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Making my first ground plane measurements, help needed

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  • Making my first ground plane measurements, help needed

    Yesterday I did my first set of ground plane measurements. But I am a little unsure if I did everything right. I am hoping some of the people here who are more knowledgeable than me could comment and perhaps give some suggestions. I need help to figure out why there is a large peak-dip-peak centred around 1200 hz on the woofer and why the measured low end is far from the simulated response.

    Speaker
    I am making a Bluetooth speaker using the Dayton DS90-8 and Dayton ND16FA-6. The enclosure is 1.61 liters (0.056 ft3) and tuned with a 2.5 cm (1 inch) diameter port with a length of 12.6 cm (5 inches). In simulations this have given me a tuning frequency of 79.1 hz and a F3 point of 74.4 hz. I have not lined or stuffed the enclosures yet.

    Setup
    The measurements were taken in a sports hall at the school where I work. The dimensions of the hall are 24x12x8 meters (80x40x25 feet). The speaker was placed 10 meters (32 feet) from the short side and 5 meters (19.5 feet) from the long sides of the hall. The mic was placed 2 meters from the speaker on the hall's center line. The speaker was placed on a wooden turntable which raised it 5 cm (2 inches) above the floor. I was sitting with my laptop 1 meter (3 feet) to the side and behind the speaker while doing the measurements.

    Measurements
    Measurements of the DS90-8 were taken using REW and an UMIK-1 using sweeps from 20-20 000 hz. Looking at the impulse graphs in REW, room reflections arrived at 22 ms. Therefore the measurements were gated from -5 ms to 22 ms. This gave me a frequency resolution of 45 hz.

    I also did measurements of the ND16FA with sweeps from 1000-20 000 hz.

    Frequency Response
    Looking at the frequency response of the DS90-8 there are a few thing I find strange. For one thing the speaker starts to roll off at 155 hz, this is far from the simulated response. Also there is a large peak-dip-peak centred around 1200 hz. The rest looks as expected and the cone breakup is visible at around 10 000 hz. However, the breakup is not as severe as in the manufacturer data. The response of the ND16FA looks all right to me. There is a roll-off towards the high end, but this roll-of is also present in the manufactures data.

    All of my frequency responses are smoothed by 1/24 octave and the manufacturer on-axis data is included in the graphs for reference.

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  • #2
    Measuring at that distance indoors is introducing the room into the equation. If you must measure indoors do it near field. For distant measurements do it outdoors. As for the LF response anomaly that would probably be explained with an impedance sweep. The dip is probably floor bounce. Typically one only uses a ground plane measurement for below the baffle step frequency. To get a full bandwidth half-space result that would be spliced to a measurement made outdoors with the cabinet on its back, the mic suspended above it, for above the baffle step frequency. 1/6 octave smoothing is preferred, because you can't hear with higher resolution that than anyway.
    www.billfitzmaurice.com
    www.billfitzmaurice.info/forum

    Comment


    • #3
      I think your measurement technique is sound, other than the fact that you're sitting very close to the speaker. Ideally, you should be farther away than the nearest wall.

      I'm willing to bet that your problems at 1200 Hz are port resonance. This should show up in an impedance measurement.

      Comment


      • #4
        Thank you for your input. A bit discouraged by my ground plane measurements, I tried doing some near field and far field responses today. Any comments are appreciated.

        The near field of the woofer was measured 0.5 cm from the baffle in between the woofer and the port. The woofer far field was measured from 50 cm. The tweeter far field was also measured from 50 cm. Woofer near field is scaled and I have not applied any baffle step corrections to the response.

        Comment


        • #5
          When you took your ground plane measurement, where was your microphone?

          Comment


          • #6
            Originally posted by 1100xxben View Post
            When you took your ground plane measurement, where was your microphone?
            I have attached a picture of my ground plane setup with the mic in place.


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            I did a simple impedance measurement using a setup from Audio Judgement using parts I had laying around. I know the measurements are not very exact, but they do show that the port resonanse is at 65 hz, the cabinet is at 135 hz, and the total system at 105 hz. This is quite a lot higher than what I designed in VituixCAD.

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            Comment


            • #7
              The mic should be rotated 90 degrees. Impedance charts don't lie, Fb is 105Hz. Either you made an error in the modeling or the software did.
              www.billfitzmaurice.com
              www.billfitzmaurice.info/forum

              Comment


              • #8
                Originally posted by solbero View Post

                I have attached a picture of my ground plane setup with the mic in place.


