BSC Versus Room Gain

[brkitup]

Hi all. I have never used bsc in any of my designs, because the room gain always seems to more than make up for any baffle loss. I've included a picture to show the in-room response of my EL-2s. Measurement was taken at a distance of 8 feet, while the speakers were sitting at their normal position at the sides of my television.

The crossover uses only 6 components (LR2 at 3k, L-pad for tweeter), since I love simple crossovers and drivers with naturally flat response, and also tend to get into a lot of trouble when I start adding lots of components.

They really sound fantastic in my living room, and have very good bass for their 5.25" woofers. The speakers sit only about 15" from the front wall, which surely contributes to their low-end response, but they sound very balanced at other points in the room also (they were at the 2010 MWAF, if anyone remembers them). Anyway, in this situation would bsc improve my design in any reasonable way? Thanks.

[carlspeak]

If your measurement is indeed the case, then BSC would do more harm than good.
BSC tends to bring down the response beyond 200-400 hz. In your case, it's already down.

[50 watt head]

If your measurement is indeed the case, then BSC would do more harm than good.
BSC tends to bring down the response beyond 200-400 hz. In your case, it's already down.

+1

[fbov]

Hi all. I have never used bsc in any of my designs, because the room gain always seems to more than make up for any baffle loss. ...Measurement was taken at a distance of 8 feet, while the speakers were sitting at their normal position at the sides of my television. .

I think you have an in-wall application, the wall in this case being your TV screen. No BSC needed, but due to your application, not the drivers, per se. I'll believe some of the hump below 100Hz is room gain related, but the lack of dip between there and 1KHz says something is filling in those frequencies. Perhaps it's a driver response characteristic, but that's unlikely.

My L/R are only a few feet from my screen, and they're flat with some BSC. My CC is under the screen, has no BSC and doesn't need it, sounds boomy with it. You don't need much structure to start reinforcing response spatially (eg. 4pi vs 2pi), and that's all BSC is trying to fix.

HAve fun,
Frank

[jclin4]

Is your TV a large flat screen? Maybe because your speakers are next to your TV, it is close to having them on an infinite baffle, therefore no need for BSC.

[brkitup]

Is your TV a large flat screen? Maybe because your speakers are next to your TV, it is close to having them on an infinite baffle, therefore no need for BSC.

I never thought about it that way. My TV is indeed a 50" flat screen. I think I'll take a bunch of measurements with them positioned away from all room boundaries and see what I get.

I've noticed that bsc is now pretty much standard on all diy designs, but I think it's funny how it's not even included as an element in older programs like X-over Pro.

[Wolf]

It's not really a separate entity though. There are 2 types of BSC; Global and Local. Global is a parallel LR filter placed in series before the entire xover. This is inferior, and adds parts that could be eliminated.

Local is generally used by making the series coil on the woofer a little larger than normal, to tilt its inherent response down to flat a little more than if not compensating at all. The tweeter reference can vary as to what is required, mainly related to baffle position and size, xover frequency, and slope.

Xover pro really does not take a lot into account when design a xover. That is also likely why BSC is an aftertought.

Later,
Wolf

[frugala]

I've noticed that bsc is now pretty much standard on all diy designs

I've wondered about that. I understand fully when speakers are in a semi-ideal room where they can be placed properly into the room - and that those are the rooms most often used for speakers. But, there are a lot of us stuffing speakers into rooms where they can't possibly be placed several feet from walls. Seems to me that there is just as much need for speaker designs with little or no BSC, as there is for full BSC designs.

[mzisserson]

I average 1-2dB on my designs because BSC is highly overrated in living rooms. Pro audio, open fields, ect, it is a must. However room gain does make up for a lot of it.

I also find the smooth power response of BW type x-overs yield less need for BSC vs. a LR type with sums to -3dB at x-over.

It is like any other aspect of a design, you compromise with it until you have it where it balances out the speaker as a whole.

I used X-over pro for years, while it does not take into account BSC, the lil' reds were highly regarded at DIY NY/NJ and was design soly in x-over pro. That said I have long since switched to Jeff's suite and will never look back.

