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Thread: Some more good stuff from DavidF

  1. #1
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    Default Some more good stuff from DavidF

    I thought it would be a good idea to post/reference a few posts Dave has made the last few days over at Ecoustics and AVS. Some very good reading and explanations:

    First, this from Ecoustics in this thread:
    http://forum.ecoustics.com/bbs/messages/1/382724.html

    Hi Christopher,

    You bring up a good point here but there are a few things worth mentioning.

    First off, the graph that you refer to is 90dB spl at 2 meters in an anechoic chamber from a single loudspeaker. This is easily equivalent to about 100dB output from a single loudspeaker in-room at 1 meter away, or 103dB output from a pair of speakers. It is also a swept sine wave source, in other words, about as abusive to a loudspeaker as you can get - and not reflective of actual music listening. (far too loud for most people)

    Additionally, the graph posted is misleading and requires explanation... Because these measurements are taken in a true anechoic chamber and this is a rear ported loudspeaker, bass response in the port range is not correctly reflected in the frequency response measurement. Much of the bass output of the port is actually absorbed in the wall treatments. THD measurement techniques at the NRC have also changed since the B&W CM1 was tested (about 3 or 4 speakers ago) They did not inform me exactly what changed so I am not sure if comparing newer graphs to older ones is valid.

    If you look at any recent measurements of rear ported speakers, you will see that the low end response does not match manufacturers specifications (usually not even close). For example, look at the B&W CM1 (http://www.soundstagenetwork.com/measurements/speakers/bw_cm1/) B&W's spec is -3dB at 55Hz while the graph indicates about -12, -13dB down at 55 Hz. A 10Hz difference. You will also notice that same huge jump in distortion at this frequency, the same with our Sierra-1 and also about the same level of discrepancy in the low frequency response compared to our published spec...

    There is a very real explanation why the distortion measurement is so high at this frequency and why it increases so dramatically. As I am sure you know, as you approach the port tune frequency, less output is produced by the woofer and more is produced from the port. In fact, at port tune frequency, the woofer is barely even moving -- with almost 100% of output being produced from the port. As mentioned above, because this is a rear ported speaker and being tested in a true anechoic chamber (I believe tuned to about 50Hz) -- the majority of the output of the rear port is being absorbed in the 3-4 foot thick foam wedges behind the speaker and since there is barely any output from the woofer at all, all that is left to measure is pure noise (the graph is THD + noise). Try placing 5-6 pillows directly behind the port of a rear ported speaker --- where’d the bass go?

    I have had a few discussions with some of the people over there regarding this, especially when it comes to including the output of the port in the frequency response measurements (simply a matter of turning the speaker around so the port faces the mic and then averaging this into the measurement) but their response was that since it is the same for all speakers, they are not willing to change. I don't agree with this though, as front ported speakers will provide better looking THD+noise graphs and deeper extension in the frequency response graph.

    Here is a link to some deep bass torture tests of the Sierra-1 conducted by Craig Chase in a large room at 3 meters away:

    http://img206.imageshack.us/my.php?image=sierra45hzox5.jpg

    Craig measured 90dB of output at 45 Hz with 2nd order disortion at 7%. Which is much more reflective of how the speaker is truly performing.

    While I am all for crossing over at around 60Hz and using a subwoofer, for music listening I think many (even most?) will find the bass of the Sierra-1 cleaner and tighter (more musical) than many subwoofers. I know I do, as do hundreds of Sierra-1 owners. Personally, I haven't used a subwoofer in quite some time. Although I felt a sub was missing when I watched 300 last night. Definitely consider adding a sub for home theater usage...

    I am not here to "ruffle any feathers", but there is a reason those distortion measurements are high and they are certainly not reflective of actual performance. Note: The NHT classic 3 you mentioned is a sealed speaker so all bass response is being reproduced by the woofer in the front and the Paradigm Signature you mentioned is front ported (the majority of port output will reach the mic before it is absorbed by the wall treatments). I would say that the only fair way to compare distortion between all three of the speakers is in a frequency range where the port is producing minimal output, from 100Hz on up. Due to the measurement technique employed, distortion and response comparisons at frequencies where the port tube is contributing to output are simply not valid.

    Believe me, I wish the NRC did things differently as I use there measurements as one point of reference for calibration of our own engineering gear...

    Hope you find this useful...
    -curtis

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    and in the same thread:

    >For people that are curious, myself included, what kind of limits would you personally place on Sierra in terms of bass response/volume levels?

