1. Join Now

    AVForums.com uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

frontfiring or downfiring?

Discussion in 'Subwoofers' started by Grand Dizzy, May 5, 2005.

  1. Grand Dizzy

    Grand Dizzy
    Standard Member

    Joined:
    Dec 11, 2004
    Messages:
    311
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    21
    Ratings:
    +1
    I'm just about to buy a sub. I'll probably go for a downfiring but I want to be certain I've made the right choice!

    Can anyone advise me?

    I'll be using it mainly for home cinema/TV, with occasional CD listening. My room is medium-sized and my floor is carpeted wood.
     
  2. eviljohn2

    eviljohn2
    Well-known Member

    Joined:
    Jul 8, 2002
    Messages:
    7,529
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    86
    Location:
    Near London.
    Ratings:
    +208
    There shouldn't be any difference as long as they've been well designed. There's certainly no way of predicting which will integrate best into your room I'm afraid. :)
     
  3. PJTX100

    PJTX100
    Well-known Member

    Joined:
    Dec 21, 2004
    Messages:
    8,132
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    136
    Ratings:
    +563
    I went for a FF BK because, well, IMO, it looks better then the DF! ;)

    ...PJ
     
  4. Grand Dizzy

    Grand Dizzy
    Standard Member

    Joined:
    Dec 11, 2004
    Messages:
    311
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    21
    Ratings:
    +1
    I agree. I much prefer the appearance of the front firing subs.

    But appearance is the least important thing to me. I want it to sound great! Any more advice?
     
  5. JBL 4645

    JBL 4645
    Banned

    Joined:
    Apr 20, 2005
    Messages:
    127
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    16
    Ratings:
    +0
    I have a JBL 4645 professional THX 18in 600watts forward direct firing…
    …I still haven’t got over the size of it…. :smashin:

    So yes forward firing rock on… :smashin:
     
  6. PJTX100

    PJTX100
    Well-known Member

    Joined:
    Dec 21, 2004
    Messages:
    8,132
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    136
    Ratings:
    +563
    Well, we've commented that sound-wise the general concensus is that there's no difference.

    Hence, it's just personal preference in terms of practicality and aestethics.

    And you've said you prefer the look of front firing.

    I think we'd be struggling to give any more advice.

    ...PJ :)
     
  7. Nimby

    Nimby
    Member

    Joined:
    Jan 27, 2004
    Messages:
    9,201
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    136
    Location:
    The Danish Bacon Factory
    Ratings:
    +669
    Though there's always chicken wire......?

    Nimby
     
  8. Sly

    Sly
    Active Member

    Joined:
    Apr 28, 2002
    Messages:
    1,465
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    51
    Location:
    west mids,England.
    Ratings:
    +48
    would a forward firing unit send less noise or vibrations to a downstairs room?
    my HC stuff is in a bedroom with floorboards.
    ta.
     
  9. Jules Tohpipi

    Jules Tohpipi
    Member

    Joined:
    Feb 4, 2004
    Messages:
    695
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    43
    Ratings:
    +146

    No, the bass driver in a sub will radiate sound equally in all directions. Hence why it doesn't matter if it is forward or downward firing.
     
  10. JBL 4645

    JBL 4645
    Banned

    Joined:
    Apr 20, 2005
    Messages:
    127
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    16
    Ratings:
    +0
    Lets think this in terms of 3-D!

    Why not a downward firing one, then if you don’t like the way it is, turn it up on its side so that it is facing the room…

    Or you can face it upwards so that is firing towards the ceiling, or build a platform high up made to size to hold the sub near to the ceiling…

    And yes it’s very doable…it’s a question of science… :thumbsup:
     
  11. Nimby

    Nimby
    Member

    Joined:
    Jan 27, 2004
    Messages:
    9,201
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    136
    Location:
    The Danish Bacon Factory
    Ratings:
    +669
    One could make a case for increased mechanical excitement of the surface on which a downfiring sub rests. (usually the floor)

    The driver attempts to accelerate its enclosure in opposition to its rapid cone movements.
    (Every action has an equal and opposite reaction)

    The forward firing sub merely tries to slide its enclosure backwards and forwards parallel to the floor.

    That said, my own SVS (downfiring) cylinder doesn't seem to produce much transmitted noise through the floor in comparison with its acoustic output. But then again the room below is open to the listening room via the open stairwell.

    Though the bass can be heard through the ceiling in the bathroom across the hall. But is it transmitted mechanically or carried through the air with acoustic waves?

