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How Much Baffle Step Compensation?

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  • How Much Baffle Step Compensation?

    I've been trying to learn about speaker building and one question (among many) that I still have is: Why would any design have less than 6db in BSC?

    Assuming we are working with a small bookshelf (6.5" woofer + 1" Tweeter - on typical bookshelf size baffle), the amplitude will begin to decrease approximately at 1000Hz, and continue decreasing to about 100hz.

    Maybe the room is suppose to help the bottom end? -but I thought the room doesn't start to help much until we get down to about 100hz.

    I'm just trying to understand that as I try to build my first bookshelf, what thought process should I follow when determining the amount to BSC.

    The woofer I plan on using in my first bookshelf build is the SB Acoustics SB17NRXC35-8, which has a sensitivity rating of 89db; however, I seen other designs out there that have developed bookshelfs utilizing this woofer with an overall rated speaker efficiency rating 85db (e.g., Jeff Bagby's Mandolin - which uses the uncoated version of the aforementioned woofer). Does that mean only 4db of BSC was employed (89db - 85db = 4db)?

    Any thoughts would be much appreciated it :-)




  • #2
    Why would any design have less than 6db in BSC?
    One: In a situation in which the placement and sensitivity difference accounts for part of the 6db acoustic difference.
    Last edited by Sydney; 09-25-2017, 11:56 AM.
    "Not a Speaker Designer - Not even on the Internet"
    "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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    • #3
      Originally posted by Sydney View Post
      One: In a situation in which the sensitivity difference accounts for part of the 6db
      Just to make sure I understand what you are saying:

      An example of this would be if I analyzed the response of the woofer on an infinite baffle (as provided by the manufacturer) and I find that the woofer's response slow increases as we move down in frequency until it ultimately starts to naturally roll off.

      In the example of SB17 (based on manufacture's specs):

      86db @ 350hz
      87db @ 250hz
      88db @ 200hz
      89db @ 175hz
      90db @ 150hz
      90db @ 125hz
      87db @ 100hz
      85db @ 80hz
      80db @ 50hz

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      • #4
        You'll get many answers to help you understand ...

        Here are the two extremes: With a small baffle in an open air environment, you need most, if not all of the 6 dB compensation. At the other end of the spectrum, is an "in-wall" woofer. The wall approximates an infinite baffle. Hence no baffle step loss at all. When placing a smaller speaker in a bookshelf, the bookshelf acts the part of a mush larger baffle.

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        • #5
          As Mike mentioned about rationale; Older Pre BSC speakers often included controls on Treble & Mid drivers to tailor output to placement as opposed to fixed internal compensation.
          I've always placed speaker close to boundaries and not had the room for freestanding speakers and now have in-wall speakers.

          "The 6dB transition theory assumes that all of the energy that is not radiated into the forward half-space is lost for ever, where common sense tells me that a large part of it will be reflected back from the rear and side walls of a normal listening room. An alternative solution is to compensate for half the subjective low frequency loss, or 3dB."

          I believe some take care of this aspect during voicing to taste and placement. *
          * See about 3/4 down:
          https://www.trueaudio.com/st_diff1.htm
          "Not a Speaker Designer - Not even on the Internet"
          "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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          • #6
            Why? Simply placement. In the last set of bookshelves that I built, the speakers will be sitting on a book shelf with the rear and sides in close proximity to the walls. If I had used the full bsc of 6db, they would be very boomy and thick in the lower mid range. By using about 3db, I avoided that and have a well balanced speaker for the location. Of course, sitting out in a room, they can sound a little thin in the bass and a bit forward in the midrange.
            https://www.facebook.com/Mosaic-Audi...7373763888294/

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            • #7
              Another situation where full BSC may not be required is placement on either side of a large flat-screen TV.

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              • #8
                I realize you mentioned a two way 7"/1" speaker... but another reason is in a 3 way tower where the woofer is close to the floor.