    I would say that in most rooms, the -3dB point is in the low 40Hz range. I have measured -3dB at 39Hz in a few different rooms but the speaker was within 18 inches of the rear wall (subject to some bass reinforcement). The limitations as far as maximum output at low frequencies are fairly close to what Craig Chase measured. At 2 meters away, with a pair of speakers, I have measured about 98dB output with a sine-wave at 44Hz, with less than 10% THD. Maximum output will decrease rather dramatically below this frequency as the woofer is now unloaded (frequencies below port tuning). For a speaker of this size and in my experience, this is impressive. I should mention that this is with a sine wave. I should probably take some measurements with our pulse generator, which is closer to music or home theater listening but I have been somewhat lazy lately.

    We do a few trick things in this woofer and the result is that the quality of the bass is very good, tight and "fast" (although I hate to use that term, but it is descriptive). A more technically correct term would be an exceptional transient response, which is why I feel many Sierra-1 owners are turning off their subs for 2-channel music; there is a more coherent and detailed sound. That is NOT to say that a quality subwoofer can not keep up with the sound quality, I am receiving reports that many can -- but they are not your typical large cabinet volume, wide-diameter ported subwoofers, which can knock a wall down, these generally sacrifice transient reproduction and phase accuracy to deliver such deep bass.

    Personally, I want to try one those Fathoms

    Nuck -

    >DF, thank you for dropping by to clarify things. With a lot of bashing and carrying on, the music seems to be lost by the wayside at times.

    Most welcome -- and yes, so much bashing everywhere. It is the nature of forums though since we are not face to face. It is quite an experience when audio hobbyists who typically bash each other online meet face to face for the first time --- It is usually all hugs and high-fives (often followed by too much alcohol) I have seen it and been part of it many times now. Imagine how BORING the forums would be if it was all this is great, that is great, great post – you are great, who wants that? The bashings, criticisms, complements, raves – it is all good Keeps things going and growing…..

    >The port issue comes up now and again, and your description of the port action is juuuust a little beyond my ken. The driver is basically unmoving, and the port takes over? If you have a 'for dummies' version of this one, I need to read it.

    That is correct, as you approach the port tune frequency, the woofer contributes less and port contributes more. At port tune, the woofer will be moving very little if at all, while it is at this frequency that the most air will be moving in and out of the port. At frequencies below the port tune, things start to change however, both woofer and port are unloaded and the woofer will act as if it is being used in free-air (no air spring to limit excursion) The neat thing about a ported loudspeaker is that because the woofer excursion decreases as you approach the port tune frequency, distortion decreases as well. Typically, the lowest distortion point for a ported loudspeaker (at least in the woofer bandwidth) is at port tune frequency because the woofer itself is barely even moving. This is one of the key advantages of a ported loudspeaker, and probably one of the most overlooked.

    Here is a very good explanation with a diagram of how it works: http://www.hometheaterhifi.com/volume_5_2/cmilleressayporting.html

    >>Again, thank you for stopping by!Leadership by example. I miss Tim, love the Lings.

    You are most welcome. Yes, I was sad to learn that Tim was moving on. This is a brutally competitive industry and it is very difficult to just pay the monthly bills, not overlooking the fact that we must try and make a few dollars too. Tim and I had a rather infamous "public" feud, but we shared many very friendly emails behind the scenes about various things, from measurement techniques to the industry in general, I shared any advice I could offer. He is the genuine article.
    -curtis

  3. #3
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    And from the AVS Sierra thread:
    http://www.avsforum.com/avs-vb/showp...&postcount=803


    Quote Originally Posted by Ascend View Post
    Thanks Stefano-M,

    Yes -- It is important when comparing measurement graphs like these to understand exactly what is being displayed. These types of graphs can be deceiving, make a loudspeaker look *worse* than another simply because the techniques being used to take the measurements work better for some and not others. That being said, the NRC measurements are still very valuable, once you are armed with the knowledge on how to interpret the data.

    Further evaluation of the phenomenon discussed at Ecoustics can be examined here:

    Comparing the on-axis graphs of the NHT classic 3 with the off-axis graphs. This is a sealed enclosure, no port.

    Classic 3 On-axis:


    Classic 3 Off-axis


    Notice the bass extension at 50Hz between all the various measurements remains the same.

    Now lets compare the bass extension of Paradigm Signature S2, a front ported loudspeaker.

    Signature S2 On-axis


    Signature S2 Off-axis


    Now let us compare to a rear ported speaker, the Sierra-1.

