HELP: xmax and open baffles

[WDYSUN]

Hello!

I wanted to builds a new loudspeaker with midrange working on an open baffle. I did several research on open baffles, but some of the technical issues still remain obscure to me. One of the obscure thing is how the open baffle determines the driver excursion.

My question is: how should I consider the xmax when choosing my midrange driver? Let us assume that midrange crosses around 300Hz and 3000Hz (old school stuffs!), in such a situation what should I require to my driver in terms of xmax?

I hope somebody out there in the cloud will help me!

Best wishes
Pierre

[CLS]

What's the size of that midrange?

I once played with a naked 12" guitar driver (no baffle) as a mid, it played down to about 300Hz pretty flat without EQ. With help of baffle, this size of mid can do 150~200Hz with little EQ and without any Xmax issue. It barely moves! (OK, that's not a common midrange size...)

In some other cases, I also tried 7" and 8" midrange on OB, played them down to about 200Hz. They produced ear-bleeding SPL (in home use) without any visible excursions.

So I don't see any problem in Xmax of a 300Hz midrange within reasonable size.

[StigErik]

In the area above the baffle cut-off frequency the OB's efficiency is actually higher than for closed box, and should provide more SPL for a given excursion than closed box.

You could try Linkwitz' Excel spreadsheet to calculate max SPL.... nice tool.
http://www.linkwitzlab.com/spl_max1.xls

[gainphile]

Yep, SL's spreadsheet above is the way to go.

To me, the requirement of surface area (Sd) and Xmax for the bass region is a revelation. I don't think there's much problem with midrange.

If you buy JohnK's tool, he also have the calculator for optimum Qts for midrange.

[WDYSUN]

Hy Guys,

thanks all for your help. The midrange I would like to use is the SEAS MCA15RCY, is the best paper cone ("true") midrange I have auditioned. Low distortion, huge dynamic (high acceleration factor), damned flat from 300 to 8KHz measured 1W/meter on a 40cm baffle! Here it is:

http://www.seas.no/index.php?option=...102&Itemid=124

I intend to use it on a 40cm open baffle, I measured the driver on the baffle from the listening position. It has -3db point around 294Hz, and then it goes down very quickly.

I want to use it with a 12" bass driver in closed box actively driven using the Beringher DCX 2496 xover with which I control xover point, delay and various equalizations. This digital xover is a killer beast.... but only in the bass region! Moreover its functions allow me to set up the bass response I had never the chance to get in the past... in-room linear from 30Hz to 500Hz with an impressive dynamic range.

The roll-off of the SEAS MCA15RCY on the open baffle is already optimal (too good to be true), but since I don't have measuring tools for distortion, I wonder whether I should put an high pass to the midrange to prevent excess excursion. Of course this would require a stepper lowpass on the woofer, but who cares... the DCX 2496 will do it!

Best Wishes
Pierre

[gainphile]

Did you measure off-axis? a 5" woofer is acoustically too small for 40cm baffle.

Other thing, did you try to feed it a tone at 100Hz or so (1 octave below your target xo frequency)? And does it produce noise?

[WDYSUN]

Originally Posted by gainphile

Did you measure off-axis? a 5" woofer is acoustically too small for 40cm baffle.

Other thing, did you try to feed it a tone at 100Hz or so (1 octave below your target xo frequency)? And does it produce noise?

Yes I measured the mid from listening position. What do you mean " acoustically too small for 40cm baffle."?


I haven't tried a pure 100Hz tone. I'll do it.

Pierre

[Rudolf]

Originally Posted by WDYSUN

The midrange I would like to use is the SEAS MCA15RCY... I intend to use it on a 40cm open baffle. I measured the driver on the baffle from the listening position. It has -3db point around 294Hz, and then it goes down very quickly.

The falling response below 300 Hz is purely due to the dipole baffle loss. The driver itself does not work less below - it has to work more!

I wonder whether I should put an high pass to the midrange to prevent excess excursion.

It is absolutely necessary IMHO. And you will need at least 12 dB/oct.

Regarding the baffle width: The diagram below shows the response of a 40 cm wide rectangular OB on axis (green) and at 30 deg (red). A baffle of that width is not optimal above 1 kHz, if constant directivity is a design goal.

Rudolf

[StigErik]

Very steep roll-off below 300 Hz could be due to the floor reflection. I found that my 8" mid was flat to 100 Hz in a ~40 cm baffle, except for a very large dip at 250-300 Hz. The only cause I found for this was the floor reflection. That's why I decided to cross over the mid at 350 Hz.

[WDYSUN]

Hi Rudolf,

Originally Posted by Rudolf

The falling response below 300 Hz is purely due to the dipole baffle loss. The driver itself does not work less below - it has to work more!

It is absolutely necessary IMHO. And you will need at least 12 dB/oct.

this seems to contradict what have been said in previous posts. I am still confused about the relation: OB vs excursion.

Originally Posted by Rudolf

Regarding the baffle width: The diagram below shows the response of a 40 cm wide rectangular OB on axis (green) and at 30 deg (red). A baffle of that width is not optimal above 1 kHz, if constant directivity is a design goal.
Rudolf

How did you get this simulations? So what would be the optimal baffle width according to your simulation?

The behaviour of OB at high frequency still remains a mystery to me. Troels Gravesen did an "OB study", have a look here:

http://www.troelsgravesen.dk/OBS.htm

In the second part he tries several combinations with the famous Vifa P13WH, well it doesn't have a 40cm baffle, but none of the response I see here are predicted from models. I honestly wonder a bit about the accuracy of the mathematical models for OB.

Best Wishes
Pierre