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-   -   Help me getting the bass I simulated (Beyma 10G40 in vented box) (http://www.diyaudio.com/forums/multi-way/139158-help-me-getting-bass-i-simulated-beyma-10g40-vented-box.html)

jm_kzo 23rd February 2009 05:58 PM

Help me getting the bass I simulated (Beyma 10G40 in vented box)
 
For this project I would like my mains based on Beyma 10G40 and B&C DE250 to go down to 80Hz at least where the sub will take over...

The Beyma in my box (30l net, 24l "actual") seems well able to achieve this (in all softawre I used).

Here is what UniBox predicts...

http://jeanmichel.cazaux.free.fr/HC/...2023022009.png


Unfortunately, I cannot get them to go as low as this.

Here is what measures tell (this is all filtered with both drivers, but I have removed response over 1K to show the thing).
http://jeanmichel.cazaux.free.fr/HC/...s%20230209.PNG

I first questioned my measurement set up, but using different method changes nothing.
I am using a Panasonic mic (WM-61A) and a "Wallin" preamp (2).

I used SpeakerWorkshop, CollEdit + Aurora and REW as well and they all tell teh same...

Bass drops as early as 330Hz and more than 3dB are lost before crossing 200Hz.

Anything obvious is wrong ?

I must say that I do not know what to explore and feel out of options...

Please help me getting teh bass I deserve (according to Unibox and WinISD) ;).

Many thanks in advance.

sreten 23rd February 2009 07:05 PM

Hi,

Seems you might have run into the baffle step chestnut .....
(though this depends how you are measuring them).

Only lower than usual due to your large baffle width.

It causes a 6dB step in the response freefield. It can be seen
on a lot of Seas bass/mid plots, for drivers measured in boxes.

Here is a classic example of 6dB baffle step for a driver that
would measure near flat into half space (very large baffle) :

http://www.seas.no/images/stories/vi...0419_p17re.pdf

~150Hz to ~750 Hz is the box baffle step.

:)/sreten.

jm_kzo 23rd February 2009 07:34 PM

I have experimented a bit more tonight and it would seem that the question is more : How do you measure bass ?

I have measure my current driver (do not remember the spec, but they *do* bass when listening)... the curve from 300Hz does look exactly the same in SW...

I tried to put the mic firther away (was using 1 meter) and I had to change the measurement window in SW...

Guess what ?
It changes the roll off point as well (starts even higher now).

How do you guys do to measure bass (I mean below 300Hz) ?

jm_kzo 23rd February 2009 07:38 PM

Quote:

Originally posted by sreten
Hi,

Seems you might have run into the baffle step chestnut .....
(though this depends how you are measuring them).

Only lower than usual due to your large baffle width.

It causes a 6dB step in the response freefield. It can be seen
on a lot of Seas bass/mid plots, for drivers measured in boxes.

Here is a classic example of 6dB baffle step for a driver that
would measure near flat into half space (very large baffle) :

http://www.seas.no/images/stories/vi...0419_p17re.pdf

~150Hz to ~750 Hz is the box baffle step.

:)/sreten.

Hi, it could well be that, but I am rather leaning towards a measurement problem.

The loudspeaker I measured tonight has very different baffle size and shows the same strange roll off.

I think this "gated" method in SW does no good for bass.

sreten 24th February 2009 11:09 AM

Hi,

Gated ? Gated measurements have a roll-off related to the window.

:)/sreten.

jm_kzo 24th February 2009 12:31 PM

Quote:

Originally posted by sreten
Hi,

Gated ? Gated measurements have a roll-off related to the window.

:)/sreten.
This is what I figured out... I remember reading gated measurement were not accurate below 200Hz, but 1)I did not really register, 2)I did not think it would have such an impact.

What would you use (freeware/open source) to do a not gated measure ? Still Speaker Workshop ?

sreten 24th February 2009 08:08 PM

Hi, use nearfield with no gating below say ~ 300Hz, :)/sreten.

cowanaudio 24th February 2009 08:30 PM

G'day Jean-Michel

You can gate your measurement, but keep the time >1sec and you shouldn't run into any problems.

Alternatively use a SPL meter and 1/6th octave test tones, near field. The analogue RS meter with Wallin mod or with correction curves works very well for the $$$. This will give you a good idea of your LF response.

Cheers

William Cowan

jm_kzo 24th February 2009 08:56 PM

Quote:

Originally posted by cowanaudio
G'day Jean-Michel

You can gate your measurement, but keep the time >1sec and you shouldn't run into any problems.

Ouch!!! Indeed, I am using 12ms, that's a big difference!!! ;).

Quote:

Originally posted by cowanaudio

Alternatively use a SPL meter and 1/6th octave test tones, near field. The analogue RS meter with Wallin mod or with correction curves works very well for the $$$. This will give you a good idea of your LF response.

Cheers

William Cowan

Could do this as well indeed...
I might give a try to SynRTA as well (or equivalent), not a big investment and it might well be good enough for the below 300Hz range.

Thanks for your advices guys,


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