A study on directivity... cones, domes, large and small, small and large baffles

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  • tktran
    Senior Member
    • Jan 2005
    • 660

    A study on directivity... cones, domes, large and small, small and large baffles

    I decided to fork out the study on directivity. And move midranges/fullranges/midwoofers etc. Basically anything that's 2" or larger.

    Let's start with the B&W 5" Continuum FST from the 803 D4, released in 2021:

    Click image for larger version  Name:	image.png Views:	17 Size:	73.9 KB ID:	951221

    Data courtesy of @Dcibel
    Here it's mounted in his test cabinet of 222mm x 414mm with a round-over radius of 10mm.

    Here are the original frequency response and distortion measurements here:
    Before Christmas I had been building a test rig. Over the holidays I was sick and without energy to do much and didn't want to leave the house, what better time to run through a bunch of driver tests. I ended up testing everything I had sitting on my shelves, as well as pulled a few drivers that I'm very familiar with as a


    Now I imported the data into VituixCAD2 to look at the directivity:

    Recall, here's the on-axis response:

    Click image for larger version  Name:	image.png Views:	12 Size:	23.1 KB ID:	951211

    Off axis:

    Click image for larger version  Name:	image.png Views:	17 Size:	104.9 KB ID:	951224

    Nothing can be done about that dip around 13KHz, but would the resonance at 16KHz benefit from a notch filter?

    Click image for larger version  Name:	image.png Views:	17 Size:	100.5 KB ID:	951225

    ​Let's straighten out the on-axis response and insert high and low pass filters for an acoustic LR4 @ 400 Hz and 4000Hz

    Before notch filter:
    Click image for larger version  Name:	image.png Views:	17 Size:	104.6 KB ID:	951222

    After notch filter:

    Click image for larger version  Name:	image.png Views:	17 Size:	100.5 KB ID:	951223


    ​Suppose we didn't try to flatten out all those lumps and bumps, but still aimed for acoustic LR4 @ 4HKhz:

    Click image for larger version  Name:	image.png Views:	17 Size:	97.0 KB ID:	951226
    ​​

    POLAR MAP for a straightened on axis, LP and HP filters for acoustic LR4 HP@ 400Hz and LP @4Khz:
    Click image for larger version

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    POLAR MAP where on axis not straightened LP and HP filters for acoustic LR4 @ 400Hz and LP @4Khz:

    ​​​​Click image for larger version

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    Last edited by tktran; 17 February 2024, 10:31 Saturday.
  • tktran
    Senior Member
    • Jan 2005
    • 660

    #2
    ONWARDS and OUTWARDS!

    Here's another B&W midrange. This time, an older model with the classic yellow woven Kevlar cone.

    This one is in a cabinet 24cm W x 30cm W x 20cm D (9.5 x 12 x 8"). Driver centered on the baffle, near bottom edge of cabinet.

    Click image for larger version  Name:	image.png Views:	2 Size:	412.2 KB ID:	951242

    ON and OFF axis, Line Chart:
    Click image for larger version  Name:	image.png Views:	2 Size:	116.4 KB ID:	951241

    Straightened on axis, with crossovers for acoustic LR4 high pass of 400Hz and low pass of 4000Hz:

    Click image for larger version  Name:	image.png Views:	1 Size:	59.9 KB ID:	951238


    POLAR MAP:
    ​​​
    Click image for larger version

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    Last edited by tktran; 17 February 2024, 10:23 Saturday.

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    • tktran
      Senior Member
      • Jan 2005
      • 660

      #3
      The same 6" Kevlar midrange in a spheroidal enclosure with diameter 21.5cm (8.5")

      Click image for larger version  Name:	image.png Views:	2 Size:	102.8 KB ID:	951240


      Click image for larger version  Name:	image.png Views:	5 Size:	54.4 KB ID:	951236

      POLAR MAP:

      Click image for larger version  Name:	image.png Views:	13 Size:	87.1 KB ID:	951262

      Notice the shape of the radiation it quite different - smoother contours lines, and although the both have -6dB beamwidth of 30 degrees at 3KHz, the region between 1Khz to 3KHz is not as wide...
      Last edited by tktran; 18 February 2024, 00:42 Sunday.

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      • tktran
        Senior Member
        • Jan 2005
        • 660

        #4
        The next FST is one from the 801 D4 and 801 D4 Signature-
        B&W mount this midrange in an spheroidal enclosure with a diameter of about 11", with large 2+ inch round-overs, then gently tapered all the way back 24" deep. The midrange enclosure alone is some 40lbs,

        Click image for larger version  Name:	image.png Views:	2 Size:	534.6 KB ID:	951244

        The whole system is some 100kg (220+ lbs). Who could blame B&W engineers for not taking polar measurements?