[Bob Brines]

The problem with allowing room gain to correct for the baffle step is that the two phenomena do not occur at the same place. Baffle step occurs in the 300-500Hz range depending on the baffle width while room gain doesn't start rolling until below ~100Hz. What you get is lots of "punch" and weak voices. A better bet is design your bass roll off to match your expected room gain and do a proper BSC.

Bob

[czag]

Bob,

I think he also is including boundary compensation such as floor bounce and/or being placed close to the back or side wall. That would definetly fill in 300hz to 500hz.

Chris

[Bob Brines]

Bob,

I think he also is including boundary compensation such as floor bounce and/or being placed close to the back or side wall. That would definetly fill in 300hz to 500hz.

Chris

Yea, but floor bounce is a suck-out usually between 150-200Hz. Asking the reflection off of the front wall to reenforce is iffy because it will be alternately in and out of phase as frequency increases. Better to damp the wall as best as one can or put the speakers hard up against the wall and then this and BSC become moot.

Bob

[brkitup]

It's not really a separate entity though. There are 2 types of BSC; Global and Local. Global is a parallel LR filter placed in series before the entire xover. This is inferior, and adds parts that could be eliminated.

Local is generally used by making the series coil on the woofer a little larger than normal, to tilt its inherent response down to flat a little more than if not compensating at all. The tweeter reference can vary as to what is required, mainly related to baffle position and size, xover frequency, and slope.

Xover pro really does not take a lot into account when design a xover. That is also likely why BSC is an aftertought.

Later,
Wolf

I didn't realize bsc could be implemented so simply. Very interesting. Thanks.

[mzisserson]

I didn't realize bsc could be implemented so simply. Very interesting. Thanks.

Most things are not too difficult, there are some who make it out as a near impossibility. Read Wolf's blogs, he has some awesome tricks around the everyday design challanges we all face.

[Jeff B.]

The problem with allowing room gain to correct for the baffle step is that the two phenomena do not occur at the same place. Baffle step occurs in the 300-500Hz range depending on the baffle width while room gain doesn't start rolling until below ~100Hz. What you get is lots of "punch" and weak voices. A better bet is design your bass roll off to match your expected room gain and do a proper BSC.

Bob

+1

You are right on the nose, Bob. I see these kinda threads from time to time. I think people tend to misinterpret the phenomena involved. Room gain is a low frequency / long wavelength phenomena involving wavelengths relative to the longest dimensions of the room. In rooms the size of typical family rooms the gain from pressurization may not begin until 50Hz. Small rooms may push this up near 100Hz, but still way too low to be associated in any way with baffle diffraction loss.

Baffle step due to diffraction, depending on baffle size, will generally slope downward from around 700-1kHz, dropping 6 to 8 dB to around 100 Hz. Room gain may boost the deep bass, but it can't compensate for baffle step at all. Moving the woofer near the floor can effectively reduce or even elliminate the loss though, as will placing the speaker very close to an entertainment center. Howevever, it has been my experience that moving a speaker close to the wall behind it has little impact on reducing the step. Despite what people tend to say, if I take a small speaker with a 9" wide baffle that is 11-12" deep and measure it in the middle of the room it will show the full baffle step in its response. If I move it within a couple of inches of the wall it does very little to reduce the step, maybe a couple of dB at the most. Dennis Murphy and I have talked about this and his experience was the same. When a baffle is narrower than the distance between the front baffle and the wall, the wall reinforcement will do little to mitigate the diffraction loss of the baffle.

I see a lot of people comment that they do not design with any, or very little baffle step compensation. However, I don't like to think in terms of whether I am designing for BSC - I am designing for flat response, and I am unable to achieve that without appropriate baffle step compensation. Speakers that I have heard that do not have much compensation implemented tend to sound thin in the lower vocal range and shouty in the upper mids due to not only the diffraction loss, but also the baffle diffraction peak, which can really emphasize the upper vocal range. My advice is to not think in terms of baffle step compensation, but simply target a relatively flat response and tonal balance.

Jeff B.

[50 watt head]

What a great thread!