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Speaker/stand/floor interaction; tweeter height

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  • Speaker/stand/floor interaction; tweeter height

    I got into an interesting discussion on another forum re: speaker stands. I use plastic milk crates to support my Harbeth M40s. The advantage of this, as I see it, is that the resulting stands are "soft." That is, the resonant frequency is so low that if you push the M40s, you can actually count the cycles. Robert E. Greene, the audio reviewer and acoustician, recommends this as a goal. His point, I believe, is that the resonant frequency of the stands should be as low as possible. The more rigid the stand, the higher the resonant frequency, and the greater the chance that it will be pushed into the all-important midrange where the ear is most sensitive.

    My milk crate stands hold the tweeters 48" off the floor, which is about my ear height when seated in the listening position. The Skylan stands mentioned in our discussion hold the tweeters about 40" off the floor.

    I don't listen at very high volumes.

    The other fellow's side of the discussion ran like this (edited for length and relevance):

    "Your milk crates made of molded plastic sheet, are a hollow dimensional object with the resonant characteristics of a stretched membrane. Unloaded, they can easily be made to resonate in a particular frequency range which is very annoying. Place a load on them (e.g., your speakers) and you merely raise the resonant frequency range. In fact, most speaker cabinets (even cheap ones) don't resonate at a high enough amplitude to cause any audible sympathetic resonance in milk crates or any other sort of stand. However, crank up the volume of the amp high enough and those crates will resonate to the point that you can feel the vibration with your fingertips against the crates when the music ranges around the resonant frequency….

    "Placing a large enough speaker on a milk crate will allow the speaker to move slightly during high excursion periods of the mid woofer and/or woofer, something which is non-existent at low to moderate listening levels, but cumulatively deleterious to sound quality at high listening levels….

    "At a minimum - and admittedly there are no verifiable test reports on this - in my estimation a speaker stand should be absolutely dead and at least twice as heavy as the speaker being supported. The heavier the better. That makes steel stands with hollow posts that can be filled with sand or clay-based kitty litter in order to drop the resonant frequency into the subsonic - although the energy is entirely absorbed anyway and never makes it into the room - ideal …

    "Coupling to a top plate vs. point contact vs. decoupling vs. hybrid coupling (the most popular example of which is Blutac) is actually dependent on speaker cabinet design. For example, Harbeth's typically lively cabinets do best when mounted on heavy stands with a top plate the same size as the bottom of the speaker cabinet, and separated from the top plate by four rubber buttons close to the corners. In that way, the deliberately designed resonant cabinet characteristics are undisturbed, the speakers can't move on their own or be readily bumped out of position, and then perform their best….

    "As for having tweeters precisely at ear height, it's simply not necessary to be so precise and it's also often somewhat self-defeating. In general, I think speaker makers try to ensure that listeners get standmount woofers/mid-woofers up off the floor by at least 12" in order to help control unwanted reflections (among other things). The Monitor 40, 40.1 and newest 40.2 are designed with tweeters that radiate significant energy angularly upward. That means (and most dealer showroom setups and dealer home demo setups of Monitor 30/40 series speakers show the point clearly) that the 14" Skylan stands that peg the 40-series tweeter at about 40" (or whatever the measurement is, exactly) is the ideal height. That's the sweet spot for the Monitor 40 series and helps the speaker do its best irrespective of flooring, floor covering, room nodes, LEDE rooms, and so on."

    I think he is making five points here:

    1. Plastic milk crates can be made to resonate at audible frequencies, and in fact do so at high listening volumes.

    2. A "soft" stand that does not hold the speaker rigidly will allow it to move slightly in response to the movement of the mid-range driver and woofer. This is bad for accurate reproduction, especially (again) at high listening volumes.

    3. Speaker stands should be extremely massive and acoustically "dead." This can be achieved by filling hollow steel stands with a very heavy and inert substance, like lead shot, sand, clay-based kitty litter, etc.

    4. Harbeth speakers, with their deliberately lossy cabinets, should be coupled to such a stand by four rubber buttons close to the corners, which will not disturb their "lossiness."

    5. Having tweeters at ear height is not that important, especially in the case of the M40s, which have tweeters that radiate significant energy upward. It is more important to have the woofers and mid-range drivers be at the correct height off the floor.

    These claims seem controversial to me, but I don't have the technical knowledge properly to evaluate them. Alan, would you care to weigh in?

  • #2
    I think there are some 'interesting' points being made here. i am completely non technical in my ability to properly answer points 1 and 2 above and would hope some of our more enlightened and technical HUG members can enter the debate and shed some light (or myth bust) these points as I am interested in this myself. However, I do have an opinion with regards to points 3, 4, and 5.

