Hi Andy,
I don t think the purpose of a closed TL is to mimic an anechoic room, more to render the backwave to an insignificant level compared to the front radiated wavefront, closer to an OB. At least this is how i see things.
In this case using wavelength to help in a case of a bandpass system may help and make some artifacts predictable (ripple in the bandpass) afaik.
I do see your point though, and your remark about stuffing make perfect sense if someone play with a seashell kind of enclosure it make sense to experiment about damping material and location in the line, but i'm sure it is better to leave the beginning of the line /rear end of driver unmuted, to let it 'breathe'.
I may be completly wrong however, this is purely intuitive.
And anyway, except for mesurements anechoic room is useless in audio: no stereophony, impossible to stay in long term...i don t know if it could be of interest for a lodspeaker box... maybe.
I don t think the purpose of a closed TL is to mimic an anechoic room, more to render the backwave to an insignificant level compared to the front radiated wavefront, closer to an OB. At least this is how i see things.
In this case using wavelength to help in a case of a bandpass system may help and make some artifacts predictable (ripple in the bandpass) afaik.
I do see your point though, and your remark about stuffing make perfect sense if someone play with a seashell kind of enclosure it make sense to experiment about damping material and location in the line, but i'm sure it is better to leave the beginning of the line /rear end of driver unmuted, to let it 'breathe'.
I may be completly wrong however, this is purely intuitive.
And anyway, except for mesurements anechoic room is useless in audio: no stereophony, impossible to stay in long term...i don t know if it could be of interest for a lodspeaker box... maybe.
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It can, to a point anyway. The main issue is that placing a high level of damping in close proximity to the drive unit can mass-load the driver's powertrain (moving components -coil, suspension, diaphragm); in extreme cases this can actually cause deformation. In less extreme cases, it simply provides excessive loading, akin to what happens if a driver is placed in an under-sized sealed box.
Actually most PMC designs are tapered lines. They just aren't very good.
The basic purpose of sealed transmission lines is to attenuate the backwave of the driver as effectively as possible. Providing the volume & axial length, taper & damping are sufficient for this purpose the specific resonant frequency of the line per se is not especially critical. However, it is valuable to understand the basic underlying physics in order to avoid errors. Thus a sealed pipe is a 1/2 wave line, an open [at one end] is a 1/4 wave. The differences in behaviour between the two are significant. In terms of sealed lines, a design with parallel walls is fine if suitably damped; a tapered design of the same Vb can help reduce the amplitude of particularly the harmonic modes further & allow a reduction in the quantity of the internal damping.
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That makes me wonder if the best solution would be an expanding rather than tapering tunnel, or expanding then tapering
I don t know which model of Pmc it is but most of them seems to use this kind of fold! Not too complicated to achieve. (Scroll down a bit)
rmaf2009
Or better this one:
ATL - Advanced Transmission Line | PMC Loudspeakers
Nevertheless, THANK YOU Scott and others, this is much clearer for me now.
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Back to midranges, say from 350Hz and up, where loading isn't needed for the low end of the passband and what we seek is to absorb the backwave... so sealed vs open TL...what's the benefit, if any?
What's the difference between a tapered TL vs a straight TL both opened at the end, both stuffed with variable density stuffing getting denser towards the opening?
What's the difference between a tapered TL vs a straight TL both opened at the end, both stuffed with variable density stuffing getting denser towards the opening?
Things would be a lot clearer if people thought about this a different way:
Long pipe to sequester rear wave
A TL (or any other enclosure except for true horns) is not used to help the sound. It is just one of several ways to sequester that pesky rear wave from a Rice-Kellogg driver. Most of the deep thought related to TLs has to do with ensuring the TL does the least harm.
As methods go, it has a number of very worthwhile advantages although not worth bothering with except for subs and even then, more beneficial when you have enough real-estate to mold the back wave to do your wishes. Like a BR, tapped-pseudo-horn or dipole, you can capture a bit of energy from the back wave although possibly at the cost to quality parameters, if that trade-off appeals to you.
B.
Long pipe to sequester rear wave
A TL (or any other enclosure except for true horns) is not used to help the sound. It is just one of several ways to sequester that pesky rear wave from a Rice-Kellogg driver. Most of the deep thought related to TLs has to do with ensuring the TL does the least harm.
As methods go, it has a number of very worthwhile advantages although not worth bothering with except for subs and even then, more beneficial when you have enough real-estate to mold the back wave to do your wishes. Like a BR, tapped-pseudo-horn or dipole, you can capture a bit of energy from the back wave although possibly at the cost to quality parameters, if that trade-off appeals to you.
B.
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I don t know if this can be a benifit but If open the TL could potentialy change the directivity behavior of the driver toward cardioid pattern. Depending on driver size it can be something to care about maybe?
About the second question I would say it slide slowly to an aperiodic behavior, but i may be wrong.
About the second question I would say it slide slowly to an aperiodic behavior, but i may be wrong.
I always prefer a low pressure box best. If you have the driver for it, or you have no need to extend the bass a well designed aperiodic enclosure is hard to beat.
Use a 4" (or 5”) plumbing pipe for a 3" (or 4”) from behind the driver thru the back panel to retrofit an existing enclosure or for ease of construction.
dave
Low pressure box = open end be it tapered or straight, correct?
Which characteristics make a driver appropriate for it?
Thank you!
TLs have horrible resonances until they are well stuffed. Various approaches and longer is better, since the only sensible use is for sub-woofers, gives you more real-estate for absorbents of various sorts since none of them has much effect at low frequencies.
B.
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Exactly. AFAIK, this is correct. I invoked the name "Nautilus" for being among the most famous of this type of enclosure design, but I certainly do not believe that the curves are anything more than aesthetic conveniences.