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Old 7th May 2006, 07:16 PM   #1
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Default Taming rising frequency response 'naturally'...

Well, I couln't stand it any longer, so I finally got an SPL meter (Ratshack digital), and before I post the results into my project thread, I need some advice. Yesterday, I moved my X-baffles with the Fostex FE108EZ sigmas into my basement office and took a bunch of near-field measurements. I measured both the horn mouth and the driver. I got some surprising and not so surprising results. Overall, this project looks very promising. There are no huge holes or spikes. The changes are quite smooth over the entire spectrum...BUT:

First the good news. It appears the horn works spectacularly well. It's only down 4 dB at 70Hz and has a rounded peak of +6 dB around 175Hz, then rolling off gradually past 500Hz.

The bad news: The midrange rises steadily from reference at 1000Hz to a peak of +16 db around 7000Hz. I knew these were a bit bright, but not that bright, and now I know why I prefer them shoved up against the wall. Again, the rise is very gradual and consistent but I feel just a bit too much overall.

My question is whether it is possible to tame this midrange rise a bit without using a notch filter or BSC? What are my 'natural' options. Would changing the compression chamber help, or perhaps the throat opening? What about playing with the line stuffing? Damping the basket? Was my testing methodology sound? I really need some experienced minds working with me here. I have a lot of unanswered questions around this initial test.

I don't necessarily want to eliminate this rise, as that is what makes this driver sound so special. I just want to tame it a bit.

Methodology: Fast C-weighting. Reference dB set at 1000Hz. 1/12 octave samples from 50Hz to 500Hz on the horn mouth and 125Hz to 17000Hz on the driver (outside those ranges, it was always down more than 10 dB, so I stopped). NCH Tone Generator software using sine wave. Meter position was 10 cm from the driver and about 1.5 inches from the center of the horn mouth, and I used a tripod. The room is pretty bare, low ceiling, sheetrock walls, and hardwood floor. About 120 sq. feet, shaped somewhat triangular. Lots of slap echos. The speaker was placed at least three feet from any wall.

Right now, all I have are columns of numbers. I hope to get them into a spreadsheet soon.

Thanks,

Doug
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Old 8th May 2006, 04:18 AM   #2
nikita is offline nikita  Canada
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Hi Doug. Just like you I'm not a great fan of "notch filter" Here's some ideas that might help.
1- Try to take the same measures you did but at half way to your listening position. (at 10cm it's almost useless)
2- Try the speakers close to rear wall (it will reinforce bass/mid-bass and should give better balance)
3- Play with the stuffing to get more bass/mid-bass.
4- Try a felt ring around the drivers (I've used it many times (with good result) to tame bright tweeters.
Perhaps it's a good idea to implement only one step at a time and evaluate the result.
Good luck and give us news on the result !!!
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Old 8th May 2006, 04:56 AM   #3
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Thank you nikita,

I'm trying really hard to get a handle on testing speakers. I read so much yesterday on the net that I gave myself a headache. I see now that near-field testing is only good to 500Hz. I was trying to eliminate room effects. At least the horn mouth results are probably accurate (which I think are great, by the way).

Okay, so first I have to retest.

Any advice on eliminating room effects? I guess I will test at 1 meter. I most want to measure the speaker itself right now, and I really don't trust anything I hear in the room the computer is located. I could move the speaker outside with about 20 feet of speaker wire. I understand outdoors is a form of anechoic chamber. Would this give me more accurate results?

Doug
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Old 8th May 2006, 05:31 AM   #4
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nikita, another question:

When you say felt ring, are you referring to where the mounting gasket goes or on the magnet?

Doug
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Old 8th May 2006, 11:25 AM   #5
lndm is offline lndm  Australia
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Quote:
Originally posted by Taperwood
Any advice on eliminating room effects? I guess I will test at 1 meter.
I find testing at 1m is a reasonable compromise. You might also look at Maximum Length Sequence testing as a way of excluding room contributions.

One test I like to include is to measure from the listening position FWIW.
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Old 8th May 2006, 02:55 PM   #6
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Default Re: Taming rising frequency response 'naturally'...

Quote:
Originally posted by Taperwood

Methodology: Fast C-weighting. Reference dB set at 1000Hz. 1/12 octave samples from 50Hz to 500Hz on the horn mouth and 125Hz to 17000Hz on the driver (outside those ranges, it was always down more than 10 dB, so I stopped). NCH Tone Generator software using sine wave. Meter position was 10 cm from the driver and about 1.5 inches from the center of the horn mouth, and I used a tripod. The room is pretty bare, low ceiling, sheetrock walls, and hardwood floor. About 120 sq. feet, shaped somewhat triangular. Lots of slap echos. The speaker was placed at least three feet from any wall.

Right now, all I have are columns of numbers. I hope to get them into a spreadsheet soon.

I too am starting to learn about speaker measurements and have had some difficulties making good measurements.

A couple of notes. With my Radio Shack 33-2050 SPL meter I use a C weighting and SLOW response. Also, the Radio Shack meters are not very linear. Are you using any corrections?

What I struggle the most with, is the distance from the driver. Here is a little excerpt from Zaph | Audio (http://www.zaphaudio.com/setup.html)

Quote:
Mic position varies with the driver tested. With tweeters, I can get as close as 4" and still have an accurate response curve. With woofers, I have to back the mic up more and more as the driver gets bigger for an accurate representation of the top end response. Tweeters can be done in one step with a small amount of gating taking care of the room reflections. Woofers however have to be done in two steps with close mic (1/4") and no gating, and spaced mic with gating. The gating varies depending on how close I can get the mic.
So 10 cm seems a little close. I suspect that a 4" full range may have to be measured in two steps and this is the problem I have also been seeing in my measurements.

Good luck and keep us posted.

Regards,
Gio.
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Old 8th May 2006, 03:12 PM   #7
mazurek is offline mazurek  United States
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Remember to do some measurements off axis (if you can produce quality measurements far enough away). A lot of drivers exhibit huge hf peaks on axis, which minimize off axis.

Unless you extremely toe in the speakers, you will be listening to them off axis.
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Old 8th May 2006, 03:16 PM   #8
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Quote:
Originally posted by Taperwood
[B]When you say felt ring, are you referring to where the mounting gasket goes or on the magnet?[B]
He may be talking about a ring of felt, cut in a bevelled star or other pattern around the driver on the front. Planet10 did the same for me on some dome mids that were a bit too forward. It looks rather ugly, but that's why I build grilles.
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Old 8th May 2006, 06:05 PM   #9
nikita is offline nikita  Canada
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Hi Doug, a felt ring is a ring of felt place around the driver's frame outside of the box, glued on the baffle. Like I said before, sometimes very effective with bright tweeters but I never used it on single fullrange!!!
Your drivers do not measure so bad, with a little rise from 2.2K and a more severe peak at 14K,
Don't loose your time in measures outside or on the moon, it's not where you're listening your speakers! I heard many speakers measuring very flat in anechoic chamber but sound awfull in my house! (including my present speakers (Magneplanar) they are very room dependant)!
Maybe your room is too lively (many windows, hard walls/floor, hard furniture) if you can put some absorbing material in your room (carpet, heavy drapes, cork panels, whatever absorbing...) and see what happens.
BTW what is your amplification system?
Good luck!
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Old 8th May 2006, 06:11 PM   #10
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Doug,

It's must be cause by your cab, because I have 3 pairs of well broken in 108eSigmas in a variety of alignments and none display the kind of rising response you have. If you want to tame it naturally, address the cause, not the result.
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