    Sierra-1 On-axis


    Sierra-1 Off-axis



    Examining the graphs more closely, you will notice that with the Paradigm, the bass extension begins to slightly decrease with the off-axis measurements. This is because the port tube, which is on the front baffle of the speaker, is now moving further away from the measurement microphone, subjecting the output of the port to more absorption, due to the effects of the chamber

    Looking at the graph of the Sierra-1, on the off-axis measurements, you will notice that the bass extension actually increases the greater the off-axis angle. On the on-axis graph, 50Hz is 9.5 dB down. On the off-axis graph, at 75 degrees, 50Hz is now 6.5dB down, a difference of 3dB (considerable). This is because at 75 degrees off-axis, the mouth of the port is much closer to the microphone, resulting in more information from the port reaching the mic rather than being absorbed. Bass extension will continue to increase as the mouth of the port approaches on-axis (0 degrees)

    The rather large "bump" at around 25Hz and then the dip at 35Hz is also an effect of a rear ported speaker being measured in an anechoic chamber. You will see something similar to this on the majority of rear-ported stand mounted speakers tested in the NRC chamber. According to this NRC graph, the Sierra-1 is -10dB at 25Hz, now THAT would really be something

    Technically, if the speaker is -3dB at 44Hz (which is accurate), the speaker would be -27dB at 22Hz (anechoic). A ported loudspeaker rolls off at 24dB octave (there are minor exceptions to this but it holds true in general) With that in mind, the on-axis full-range graph posted on our website is far more accurate with regards to expected bass performance below 100Hz than is the measurement taken at the NRC. (our graph indicates -25dB at 22hz, very close to the -27dB I mentioned above)

    Measurements are indeed an important aspect of performance, but they do require a complete understanding of exactly how the measurement was obtained and precisely what you are looking at. Much of what I do involves interpreting measurements; validating the results, evaluating potential problems and much more.

    For example, just from the simple on-axis measurement of the Sierra-1, I know what the approximate width of the front baffle is. Look at the on-axis graph. Notice the dip at approximately 3.5kHz. Examine the off-axis graphs, notice that the same dip is not there. This tells me that the dip is not caused by the tweeter itself, it is a reflection and because the dip disappears at as little as 15 degrees off-axis, I know that it is not audible, even though it looks "ugly". Experience tells me that this is an edge reflection, an effect caused by diffraction. 3.5 kHz equals a wavelength of ~3.875 inches. So the output of the tweeter is reaching an edge at 3.875 inches, or in other words, the tweeter is spaced approximately 3.875 inches from an edge. Since the tweeter is in the middle of the baffle, 3.875 * 2 = 7.75 inches (the width of the front baffle). While not 100% precise, if we then subtract half of the 1/4" radiused edges on either both edges, which makes the front baffle appear larger than it is on measurements, we end up with precisely 7.5" Which is the precise width of the speaker

    Again, there is much information that is revealed from measurements but you have to know how to evaluate it, how it was obtained and most importantly, a quick glance is not nearly enough time to draw any conclusions....

    Hope some of you can find this at least somewhat interesting
    -curtis

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    this is great stuff!
    Jon O.

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    "While I am all for crossing over at around 60Hz and using a subwoofer, for music listening I think many (even most?) will find the bass of the Sierra-1 cleaner and tighter (more musical) than many subwoofers. I know I do, as do hundreds of Sierra-1 owners. Personally, I haven't used a subwoofer in quite some time. Although I felt a sub was missing when I watched 300 last night. Definitely consider adding a sub for home theater usage... "



    David, assuming you use a H/K, I believe I know how you can set up your H/K to allow you to only hear only bass from your Sierra's on music, but where you can also hear home theater bass from both your LFE and other channels (center, surrounds) without you having to "add and disconnect" your sub as you go from music to HT.

    To do this, in the "speaker size" menu of the H/K, choose Large for the Sierra's and set the "subwoofer" to simply LFE (only). This way, in 2-channel music, there will be no bass information sent to the sub. However, in DD 5.1/DTS/ or any multi-channel modes (like Logic 7 or DPLII), you'll hear bass for both LFE channels (DD/DTS), and also bass that's meant for the center or surrounds provided that you have those speakers set to "small".

    In any event, there should be no reason to have to connect and disconnect a sub with a H/K as you go from music to home theater and back.

    I imagine many other receivers have an equilivent work-around.
    Sierra-1 - Mains+Center
    Surrounds - HTM200SEs (x4 in back, and x2 Atmos)
    Sub - SVS PB-2000
    Receiver - Onkyo TX-RZ1100
    Oppo Darbee Edition Blue Ray
    Sony 4K blu ray player

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