    Summing up and all in all: It's swings and roundabouts.
    So I should forget everything I've said so far.

    Hope this helps? :)

    Nimby
     
  12. Mroizouk

    Mroizouk
    Standard Member

    Joined:
    Jan 11, 2005
    Messages:
    333
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    18
    Location:
    London
    Ratings:
    +1
    You can always isolate the sub to keep the sound in your listening room. I've got a downward-firing svs and was worried about the transmission of noise through the floor, but it's not appreciable worse than a forward-firing sub. Now its isolated though, I can turn it up pretty loud before it bothers anyone downstairs.
     
  13. bob1

    bob1
    Well-known Member

    Joined:
    Nov 7, 2002
    Messages:
    4,326
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    86
    Location:
    Sheffield
    Ratings:
    +129
    :lease: :rotfl:
     
  14. Sly

    Sly
    Active Member

    Joined:
    Apr 28, 2002
    Messages:
    1,465
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    51
    Location:
    west mids,England.
    Ratings:
    +48
    yes you should cuz i have! :rolleyes:
     
  15. Sly

    Sly
    Active Member

    Joined:
    Apr 28, 2002
    Messages:
    1,465
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    51
    Location:
    west mids,England.
    Ratings:
    +48
    how have you done this , and has it made a lot of differance?
     
  16. Mroizouk

    Mroizouk
    Standard Member

    Joined:
    Jan 11, 2005
    Messages:
    333
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    18
    Location:
    London
    Ratings:
    +1
    Hey sly

    I used some sorbothane feet and a concrete slab under the sub. If you search for isolation on this forum you'll find other solutions people have come up with, but I found this worked well for me.

    It makes it a good bit quieter elsewhere, but the main benefit was to stop the really low stuff rattling the plumbing and floorboards! Sounds tons better now.
     
  17. Grand Dizzy

    Grand Dizzy
    Standard Member

    Joined:
    Dec 11, 2004
    Messages:
    311
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    21
    Ratings:
    +1
    PJTX,

    I say you can never have too many opinions! If I only asked one opinion of everything, I would have a biased view of the world!
     
  18. Jules Tohpipi

    Jules Tohpipi
    Member

    Joined:
    Feb 4, 2004
    Messages:
    695
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    43
    Ratings:
    +146
    This brain-dump is partially to help out someone with a sub, but also to help Nimby out on his new found quest to understand Newtonian physics (forces and waves) – as he clearly didn’t take much interest at school !! (Sorry!)

    Subwoofers: because the energy levels are very low here, then we can all but eliminate the effects of thrust; mainly because a speaker is not very efficient at thrusting its cabinet.

    However, we will be generating quite a bit of resonance in the cabinet, and resonance usually has quite a bit of motion associated with it. It’s the nature of resonance that you don’t need a great deal of energy to get it started. Any resonance motion generated in the cabinet will be transmitted to the floor via the subwoofer’s feet/spikes. Thus it matters not if the sub is forward or downward firing.

    It’s a very good idea to try and mechanically isolate the sub from the floor, as it is very easy to get a wooden floor droning along with the music – along with any flimsy walls attached to that wooden floor – or any flimsy ceilings attached to that flimsy wall, attached to that flimsy floor. Things can get very quickly out of hand in the wrong circumstances – causing a one-note drone to be produced in the listening room and any adjacent rooms. Note that when you sit a subwoofer on a flimsy floor, you are 90% towards making a big do-it-yourself NXT speaker (so long as you have enough resonance being generated in your cabinet). NXT speakers radiate sound from top and bottom surfaces equally, and don’t need much energy to get them working, so don’t underestimate this effect. You can reduce this effect by minimising the contact area to the floor (spikes), or by having a lossy interface between cabinet and floor (sorbothane).

    The next thing we need worry about is a flimsy floor/wall/ceiling being excited by direct air pressure. At the long wavelengths produced by a subwoofer, then this effect is going to be noticeably smaller than that mentioned above (for reasons explained later below). The only solution is to have less flimsy stuff in your house. A bass driver in a sub produces sound equally in all directions (it isn’t in the slightest ‘beamy’, like a mid driver or tweeter), so it matters not here if you have a downward or forward firing sub – both are in very close proximity to the floor.