        But what does using a spheroidal enclosure do?

        Here we have taken the midrange and placed into a spheroidal enclosure.
        This one is made of fiberglass and is 8.5" in diameter, and effective round-over is only ~1".
        The upside is that it's much lighter, so that can it can be placed onto a turntable for polar measurements:

        Click image for larger version  Name:	Front.jpg Views:	2 Size:	2.18 MB ID:	951245 Click image for larger version  Name:	Diagonal 1.jpg Views:	2 Size:	2.29 MB ID:	951246 Click image for larger version  Name:	Diagonal 2.jpg Views:	2 Size:	2.29 MB ID:	951247

        On and off-axis measurements, Line Chart:

        Click image for larger version  Name:	image.png Views:	2 Size:	95.6 KB ID:	951248

        Straightened and corrected for acoustic LR4 @ 400Hz and 4KHz:

        Click image for larger version  Name:	image.png Views:	2 Size:	87.3 KB ID:	951249

        Resonance is 40dB down. What about a notch filter?
        Perhaps unnecessary, but this is what it would look like with the notch applied:
        Click image for larger version  Name:	image.png Views:	2 Size:	85.7 KB ID:	951250


        POLAR MAP on a 30dB Y axis

        Click image for larger version  Name:	image.png Views:	53 Size:	84.3 KB ID:	951290​​

        ​​


        Edit: corrected graphs, dimensions. 360 degree measurements to come…
        Last edited by tktran; 23 February 2024, 06:22 Friday.

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        • tktran
          Senior Member
          • Jan 2005
          • 660

          #5
          NEXT UP:

          A 3" dome that weight 20lbs. It's the the Volt VM752. This one definitely deserves a photo:

          Click image for larger version  Name:	Volt.png Views:	61 Size:	553.9 KB ID:	951256


          It's designed with a shallow waveguide, to be mounted from the rear onto a 3/4" baffle, sitting flush. So we have a situation that is somewhat the the opposite of the B&W driver above- ie. instead of having a round-over starting immediately at the frame and swooping backwards away from the cone, we have a waveguide swooping forwards from the dome and then meeting a flat baffle.

          So how does this shallow waveguide + baffle affect the directivity/dispersion?

          Here's a speaker designed by Matthias Brennwald, the Open Source Monkey Coffin project: GitHub - mbrennwa/osmcdoc: Documentation for the Open Source Monkey Coffin

          The center of this 3" dome is mounted 14.5" wide (368mm W x 682mm H) cabinet, offset 1 11/16" from center.
          Here's the on and off axis responses taken from the .FRD files shared by Matthias Brennwald:

          Click image for larger version  Name:	On axis.png Views:	59 Size:	88.6 KB ID:	951257

          What is that dip around 2.4KHz?

          The project documentation said "The on-axis response of the Volt VM752 midrange shows a dip at 2.3 kHz. This dip disappears in the off-axis curves, which indicates that this is an uncritical diffraction artifact related to the driver/waveguide geometry rather than a problematic resonant effect."

          I've applied a filter to result in an acoustic LR4 high pass 300Hz and 3KHz low pass to the on-axis. The pink is what it will look like:

          Click image for larger version  Name:	Corrected.png Views:	61 Size:	30.9 KB ID:	951259

          Click image for larger version  Name:	300 3K off axis.png Views:	59 Size:	96.1 KB ID:	951258




          So what does the POLAR MAP look like on a 30dB scale?

          Click image for larger version  Name:	image.png Views:	60 Size:	94.5 KB ID:	951260​​
          Last edited by tktran; 25 February 2024, 06:08 Sunday.

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          • tktran
            Senior Member
            • Jan 2005
            • 660

            #6
            NEXT UP:

            4" cone midrange- Faital 4FE42 in 6" spheroidal enclosure:

            On and off axis measurements:

            LINE CHART:
            ​​

            POWER and DI chart:
            ​​
            ​​

            Frequency response now baffle step loss compensated, neutralized and ​with crossovers for acoustic LR4 high pass 300Hz and low pass 300Hz: In a passive XO, this gives a midrange sensitivity of a healthy 90dB/2.83V (8 ohm driver)

            ​​​​

            ​​

            Last edited by tktran; 01 April 2024, 18:54 Monday.

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