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Near field measurements with phase plugs?

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  • Near field measurements with phase plugs?

    Happy to recently find the Bagby white paper on how to measure in-room responses with OmniMic. When measuring near field response, how far away should the microphone be when you have a driver with a phase plug? For example, I have some TB W4-1337SDF full range, 4" speakers. Per Jeff's recommendation the mic would need to be 1/10th the distance of the radius of the radiating area. In the case of these W4's, there's a phase plug protruding from the center much farther than the recommended distance from the actual cone. Also wondering what impact a phase plug may have on near-field measurements in general... Thanks guys!

  • #2
    Near field response is generally for obtaining accurate low frequency response of the speaker. Essentially, the phase plug has no effect here, just move the mic over to the side a bit and measure away. Since you have the mic, you really should be able to answer the question for yourself. Put the mic in front of the cone 1/4" away in a few different locations and measure, how does the response from <500Hz compare?
    Don't waste your money on a new set of speakers, You get more mileage from a cheap pair of sneakers. Next phase, new wave, dance craze, anyways it's still rock and roll to me!

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    • #3
      I asked the same question of him at one of his seminars. His response was basically, just get it as close as possible without touching it. I assumed he meant to still center it but he didn't state it. dcibel has a good point... that you could experiment with both and see if there are any differences. I bet it's minimal, but not sure.

      TomZ
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      • #4
        Thanks guys, I appreciate your insight on this. I'll do some experiments on my own to get a feel for what's actually happening. Come to think of it, I have some 4, 5 and 7" drivers with phase plugs, so as I was reading the instructions about placing the mic in the center of the dust cap, this was one of the first questions that came to mind...

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        • #5
          This thread is a few months old, but I recently tested this and just wanted to throw in my 2 cents that it doesn't make that much difference where the mic is located when you're doing nearfield measurements as long as it's close. I've been measuring the RS180P and RS225P drivers (with phase plugs) quite a bit lately with my latest speaker project. There is a subtle measurable difference between the response of the driver when measured directly in front of the phase plug vs. offset 1-2" in either direction but only above about 2 kHz. Your mileage may vary. See attached plot. Note my test method was as follows: mic placement directly in front of the phase plug, within 1/2" from the tip, and then 4 additional measurements with the mic up/down/left/right of center about 1-1/2" (without changing the distance to the driver). There is no filter or EQ in this data. The light teal line is the measurement directly in front of the phase plug. There is a suckout or null in all of the measurements in my setup at 1.4 kHz, not sure exactly what it is from, but it was not influenced by the location of the mic (off-center or not) at this distance from the driver. At 1 meter distance the suckout/dip goes away, but the cone break-up modes really start to show up.

          Click image for larger version

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          • #6
            Originally posted by danmarx View Post
            This thread is a few months old, but I recently tested this and just wanted to throw in my 2 cents that it doesn't make that much difference where the mic is located when you're doing nearfield measurements as long as it's close. I've been measuring the RS180P and RS225P drivers (with phase plugs) quite a bit lately with my latest speaker project. There is a subtle measurable difference between the response of the driver when measured directly in front of the phase plug vs. offset 1-2" in either direction but only above about 2 kHz. Your mileage may vary. See attached plot. Note my test method was as follows: mic placement directly in front of the phase plug, within 1/2" from the tip, and then 4 additional measurements with the mic up/down/left/right of center about 1-1/2" (without changing the distance to the driver). There is no filter or EQ in this data. The light teal line is the measurement directly in front of the phase plug. There is a suckout or null in all of the measurements in my setup at 1.4 kHz, not sure exactly what it is from, but it was not influenced by the location of the mic (off-center or not) at this distance from the driver. At 1 meter distance the suckout/dip goes away, but the cone break-up modes really start to show up.

            [ATTACH=CONFIG]n1332947[/ATTACH]

            The 1.4K dip is probably a resonance in the surround which creates a dip from being out of phase with the cone.

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