    Point 3: Well good luck using those 'extremely massive' speaker stands with M40.2's. if, as has been stated, the stand should be at least twice the weight of the speaker!! Personally, I think this bears no merit whatsoever and my preference has been to go with open frame solid steel stands from Something Solid and make sure of levelling and floor contact at all points for a secure base.

    Point 4: I don't think that anyone would choose to place their Harbeth speakers on the 4 rubber buttons for any reason other than it being a highly sensible approach (as for all stand-mount speakers). Harbeth themselves caution their customers against using blutac because of the risk of veneer damage.

    Point 5: Utterly wrong in my opinion, and certainly with Harbeth speakers. Alan himself has made it perfectly clear himself that the crucial thing with stands is to make sure they place the speaker at ear height in relation to the tweeter. After that, get whatever stand takes your fancy or fits with your furnishings and decor.

    Regards, Mike.

    Comment


    • #3
      Stands and vibration

      Originally posted by MikeM View Post
      Point 5: Utterly wrong in my opinion, and certainly with Harbeth speakers. Alan himself has made it perfectly clear himself that the crucial thing with stands is to make sure they place the speaker at ear height in relation to the tweeter. After that, get whatever stand takes your fancy or fits with your furnishings and decor.

      Regards, Mike.
      100% correct. That is precisely my position. It's a complete misunderstanding of basic loudspeaker design to not appreciate that for the best possible sound (with every speaker I've ever encountered) the listener should aim to get his ear level with the tweeter. And the M40 doesn't radiate sound 'upwards' - to do that the tweeter would have to be pointing, er .... upwards. Which it isn't. It's pointing parallel to the floor. And if you are deadly serious about Reference Standard sound, that's the very optimum listening position, precisely where my measuring microphone was during design.

      Incidentally, this week I've purchased six B&K accelerometers and their preamplifiers* for a little project I have in mind. As you'll read, they are the standard method for detecting vibration in industry. It would be a trivial, but interesting, exercise to apply them to various points of a speaker stand to explore just how much the stand moves in 3D relative to the speaker, if at all. And then to try and relate that tiny vibration of the stand (if measurable) to the sound output the motion of such small surface areas could theoretically generate, be it even measurable.

      I think we'd have to agree, as logical level headed chaps, that if, say, the vibration of the stand, as measured with precision equipment, contributes, say, 0.1% (or even 1%) to the sound output from the speaker sitting above it that no human can detect that small a sound. And hence, the 'sound' of a speaker stand would be in the imagination. Still hear a difference in sound quality between stands? Fair enough: but we need to look elsewhere for the source of that. And stand height difference would most likely be the explanation.

      * Preamplifiers: I stumbled across an old post here where the contributor completely failed to appreciate what a preamp does. All it does - all it exists to do - is to apply a small amount of amplification to its incoming signal to make that signal a better match to a power amplifier's input, if necessary. That's it. No more than that. So it beggars belief that somehow a preamp can apply magic dust to the audio on it's way to the power amp and speakers. It can't. Or rather, if it does, it fails to meet the definition of being a high fidelity component!

      Image added after posting: ("sent to server" visible or not?)
      [Image now deleted]


      Here is an 'LS3/5a' being measured with my B&K system. The reference microphone is exactly level with the tweeter (although from this angle the mic looks a little high, that's an optical illusion).
      Alan A. Shaw
      Designer, owner
      Harbeth Audio UK

      Comment


      • #4
        I like the idea of milk crates for the M40.2. The dimensions are ok and should raise the speaker to a suitable height and they are quite safe. You could add additional mass/weight quite easily with a brick or two. Only downside is that they ain't pretty.

        I lost interest in that post when it was said the M40.2 tweeter radiates "significant energy upward".

        Comment


        • #5
          {(Test) Here is a fresh post with the image that I tried to add to post #3. Attached using the camera icon, and then located image in the pre-loaded Image Library}.

          Viewable?
          Alan A. Shaw
          Designer, owner
          Harbeth Audio UK

          Comment


          • #6
            Originally posted by hifi_dave View Post
            I like the idea of milk crates for the M40.2. The dimensions are ok and should raise the speaker to a suitable height and they are quite safe. You could add additional mass/weight quite easily with a brick or two. Only downside is that they ain't pretty.

            I lost interest in that post when it was said the M40.2 tweeter radiates "significant energy upward".
            Yes, but which milk crates, David? Would that be the ones that deliver a full fat sound with cream on top or the semi skimmed ones for a lighter, less full sound . I know what you mean about what they would look like - they wouldn't make it past your eyes!! Ok, that's my joke for the week, better stick with the day job!

