Polk RTA 15TL black rectangle above tweeter

KENNYY
KENNYY Posts: 1
in Vintage Speakers
Just had an opportunity to pick up a pair of RTA 15TLs and I noticed they have these little plastic rectangles above the tweeters.

I'm talking about these parts here (different speakers, not the ones I'm looking at)
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I couldn't figure out a good way to look up what these are, so I thought I'd ask.

Are these important for the speakers' function? I saw some images of the speakers without them as well. Not entirely sure if I'd consider them cosmetic either.

Thanks!
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Best Answer

  • Gardenstater
    Gardenstater Posts: 4,459
    Answer ✓
    It was supposed to control the dispersion of the tweeter to address some reflections due to diffraction. I feel you'd be much better off with anti-diffraction wool pads around the tweeter.
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Onkyo A-8017 integrated
    Logitech Squeezebox Touch Streamer w/EDO applet
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    Custom Steve Wilson 1m UPOCC Interconnect
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14awg, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
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Answers

  • F1nut
    F1nut Posts: 50,537
    Are these important for the speakers' function?
    Yes, leave them in place and forget the wool pads notion.
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."

    President of Club Polk

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  • xschop
    xschop Posts: 5,000
    I removed the flimsy plastic reflectors from the 8TLs because I believe the RDO throat profile negates the need for them.

    That, and deflectors are for amateurs and punks.
    Don't take experimental gene therapies from known eugenicists.
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  • F1nut
    F1nut Posts: 50,537
    The plate is used to prevent an interaction between the tweeter and the driver above it. It's got nothing to do with throat profile.
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."

    President of Club Polk

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  • Gardenstater
    Gardenstater Posts: 4,459
    edited July 2022
    The question is, does a waveguide control dispersion, and thus diffraction? I think it does. The other question though is, what frequencies is such a short waveguide effective upon? Probably just the higher ones that the wavelength is only 8x (or so) the length of the feature.
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Onkyo A-8017 integrated
    Logitech Squeezebox Touch Streamer w/EDO applet
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    Custom Steve Wilson 1m UPOCC Interconnect
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14awg, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
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  • Gardenstater
    Gardenstater Posts: 4,459
    Probably just the higher ones that the wavelength is only 8x (or less) the length of the feature.

    Fixed it.
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Onkyo A-8017 integrated
    Logitech Squeezebox Touch Streamer w/EDO applet
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    Custom Steve Wilson 1m UPOCC Interconnect
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14awg, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
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  • skrol
    skrol Posts: 3,375
    The attached has the tech info on the RTA-8TL, 11TL and 15TL. It describes what the "diffraction spoiler" on page 3.
    Stan

    Main 2ch:
    Polk LSi15 (DB840 upgrade), Parasound: P/LD-1100, HCA-1000A; Denon: DVD-2910, DRM-800A; Benchmark DAC1, Monster HTS3600-MKII, Grado SR-225i; Technics SL-J2, Parasound PPH-100.

    HT:
    Marantz SR7010, Polk: RTA11TL (RDO198-1, XO and Damping Upgrades), S4, CS250, PSW110 , Marantz UD5005, Pioneer PL-530, Panasonic TC-P42S60

    Other stuff:
    Denon: DRA-835R, AVR-888, DCD-660, DRM-700A, DRR-780; Polk: S8, Monitor 5A, 5B, TSi100, RM7, PSW10 (DXi104 upgrade); Pioneer: CT-6R; Onkyo CP-1046F; Ortofon OM5E, Marantz: PM5004, CD5004, CDR-615; Parasound C/PT-600, HCA-800ii, Sony CDP-650ESD, Technics SA 5070, B&W DM601
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  • xschop
    xschop Posts: 5,000
    Initially, I remember the flimsy "spoilers" vibrating on heavy bass, that's why I removed them, irregardless of the upgraded RDO's. I do understand Polk's original diffraction mitigation attempt here, but don't care for their choice of material in this particular application.
    Don't take experimental gene therapies from known eugenicists.
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  • Gardenstater
    Gardenstater Posts: 4,459
    Thanks for posting that skrol, I do not think I had seen it. They make a point of saying they sound better with the grilles on than without and that the interior corners were rounded and the dimensions were chosen carefully. I would *assume* that the following freq. response is with the grilles on then. Pretty significant difference with the spoiler. It seems like they are saying it is for diffraction and also for breaking up the midwoofer's symmetry.

    vichmu35acxe.jpg
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Onkyo A-8017 integrated
    Logitech Squeezebox Touch Streamer w/EDO applet
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    Custom Steve Wilson 1m UPOCC Interconnect
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14awg, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
    · Share on FacebookShare on Twitter
  • skrol
    skrol Posts: 3,375
    xschop wrote: »
    Initially, I remember the flimsy "spoilers" vibrating on heavy bass, that's why I removed them, irregardless of the upgraded RDO's. I do understand Polk's original diffraction mitigation attempt here, but don't care for their choice of material in this particular application.

    You have me wondering if they used different, flimsier, materials prior to mine. Mine are rather sturdy and I haven't noticed any vibration.
    Stan

    Main 2ch:
    Polk LSi15 (DB840 upgrade), Parasound: P/LD-1100, HCA-1000A; Denon: DVD-2910, DRM-800A; Benchmark DAC1, Monster HTS3600-MKII, Grado SR-225i; Technics SL-J2, Parasound PPH-100.

    HT:
    Marantz SR7010, Polk: RTA11TL (RDO198-1, XO and Damping Upgrades), S4, CS250, PSW110 , Marantz UD5005, Pioneer PL-530, Panasonic TC-P42S60

    Other stuff:
    Denon: DRA-835R, AVR-888, DCD-660, DRM-700A, DRR-780; Polk: S8, Monitor 5A, 5B, TSi100, RM7, PSW10 (DXi104 upgrade); Pioneer: CT-6R; Onkyo CP-1046F; Ortofon OM5E, Marantz: PM5004, CD5004, CDR-615; Parasound C/PT-600, HCA-800ii, Sony CDP-650ESD, Technics SA 5070, B&W DM601
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