Driver acoustical roll off + cabinet roll off + electronic roll off

Discussion in 'DIY Corner - Tips & Techniques' started by jinjuku, May 1, 2013.

  1. jinjuku Moderator

    jinjuku
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    When you take into consideration (and use it as much as you can) the driver and cabinet acoustical roll-off and add an electronic crossover how is it summed? Simply additive?
  2. Swerd Audioholic Ninja

    Swerd
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    Are you asking how can you predict the roll-off of a driver when it's in a cabinet and with a crossover? Is it simply additive, less than, or more than additive? The short answer is I don't know.

    I don't expect a precise prediction of a driver's acoustical roll-off early on while designing a speaker. I start with educated guesses while selecting a woofer, based on the manufacturer’s published frequency response curves on- and off-axis, T/S values, etc. I use that info to decide on cabinet dimensions for bass response (predictions work well for that). Once a cabinet is built, I mount the woofer and take measurements with a variety of simulated crossovers and see what real results look like.

    I do something similar for a tweeter. I select based mainly on Fs and frequency response and the cabinet width, as well as knowing of what kind of high frequency behavior I can expect from the woofer I have in mind.

    This requires measurement and crossover design software that allows easy trial and error. That way you measure as you go, instead of relying on predictions. Obviously, this works better with time and experience, as you develop a sense of what works and what doesn’t work. It also helps if you have a friend who owns all the software and gear and knows how to use it ;).

    Did this answer you question? I think you already understand this.
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  3. jinjuku Moderator

    jinjuku
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    Thanks Swerd. I just wanted to make sure I wasn't missing anything in my understanding.
  4. Swerd Audioholic Ninja

    Swerd
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    It certainly helped me understand this better once I saw Dennis Murphy's set up in use. He uses Praxis for measuring speakers and LSPCad for designing crossovers Liberty Instruments, Inc. Home Page.

    To get the full benefit, you have to have a full-duplex sound card. That way, in a brief moment, LSPCad generates a pink noise test tone, and plays it through your speakers via a simulated crossover, while at the same time it records the sound your speakers make and displays the resulting frequency response curve. You start with a mouse click, and quickly see the measured FR curve. You can vary elements in the crossover (all simulated in LSPCad) and quickly see the results. It makes trial and error so quick that any other method of designing seems painfully slow.
  5. jinjuku Moderator

    jinjuku
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    I have a Dayton Omni-Mic V2 measuring system for starters. I currently have the Dayton RS180S and will be getting the RS28F and wave guide. Going to use the Behringer DCX2496 for X-Over and 4 channels of amp to start.

    Just need to figure out what I want to use for a test baffle.
  6. Swerd Audioholic Ninja

    Swerd
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    Skip a test baffle and use a cabinet of suitable volume for the RS180S (one or two?) and wide enough for the wave guide.

    There will be baffle step issues. Might as well measure them and make adjustments while designing the crossover.

    In theory the wave guide should minimize cabinet edge diffraction issues. As we all know… In theory there is no difference between theory and practice, but in practice there usually is :D. Mount the tweeter and wave guide on a cabinet of the size you intend to work with and measure how it works.
  7. GranteedEV Audioholic Ninja

    GranteedEV
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    For an individual driver, it is simply additive. However it also depends on the pole and Q for the electronic crossover, which may differ from the fs and Q of the driver. Depending on the desired frequency response, you may have to stagger your slopes - IE use different, cascading 6db electronic filters.

    The driver's acoustical rolloff at the bottom end is not different from the cabinet's acoustical rolloff. The cabinet is simply altering the driver's rolloff.

    The driver's acoustical rolloff in its upper ranges is affected by

    - the baffle step which imparts a rise in frequency response of around 4-5db, but this ends rather than continuing on on up in frequency. Often the way to deal with this is to include the compensation into the low pass of the woofer and use multiple poles.
    - inductance and mass will accelerate rolloff, but you probably won't have to worry about this with the dayton drivers.
    - cone and surround resonances with give uneven rolloff and decay. You want to push these well down (IE 30 to 40db down, and response should follow the intended rolloff)

    For multiple drivers, the degree of summation however depends on the phase difference.

    +1 on this

    Measure the drivers all in the same box, and then aim for a specific driver interaction in the simulations. Are you using Active Crossover Designer?
    Last edited by a moderator: Oct 19, 2014
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