                I never have a "targeted" amount of BSC. I always build my enclosures, then measure them in their actual intended home position, then design and voice a balanced crossover.
                Craig

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                • #9
                  The room never helps! ;) I kid. It can help in isolated bandwidths but overall it is a detriment. I Jeff's mondolins the crossover may be bringing up the sensitivity. The low pass often increases sensitivity just above the woofer's impedance peak. Sometimes as much as 2db. Sent from my SGH-I337M using Tapatalk
                  https://www.youtube.com/channel/UCm2...oSKdB448TTVEnQ

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                  • #10
                    You guys are awesome! Thanks for responding - this helps. Sent from my iPhone using Tapatalk

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                    • #11
                      I am not saying this applies to all DIY designs, but I experimented with a few similar small 2 way speakers, placed a good bit out into the room, and no BSC was thin and strident.

                      But the FULL 6 db compensation was much fuller, but also kinda duller and not balanced sounding to me.
                      I finally tried about 3-4 db and that was closer to ideal.

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                      • #12
                        Originally posted by dkalsi View Post

                        Assuming we are working with a small bookshelf (6.5" woofer + 1" Tweeter - on typical bookshelf size baffle), the amplitude will begin to decrease approximately at 1000Hz, and continue decreasing to about 100hz.

                        Maybe the room is suppose to help the bottom end? -but I thought the room doesn't start to help much until we get down to about 100hz.
                        Room gain and BSC are two different things. BSC has to with radiation patterns of the frequencies. Any frequencies that have lengths shorter than the width of the baffle will have most of their energy radiated forward from the front of the speaker (the baffle is in the way so they cannot go backwards). The wavelengths that are longer than the baffle will go around the the sides of the baffle meaning the energy is spread out in more directions. This means that since the energy is spread out it has less energy at any specific point (such as listening position) and since the higher frequencies are less spread out they are louder at the listening position.

                        Room gain does boost the low end but has nothing to do with the causes of baffle step.

                        As PWR RYD said it is best to measure the speakers in the box. The amount the baffle is affecting the response will show up in the measurements. Then you just knock the upper end down to match the lower end using various crossover techniques. I remember Jeff Bagby said basically said that people get too wrapped up worrying about BSC when designing a speaker with real world measurements.

                        The only reason to worry about adding BSC is if you are simming a xo without the ability to add baffle affects to the sim. This should not be necessary with the free programs we have today that can sim and add baffle affects.

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                        • #13
                          Originally posted by kevintomb View Post
                          I am not saying this applies to all DIY designs, but I experimented with a few similar small 2 way speakers, placed a good bit out into the room, and no BSC was thin and strident.

                          But the FULL 6 db compensation was much fuller, but also kinda duller and not balanced sounding to me.
                          I finally tried about 3-4 db and that was closer to ideal.
                          I am curious how you are figuring how much baffle step you have? Were you measuring or simming?

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                          • #14
                            Originally posted by killa View Post

                            I am curious how you are figuring how much baffle step you have? Were you measuring or simming?


                            Changing resistors and measuring.

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                            • #15
                              Originally posted by kevintomb View Post
                              I am not saying this applies to all DIY designs, but I experimented with a few similar small 2 way speakers, placed a good bit out into the room, and no BSC was thin and strident.

                              But the FULL 6 db compensation was much fuller, but also kinda duller and not balanced sounding to me.
                              I finally tried about 3-4 db and that was closer to ideal.
                              This is usually what I find, as well. No BSC, even up against the wall, sounds thin to me. Full BSC often sounds dull and a hair muddy. But some I've built use 3-4 db, some use up to 5, one used full 6 db but it was a 4" woofer that I was asking to play down to 50 hz in a tiny box with sufficient volume and oomph. It worked and wasn't for critical listening and was inefficient. When modeling box response, sometimes doing something like decreasing the box size a little will put a small lump in bass response between, say, 80 and 200 hz. Such a woofer and box combo might require less BSC. So, as to echo many who have already spoken, it depends on the woofer, the design, the placement, and your own tastes.

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