            Comment


            • #7
              Originally posted by A.S. View Post
              Viewable?
              Yes viewable for me, both as a thumbnail and when clicked, into a modal popup window. Logged on.

              Comment


              • #8
                Originally posted by A.S. View Post
                {(Test) Here is a fresh post with the image that I tried to add to post #3. Attached using the camera icon, and then located image in the pre-loaded Image Library}.

                Viewable?
                Yes Alan, it is now thanks.

                Comment


                • #9
                  Yes the image in post #6 is viewable in the actual message, and when clicked upon it opens a larger image. Usually neither of my accounts Jeff_C or JC-test allow me to view images whether logged in or not. For instance image link in post #3 is not viewable.

                  This method of adding images allows it to be viewed by me whether logged in or not,

                  Comment


                  • #10
                    Originally posted by A.S. View Post
                    {(Test) Here is a fresh post with the image that I tried to add to post #3. Attached using the camera icon, and then located image in the pre-loaded Image Library}.

                    Viewable?
                    Yes visible to me Alan.

                    Don

                    Comment


                    • #11
                      Yes, viewable to me.

                      Comment


                      • #12
                        Originally posted by A.S. View Post
                        Stands and vibration

                        Incidentally, this week I've purchased six B&K accelerometers and their preamplifiers* for a little project I have in mind. As you'll read, they are the standard method for detecting vibration in industry. It would be a trivial, but interesting, exercise to apply them to various points of a speaker stand to explore just how much the stand moves in 3D relative to the speaker, if at all. And then to try and relate that tiny vibration of the stand (if measurable) to the sound output the motion of such small surface areas could theoretically generate, be it even measurable.

                        I think we'd have to agree, as logical level headed chaps, that if, say, the vibration of the stand, as measured with precision equipment, contributes, say, 0.1% (or even 1%) to the sound output from the speaker sitting above it that no human can detect that small a sound. And hence, the 'sound' of a speaker stand would be in the imagination. Still hear a difference in sound quality between stands? Fair enough: but we need to look elsewhere for the source of that. And stand height difference would most likely be the explanation.
                        If you've not already seen it, you might be interested in an investigation conducted by Stereophile (some time ago) into the effects on speaker performance of various speaker stands.
                        http://www.stereophile.com/features/806/index.html
                        Also explored were the effects of different means of coupling or decoupling the speaker to/from the stand using either rigid spikes or a more compliant interface, such as Blu-Tack or rubber feet.
                        In addition to measuring any vibrations conducted to the stands, you may wish to check for any changes in the output of the speaker cabinet panels when using different types of stands and/or interfaces. Likewise with the stand spiked to the floor, or not.

                        Comment


                        • #13
                          Originally posted by IMF+TDL View Post
                          If you've not already seen it, you might be interested in an investigation conducted by Stereophile (some time ago) into the effects on speaker performance of various speaker stands.
                          http://www.stereophile.com/features/806/index.html
                          Also explored were the effects of different means of coupling or decoupling the speaker to/from the stand using either rigid spikes or a more compliant interface, such as Blu-Tack or rubber feet.
                          In addition to measuring any vibrations conducted to the stands, you may wish to check for any changes in the output of the speaker cabinet panels when using different types of stands and/or interfaces. Likewise with the stand spiked to the floor, or not.
                          Well, the last sentence of the first paragraph in the link quoted is not at all accepted wisdom. The opposite conclusion is the conclusion of the work undertaken at the BBC Research Department, and well documented too.
                          Alan A. Shaw
                          Designer, owner
                          Harbeth Audio UK

                          Comment


                          • #14
                            Originally posted by RZangpo2 View Post
                            "Your milk crates made of molded plastic sheet, are a hollow dimensional object with the resonant characteristics of a stretched membrane. Unloaded, they can easily be made to resonate in a particular frequency range which is very annoying. Place a load on them (e.g., your speakers) and you merely raise the resonant frequency range. In fact, most speaker cabinets (even cheap ones) don't resonate at a high enough amplitude to cause any audible sympathetic resonance in milk crates or any other sort of stand. However, crank up the volume of the amp high enough and those crates will resonate to the point that you can feel the vibration with your fingertips against the crates when the music ranges around the resonant frequency.".
                            Taking the last sentence at face value, what about all the other mechanical vibrations in the room that are excited by high sound pressure levels? Won't they completely mask any potential acoustic contribution from the stands?

                            Comment


                            • #15
                              The BBC research report published by Harwood and Mathews in 1977, "Factors in the design of loudspeaker cabinets", can be found here.

                              Comment

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