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Thread: Placing your speakers atop a stand - acoustic considerations

  1. #1
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    Default Placing your speakers atop a stand - acoustic considerations

    As if there weren't enough things to worry about .... here is a new one! Now first to say, this is just for your general curiosity; I can show you later why well established acoustic theory shows that the normal human ear is unlikely to actually hear this effect, so don't waste too much time convincing yourself one way or another! But certainly, when making precision measurements with a dumb microphone, it certainly can detect and display the reflection. But it can't evaluate how it would sound.

    Now I have a permanent acoustic measurement set-up here in this beautiful, restored 16 century barn, I'm able to explore micro-issues which I couldn't before. And one came to light yesterday - why there is a spike of wide-band (up to about 10kHz) energy that follows the initial impulse about 1mS (one thousandth of a second) later. Could it be the grille? No, removing the Harbeth SuperGrille proves that has nothing to do with that. So what could it be? Well, the answer is that the humble wooden plinth the speaker sits on is the culprit. With the bottom edge of the speaker flush with the speaker's lower lip - which common sense says is the correct position, and looks attractive too - there is a distinct reflection - see picture - but with the speaker pulled forward by about 25mm, reflection eliminated. Incidentally, draping the edge of the platform with VetBed (the white furry material - an excellent general purpose absorber in the higher frequencies) made no worthwhile difference at all.

    So - maybe we should bring our speakers just a fraction forward of the stand edge, or is this a degree of precision far beyond what we can hear? Better to consider only cosmetics and stability perhaps.

    >
    Attached Images Attached Images
    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK

  2. #2
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    Default Difference in Initial Impulse Energy

    Wow. Thanks for that. I take it the microphone was at constant distance from the speaker during the test? I ask because I see that the frequency spectrum of the energy of the initial impulse is also less, when the speaker is pulled forward of the plinth.

    {Moderator's comment: this was not a precision measurement. Don't scrutinise it too carefully as it was just intended to give a general idea.}
    Ben from UK. Harbeth P3ESR owner.

  3. #3
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    Default Deep thoughts about speaker baffles

    As far as I can remember, the lossy (screw-attaced) rear of a Harbeth cabinet does have a direct effect on it's overall energy balance.
    The baffle and it's surround (picture-frame if you will) is more inert thanks to double thickness MDF.

    Perhaps moving the P3ESR derivate (as it looks to be) forward on a platform which is not capable of fully supporting the speaker's footprint, one goes from a situation where the already inert front portion of the speaker is well supported (hence damped) and the more 'live' rear section hangs over the edge, to a reversed situaton with full support for the most lossy part of the cabinet.

    I.e. is it purely an acoustic matter (diffraction) or maybe a mechanical matter as well?

    >An idea: why not make a new support platform which exactly matches this speaker's footprint? If that gives more or less identical or comparable measuring results, it would point to the rear needing full support and not the platform needing to be an inch back from the baffle's edge (as one can see in the photograph, this coincides with the rear of the cabinet joining the current platform's rear edge).
    Last edited by Spindrift; 16-09-2011 at 08:11 AM. Reason: added idea

  4. #4
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    Default Stand design and acoustics

    Stand design and how the speakers are supported do make a difference. At least in my case for the P3ESR.

    By adding isolating supports between the speaker and the stand top mount, I have reduced vibrations being transferred to the stands (you can easily feel it with your hand). In doing so, there is indeed an improvement in the purity of the sound. At the end of the day, the objective is about hearing sound coming from the speakers and allowing them to perform efficiently.

    I believe how much difference this will make also depends on the room acoustics. A room with poor acoustic is likely to see bigger difference as such stray sound sources are reflected more.

  5. #5
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    Default STand top plate footprint

    There are stands that match the exact footprint of each Harbeth model. I know for certain that the Foundation open frame stands are built to Harbeth spec, and I would imagine that the Skylan stands are as well.

  6. #6
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    Default Stand isolator supports

    Dear engjoo,

    What kind of "isolating supports" did you use?

  7. #7
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    Default Skylan

    I am using what my Harbeth dealer had loaned me. He told me it is supplied by Skylan.

    It is a combination of different materials comprising mainly of what I would described as a pair of very dense yet light material bonded together. On the top, it has one of those base protectors which are soft to provide some grip and protection to the speakers.




  8. #8
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    Default Decoupling from the stand

    There is no loudspeaker built that doesn't benefit from some form of decoupling from the surface it is placed upon or attached to.
    The traditional view of using spikes has been a blind alley that the industry has lead us down.

    Alan could probably verify by using an acceleromator that decoupling enhances the thin walled cabinet effect. The main effect is to speed up the bass and reduce the box effect that cabinets impose. Harbeth speakers are not known for being fast, and should benefit accordingly.

    Probably sorbothane would be the ideal substance, but I suspect any form of rubber eg tap washers, would work well.

  9. #9
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    Default Stand decoupling

    Quote Originally Posted by jair44 View Post
    There is no loudspeaker built that doesn't benefit from some form of decoupling from the surface it is placed upon or attached to.
    The traditional view of using spikes has been a blind alley that the industry has lead us down.

    Alan could probably verify by using an acceleromator that decoupling enhances the thin walled cabinet effect. The main effect is to speed up the bass and reduce the box effect that cabinets impose. Harbeth speakers are not known for being fast, and should benefit accordingly.

