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88.5dB sensitivity is excellent? (yes, I understand A2rx-c has a true 8 ohm load)

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  • 88.5dB sensitivity is excellent? (yes, I understand A2rx-c has a true 8 ohm load)

    The quote at the bottom is from Jon Lane regarding sensitivity of the A2rx-c.
    It is in response to a post I made saying that a lower tier AVR shouldn't have problems driving these speakers in a normal setting (the average Joe listening at average living room sound levels in an average sized room) but the AVR may struggle if the listener wants very loud SPLs and/or needs to fill a large space.

    I said this was due to the speakers lower sensitivity (I did not say they have abnormally low sensitivity).

    I agree with most of what Jon is saying... yes, the sensitivity of these speakers is comparable to speakers of its class.... the true 8ohm load makes the amps job easier.... speakers with lower impedance are more taxing on amps.

    But I need clarification with the notion that 88.5dB/8ohm speakers are "easy to drive and have excellent sensitivity." I've always understood and agree with this... sensitivity in the low 80's is considered very low, sensitivity in the upper 80's is considered low, sensitivity in the low 90's is good, and sensitivity in the upper 90's is excellent.

    Then the statement of "it's only by comparison to a 4-6 ohm speaker that you'll notice having to use a little more volume control..."
    It's not only in comparison to a lower impedance speaker, sensitivity plays a huge role in how loud the speaker plays. When comparing 8 ohm speakers that have a sensitivity of 88.5dB and something that does have a good 92dB, there will be a big noticeable difference in volume levels. In other words, the 88.5dB speaker is going to need the AVR volume knob turned up higher than with the other speaker to get the same SPLs.

    Jon's statement:
    "For reference, the in-use sensitivity of the A2rx-c is very comparable to speakers of its class - it's in the upper 80's. The A2rx-c is also a true 8 ohm load, which makes the amplifier's job easier.Speakers with lower impedance - including many only rated at 8 nominal ohms - will actually draw more current from an amplifier, potentially taxing it, especially at its higher volume settings.
    The short version is hook em up and turn em up; they're easy to drive and have excellent sensitivity. It's only by comparison to a 4-6 ohm speaker that you'll notice having to use a little more volume control, but not more power."
    Last edited by gdstupak; 07-23-2015, 09:46 AM.

  • #2
    Originally posted by gdstupak View Post
    ...I need clarification with the notion that 88.5dB/8ohm speakers are "easy to drive and have excellent sensitivity." I've always understood and agree with this... sensitivity in the low 80's is considered very low, sensitivity in the upper 80's is considered low, sensitivity in the low 90's is good, and sensitivity in the upper 90's is excellent.
    The calculated power sensitivity of the A2rx-c is 88+dB down to its bass cutoff. The in-room sensitivity will be a little less. If we assume an average 86-87dB sensitivity, the A2rx-c, which uses two identical midwoofers in parallel, is appreciably louder than a single woofer speaker in the same approximate class, meaning the majority of single 5.25" and single 6.5" models with comparable bass extension.

    A typical single 5.25" speaker of approximately 50Hz cutoff will, regardless of brand, technology, advertised claim, or nearly any other factor, have a similarly-rated efficiency of roughly 85dB. Note that I've changed from sensitivity to efficiency.

    To increase this speaker's sensitivity, all we have to do is halve the impedance (and adjust the crossover suitably). All other factors remaining the same, now the same basic speaker's voltage sensitivity rises to about 88dB but its sensitivity stays at roughly 85bB.

    The difference between efficiency - which is a much more useful term - and sensitivity is that the former relates the baseline output to 1W consumed in the speaker. Sensitivity relates it to 2.83v driving the speaker's inputs, which can then range from 1W consumed in a 8 ohm speaker to 2W consumed in a 4 ohm speaker. The lower the load impedance naturally the more current is drawn, increasing the power consumed. (Assuming a competent amplifier, that is.)

    This is why you have to simply increase the volume setting for the higher impedance speaker, and why the lower impedance speaker will then run the amplifier out of power at almost exactly that volume control difference at the loud end of the scale. No free lunch! The volume control does not strictly set power output; it adjusts the voltage gain at the input of the amplifier which in turn causes the amplifier and speaker load impedance to establish power consumed and loudness achieved.

    Considering that the 8 ohm speaker is easier to drive, the 8 ohm speaker has the advantage of having virtually identical useful efficiency to its 4 ohm sibling - how much sound for a fixed power consumption - while presenting the amplifier with a load the amplifier, typically, "prefers" to drive.

