SB acoustics SB12MNRX25-4 "full range" Build thread - Page 4 - diyAudio
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Old 20th January 2013, 04:29 AM   #31
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yeah these are actually going pretty fast by my standards I joined here back in August 2003 to ask some questions about the speakers I was thinking about building. I finally "finished" them in Oct 2011. I put finished in quotes because I never got around to getting the felt that I wanted to put on the baffles or the proper covers that was to integrate with the felt

I've just taken the clamps off but I think I really need a brace between the sides. The 6mm mdf is pretty hollow sounding which I was expecting. I've got some small diameter dowel around here somewhere if I can dig it up. I did find the speaker terminals which is good. Top and bottom sound good on a rap test.

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Old 20th January 2013, 08:28 AM   #32
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I couldn't resist. I had to clamp the back on and hook up the driver with alligator clips and have a listen. Originally I was going to do some impedance measurements to check that the two pieces of dowel I put in were not causing any problems, but I didn't get that far

What can I say... This project is definitely worth finishing!! The speaker sounds a bit unbalanced, but that is to be expected as I have no Baffle Step compensation at this stage, however the vocals on Ella and Louis sounded VERY good indeed. apart from the slightly more pronounce midrange and treble they sound very close to my MTM's. In fact I had one of the MTM's playing with the little SB on top of the other replacing it, and the image stayed dead centre and it was not perceptible that there was anything different to normal! I think that the sensitivity of my MTM's must be pretty much the same as these little speakers (which means that the mtm's are a tad less efficient than I thought).

The only other thing was that I could notice a slight tizziness in the top end, which is probably the breakup, but it was not at all bad. I'll wait till I've done some measurements (and played with BSC) before trying to do anything about that though.

For a set of computer speakers for my 5YO these are going to be very very good indeed!

Tony.
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Old 21st January 2013, 08:51 AM   #33
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OK first measurement Bear in mind that this is just with the rear baffle being held in place by a single F clamp and not properly sealed. The other thing to bear in mind is that this is a completely bare box. No lining of any sort just bare MDF. It looks from the impedance curve that the tuning is pretty close to my target of 70Hz.

I'll take out the braces I added and measure again and see if the twitches in the impedance curve at just over 1 and 1.5Khz are being caused by the braces. Apart from another small glitch at around 9.5K which probably corresponds with the breakup on the manufacturers curves the impedance plot I think it looks very good indeed

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Old 21st January 2013, 10:02 AM   #34
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Well it looks like the giltches are not the bracing, but by a kind twist of fate I must have over compensated for the volume slightly, and with the bracing (which I hadn't originally allowed for) in place the volume is just about perfect for the 70Hz tuning frequency. Without it it is about 2.5 hz lower.

I'm actually thinking that the sharp glitch at just above 1Khz might be the F clamp, as no doubt the position of it on the front baffle moved a bit from the first measurement, and coincidentally the glitch moved a bit as well.

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Old 21st January 2013, 10:16 AM   #35
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Quote:
Originally Posted by wintermute View Post
the sharp glitch at just above 1Khz
It is about the right frequency to be the transition from pistonic behaviour to resonant behaviour.

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Old 21st January 2013, 11:11 AM   #36
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ok thanks Dave. in all it is a pretty small gitch I think. I'm not too worried about it at the moment.

I've just tried doing some accoustic measurements and well given the setup I don't think I should read too much into them, but they aren't particularly good. I'm in a 3 X 4M room with furniture everywhere and the speaker sitting on the shelf of my desk. I tried doing nearfield, but I'll have to try slpicing the port and woofer measurements together. It certainly doesn't look like I expected though.. big rise (like almost 10db) around 125Hz. box is not properly sealed so that may have something to do with it I guess, or perhaps I have damaged my mic again

It might be a while before I get some proper measurements, but I think I should at least finish the box before doing that Measurement shown was taken at about 50cm and is 1/8th octave smoothed. It looks like what I was hearing as needing correcting in the top end was correct too.

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Old 21st January 2013, 12:53 PM   #37
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The rise at 125Hz is too steep to be the box tuning, as your letting in some of the room, it is likely to be that. The measurements do show a gradual decline below 1k, most likely due to bafflestep and then a small rise in the top end, which may not need addressing as it will most likely vanish off axis.

My only concern is that your daughter, with the young ears, may find them dull sounding, it might be unlikely that she notices though.
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Old 21st January 2013, 07:17 PM   #38
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I'm a bit perplexed about the resuilt actually. I did a 1cm nearfield measurement and it also has a big (much bigger than in the 50cm measurement) sharp hump at 125Hz. It does look way too steep to be the box tuning, and is absolutely nothing like the modeled response. having another look at the impeadance curve the second (very high) spike is at pretty much 125Hz.

Leaks or not there shouldn't be this sort of effect. It may be a case of the driver behaving very different to when modeled, or I may have really screwed something up Funny thing is I didn't notice it at all when listening (though I may now).

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Old 21st January 2013, 08:05 PM   #39
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Hear are the nearfield measurements I took last night. taken at around 5mm Blue is the driver red is the port. One thing that I think this shows is that the little glitch in the impedance curve just above 1Khz is actually the port resonance

I've never done a slot port before, so I don't know whether my moddeling using a standard round port (as unibox doesn't have an option for rectangular ports) will have thrown things off completely.

Interestingly if I increase the series resistance from the modeled 0.2ohms to four ohms I start to get a pronounced hump around 125Hz. If I increase it to 10 ohms I get an almost perfect match for the woofer nearfield measurement. Perhaps I need to retry with proper wiring rather than alligator clips

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Old 21st January 2013, 10:00 PM   #40
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Is Australia a 60hz electricity country? 120hz may be some noise in your measurement setup. You're measuring at a fairly low level. I'd crank it up. And if it's a laptop, unplug it.
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