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                I did a simple impedance measurement using a setup from Audio Judgement using parts I had laying around. I know the measurements are not very exact, but they do show that the port resonanse is at 65 hz, the cabinet is at 135 hz, and the total system at 105 hz. This is quite a lot higher than what I designed in VituixCAD.

                Click image for larger version

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                I wanted to make sure you had your microphone placed on the ground, which you do. Since that is the case, you do not have any floor bounce until you get to extreme high frequency (due to the microphone element being a cm or so off the ground).

                Is your enclosure volume 1.61 liters total, or 1.61 liters per half? Do you have a divider in the middle?

                Comment


                • #9
                  Something is wrong there.
                  WinISD says IF you had a 1"id x 5" long vent in a 0.056cf box, the tuning would be at your targets. Physics doesn't lie.
                  Your Z plot (which I believe is correct) shows what your port would look like in a box HALF that size.
                  Are the (2?) vented portions (one for ea. port/woofer) of your cab only the front half of each side (w/the rear halves for electronics?)?
                  Just off-the-cuff, the outside dims of your box LOOK like maybe 6" x 8-9" x 16-18". That'd be about 0.3-0.4 cu.ft. total. What's up??
                  (do you have a pic w/the top off?)

                  Comment


                  • #10
                    Originally posted by Chris Roemer View Post
                    Something is wrong there.
                    WinISD says IF you had a 1"id x 5" long vent in a 0.056cf box, the tuning would be at your targets. Physics doesn't lie.
                    Your Z plot (which I believe is correct) shows what your port would look like in a box HALF that size.
                    Are the (2?) vented portions (one for ea. port/woofer) of your cab only the front half of each side (w/the rear halves for electronics?)?
                    Just off-the-cuff, the outside dims of your box LOOK like maybe 6" x 8-9" x 16-18". That'd be about 0.3-0.4 cu.ft. total. What's up??
                    (do you have a pic w/the top off?)
                    Attached are some pictures of my enclosure. Each chamber is 1.69 l in volume. With the drivers, ports, battery packs and crossovers subtracted the total volume should be 1.56 l.

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                    I am having trouble making sense of the numbers as well. In VituixCAD I have simulated the DS90-8 in a bass reflex enclosure with volume 1.56 l and a port of 2.4 cm diameter and a length of 11 cm. This is the same size as my enclosure. This yields an Fb of 82 hz and a F3 of 80 hz.

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                    However, on the product pages at Parts Express for the driver a bass reflex enclosure of 3.39 l and a port of 2.5 cm diameter and length 15.2 cm is recommended. I have simulated this enclosure and it doesn't make sense. This info is given under Product Details. To make things more confusing, under Product Specifications a bass reflex volume of 1.41 l is given as optimal, which is similar to my simulations.

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                    • #11
                      Scaling off the DS90 dia., your box's ODs LOOK like about 5"x5"x13" ?
                      I can pretty much match your Z-plot (in your ported chamber - using WinISD) using the DS90 in 1.6L,
                      IF I cut your port length in HALF (5" down to 2-1/2"), which raises the tuning to around 105Hz.
                      I do NOT get this, at all ? ?

                      Comment


                      • #12
                        Originally posted by Chris Roemer View Post
                        Scaling off the DS90 dia., your box's ODs LOOK like about 5"x5"x13" ?
                        I can pretty much match your Z-plot (in your ported chamber - using WinISD) using the DS90 in 1.6L,
                        IF I cut your port length in HALF (5" down to 2-1/2"), which raises the tuning to around 105Hz.
                        I do NOT get this, at all ? ?
                        Sorry I didn't include the dimensions of each chamber. They are 133 mm (5.2 in) W, 95 mm (3,7 in) H, 133 mm (5.2 in) D. I have also lined the chambers with 1 cm thick acoustic felt after the pictures in the previous post were taken.

                        I am making an improved impedance rig for REW which should be complete in a few days. I will measure one of the drivers then and calculate the T/S parameters for the DS90-8. I think that either the manufacturer's data for the DS90-8 are inaccurate, the port is placed to close to the cabinet walls, or there is some serious air leakage somewhere.

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