Glad you joined. What's the downside of a rectangular or square pyramid vs a triangular or five-sided?
Hi Lewinski,
I will give you my own pov/experience about that: the issue is all related to paralell wall and to a lesser extent about axial symmetry in the enclosure.
Paralell wall give rise to strong modal behavior so there will always be some issue in very defined frequency area. This can to an extent be tamed by absorbtion usually but i find this give a kind of signature in the frequency area in question (and harmonics related to them). I find them to have some cumulative effect and are spoted because what i think is some cumulative and monotonic kind of effect.
Once you use non parallel architecture the modes are less predictable but usually spread over larger frequency area (spacing of axial, longitudinal and oblique modes may be more randomly distributed and less related one to anothers) and the signature is less easily identified imho.
That being said a closed TL show this kind of behavior too as the graph posted by Scott show clearly, even with a tappered line however with a tapper and with a bit of driver offset they can be made not as related to the (low freq) bandpass frequency (as in the non tappered one) so probably easier to deal with as less monotonicaly related to the bandpass freq limit ( so less easily heard).
I will give you my own pov/experience about that: the issue is all related to paralell wall and to a lesser extent about axial symmetry in the enclosure.
Paralell wall give rise to strong modal behavior so there will always be some issue in very defined frequency area. This can to an extent be tamed by absorbtion usually but i find this give a kind of signature in the frequency area in question (and harmonics related to them). I find them to have some cumulative effect and are spoted because what i think is some cumulative and monotonic kind of effect.
Once you use non parallel architecture the modes are less predictable but usually spread over larger frequency area (spacing of axial, longitudinal and oblique modes may be more randomly distributed and less related one to anothers) and the signature is less easily identified imho.
That being said a closed TL show this kind of behavior too as the graph posted by Scott show clearly, even with a tappered line however with a tapper and with a bit of driver offset they can be made not as related to the (low freq) bandpass frequency (as in the non tappered one) so probably easier to deal with as less monotonicaly related to the bandpass freq limit ( so less easily heard).
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Thank you krivium. I was following the rationale if the first two paragraphs and then you started your 3rd with
Weren't the first 2 paragraphs referring to closed TLs as well?
In light of your remarks I would conclude the circular cross section B&W uses would be the worst in that all such modes will coincide because of the single diameter. Same for Vivid audio.
Weren't the first 2 paragraphs referring to closed TLs as well?
In light of your remarks I would conclude the circular cross section B&W uses would be the worst in that all such modes will coincide because of the single diameter. Same for Vivid audio.
Hi Lewinski,
In fact in the 2 first paragraph i was refering to "regular" (rectangular) box (closed or not) and the fact that in this case it is usual to use a lot more absorbent material than what i ve seen in TL (1/4 wl, so ML-TL).
As far as i understand for a closed TL this is the same, less absorbent material used to control the resonnance compared to a "regular" closed box. In my view this is benificial but your point of view may vary.
Yes theoricaly a circular non tapered cross section is a worst case scenario. But in practice this may be a valid approach, i ve seen some project based upon tube and outcome seemed to please the owners ( i ve never heard a design like this so can t speak about them from experience).
Thanks to this thread i may understand why the owners of such system are pleased even if they based the design more on a typical closed box (volume needed for the driver only) but with progressive stuffing of the line.
Anyway, about B&W there is multiple version of the circular chambers used, some have a tapered line after the 'bowl' shaped chamber (like the Nautilus second lowest way iirc), some use an oval/teardrop shaped chamber (and specificaly shaped motor piece) to redirect backwave and rely on diffraction to control standing wave from their technical(?) description.
I must admit i m a bit sceptical about the second solution because of the supposed bandpass of the way (wavelength at work are probably to long to be affected). But i don t know maybe other principle are at work...
Whatever i ll try a closed TL in comparaison to a standard "regular" closed box of same overall volume with a full ranger (M.A. alpair7 1st gen which wait for a new box) and make my own conclusion from there.
There is an interesting thread in the fullrange subsection about a fullranger driven by current amplifier and if you follow the link there is some plans of box which use a closed TL (non tapered but it shouldnt be to difficult to modify it to allow tappering).
Edit : this thread,
http://www.diyaudio.com/forums/full...r-aid-closed-channel-damping.html#post5467484
Scroll down for box plan:
Clean-current speaker project | Current-Drive - The Natural Way of Loudspeaker Operation
In fact in the 2 first paragraph i was refering to "regular" (rectangular) box (closed or not) and the fact that in this case it is usual to use a lot more absorbent material than what i ve seen in TL (1/4 wl, so ML-TL).
