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Thread: When Measuring a Woofer's Response, Where Do You Place the Mic?

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    When Measuring a Woofer's Response, Where Do You Place the Mic?

    I've been designing a set of speakers/crossovers and have been dealing with the woofer's response.

    The woofers (10" JBL 2251J) are in ~1/2 cu ft ported cabinets. These are to be used as midrange speakers with an 18" for the woofer and a Heil AMT for the tweeter

    I can set the crossover values to get the 2251s to better than +- 3 dBA from ~120 Hz - 4K, in my room, when the mic is placed <1" of the woofer dome, within the "confines of the cone" itself.

    However, if I pull the mic back, even just a couple inches, it's a totally different story where the bass end fall off like a stone. And of course, the further back you measure, the more the room influences the results.

    I wish I had an anechoic chamber, but that won't happen. I may be able to take it outside and lie it on its back raised a bit so the port would be open in the back.

    But even then, how close should the mic be to the cone and where/how did JBL take the measurement???

    Thanks


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    Senior Member Ian Mackenzie's Avatar
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    Hi Tod

    This is a normal problem with near field measures

    One way around this is to do a measurement as you have done of the woofer and then put the mic over the port and splice the two measurement together

    But due to the port radiating area done calculations are required to adjust the measurement for the real spl if hhs port.

    One option is to do a simulation of the driver in BassBox of the ported enclosed. This is normally fairly accurate.

    Alternatively, attempt a ground plane measurement on your driveway outside

    Place the enclosure on its side on the driveway away from any buildings

    Place the mic on the driveway 2 m from the enclosure

    Tilt the enclosure so the centre of the cone is on axis with the position of the mic

    You will get a 6 db increased measurement overall due to the ground acting as a reflection of the direct sound from the driver and this is normal

    There are some limitations

    The ground plane measurement is not the same as a half space measurement where impact of the baffle radiating area can influence the baffle step response

    See this link for further discussion
    https://www.prosoundtraining.com/201...-measurements/

    http://www.mh-audio.nl/Groundplane.asp

    Below is a link to a Jbl spec sheet of a typical driver

    Refer to comments about measurement
    http://www.jblpro.com/pub/obsolete/2118.pdf

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    Thanks.

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    Senior Member ivica's Avatar
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    Quote Originally Posted by toddalin View Post
    I've been designing a set of speakers/crossovers and have been dealing with the woofer's response.

    The woofers (10" JBL 2251J) are in ~1/2 cu ft ported cabinets. These are to be used as midrange speakers with an 18" for the woofer and a Heil AMT for the tweeter

    I can set the crossover values to get the 2251s to better than +- 3 dBA from ~120 Hz - 4K, in my room, when the mic is placed <1" of the woofer dome, within the "confines of the cone" itself.

    However, if I pull the mic back, even just a couple inches, it's a totally different story where the bass end fall off like a stone. And of course, the further back you measure, the more the room influences the results.

    I wish I had an anechoic chamber, but that won't happen. I may be able to take it outside and lie it on its back raised a bit so the port would be open in the back.

    But even then, how close should the mic be to the cone and where/how did JBL take the measurement???

    Thanks


    Hi todd,

    As You have relative small box for the speaker, You can put the box on the shelf about in the middle of the wall and move back the mic 1~1.5m, and do the measurements. Use the gating time to reduce the reflections from the walls. to see the influences of the reflections on the LF, move the mic about 0,5m left-right-up-down and get the mean value of the measurements.

    regards
    ivica

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    Could work but would have to be far enough so as to not influence the rear port.

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    Senior Member Ian Mackenzie's Avatar
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    http://www.audioheritage.org/vbullet...-2251-Variants

    Some more info above. A few have gone this road before

    What are the dimensions of the front baffle of the mid dog box?

    Is it a satellite or part of a larger system?

    If it is just the driver dimensions then it might cause a step response but depending on where you actually mounted the enclosure would have an over riding influence on the low end

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    Separate cabinets ported in rear, 11.9" x 11.9"



  8. #8
    Senior Member Ian Mackenzie's Avatar
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    Looks like a B&W 801 on steroids..LOL

    I would not worry too much.

    The problem is that any thing nearby will impact on your measurements at those frequencies and mess it up. The near field is as good as you can do unless you can drag its outside. Gated measurements are both unreliable and somewhat impractical in the 100 hz area due to the distance required for a gated clean impulse.

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    How to do it, from D.B. Keele

    From D.B. Keele's website:

    6. "Low-Frequency Loudspeaker Assessment by Nearfield Sound-Pressure Measurement," J. Audio Eng. Soc., (April 1974).

    A loudspeaker test technique is described which depends on nearfield pressure measurements
    made in a nonanechoic environment. The technique allows extremely simple
    measurements to be made of frequency response, power response, distortion, and
    electroacoustical efficiency.

    Ruediger

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