    The final thing: the longer the wavelength (of any type of wave), then the less likely it is to be impeded by an object. Or put another way, the longer the wavelength, the easier it will pass through an object – such as a wall. Quick example from another field: you are an astronomer and you want to take a peek at a star. However, there is a big dust cloud halfway between you and the star preventing you from seeing it – so what do you do ? Switch to infra-red instead and you’ll see it perfectly. The energy the star transmits in the infra-red (longer wavelength) passes straight through the cloud. Similarly, this is why you can hear bass through walls, and why it doesn’t really matter if you stick a sub behind a sofa, or why thin speaker grilles don’t make much difference to the sound (except maybe in the extreme HF). It’s like the object isn’t there at all.

    This is why it takes an inordinate amount of effort and material to properly sound-proof a room. But you will crack a great deal of the problem if you isolate the sub from the floor. A sub spiked (small contact area) onto a heavy concrete slab (stability) in turn sitting on sorbothane feet (lossy) onto the floor may get you a divorce, but will go a long way toward retaining the friendship of your neighbours – certainly if you’re living in a flat.
     
  19. Nimby

    Nimby
    Member

    Joined:
    Jan 27, 2004
    Messages:
    9,201
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    136
    Location:
    The Danish Bacon Factory
    Ratings:
    +669
    Brain dump? Seems appropriate. Don't apologise though. It's decades since I had a formal physics lesson. We also learnt social etiquette. But we'll gloss over your lack of attention in that particular class.

    Really? Try telling that to anyone owning an IB line array. Do 2" amplitude wall vibrations, popping nails and loosening plasterboard mean anything in your rather odd dimension? The sole purpose of an IB manifold is to cancel physical vibrations from cone movements. It cannot be said that this fudge offers the best sound quality. But users persist in their use for the considerable mechanical advantages offered.

    Are we talking harmonics here? Panel sizes are usually too small and stiff for most subwoofers to have much resonance at the fundamantal frequencies. Your "resonance" is simply mechanical vibration (motor noise) from cone accelerations. Any subwoofer worth the name is massively built and structurally stiffened to resist compression resonance in the panels.

    An NXT panel vibrates equally (and rather uniquely) in-phase from both sides.

    A floor set in sympathetic vibration by sound waves vibrates in phase from one side and out of phase from the other. It is a simple vibrating panel.

    I heard that NXT panels are crap at low frequencies. So poor analogy I'm afraid.

    Your suggestion that a room can "drone" at one frequency due to cabinet "resonances" doesn't sound likely. Unless you are discussing cheap one-note subwoofers with very limited LF reach and rather lightweight cabinets?

    Room resonances due to acoustic standing waves might well and often do produce standing wave "resonances". But those that are actually noticable would be most likely at fundamental frequencies heard by the listener as a one-note boom. Why would a cabinet resonate at one particular frequency at sufficient amplitude to produce a one-note drone in the floor or walls? Let alone the ceiling.

    Now we're getting somewhere. Or would be, if only you were right. At low frequencies I can easily excite every surface in the house with my big SVS. Perhaps oddly, it is damn sight more difficult to excite them at higher frequencies. But what do I know? Your mileage obviously varies.

    Omnidirectionality is a well known phenomonem of low frequencies. It is how LF waves can bend around objects and makes modest baffles cancel by antiphase and reinforce by the reproduced sound waves being in phase on either side of the cone.

    Where is the transformation in acoustic sound waves analageous to visual v infra red in the optical spectrum? Okay, so walls and floors are more transparent to low frequencies. But what has this to do with IR astronomy? Low frequencies also travel well compared with higher frequencies which are more easily absorbed.

    It is why large, firest-living mamals have often well-developed deep voices to allow their calls to carry over long distances to rival males and potential mates. But I digress.

    Isn't the primary purpose of the slab simply to add inertia in order to reduce excitement from the rapid cone accelerations (motor noise) and as a sink for any structural harmonics in a cheap sub?
    The spikes are simply a way to minimise cabinet rocking due to cone accelerations. Though isolation of a sub is not the same as isolating a loudspeaker being fed full range audio frequencies. Spikes are certainly not recommended by the designers and directors of the SV range of subwoofers.

    Any compliance placed beneath the slab will help to isolate any " motor noise" in the concrete slab. (and any remaining compression artifacts in a poor subwoofer)

    It would be a poor subwoofer which didn't irritate neighbours most by air-borne sound waves more than it did by resonance through the floor. My own subwoofer can irritate the hell out of me when I'm in the workshop twenty yards from the house and my wife is enjoying her "music"! This has nothing whatever to do with structural "resonance". Nor do cars with pounding subs passing 200 yards away on the road have resonant induction through our floors. Nor do dance nights at the sports hall half a mile away as the crow flies. Nor my distant neighbour's mini-stack boombox with which he entertains himself (and possibly his guests) at his modest family garden parties.