As far as i understand for a closed TL this is the same, less absorbent material used to control the resonnance compared to a "regular" closed box. In my view this is benificial but your point of view may vary.
Yes theoricaly a circular non tapered cross section is a worst case scenario. But in practice this may be a valid approach, i ve seen some project based upon tube and outcome seemed to please the owners ( i ve never heard a design like this so can t speak about them from experience).
Thanks to this thread i may understand why the owners of such system are pleased even if they based the design more on a typical closed box (volume needed for the driver only) but with progressive stuffing of the line.
Anyway, about B&W there is multiple version of the circular chambers used, some have a tapered line after the 'bowl' shaped chamber (like the Nautilus second lowest way iirc), some use an oval/teardrop shaped chamber (and specificaly shaped motor piece) to redirect backwave and rely on diffraction to control standing wave from their technical(?) description.
I must admit i m a bit sceptical about the second solution because of the supposed bandpass of the way (wavelength at work are probably to long to be affected). But i don t know maybe other principle are at work...
Whatever i ll try a closed TL in comparaison to a standard "regular" closed box of same overall volume with a full ranger (M.A. alpair7 1st gen which wait for a new box) and make my own conclusion from there.
There is an interesting thread in the fullrange subsection about a fullranger driven by current amplifier and if you follow the link there is some plans of box which use a closed TL (non tapered but it shouldnt be to difficult to modify it to allow tappering).
Edit : this thread,
http://www.diyaudio.com/forums/full...r-aid-closed-channel-damping.html#post5467484
Scroll down for box plan:
Clean-current speaker project | Current-Drive - The Natural Way of Loudspeaker Operation
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Thinking about testing a sealed TL for sequestering the back wave in a midbass section. Am I on the right track?
TL length: 1/4 wavelength. Not 1/2 WL, right?
Midbass will play 60 to 350Hz. Since 1/4 wavelength at 60Hz is 143cm. That is the minimum length I'm solving for.
Folded line OK
1.4m is too long to make it a straight line, so folding is in order. At these frequencies super smooth folding like the Nautilus isn't required, but better to make the first section of the line as long as possible before the first bend and avoid 180 degree bends; 90 degrees much preferred.
Rectangular cross-section OK
At these frequencies the cross-section shape is not critical. Rectangular is OK.
Taper?
While exponential was mentioned as optimal, sounds like a linear taper would be almost as effective and much easier to implement. How about the taper rate? Would taper/constant section/taper be a no-go, a non-issue, or just don't know? Seems an initial section with a fast taper, followed by sections with slower taper would best approximate to an exponential, so wouldn't hurt.
Any consideration towards Qtc?
For my 10" midbasses a Qtc of 0.7 is achieved with 11 liter cavity, so a fast initial taper would be required and then transition to a slow taper to reach the 1.4m length required.
Same 10" need 28 liter cavity for Qtc=0.5, so allowing for a much slower taper and of course a much larger cavity.
TL length: 1/4 wavelength. Not 1/2 WL, right?
Midbass will play 60 to 350Hz. Since 1/4 wavelength at 60Hz is 143cm. That is the minimum length I'm solving for.
Folded line OK
1.4m is too long to make it a straight line, so folding is in order. At these frequencies super smooth folding like the Nautilus isn't required, but better to make the first section of the line as long as possible before the first bend and avoid 180 degree bends; 90 degrees much preferred.
Rectangular cross-section OK
At these frequencies the cross-section shape is not critical. Rectangular is OK.
Taper?
While exponential was mentioned as optimal, sounds like a linear taper would be almost as effective and much easier to implement. How about the taper rate? Would taper/constant section/taper be a no-go, a non-issue, or just don't know? Seems an initial section with a fast taper, followed by sections with slower taper would best approximate to an exponential, so wouldn't hurt.
Any consideration towards Qtc?
For my 10" midbasses a Qtc of 0.7 is achieved with 11 liter cavity, so a fast initial taper would be required and then transition to a slow taper to reach the 1.4m length required.
Same 10" need 28 liter cavity for Qtc=0.5, so allowing for a much slower taper and of course a much larger cavity.
Tl length: .Edit : no if you go sealed the length should be 1/2 wavelength, so 286cm
Folded line : i won t be too worried about the bent, look in previous post in the thread' Dave gave an ok for a 120 to 1k line with multiple 180bends.
Rectangular: yes absorbtion should be ok to tame artefacts,
Taper: i agree with you.
Qtc: use a linkwitz transform! Any qtc, any fs (given reasonnable limits and a little max spl loss if you want to go lower).
Folded line : i won t be too worried about the bent, look in previous post in the thread' Dave gave an ok for a 120 to 1k line with multiple 180bends.
