to 2 kHz, which is below the crossover point. So All I'm seeing is an unsmoothed room response that reflects the inherent response of the CA15 woofers in your room. There are all kinds of cancellations and room modes. I don't think you can conclude much of anything from the graph.

Nuance · « **Reply #2 on:** 13 Jul 2009, 12:22 am »

Agreed. Please re-measure up to the full 20Hz. As for the graphs at hand, you're seeing cancellations and room interaction, just as Dennis said. This can be fixed with or without EQ (meaning room treatments, experimenting with placement, etc, which is better IMO).

Anyway, let's see that fill 10Hz-20KHz plot (flat to 10Hz is VERY nice. Rythmik makes great subs).

elee532 · « **Reply #3 on:** 13 Jul 2009, 01:12 am »

Yeah, I picked up the Rythmik used for about $450 direct from the guy who built it. Even he is impressed by how flat my response is down to 10Hz.

Anyway, here is all the way up to 20Khz...

elee532 · « **Reply #4 on:** 13 Jul 2009, 01:15 am »

Oops. Duplicate post. Text deleted.

DMurphy · « **Reply #5 on:** 13 Jul 2009, 01:19 am »

Quote from: elee532 on 13 Jul 2009, 01:15 am

Oops. Duplicate post. Text deleted.

These REW measurements are making me crazy. There's got to be a way to invoke smoothing, or move the mic up to within quasi-anechoic measuring distance. Can anyone out there help us? How do you use this thing?

elee532 · « **Reply #6 on:** 13 Jul 2009, 01:27 am »

Quote from: DMurphy on 13 Jul 2009, 01:19 am

These REW measurements are making me crazy. There's got to be a way to invoke smoothing, or move the mic up to within quasi-anechoic measuring distance. Can anyone out there help us? How do you use this thing?

You kind of beat me to the punch Dennis. It was suggested to me that the dramatic up and down was comb filtering and that I should apply smoothing to the graph. Here it is with 1/6 octave smooting applied...

DMurphy · « **Reply #7 on:** 13 Jul 2009, 01:38 am »

Well, that's progress. But something's still wrong. I couldn't get that kind of elevelated response out of an 0W2 or lcy ribbon unless I had an active equalizer.

elee532 · « **Reply #8 on:** 13 Jul 2009, 03:53 am »

Quote from: DMurphy on 13 Jul 2009, 01:38 am

But something's still wrong.

Thoughts?

Thanks!

TJHUB · « **Reply #9 on:** 13 Jul 2009, 03:08 pm »

Quote from: elee532 on 13 Jul 2009, 03:53 am

Quote from: DMurphy on 13 Jul 2009, 01:38 am
But something's still wrong.

Thoughts?

Thanks!

What are you using for a mic?

elee532 · « **Reply #10 on:** 14 Jul 2009, 12:45 am »

Quote from: TJHUB on 13 Jul 2009, 03:08 pm

What are you using for a mic?

Radio Shack digital SPL meter.

TJHUB · « **Reply #11 on:** 14 Jul 2009, 01:24 am »

Quote from: elee532 on 14 Jul 2009, 12:45 am

Quote from: TJHUB on 13 Jul 2009, 03:08 pm
What are you using for a mic?

Radio Shack digital SPL meter.

Then that is your issue. The RS meters are only accurate up to about 3kHz and should never be used for full range measurements. You would need to pick up a mic like the Behringer ECM8000 and some sort of sound card or mixing board with mic power.

Basically your measurement is inaccurate due to the RS meter. Go to the hometheatershack for more info.

This is good news, now Dennis' head won't explode.

Danny Richie · « **Reply #12 on:** 14 Jul 2009, 01:47 am »

If I may....

Find something to set one of the speakers on (one speaker) like a box or something to get it up off of the floor and away from any walls.

Get 1 meter away with the mic, on tweeter level, and then shoot a response.

Then the most important part: Gate the time window to limit only about a 4ms time window. The first arrival will take about 2.75ms to arrive, so start it just before that (about 2.7ms), then cut it off at 6.7ms.

Also set your range to go from 200Hz and up. At one meter you are not far enough away to accurately measure below that anyway.

An in room response measurement, like you are taking, will only tell you two things. It will tell you if the subs are in phase with your main speakers, and it will tell you what the room effects are and where.

Please note though that you can not look at a room response of both speakers playing together, at the same time, with the same signal. If you run them both together, at the same time, then one will just cancel out the other in some places, while causing peaks in other places. And then you'll get a mess much like what you have posted.

DMurphy · « **Reply #13 on:** 14 Jul 2009, 03:22 am »

Thanks Danny. And then there's the Rat Shack "mic" issue. But still, if you average together Audiocrazy and elee532's measurements, you get a perfectly flat line. So I guess that takes care of that. aa

Nuance · « **Reply #14 on:** 14 Jul 2009, 03:23 am »

I agree with both TJHUB and Danny. Although, I got lost at the "so start it just before that (about 2.7ms), then cut it off at 6.7ms" part.

For what it's worth, Terry (TJHUB) and I measured my SongTower RT's at 1 meter with his Behringer mic and is was nice and flat from 200Hz to 20KHz. No worries Dennis (or the others); the speakers are just fine.

DMurphy · « **Reply #15 on:** 14 Jul 2009, 01:21 pm »

Danny is just saying that the sampling window should only include the actual test signal as it arrives at the mic--not noise before or reflections after. So setting the window at 2.7 ms will get things going just before the signal arrives, +plus 4 ms for the signal itself=6.7 ms.

audiocrazy · « **Reply #16 on:** 14 Jul 2009, 03:32 pm »

I will be positing FR of my ST's with the professional calibrated ECM8000 in a few days.
In my opinion the REW graph are not very accurate in the top end of spectrum as there are many variable that affects the graph.

Rick Craig · « **Reply #17 on:** 14 Jul 2009, 04:10 pm »

Quote from: elee532 on 13 Jul 2009, 01:12 am

Yeah, I picked up the Rythmik used for about $450 direct from the guy who built it. Even he is impressed by how flat my response is down to 10Hz.

Anyway, here is all the way up to 20Khz...

Do a nearfield response curve on the subwoofer with a calibrated microphone. You'll find the real bass extension with that method and see that the response isn't flat to 10hz. The meter and your software will overstate this and give you an "optimistic" graph.

Nuance · « **Reply #18 on:** 14 Jul 2009, 05:43 pm »

Quote from: DMurphy on 14 Jul 2009, 01:21 pm

Danny is just saying that the sampling window should only include the actual test signal as it arrives at the mic--not noise before or reflections after. So setting the window at 2.7 ms will get things going just before the signal arrives, +plus 4 ms for the signal itself=6.7 ms.

Gotcha. I have no idea if you can do that with REW, but thanks for the explanation.

Rick, correct me if I am wrong, but the subwoofer is reaching down that low due to room gain, no? At a nearfield measurement position he won't see flat depth like that, but rather a slow roll-off starting much higher. This would be more like an anechoic measurement. Is that correct?

Danny Richie · « **Reply #19 on:** 14 Jul 2009, 06:59 pm »

Quote

At a nearfield measurement position he won't see flat depth like that, but rather a slow roll-off starting much higher. This would be more like an anechoic measurement. Is that correct?

On any of our servo controlled woofers he will get a near field response that is flat to 20Hz and the -3db down point will be in the teens.

It's a bit worthless though as he is never going to listen to the sub with his ear in it. What it is doing in the room is much more important. He can see where it crosses to each speaker, make phase adjustments, and see if there are any peaks or dips. A peak or a dip can then be EQ'ed out with the EQ system built into the amp.