    Last and not least. Please explain why a subwoofer "walks" if, as you claim, there is "low energy" and "a speaker is not very efficient at thrusting its cabinet". Why are these statements so at odds with (our) reality. :)

    Regards
    Nimby
     
  20. eviljohn2

    eviljohn2
    Well-known Member

    Joined:
    Jul 8, 2002
    Messages:
    7,529
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    86
    Location:
    Near London.
    Ratings:
    +208
    It's worth remembering that there are two methods of transmitting energy into the room. There are the mechanical sound waves induced by the pistonic driver movement and the mechanical waves transmitted from the speaker cabinet.

    There is nothing we can do to minimise the induced sound waves which pressurise the room - in fact that's exactly what we're trying to achieve.

    We can prevent the cabinet vibrations from transmitting into the surrounding floor (floor vibrations are very annoying and make the direction localisable as they reach each foot at a discernable time in my experience). In principle this is the idea behind "decoupling" although a unit properly coupled to the floor will exhibit similar properties. The mass loading of the speaker helps to control cabinet resonance and nothing else in my opinion.

    I don't think that the argument of the driver going forwards whilst the cabinet travels backwards carries much weight either. Whilst it's true that this will happen, it is also true that momentum will be conserved. In the free-space idealised case where this effect will occur more than when the unit is actually resting on something it still becomes negligible after a few basic calculations:

    Considering my XLS200 subwoofer's as an example, we need to consider the forward travelling momentum of the driver which is specified as the MMS in it's parameters. In my case this is 135.3g, we'll also consider that the unit has travelled 6mm forwards (Xmax is 12.5mm so this is quite feasible).
    The remainder of the unit weighs about 17000g. It follows that the unit will recoil 6mm x 135/17000 = 47.6microns. Since this is about half the width of a human hair I for one don't think it's worth worrying about. Also remember that the cone will be travelling in the opposite direction too so any movement should largely cancel out.

    Electromagentic and mechanical waves can be generally analysed in exactly the same way although the astronomical analogy isn't entirely accurate although makes the point quite well. :)
     
  21. Grand Dizzy

    Grand Dizzy
    Standard Member

    Joined:
    Dec 11, 2004
    Messages:
    311
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    21
    Ratings:
    +1
    Fascinating stuff, thanks! :)
     
  22. Jules Tohpipi

    Jules Tohpipi
    Member

    Joined:
    Feb 4, 2004
    Messages:
    695
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    43
    Ratings:
    +146
    Crikey, hold on to your knickers there Nimby!! The eclamation marks early on were intended to convey a bit of fun, rather than anything too aggressive. :)

    I think you are somewhat taking things out of context. So you are therefore somewhat right, and so am I in my context - the same physics can be applied to different scenarios.

    Correct that we are talking about panel vibration more than panel resonance - slightly sloppy in my words there.

    NXT panels are only crap at low frequencies when they are small. They are relatively inefficient at low frequencies - true - but will produce some LF if the panel is big enough. Remember where this all originally came from - vibrations in the motors of helicopters causing the cockpit panels to vibrate in sympathy thus making the cockpits noisy. This is what the MoD were specifically trying to tackle - they wanted the panels to vibrate less. Even though the motors were on mounts, and the panels clamped around their circumference, and the excursion on parts of the panel were microscopic, the panels were found to be acting like a (new form of) loudspeaker. The excursion on an NXT exciter is tiny, and many orders of magnitude shorter than a conventional speaker, but doesn't play correspondingly quieter. This discovery as to why their cockpit panels were so damn noisy (without pistonic motion) is what was sold to Verity. We do need to be talking about crappy floors though.

    You are quite correct that these effects will be reduced with a well designed subwoofer.

    It is a simple fact of physics that longer wavelengths interact less with things in their way than shorter wavelengths. They do move, but in perfect sympathy. They don't produce a different sound of their own - as it were, they will behave more like the medium itself - and one can hardly accuse the air of making a drone. This is why whales use LF and bats use ultrasonics to be sure of what is close to them. At LF a wall presented with a sound wave will move more like the medium, allowing transmission straight through.

    You are quite correct that neighbours in *separate* buildings will be more annoyed by the sound waves of a sub, rather than the soundwaves coming from your vibrating floor. Did I not say "If you live in a flat" at the end?

