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Q12 12" Sub

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Steve_B View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steve_B Quote  Post ReplyReply Direct Link To This Post Posted: 20 August 2009 at 7:45pm
Here is a plot of the frequency response of the bms12 in a reflex cabinet. Measurement was done outside with both the speaker and microphone placed on the floor.

edit: forgot to mention this is pre eq.



Edited by Steve_B - 20 August 2009 at 7:49pm
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Tony Wilkes View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tony Wilkes Quote  Post ReplyReply Direct Link To This Post Posted: 21 August 2009 at 10:26am
Steve, Very similar curve to that I have measured. The hump on my box is very slightly lower and centred on 60hz which is caused by the slightly bigger box of coarse. Only a tiny bit of EQ needed to get it flat to 40hz .

What program do you use to display the FR graphs etc?

Tony
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steve_B Quote  Post ReplyReply Direct Link To This Post Posted: 22 August 2009 at 4:50am
Tony,

It is an older version of Smaart. I got it in exchange for some work I did for an acquaintance who has moved on from his audio obsession and no longer needed it. Prior to that I used to borrow Spectralab from work. As I now consider audio to be more of a hobby than job I couldn't really justify a big spend on software for the amount of speakers I now build. I was just fortunate to know the right person at the right time.

If you are measuring outside or in an anechoic environment using a pink noise source, then you know that the reference source is flat and a RTA read out gives the same response as a transfer function. The phase measurement makes setting up crossover points easier and the coherence plot highlights where the curve is giving a false reading, but by changing the test setup and getting similar readings you can be confident that your measurements are good. Anything with a higher res than 1/12 octave should get close. In most cases if you set up the same cabinet three times you end up with three slightly different settings on the crossover eq. If the measurements are wildly different something is wrong.

One of the problems with aligning subs with mid/tops on a tripod stand is not being able to set up consistently at gigs. I usually do a few measurements putting the cabs in different positions to try an get a feel for what is happening. At pub gigs there is never a chance to do any alignment in situ.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote stevie Quote  Post ReplyReply Direct Link To This Post Posted: 28 August 2009 at 3:03pm
It's difficult to tell from this distance, but tuning the box lower might flatten the peak at 60Hz and produce some more useful output below this.
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Tony Wilkes View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tony Wilkes Quote  Post ReplyReply Direct Link To This Post Posted: 28 August 2009 at 5:24pm
Both myself and Steve_B deliberately wanted the peak which gives about 2db at just about the most influential bass frequency. It is a trade off between extension and size but one which seems to work very well indeed.

If you just want extension then just build a 60L box tuned to 28hz and they will go lower than most 18" ers   i.e. -3db @ 32hz.

Tony
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steve_B Quote  Post ReplyReply Direct Link To This Post Posted: 29 August 2009 at 2:55pm
In my opinion people get too obsessive about getting the flattest most extended response they can from a cabinet design. Don't forget that the loudspeaker is acting as a high-pass filter. If you add an electronic filter in series the response is combined. With modern electronics eqing a flat response is easy. The effects can be simulated in winisd.

Sticking with the bms 12" driver this first image simulates three boxes. One is close enough to the box that I built (blue trace). The second is the same box with a -1.0dB cut at 55Hz (red trace) and the third shows a 74 litre box tuned to 32Hz. From these simulations the options seem to be a small box or an extended response. For me all things equal extending the response down to 32Hz from 38Hz wouldn't be worth it anyway for the bigger sized box.



This second graph shows the same three boxes but now plots the maximum spl based on power and xmax. Although the yellow trace (big box) has an increased output below 38Hz the maximum output above 38Hz is less than the small box. Note that the maximum output of the two smaller boxes is identical (you can only see one trace) with the -1.0dB filter at 55Hz the power is reduced at the same point that the output is limited by xmax. For most PA and disco applications where the loudspeaker is operating close to capacity the smaller box will give a better bass output than the large cabinet.


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stevie View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote stevie Quote  Post ReplyReply Direct Link To This Post Posted: 29 August 2009 at 4:39pm
If you stick with the 48 litre box but tune at 42Hz instead of 47Hz, you will lose the peak at 60Hz and gain another 2dBs at 40Hz - that's what I meant. I've got it plotted in BassBox but I don't know how to upload images to the forum. If you want the peak at 60Hz, fair enough.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote stevie Quote  Post ReplyReply Direct Link To This Post Posted: 29 August 2009 at 4:42pm
Please put a smiley on the end of that last post - I'm really not trying to teach grandma to suck eggs.Emoticons

Edited by stevie - 29 August 2009 at 4:43pm
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Steve_B View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steve_B Quote  Post ReplyReply Direct Link To This Post Posted: 29 August 2009 at 5:04pm
Prediction programs are not that accurate. One parameter that I have never seen discussed here is Ql or box loss. This can only be measured after you have built the cabinet so any prediction software has to use an estimated value. Each software designer might use a different value. The graph of my original cabinet was tuned to 38Hz not 47Hz. Using the values you have given winisd predicts an even bigger peak this is what I would expect. A 47Hz tuning would give an even bigger peak.



The original image I posted was a measured response. It conforms closely with the predicted response in winisd which gives me confidence in the programs accuracy.

To show images you need to upload them to a website and then provide a link. If you don't have your own webspace there are loads of sites that host images for free. For posting on speakerplans I use tinypic.com. It will automatically resize your image to fit different formats and provides the tagged link which you just copy and paste into your message.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jasonstry Quote  Post ReplyReply Direct Link To This Post Posted: 29 August 2009 at 8:44pm
Tony,
How much would it cost me to buy some of these boxes, completely built, loaded, painted etc.
Cheers.
Andy
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Post Options Post Options   Thanks (0) Thanks(0)   Quote space-face Quote  Post ReplyReply Direct Link To This Post Posted: 06 October 2009 at 8:51pm
Anyone know where to get the BMS 12N630 in the uk?
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darkmatter View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote darkmatter Quote  Post ReplyReply Direct Link To This Post Posted: 06 October 2009 at 9:28pm
Tony Wilkes can supply BMS drivers
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