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Friend's project..

Discussion in 'DIY Projects' started by TKoP, Aug 29, 2017.

  1. TKoP

    TKoP Well-Known Member

    A friend of mine went off and built himself a bluetooth speaker by himself.
    This is the speaker driver he got.

    I wanted to scrape off the rust real quick, and see if I still had some ability here..

    1. Full range driver, so no crossover needed
    2. That being said, the freq response above 10k spikes up, so I'd want to maybe design some sort of notch filer (did I get that right) to even out that response, if not dampen it down
    3. Internal box dimensions would be 7.41" x 11.86" x 4.45"
    4. Vent is 2" diameter and about 3.82" long
    5. Stuff it some sort of filler

    Any thing I might be missing?
     
  2. Flint

    Flint "Do you know who I am?" Superstar

    1. Correct
    2. Don't trust the on-axis response from the manufacturer. Here's some math - The rated area of the effective cone is 57 cm sq. That makes the diameter 8.52 cm which is the same as 3.35 inches. The speed of sound is 13,536 inches per second, so the effective diameter of the cone is the acoustic wavelength of 4,041 Hz. The half wavelength is 2.020 Hz. The governing rules of directionality of a vibrating piston dictate that above about 2 kHz this particulardriver will start becoming a beaming source, not an omnidirectional source. So, a simple on-axis response will not represent the in-room power response. Instead a power response curve, like the one Harman has instituted on the research of Dr. Toole, would better reflect how the sound is balanced in the room for a listener. Given the insane dispersion patterns a 3.35 inch cone will have above 8 kHz, there could be enough treble to satisfy a listener, but any movement of the head will impact that sound. Also, room echo reflections will be weaker in the high treble than in all the other frequencies below 2 kHz. So, I would build the speaker, test it in various listening environments, and only add filters after experiencing the room sound.
    3. That looks about right.
    4. Can't argue with that.
    5. Filler should be an after thought. If the cabinet walls are solid and free from resonances, the tuning/port resonances will be more efficient than if you add stuffing. However, you may need some stuffing to soften internal midrange reflections. Experiment and learn what works best. If you do stuff, the fancy Acoustistuff product from Parts Express is worth the price over fabric store quilt batting.
     
  3. Flint

    Flint "Do you know who I am?" Superstar

    upload_2017-8-29_18-47-21.png

    Here's the measured response of a higher end 4" fullrange speaker from Mark Audio. Notice that on-axis the response above 6 kHz has all sorts of spikes and valleys but is generally higher than the 500 - 5,000 range. However, off-axis by a mere 15 degrees the response is closer to something you'd want to listen to in real life. Meanwhile 30 degrees off-axis the response is quite muted above the 4,100 Hz point where the beaming starts. They get decent off axis response in the treble through cone resonances and a protruding dome-like dustcap.
     
  4. Flint

    Flint "Do you know who I am?" Superstar

    Here's a more comparable driver from Peerless:

    upload_2017-8-29_18-54-22.png

    Same deal, here. The amplitude performance above about 2,300 Hz, where the beaming clearly starts, is vastly different above 6,000 Hz.
     
  5. Flint

    Flint "Do you know who I am?" Superstar

    Be sure to study the scale on each of those charts before making assumptions on their sound. The Mark Audio uses a wider scale so the drop in output looks less steep compared to the Peerless. Also, the Peerless does not offer a 5 degree off-axis measurement, choosing instead a 60 degree off-axis curve.
     
  6. Randy

    Randy Well-Known Member Famous

    Cool.
     
  7. TKoP

    TKoP Well-Known Member

    One thing to clarify, this was a dual driver system..

    Hopefully that still means that I got it mostly right. I think I follow the off-axis response issue. Depending on the size of the driver itself, it "beams" rather than, i don't know, "radiates" from the driver itself. How do you compensate for that in a regular speaker? or is it just another limitation we have to live with.

    And i think i get the issue with the batting as well. I had it in my brain that it was always good to have to suppress the internal reflections from impacting the speaker cone. Good to know that it's not always the case.

    For reference, here is the frequency response chart
    [​IMG]

    And vaguely this is what his box looks like --- he didn't recess the drivers and the wood used was different, but overall, it looks close
    [​IMG]
     
  8. Flint

    Flint "Do you know who I am?" Superstar

    How do you compensate for beaming? Well, with a cone speaker you cannot do much to change the polar patterns of the high frequencies. I've seen some ingenious side-firing 360 degree output wave guides, like you see used with the Amazon Echo speaker or the B&O point source stuff, but generally those are average in performance and hard to get good results as a DIY project. The breakup of the cone is what allows for high frequency output way above 10 kHz and as such the output pattern can be crazy and strange. Personally, I'd just make it and go with it. If you find it too bright you can either EQ the source, add a passive filter, or you can put a physical "phase plug" type of thing directly in front of the cone, but chances are that won't be necessary.

    Paraphrased, here's what Dr. Small of the T/S parameter standard for drivers has to say about small speakers --- as long as the treble extension and bass extension are equally limited, the overall experience for the listener will be good. In other words, don't make a speaker that cannot play above 10kHz but does well below 40Hz. So, if this speaker is limited to 100Hz to 10kHz. it should sound pretty decent.
     
  9. malsackj

    malsackj Well-Known Member

    So that would be like a bose 802 ?
     
  10. malsackj

    malsackj Well-Known Member

    Or how about the supposed beam blockers guitar player use Large metal disks placed in front of the speaker cone.
     
  11. Flint

    Flint "Do you know who I am?" Superstar

    Basically a phase plug like metal domes require?

    Yeah, but those introduce their own problems. I recall seeing some PA systems with low compression horns which used a cone woofer/midrange which would beam too much before the crossover filter was affecting the signal which used cool looking egg or tear-drop shaped phase plugs in front of the woofer to reduce the beaming - but again they had issues with fidelity which was acceptable because of the vastly increased efficiency and dispersion control the horn provided.
     

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