    And finally, speaker cabinets much prefer to vibrate rather actually be translated in position. When a diaphragm/voice-coil/former is being thrust forward, so the permanent magnet wants to move backward. The magnet is bolted to the chassis, and the chassis to the baffle. This will cause a cyclical motion in the baffle because the inertia of the entire sub is too great to allow it to move. As the sub can't move then it translates the energy into a vibration instead - the bending of the baffle. The sub would be thrust backwards and forward (by a tiny amount) were it sat on a frictionless surface and had a perfectly stiff cabinet. However it doesn't, so the energy goes into bending the baffle instead - because the sub won't move. It's a very subtle difference. The resulting vibration in a front firing sub will be transmitted along the baffle, to the bottom panel, and on to the floor - in an up/down motion. (This is why we are nearly talking about the same thing but not quite - your example is more idealised). A sub is excellent at vibrating its panels.

    So, as the MoD's research labs (DERA) discovered in their cockpits, and what piano manufacturers have known for years, take something that's mildly vibrating and contact it at a couple of points to a large board (wood in the case of a piano, and wood in the case of wooden floors), then you get a merry (non pistonic) song. Do it at high enough energy, and with enough drivers, and you might indeed get a nail to vibrate out of a panel.

    But you can have a go at stopping this effect. However, as you and I both said, regardless of all that, there's little to stop the sub's soundwaves being heard halfway down the street.

    :)
     
  23. Nimby

    Nimby
    Member

    Joined:
    Jan 27, 2004
    Messages:
    9,201
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    136
    Location:
    The Danish Bacon Factory
    Ratings:
    +669
    I was brought up to believe that exclamation marks represent somebody shouting!! Smilies or emoticons represent irony or humour where the words do not convey such a meaning directly. Your words suggested not a hint of humour. Though sarcasm could well be implied. Sarcasm is humour directed at, and at the expense of, the other participant. :p

    Will one or both of you try and explain why you think some subs "walk" if it not a direct reaction to piston motion? Are you suggesting some sort of micro-caterpillar traction from some sort of resonance/flotation effect? You are also ignoring the braking effect of the amplifier on cone motion once maximum excursion is achieved. It does not simply cancel out just because the cone moves back the other way.

    Remember that most room floors are quite level within reasonable limits. While most subwoofers have feet near the corners of the bottom panel where panel resonance is greatly diminished.

    Simple vibration (call it resonance if you like, it makes no odds to me) would suggest microscopic bouncing-up-and down on the spot. But some subs actually move across the floor. Where does this lateral motion come from if not from a reaction to piston acceleration? :eek:

    Why do you think mass loading helps some subwoofers? Do you feel it is entirely because it is deadening resonance in the top panel? Then why not attach concrete slabs to the side panels as well? Easily achieved with a couple of loops of string. (But watch your nice finish) :rolleyes:

    Thanks
    Nimby
     
  24. eviljohn2

    eviljohn2
    Well-known Member

    Joined:
    Jul 8, 2002
    Messages:
    7,529
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    86
    Location:
    Near London.
    Ratings:
    +208
    I have to admit that having been through 3 forward-firing subs I've not had any problems with them walking, I don't see why that isn't as likely to be induced by the mechanical vibrations of the cabinet rather than the driver "pushing" the unit along. If a sub's panels do vibrate then they will generally vibrate in a similar way all of the time - do these walking units always go in a dead straight line? And are downward-firing units immune to this effect?

    I do feel that mass loading only effects the vibration of the top panel (which may in turn help to "solidify" some or all of the other panels). Similarly, I would predict that adding additional material to the side panels would reduce these vibrations in the same way although we wouldn't have gravity on our side for this case. :laugh:

    You are of course right that we're not considering pure resonance in this case and that Lost Cause should have used smilies in his post. :)
     
  25. Nimby

    Nimby
    Member

    Joined:
    Jan 27, 2004
    Messages:
    9,201
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    136
    Location:
    The Danish Bacon Factory
    Ratings:
    +669
    Since neither resonance nor reaction can be proved to be responsible we can agree to disagree. :)

    It make a change to discuss something in some depth instead of the usual: "Which budget sub under a fiver for music?"

    If Lost Cause hadn't forgotten his smilies I probably wouldn't have bothered to argue. :devil:

    Regards
    Nimby
     
  26. Jules Tohpipi

    Jules Tohpipi
    Member

    Joined:
    Feb 4, 2004
    Messages:
    695
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    43
    Ratings:
    +146
    OK, excellent stuff.

    Firstly, if you have something that is vibrating purely up and down - unsecured to a surface - then it will tend to walk a bit laterally one way or t'other, because no motion is perfect.

