Baffle Step Compensation ?

[Ecir38]

I see crossovers built with different kinds of BSC built into them. What I have gathered so far is that the type you use is determined by the location of the speaker to its boundaries (on wall, close to wall, two feet from wall, close to TV, etc.).

Here are just to name a few different examples of the different ways crossovers are explained by:

Full BSC
With BSC
No BSC
Reduced BSC
Light droop
Low BSC
Less BSC

Needless to say this is a little confusing due to my lack of experience even though some are just common sense.

Can someone explain which ones mean the same thing and a short explanation of each with the recommended placement? (If you know any other terms used please add them)
i.e.
Full BSC = With BSC (more than a 2’ boundary)
No BSC = (wall mounted)
Reduced BSC = Light drop, Low BSC, Less BSC (

I do realize that how much the BSC is reduced will determine different placements from a boundary. I think this is expressed in the amount of db’s taken out of Full BSC.

Thanks, BR

[Dennis H]

Full BSC is 6dB. Less is, well, less than 6dB. It's more useful to talk in dB than adjectives. Then everyone knows exactly what you mean.

So why 6dB? Think of it this way. High frequencies will mostly radiate forward and very little to the rear. Low frequencies radiate in all directions. Like when you walk behind a speaker and the lows are still there but the highs go away. Assume you are outdoors. Since the lows are going in all directions, you need to boost them so they will sound equally as loud as the highs when you are in front of the speaker. Twice the total sound pressure is 6dB SPL. Half goes forward, half goes backward.

Now, if the speaker is near a wall, the bass doesn't just disappear to the rear like it would outdoors; some bounces off the wall. So you might choose less than the full 6dB.

If the speaker is mounted with the baffle flush to the wall, all the sound can only go forward, both the lows and the highs. So you do don't need any BSC.

[Ecir38]

Wow, what a great explanation Dennis. This will keep me from asking allot of dumb questions in the future

Thanks, BR

[DougyD19]

Ok, dumb question....Why do we care about how it sounds behind the speaker. You are supposed to be in front of it anyway right?

[Ecir38]

I'll take a stab at that. The speaker won't sound as good with full BSC if it is closer than 2' from a boundary.

BR

[Landroval]

Ok, dumb question....Why do we care about how it sounds behind the speaker. You are supposed to be in front of it anyway right?

Yes.. BSC is used to make the level of highs and lows the same in the front of the speaker (and in the general listening area). E.g:

If speaker is far away from walls:
In front of the speaker / without BSC: Highs will play +6dB louder than lows
In front of the speaker / with full BSC: Highs and lows will be equal

If speaker is on-wall/in-wall:
In front of the speaker / without BSC: Highs and lows will be equal
In front of the speaker / with full BSC: Lows will play +6dB louder than Highs

All this because of the different sound radiation behavior depending on the wavelenght -> 2pi or 4pi.

[cjd]

Ok, dumb question....Why do we care about how it sounds behind the speaker. You are supposed to be in front of it anyway right?

You don't. What you DO care about is radiating pattern, because output is a static sound PRESSURE level - if you take pressure X and apply it forward vs. forward and backward, you get twice as much. So it's not the backward part we care about here - it's the fact that it's not all forward, which means we lose SPL in the forward radiation pattern below baffle step. Ultimately this means we'll have 6dB less SPL when measured in front. Speakers should have this built in - it's yet another reason a textbook crossover doesn't really do what you need it to do.

Trouble exists if you start placing a speaker near a boundary, where some of that rear-radiating pressure will be directed forward - if you were to go in-wall you would have ALL of it forward, meaning you would now have 6dB too much bass. Near wall can be varying degrees - placing a center on a TV, for example, will give you a different step and different amount of relative compensation.

The goal is flat response. The trouble is bass frequencies respond to their environment, making response quite variable depending on where you place a speaker. So we like to specify - no BSC = in an entertainment center perhaps, in a fairly shallow cabinet on-wall, or even in-wall. Partial BSC is somewhat flexible but means near-wall or on-TV or... Full BSC means you'll get bloated bass if the speakers aren't out in the room some.

