I disagree that the "modern approach" is as you state or that the line's 1/4-wave resonance is not an overwhelming design criterion. To put it simply, if one goes to the bother of building a tapered line, for instance, with its increased complexity, why would you not want to have its terminus provide significant bass support? If all one wants is a fairly damped speaker box, why not just make it a sealed one?
Paul
Paul
I understand what you are saying, that a tube tuned to 50 Hz will contain moving air at say 51, 52, 53 hz etc or whatever is applied to it, but it will only reasonate at 1/4 the wavelength (and odd multiples thereof ie 150Hz)
I made this mistake, I thought that if you doubled the length of a TL then it would become a 1/2 wave reasonator, but it doesn't.
A tube closed at one end will only be a 1/4 wave resonator, and its length will determine that frequency. It wont reasonate at 1/2 the frequency, nor 1/8 the frequency, just 1/4 and then odd multiples of that.
This is explained more here
Resonances of open air columns
I think the idea of it being tuned to the drivers Fs is that when the driver itself is resonating (an increase in volume) then the air moving in and out of the port is out of phase, resulting in a reduction or balancing out of the volume, flattening that peak in the drivers response. But at lower frequencies the port is in phase, reinforcing the response below the drivers cut off point.
The problem being that as you increase the air flow to 'boost' the lower freqs, you also increase the out of phase freqs too, resulting in that ripply response that isn't so ideal....
Called an acoustic labyrinth. On a quick scan, it seems that its length was set to a half-w/l.
http://p10hifi.net/planet10/TLS/downloads/Acoustic-Labyrinth-Olney.pdf
dave
A TL can be used to flatten the Fs peak and can be considered a low pressure enclosure.
dave
Dave took your advice on board
seen the Alpair 6 gen 1 going cheap ($160 for 4) - not sure how much to ship to uk yet. Did some research on a replacement bass unit, v tempted by the HiVi M8a which I can get for less than £50 each in the UK. The M8a looks to work great in a sealed and the efficiency is a little higher than the SDX7. Altogether I am getting 6 great drive units for hopefully just over £250 inc delivery. Bit of a bargain.
Will put the 2x2 alpairs in TL's and the 2x HiVi's in sealed cabs, undecided on x/o
seen the Alpair 6 gen 1 going cheap ($160 for 4) - not sure how much to ship to uk yet. Did some research on a replacement bass unit, v tempted by the HiVi M8a which I can get for less than £50 each in the UK. The M8a looks to work great in a sealed and the efficiency is a little higher than the SDX7. Altogether I am getting 6 great drive units for hopefully just over £250 inc delivery. Bit of a bargain.
Will put the 2x2 alpairs in TL's and the 2x HiVi's in sealed cabs, undecided on x/o
lol planet10 u don't speaker that make me don't know anything. 1/4 TL is the best version. i see the Bookselve the pmc speaker. many versions are less than 1/4 wt. see the respond freqgang is okay. 1/8 wt i will try for Pa speaker but don't know is it good idea.😕
You do want the line to contribute output, but in the end you must find a compromise between line output and the higher modes and their negative contribution. The Augspurger paper shows this well. Tapering can help improve the compromise by pushing the upper modes a little higher. Damping the line does make the system tend towards the sealed box performance and Augspurger only found a few dB improvement in bass output level, over the sealed box, for his optimally damped cases.
In some regards it behaves like a vented box with tuning due to line length rather than the lumped parameter constants of the standard reflex port. Being a function of length brings the upper modes along as well. By the way a standard vented box will also have upper modes related to port length, but they tend to be fairly high in frequency and easily reduced by cabinet damping.
My point is that the early days of vented boxes the premise was always to place the cabinet resonance at the system or woofer resonance. Modern analysis showed that was a false premise. Once optimally designed you can move the free air resonance of the woofer around considerably and not change system performance appreciably (compliance changes only). There is no relationship between woofer resonance and optimum vent frequency.
Designing a TL by setting the length to give 1/4 wave resonance at woofer fs is also an oversimplification and unecessary constraint. Unlike Olney in the 1930s, we aren't trying to restrain the motion of a high Q woofer. We are trying to create a woofer/box system with better performance than the baseline closed box system.
David S.
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There are always compromises that have to be accepted but I do want to clarify a point: Making the 1/4-wave resonant frequency of the line, however the line is configured, is seldom an optimum choice. It may be for a specific driver but not always. You have to optimize the line's resonant frequency based on both Fs and Qts of the driver, sometimes making the line's Fr higher than Fs, sometimes lower, and sometimes the same. I assume you know and agree with this. Each TL designer has their own prejudices, of course, about what an optimum overall response is.
Paul
Paul
I'm a little confused by the arguments for and against the importance of line length. I assumed that the length was critical for controlling cone excursion (in an open line at least) and had to correspond to a wave length, whether quarter, half, whatever.
My interest on closed lines are about the absorbing of the back wave and not for boosting bass. I think I am right in saying that a well stuffed line above say, 150hz, has no set length altho a minimum length should be considered..
My interest on closed lines are about the absorbing of the back wave and not for boosting bass. I think I am right in saying that a well stuffed line above say, 150hz, has no set length altho a minimum length should be considered..
Hi,
It is. For open you get a halfwave reversal at the mouth and the
reflection returns to the driver 1/4 +1/2 +1/4 = one wavelength back
at the driver, causing an excursion null similar to vented alignments.
It you critically damp the line and exit it does not matter whether
its open or closed. But if its closed and undamped the resonance
will be an octave higher than open, open is a lot more useful.
rgds, sreten.
It is. For open you get a halfwave reversal at the mouth and the
reflection returns to the driver 1/4 +1/2 +1/4 = one wavelength back
at the driver, causing an excursion null similar to vented alignments.
It you critically damp the line and exit it does not matter whether
its open or closed. But if its closed and undamped the resonance
will be an octave higher than open, open is a lot more useful.
rgds, sreten.
sreten half wave is very long. 1/4 wt i ever listen but on my pics i ever listen and think that i like more than 1/4 wt. i want to build with the other driver but don't know how to do. this speaker from thailand. it's very short line. use tdl driver 8dc-442. -3db about 28 hz the first line out behind the driver. it's cover by thick cloth(red) which [protect the high temp in car] i want to do like this if someone know plese tell me.
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If you stuff a 1/4 wave closed and cut off the bass with a crossover, thats ok, right?
Depends what frequency you're tuning the pipe to. 1/4 wavelength doesn't mean anything without that particular detail. 😉 Nor does 1/2 wavelength, or any other fraction of a wavelength for that matter.
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