    Secondly, and different to above, remember I said in the last post that on a frictionless surface, with a perfectly stiff baffle, then you will get 100% thrust (for want of a better term) back and forth from the cabinet ? The net effect should be zero motion though, so long as the cone moves at equal speed in both directions.

    Lets suppose I was stood on an open carriage on a railway track. I am holding a large, stiff, thin board. I hold the board to my chest. Then I extend both my arms out forward as fast as I can. There's air pressure on the board, my feet are perfectly secured to the floor of the carriage, and the carriage moves backwards. I have just propelled myself. But, and this is a very big but, if a return the board back to its starting position (in an exact equal and opposite manner) then I'll be pushing on the air in the opposite direction (the air is uniform) and godammit if I'll not be back where I started.

    It is irrelevant where the energy is coming from to move the board or the characteristics of that energy - my body, a machine, etc. That is based upon the assumption that one's intention is to move the board back and forth to replicate a *perfect* wave. In this circumstance the end result of the two motions will be return you back to where you started - regardless of the system driving the board, or any of its characteristics.

    One would term that repetitive motion back and forth a vibration (a wave), because there is no net movement.

    If your subwoofer is not perfectly secured, and the surface it is sitting on is grained, or otherwise resisting motion more in one direction, then you may well find it tends to move in one overall direction - at that point you have to analyse the whole system and discover where the losses are taking place.

    The system is not perfect, of course, and the baffle will bend and vibrate also as the magnetic field generated in the voice coil tries to throw the permanent magnet backwards and forwards (this is where the push/pull interface is) - thus moving the driver's chassis similarly. That's bolted to a baffle, which will likely bend and transmit the vibrations through the surfaces of the cabinet. And that will be translated to the floor. As the wooden surfaces are quite big, then you might get a fair bit of vibration - though that depends on the system again.

    In hindsight I shouldn't have mentioned the concrete slab, not least because I hate the things. I know they don't surpress resonance - rather they just change the resonant frequency. But the other poster mentioned his concrete slab, and with spikes going onto it, then at least it is reducing the contact area of sub to floor - and the post was getting long enough as it was, so I didn't go there. So you were right to pick up on it.
     
  27. Jules Tohpipi

    Jules Tohpipi
    Member

    Joined:
    Feb 4, 2004
    Messages:
    695
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    43
    Ratings:
    +146
    Oops, two posts submitted whilst I was typing mine. Just in case it looks a bit out of sequence.

    :) :)
     
  28. Nimby

    Nimby
    Member

    Joined:
    Jan 27, 2004
    Messages:
    9,201
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    136
    Location:
    The Danish Bacon Factory
    Ratings:
    +669
    There might be no net movement with a sine wave. :)

    But what about a violent transient like a gunshot? :devil:
     
  29. eviljohn2

    eviljohn2
    Well-known Member

    Joined:
    Jul 8, 2002
    Messages:
    7,529
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    86
    Location:
    Near London.
    Ratings:
    +208
    No net movement even with a sine wave would still be no good as although the unit wouldn't "walk" it would still take the edge off of the drivers movement.

    I stand by the conservation of momentum argument that I put forward and calculated in an earlier post that the movement of the driver does not move the cabinet a perceivable amount. Of course for a really lightweight cabinet combined with a very high excursion and MMs it may be noticeable if the unit were suspended in free space. :)
     
  30. Jules Tohpipi

    Jules Tohpipi
    Member

    Joined:
    Feb 4, 2004
    Messages:
    695
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    43
    Ratings:
    +146
    Yes, quite. I don't think we are talking about a lot of movement either way.

    But a sub usually sounds much better in a room if it is spiked (IMO), rather than just resting straight on the floor. Resting straight on the floor seems to cause a drone in the room. So what's causing that ? Now, as the weight of the sub means it can't be moving much (as you calculate), so one has to presume that it is panel vibrations that are setting the floor and adjoining surfaces off. And that using the spikes reduces the contact area, and hence the level of sound transmitted downwards.

    That's my complete guess - you'd have to measure the individual effects to know for sure, and I haven't ever done that.

    BTW, a gunshot is still an equal and opposite thing, as all sound is. Its sound would be broken down into a series of sine waves if you analysed it.

    Just out of interest EvilJohn - I'd be keen to know how my astronomy analogy went a bit off. My knowledge is far from complete in that area - perhaps I misunderstood what I read on this, and if so, would appreciate being put back on track.

    Good chat.
     

Share This Page

Loading...