C

[Ecir38]

Thanks guys for breaking it down a little more, this helps allot.

BR.

[truckman]

I'll take a stab at that. The speaker won't sound as good with full BSC if it is closer than 2' from a boundary.

BR

Dumb question. Is the 2' measured from the front of the baffle or the rear of the cabinet?

[cjd]

Front of the baffle is where the step occurs, so...

[nelsondog]

My turn, My turn... I've got a dumb question!
What about rear ported designs? I plan on mounting my set near the ceiling with speaker brackets tilted downward toward the listening position... Would it not be better to have the port firing from the front baffle instead of exhausted from the rear of the enclosure? or, Are we now introducing cancellation issues?... Can we/do we need to, build front ports instead of rear?
Brian

[ThomasW]

The port location isn't related to BSC.

As long as there's adequate room for the port to "breath" meaning it's not obscured by a boundry, furniture or some other object, it doesn't matter where in the box the port is place. It's a good idea to keep it away from the tweeter.

Now lets keep this thread on topic. Additional questions regarding port location or topics not related to BSC should go in appropriate threads.

[cjd]

Ports are almost always outputting frequencies below step frequency so they radiate spherically - they're also tied to woofer output below step generically, so when you port a woofer it outputs at an appropriate level. Step is included (or not).

Placement is so much less critical because it radiates spherically though.

Maybe this helps some of the mess fall into place for folks which is why I bring it up.

C

[garto]

So does this mean that if I tweak BSC in a speaker it should sound the same 3 feet from the wall as it does 1 inch? My next question is how can I change the field of sound, say I use bookshelfs as computer speakers (sound stage must be within 2 feet of speaker). Is this also a function of BSC?

Thanks,
Chris

[cjd]

So does this mean that if I tweak BSC in a speaker it should sound the same 3 feet from the wall as it does 1 inch?

Not quite sure I follow you here, but I'll attempt to answer. Assuming we limit the scope of changes in sound to those introduced by baffle step and the potential "extension" of baffle by near-wall placement, you would need *different* baffle step compensation for a 3' placement vs. a 1" placement. But you probably would not need a change from 3' to 20'. A good crossover does not have baffle step compensation as a discrete circuit either - it is simply incorporated in the design.

My next question is how can I change the field of sound, say I use bookshelfs as computer speakers (sound stage must be within 2 feet of speaker). Is this also a function of BSC?

Sound stage is completely artificial and is dependent on a great many things, among them the specific recording, and your propensity to give in to that particular insanity. Most quality bookshelf speakers should do well in nearfield listening, though some of that depends on scale - the closer you sit, the more problems distance between drivers can cause because you're changing the sum distance and working farther (potentially) into off-axis while being ON axis.

In the hopes to make this more clear - when I built my MTM's I measured at 1meter minimum. When I built the WWMTM's minimum was 2meter. At 1M the bottom woofer was almost 45 degrees off-axis!

C

[garto]

Thanks, that actually makes a lot more sense now. So if I built a smallish bookshelf, I could tweak the crossover (specifically the BSC portion which is integrated into the crossover anyways) so that I could place it closer to a wall. And choosing a bookshelf design with speakers that are closer together will help even more.

Sorry I was just talking to myself. But thanks CJD, where did you learn most of your stuff anyways? Do you do a lot of speaker testing yourself with your own equiptment out of your own pocket or are you in the industry or were part of the industry?

Chris

[cjd]

You would probably not be able to do any crossover tweaking yourself for changes in bafffle step where it is included in the crossover directly. There are many small(ish) options that may work for you with minimal baffle step (or no baffle step). Depends how small.

And, this is 100% hobby. The last pair of speakers I built were never a design I intended to build myself, only set me back ~$1200 or so. Worth it and I'm quite glad I have 'em but...