Rectangular: yes absorbtion should be ok to tame artefacts,
Taper: i agree with you.
Qtc: use a linkwitz transform! Any qtc, any fs (given reasonnable limits and a little max spl loss if you want to go lower).
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A 1/4 or 1/2 wavelength is irrelevant because all the sound is going to get absorbed. The length needs to be long enough to absorb all the sound given the stuffing. The stuffing should start light and get progressively heavier to avoid significant reflection. The line should taper in order to increase the velocity and hence the effectiveness of the stuffing.
Folding is not ideal because some sound will be reflected at the discontinuity even if the majority continues down the line. A smooth and gentle curve would be preferable.
Yes but the highest frequency behind the cone needs to form a plane wave at the mouth of the line.
The taper and the increase in stuffing density are intimately related. The combination should seek to minimise the amount of sound reflected due to the changing impedance. The maths is fairly complicated but widely discussed but it will depend on the particular properties of the stuffing used. I am not aware of a direct example.
The normal sealed/vented box maths assumes the air is a zero dimensional lump. This is not the case for a pipe/line which is usually assumed to be one dimensional with varying parameters (cross section, damping,...).
An additional concern could be forming a plane wave to send down the pipe/line with struts and magnets in the way. This probably won't be as complex as a phase plug for a horn (which doesn't have struts and magnets in the way!) if the shortest wavelength is kept significantly larger than the diameter of the pipe.
Hornresp is your friend - it can provide answers to all the above questions. As far as the Nautilus loudspeaker is concerned, from an acoustical engineering viewpoint the shape of the driver enclosures is more about style than substance, despite what the marketing material might say. The speaker certainly looks very nice, though
.Attachments
Sorry for reviving an old thread, but I'm not sure I understand. Are you saying diffraction is not an issue because the radiating surface is large? I'm sorry I am new to diy audio
I can't say I see eye to eye with Keyser's post. If one were to use a circular baffle with a sharp edge, diffraction effects would or could indeed be terrible (but very early in time). The Nautilus discussed here doesn't have circular baffles with sharp edges though. In fact, it's shape flows gently from the cone outwards. One could view it as a horn/waveguide turned inside-out. This spreads any kind of diffraction ripple and minimizes it where it counts, in the frequency curve on and off axis. The transition from 2Pi to 4Pi is as smooth as it can be with a baffle like that. A good simulation program, like ABEC can simulate the difference between a circular baffle with sharp edges and a more flowing shape with a sphere like shape that holds the driver like we have here.
To help smooth that flowing shape even further, the midrange of the B&W speakers of subsequent models (like the 800D1) had a surround that doesn't stick out.
In the days of the Nautilus design, still with Laurence Dickie on board, B&W had a lot of attention within their designs to avoid the negative effects of diffraction, using smooth shapes like these. Dickie resumed to design speakers with features like that at Vivid Audio.
Yes, all four compartments are sealed, and the top left ( on the left hand channel ) is connected in series with the bottom right, the bottom left is connected with the top right, but the bottom left is shorted by 2 uf capacitance, and then both pairs wired in parallel; they're connected like this to avoid beaming, and it seems to work. I intend on fitting an acoustic ( and cosmetic ) hood.
Could it be the directivity is responsible for great imaging, was the previous speaker alike in this respect?
While it is possible sound inside the speaker affects output and could have effect on stereo image it the sound ought to be similar on both sides of stereo (given cabinets are similar enough) and thus not affect too much on imaging? On the other hand directivity has direct influence on image as reflections in room change, among other things.
Its very hard to separate various aspects/issues and draw conclusion what is causing what perceived quality increase or decrease, unless isolated somehow. It is great though that you have made progress and better sound
While it is possible sound inside the speaker affects output and could have effect on stereo image it the sound ought to be similar on both sides of stereo (given cabinets are similar enough) and thus not affect too much on imaging? On the other hand directivity has direct influence on image as reflections in room change, among other things.
Its very hard to separate various aspects/issues and draw conclusion what is causing what perceived quality increase or decrease, unless isolated somehow. It is great though that you have made progress and better sound
Well, I can categorically say - NO! - the previous speaker with the folded wedge was an omni, the driver was mounted "magnet out " so the cone radiates the sound very evenly, so the exact opposite of directional, and that imaged very well also; the only thing in common ( apart from they both need a sub to fill in the bass, the omni much more than the quad cube ) is the folded wedge. They're only folded for convenience of construction, I did make two triangle speakers. Of course it could be a coincidence that two different speakers with wedges have good imaging, perhaps it's easy to get an omni, or directional speaker to image, perhaps it's just more technical to get a " middle ground " speaker to image well.
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