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Thread: Audible Physics XR6.5M

  1. #11
    Tester Extraordinaire ErinH's Avatar
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    I'm not using SE for impedance/t/s. I'm using the woofer tester 2. Also, the FR's are not done to show sensitivity because the response isn't done at any fixed voltage. So, the sensitivity should be taken from the wt2 measurements and/or the mfg measurements. Not sure if you did, but just making sure that you know that.

    I agree, if the sensitivity is taken over the entire bandpass the resultant sensitivity is skewed. However, as I understand it, sensitivity is not affected by FR; rather other t/s parameters and is a function of efficiency. I tried to find the answer wholly, and actually, the wiki site has a pretty good explanation and this equation for efficiency.


    Now, back to what you're mentioning... where is this sensitivity taken at, in order to give us a reference? If I had to guess, I'd say Fs. I kind of figured it took it from the Fs and it looks like it might if you check this link:
    Thiele/Small - Wikipedia, the free encyclopedia
    I don't think there's ever been a case where someone really would need to know but this is a new arena since the response above 4khz is 15dB higher at max magnitude (~8khz). So, with that said, my gut tells me that the sensitivity rating is likely for the sub-4khz response and the above 4khz is just that much higher. The only way for me to know for sure is to test at 1w/1m which isn't hard to do. The problem then is that the mic isn't calibrate for SPL levels either so it's just another wrench in the mix. But, yea... interesting topic for sure.

    The results from a bandpassed HD test shouldn't be terribly different. The results are a relational response rather than absolute. So, basically the software is playing a tone and recording the fundamental (typically about -10 to 0dB) and then the orders of distortion. I'm not saying the results wouldn't be different at all. However, given the way SE works, I don't see it being a large enough difference to matter or even be worthwhile. In past measurements I found this was the case (when I was trying to figure out where I wanted to test). I don't recall coming across a case where the bandpass changed the HD at X frequency. You can get a feel for this in the two different HD plots I posted. Even though the results are 5dB different, the bandpassed results still show the same relational traits.
    I think I understand where you're coming from with this response, though and can see why you might be inclined to think that. But, remember that SE tests HD in a stairstep type fashion where it plays a tone for about 1 or 2 seconds while recording distortion, then steps up about 50hz to the next tone until it gets pretty high up then starts spacing out about 100hz (I think... it may still be 50hz because it takes about 5-7 minutes IIRC for a single test). The omnimic is a quick sweep so is not as accurate (or at least not as repeatable in my findings) which is why I chose to send it back.


    Edti:
    I can't figure out how to remove those darn links in the sentences.
    Edit #2: Remove links... who wouldathunk. *doh!*
    Last edited by ErinH; 05-16-2011 at 12:23 AM.
    Your ears: The best tools you have... and they're free, too!

  2. #12
    Senior Member cvjoint's Avatar
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    I remember you setting the spl level using the T/S parameters from the WT2. The output would then be based on the small parameters measured there. Very illuminating, I was not aware of how sensitivity gets measured. The second relationship on Wiki is probably the most useful one, and that one does have FS in it. The one you posted gives the 1% efficiency level for standard drivers. In this case I think bandpassing might look the same, I mean you would feed it the same amount of power. It may that the cone resonance and other various parts would play in at a different magnitude, but yeah sensitivity wise it would be a copy.

    The fundamental in the HD plots is still a mystery to me. Why doesn't it look like the FR? I turned the game on my sub from min to max and I think the HD plot didn't change at all! My harmonics results also end even more prematurely than yours, THD ends at 9khz I think.

  3. #13
    Founding Member Subwoofery's Avatar
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    Copying paste from Andy's post regarding efficiency. Even he states that efficiency ratings are mostly calculated:
    Quote Originally Posted by Andy Wehmeyer
    Arrrrrgh...why do I let myself get sucked into this?

    First, sensitivity is often calculated rather than measured and the calculation is based on the reference efficiency of the speaker, which is also calculated. The calculated sensitivity is "mid-band" sensitivity and indicates the output in the region where the speaker's response is flat. This is why sensitivity specs aren't particularly useful for determining just how loudly a subwoofer will play--we don't use subwoofers in that region. We use them in the stop band--where the output is not flat.

    Anyway, the formula for reference efficiency is:

    Reference Efficiency= (2.7*10^-8(Fs^3(Vas)))/Qes

    1W/1M sensitivity= 112-10log10(1/efficiency)

    No engineer worth his salt would attempt to build a system using only these calculations and would measure the speaker's frequency response for a view in finer detail.

    In my opinion and considering you'll have a separate amplifier to drive these speakers, the choice should be made by looking at a frequency response plot to determine that the speaker actually has useable output in the band for which you'll rely on it.

    This idea that the design of the entire system should be based on the maximum undistorted peak output is erroneous, since that distortion is inaudible until there's a lot of it. It's like me deciding I'm going to design a pick-up truck that will allow me to transport 12 pigs home from the fair in case I want to eat them, even though I have no facility to keep them or the stomach to butcher them or have them butchered on my property. It's a complete waste of time unless the exerecise is designed as simply educational.

    What separates people from animals is that people have a canon of knowledge gathered over many years and stored in books, on the internet, in the minds of experts and in plenty of other places and on other media. If this exercise is intended to be educational, then I think you should be a bit more responsible for your own education and for doing some of your own research.

    I'm tired of arguing about the truth and tired of digging through the canon so I can transcribe the information for your benefit so you can argue.
    Kelvin

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