    Probably sorbothane would be the ideal substance, but I suspect any form of rubber eg tap washers, would work well.
    Yes. I believe thin walll cabinet designs will benefit more from being decoupled to its support than other speaker designs.

    {Moderator's comment: this is an entirely understandable human emotional response. Sadly, we doubt that there is any science behind this belief.}

  10. #10
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    Default Sorbothane

    Sorry for being off topic.

    Check out the Sorbothane website homepage! It's dripping with non-referenced, no context bunkum! I see this nonsense everywhere; it's what makes the objective, honest approach of this (Harbeth) company so refreshing.

    By the way, I'm not dissing the Sorbothane product, only the way it's presented.

    Back to the topic, to couple or decouple, I've been unable to observe a difference one way or another, either with thin-wall Harbeth or thick-wall Proac standmounts. I understand the theoretical benefits of coupling (to lessen the tendency of the speaker box to move backwards during a bass impulse according to Newton's third law) and decoupling (to kill the resonances of the stand and floor) but cannot nail a difference in my living room.

    I'm doubting myself slightly. I haven't tested anything audio for months; I will test the couple/decouple thing again tonight. My P3s are not coupled at the moment - the stands are loose on carpet and underlay. I will add spikes, press them into the floorboard and play a recording with which I'm very familiar that has plenty of bass, right at the limit of the P3's frequency range.
    Ben from UK. Harbeth P3ESR owner.

  11. #11
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    Default Isolation - a matter of taste

    To couple or isolate is a matter of taste. I have tried and used countless designs of stands over the years and the best I have found with Harbeth speakers are the Something Solid XF. These are a relatively lightweight, rigid and open frame with Carbon Fibre pucks to couple the speaker to the frame without damaging the cabinet.

    De-coupling, with the XF, adversely affacts the performance.

  12. #12
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    Default Forward tilting

    I prefer to tilt my C7ES3s slightly forward. The top of the speaker is little closer to the listener than the bottom. This results in improved clarity and extra sweet treble. Less toe in does not bring the same effect. That is at least how I feel.

    This could be specific to my personal taste or room acoustics. The ceiling is concrete, and I thought the forward tilt position might reduce the reflection from the ceiling. It could be something else as well. I do not know really.

    What do you think could best explain my findings? It would be interesting to know if anyone else has similar experience. If you are curious to experiment, be careful not to drop your fine speakers!

  13. #13
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    Default Tweetrs and stand heights

    You are pointing the HF units more directly towards your ears. A similar effect might be obtained by toeing in the cabinets and/or by employing stands which place the HF unit at the same height as your ears when seated.

    Whenever I sell speakers and stands, I obtain the customer's seated ear height and get the stands made to suit.

  14. #14
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    Default Tweeters now above ears

    Thanks Dave. I think your explanation made it clearer. The tweeters seem indeed to be above my ears when I get comfortable on the couch. I will later experiment with the height of the stands a bit.

    Hopefully the bass response will not rise too much.

  15. #15
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    Default To Couple or Decouple

    Quote Originally Posted by BAS-H View Post
    Sorry for being off topic.

    Check out the Sorbothane website homepage! It's dripping with non-referenced, no context bunkum! I see this nonsense everywhere; it's what makes the objective, honest approach of this (Harbeth) company so refreshing.

    By the way, I'm not dissing the Sorbothane product, only the way it's presented.

    Back to the topic, to couple or decouple, I've been unable to observe a difference one way or another, either with thin-wall Harbeth or thick-wall Proac standmounts. I understand the theoretical benefits of coupling (to lessen the tendency of the speaker box to move backwards during a bass impulse according to Newton's third law) and decoupling (to kill the resonances of the stand and floor) but cannot nail a difference in my living room.

    I'm doubting myself slightly. I haven't tested anything audio for months; I will test the couple/decouple thing again tonight. My P3s are not coupled at the moment - the stands are loose on carpet and underlay. I will add spikes, press them into the floorboard and play a recording with which I'm very familiar that has plenty of bass, right at the limit of the P3's frequency range.
    Well, I sat through the above test. I added spikes, then removed them and decoupled the stands from the floor further with 15mm pipe insulation. Disappointingly, in my room and my speakers, no difference was detected. Perhaps a difference may exist with larger speakers at higher volumes.
    Ben from UK. Harbeth P3ESR owner.

  16. #16
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    Default

    Quote Originally Posted by A.S. View Post
    I can show you later why well established acoustic theory shows that the normal human ear is unlikely to actually hear this effect, so don't waste too much time convincing yourself one way or another!
    (...) there is a spike of wide-band (up to about 10kHz) energy that follows the initial impulse about 1mS (one thousandth of a second) later. (...) With the bottom edge of the speaker flush with the speaker's lower lip - which common sense says is the correct position, and looks attractive too - there is a distinct reflection - see picture - but with the speaker pulled forward by about 25mm, reflection eliminated. (...) So - maybe we should bring our speakers just a fraction forward of the stand edge, or is this a degree of precision far beyond what we can hear?
    >
    Resurrecting an old topic, did someone notice any audible differences bringing the speaker forward as mentioned? Most of the stands designed for Harbeths have top plates more or less equal to the base of the speakers, while many other manufacturers use top plates smaller than the base of the speakers, I wonder if it has something to do with this kind of reflections.

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