Questions Computing Baffle Step Compensation circuit

[chasingame]

Hello,

It was recommended that I read and follow this calculator for Baffle Step Compensation circuit.

http://diyaudioprojects.com/Technica...it-Calculator/

My question is "how do I know how many decibels to input in the formula?"

I have read that maximum BSC can be 6db. So I assume the values are from 1 to 6 db. How do I pick a value ?

Also, if I decide to use two of the mid woofers in each enclosure, is the BSC still the same ? The formula requests the R(e) but does this change if I use two drivers instead of one ?

Also, as to SPL... if the SPL on a single midwoofer is 86, when I add two in the enclosure, overall, I get 92 (+6db I assume) then if I choose 4db for BSC this puts me back at 88db. If my tweeter is rated at 90db SPL, would I assume that I should attenuate the tweeter by 2db to equalize the response ?

[augerpro]

Originally posted by chasingame

I have read that maximum BSC can be 6db. So I assume the values are from 1 to 6 db. How do I pick a value ?

The answer depends on how close to adjacent boundaries the speaker will be and your own taste. Typically a speaker a good distance from the walls uses about 3-4 dB of BSC. One designed to be close to (or on) wall may have 0-1 dB of BSC. There is no easy answer without having some experience with your particular room and taste, but those guidelines should help you decide for your first project.

Originally posted by chasingame
Also, if I decide to use two of the mid woofers in each enclosure, is the BSC still the same ? The formula requests the R(e) but does this change if I use two drivers instead of one ?

Also, as to SPL... if the SPL on a single midwoofer is 86, when I add two in the enclosure, overall, I get 92 (+6db I assume) then if I choose 4db for BSC this puts me back at 88db. If my tweeter is rated at 90db SPL, would I assume that I should attenuate the tweeter by 2db to equalize the response ?

BSC is mainly dependent on the baffle dimensions, so yes the BSC would be the same for having two drivers. There is some mutual coupling of multiple drivers, but this is complex and beyond the scope of novice build so I wouldn't worry about if I were you.

Also you sensitivity calculation is exactly right, keep in mind that though that the +6 dB for having two woofers assumes they are wired in parallel. If you wired them in series there would be a net change of 0dB compared to the single woofer sensitivity.

[augerpro]

Re would be whatever the woofers are wired together, so if each had an Re of 6 ohm and you wired them in parallel the Re would be 3 ohm.

[chasingame]

Ah.... thanks. Yes the plan was parallel wiring.

So the BSC computation for two parallel drivers is different since the R(e) is now different. I was afraid of this and hence my question.

[richie00boy]

Alternatively, do it actively and have it adjustable

[ScottG]

Use this:

http://www.tolvan.com/edge/help.htm

You can play with the values for the compensation circuit to see changes in response. (..make sure you input the correct impedance for the circuit's pass-band.) Look to the "negative" blue line to see the filter. The red line shows you the effect of the filter for that particular driver + baffle. (..make sure you don't have open baffle selected, unless of course the design is an open baffle/no enclosure.)

[panomaniac]

3dB works for most speakers and rooms.

[sandyhooker]

Go to MJ King website ..He provides info and formulas..

[chasingame]

Thanks everyone. My head is about to explode with information but it is slowly starting to make some sense. I can see I will never be able to do a "good" job with our measurement equipment.

I wish I could travel to some "institute" or even some engineers house and take a 1 week crash in speaker building/design. I got the wood working down pat as I have been doing this for years.

got any suggestions? I would like to come away with the skill set and the list of equipment to do the job right. My goal would be to "ramp up" and not spend years fiddling on the internet...

[ScottG]

Originally posted by chasingame
If my tweeter is rated at 90db SPL, would I assume that I should attenuate the tweeter by 2db to equalize the response ?

You'll get a *theoretical* +6db by wiring in parallel with a pure voltage amplifier (i.e. a typical solid state amp that doubles the watts from 8 ohms to 4 ohms - in the case of a net average 4 ohm load.) Chances are you won't get quite +6 db, but close.

HOWEVER, in relation to your BSC circuit you must have the full average in spl maintained throughout that region (..for perfect "matching" of the BSC circuit). Chances are that unless the driver has a weaker motor (with a higher qe and resulting qts), AND sufficient enclosure volume, that the driver(s) will not have a "flat" response (on even an infinite baffle), in the BSC region/pass-band. (This is all something you can model in software like Unibox and WinISD.) A quick model can give you some perspective on the sp-loss (or gain) for a given enclosure with driver(s).

As for the tweeter - this depends a great deal on the lower freq. response (high-pass crossover), AND the contribution of the baffle. This is where the Edge can help quite a bit. The Baffle Diffraction Simulator can ALSO model for chamfered edges. Depending on that baffle and crossover the response may not meet the target "90" db at that freq. (..even though it's rated at that, or is that on an infinite baffle).

Originally posted by chasingame

got any suggestions?

[/B]

Read, and re-read the loudspeaker articles from Stereophile. 1st look over the objective data (measurements), particularly freq. linearity (with dispersion), and in-room response. Then read over the subjective reviewer comments. Then go back again and read the measurement section, comparing subjective to objective. Finally, compare the measurements of various designs - and look for commonalities to generally good designs - especially in respect to freq. response (and dispersion) in relation to in-room response (with a nice linear/flat in-room response as the ideal).

http://www.stereophile.com/floorloudspeakers/

http://www.stereophile.com/standloudspeakers/



Further:

Here is a source and link to freeware programs like the, Unibox, Baffle Diffraction Simulator, and the Room Response Calculator:

http://www.rjbaudio.com/Audiofiles/FRDtools.html

This should get you started without having to spend anything. Note however that it isn't entirely complete.

There are 4 significant effects to note with respect to the room and you relative to the loudspeaker (and even specifically the drive units):

1. Room gain
2. Modes
3. Boundry gain
4. "Floor bounce" aka "the Allison effect".

#2 you generally do *not* factor into the design. The rest you should factor into the design. 1 & 2 can be modeled with the Room Response Calculator. Among more sophisticated modeling programs, I believe that MJK's worksheets account for 1, 3, and 4 - and I think he is offering a low cost license for them again. (His enclosure modeling is also superior to something like Unibox, and actually just about anything out there.)

If you want to get further into this hobby (than modeling), then consider an "all-in-one" software package like Sound Easy (with appropriate hardware for measuring).

Note that measurement capability is pretty much required for any good design. While you can do this with Speaker Workshop, you are probably better of with an inclusive package like Sound Easy.

Sound Easy will cost you considerably more, but you can read all of the manual for free ahead of time:

http://www.interdomain.net.au/~bodzio/

For a low fee you can "wade-in" with JohnK's Sound Easy Design Guide (*highly* recommended):

http://www.musicanddesign.com/Guide.html



Finally,

If you find all of that too daunting then consider a proven design. There are quite a few excellent designs out there now. Here are some sources:

http://www.htguide.com/forum/forumdisplay.php4?f=39

http://www.zaphaudio.com/

http://www.troelsgravesen.dk/Diy_Lou...r_Projects.htm

(..with plenty of other well documented designs on the web to choose from. If you find drivers you like then consider doing a web search and see if any design utilizes them - you may find something you like.)

Generally do NOT deviate from the designs, IF you intend to use them.



Good Luck!