    As to how to subjectively categorize a speaker's efficiency, the typical 5.25" single woofer speaker is not a particularly efficient device, granted. 85dB is what most will consider low efficiency, however there are thousands of competent amplifiers that can still drive it to excellent playback levels. Power is cheap, after all.

    Doubling the acoustical size of this speaker - which the A2rx-c does - adds +3dB. I'd consider this excellent efficiency for and in either speaker's class. Naturally, we wouldn't expect, say, 92dB from a 5.25"-based speaker or even from a dual 5.25" speaker (unless we were willing to relax the speaker's bass performance and raise it's cutoff to perhaps 120Hz. Then our designer would have given us the option of a speaker with higher efficiency but over a narrower bandwidth. We'd have a midrange system and not a relatively full band speaker system.)

    Moving to a much larger speaker like a dual 8" tower, however, will raise both efficiency and bass output, and we could start to think about ratings over 90dB. Moving to a single 15" could get us into the low to mid nineties - again assuming our 40 or 50Hz bass cutoff - and moving to something like a quad of 15" could push our sensitivity into the upper nineties.

    But for literally any speaker in the A2rx-c class, executed in a comparably sized enclosure and having a comparable bass cutoff frequency, a power efficiency of 87dB or more is excellent. Speakers may be advertised with much higher numbers, but the physics behind their acoustics will not be affected.

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    • #3
      Just making sure I'm understanding...

      The A2's 88.5dB sensitivity beats the average for this class which makes it excellent.
      You are not saying that the A2's 88.5dB sensitivity is as excellent as a competitors 92dB, you are saying that it is hard to get 92dB in this class.


      As always, thank you for taking the time for explaining your process.

      Another question regarding sensitivity/efficiency (maybe I should already be able to determine this from your previous explanation but I'm still sorting it all out)...
      Does the single woofer vs double woofer design have different effects on sensitivity as more wattage is applied (can sensitivity be increased as power is increased)?
      In other words (with an example):
      Using the A1 and A2 speakers. The A1 has a sensitivity rating of 85dB (1w/1m). The A2 has a rating of 88.5dB.
      If you increased the power from 1w to 3w, would the dB levels raise equally for each speaker, or would one speaker's dB level increase more than the other?

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      • #4
        A speaker's acoustical class hinges on it's internal air volume as it's tuned - hopefully, properly - against the total swept cone area of the bass driver(s). That tuning also includes the speaker's cutoff frequency. There is strict a relationship between the bass driver's parameters and how this tuning is done, and how large the box becomes and how low the bass goes.

        From these variables the speaker's output characteristics and efficiency are determined. As mentioned, there's no way to get around this iron rule.

        A speaker of a certain class cannot exceed it's rather narrow range of output characteristics, at least not without trading bass cutoff for efficiency or for air volume, with an important nod to the total swept cone area, all of which set this class.

        Change one and the others must change. Change one and the class changes. The A2rx-c's net size, total swept bass driver area, and bass cutoff frequency determine its class.

        This tells us that with a one watt input and XYZ dB for any particular class, it's impossible for a competing speaker in the same class to achieve XX+3, +6, or +9dB, etc. We won't find a legitimate claim for Model B to exceed Model A in the same class by the efficiency difference between 88dB and 92dB or four dB. The 92dB speaker must sacrifice something to play what's actually more than twice as loud. It'll need much more internal air volume, probably a number of times more swept cone area, and possibly less bass extension.

        The physics are set in concrete, not in technology, brand, or claim.

        Your other question relates to the dynamic output of the particular design. The A1rx-c and A2rx-c will track upward in loudness similarly as signal voltage across their inputs increases, up until the point where heat in the single voice coil in the A1rx-c starts to change the woofer's parameters, or its large cone excursion starts to become non-linear. (SplitGap has a much higher threshold but it's still going to reach a thermal and/or mechanical limit eventually.)

        As this approximate threshold approaches, the twin coils in the A2rx-c will allow it to absorb roughly double the wattage and maintain linearity much longer (louder). The A2rx-c has the benefit of a higher initial 1W loudness, and it also has the benefit of more loudness as the speaker starts to be played loudly. This may account for all the three channel A2rx-c systems in use...

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        • #5
          Understood.
          Thank you.

          I'm good for now, so ignore your computer social media and phone, and go back to getting those new models ready.

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