Geoff,
Looking at your link, I found the typical equations for speed of sound that can also be found (and derived) in most physics or acoustics texts. If you look at any of those equations, do you see any correction term for geometry? Speed of sound in air is speed of sound in air be it a plane wave or a spherical wave. Not much else I can say.
Looking at your link, I found the typical equations for speed of sound that can also be found (and derived) in most physics or acoustics texts. If you look at any of those equations, do you see any correction term for geometry? Speed of sound in air is speed of sound in air be it a plane wave or a spherical wave. Not much else I can say.
Thanks for re-affirming my sanity Martin. I was wondering (though not too much) if I must be loosing it.
Air pressure and velocity might change; look at ground effect used in 1970s era F1 cars (an obscure example, but I happen to be interested in it). But the speed of sound in air itself isn't going to change except by the usual well known causes. Those might have a small effect inside a cabinet, but the significance of these is not exactly high, given the fact that we have compared the measurements of speakers to their predicted MathCad responses and (surprise!) they are as identical as you are ever likely to get. Both ways -speakers designed in MathCad, then built, and simulating existing cabinets which have been measured, and comparing the predicted response. Draw your own conclusions.
Air pressure and velocity might change; look at ground effect used in 1970s era F1 cars (an obscure example, but I happen to be interested in it). But the speed of sound in air itself isn't going to change except by the usual well known causes. Those might have a small effect inside a cabinet, but the significance of these is not exactly high, given the fact that we have compared the measurements of speakers to their predicted MathCad responses and (surprise!) they are as identical as you are ever likely to get. Both ways -speakers designed in MathCad, then built, and simulating existing cabinets which have been measured, and comparing the predicted response. Draw your own conclusions.
I've done some baffle sims, and with a width of 40cm right into a corner (and plenty of height) I'm expecting a rise of 3dB/octave from around 200Hz down for two octaves. At that point, I expect to find room gain starting. This roughly mirrors my drivers natural rolloff (fd=28Hz, Qtd=0.28) all the way down.
It would seem I'd want the rear energy to just disappear. I have read to the effect that a Qtc of less than 0.5 can give sluggish results, but I can't comment on this. I believe I could build a non-resonant tl system that would give a response like that of the raw driver on an IEC baffle. I hope it may effectively dissipate the rear energy in a good way. It would certainly give me options by way of the stuffing, if the bass turned out to be too light, and these things happen.
I've been thinking of a line of 75cm with the driver at 1/3 of the length, a 6:1 taper, a volume of about 13l (where Vad=48l), and a stuffing of more than 1lb/ '^3 of fibreglass.
Geoff H,
I don't mean to be presumptuous, but are you perhaps describing the venturi effect at DC?
It would seem I'd want the rear energy to just disappear. I have read to the effect that a Qtc of less than 0.5 can give sluggish results, but I can't comment on this. I believe I could build a non-resonant tl system that would give a response like that of the raw driver on an IEC baffle. I hope it may effectively dissipate the rear energy in a good way. It would certainly give me options by way of the stuffing, if the bass turned out to be too light, and these things happen.
I've been thinking of a line of 75cm with the driver at 1/3 of the length, a 6:1 taper, a volume of about 13l (where Vad=48l), and a stuffing of more than 1lb/ '^3 of fibreglass.
Geoff H,
I don't mean to be presumptuous, but are you perhaps describing the venturi effect at DC?
Hi Scottmoose,
In radio antenna work, 1/4 and 1/2 wave sections of cable are used as impedance matching devices, filters and phase inverters, exactly what the acoustic models do. I have done many for RF.
The first 1/4 wave (acoustic) line I am aware of was about 1875, the first acoustic frequency meter.
The electical equivalent consisted of 2 parallel wires and a shorting bridge. IIRC called Lecher wires, used for measuring frequency, about 1890. www.argo2012.de/info/englisch/lecherengl.htm
Hi Martin,
That link was not the one I intended. I was after a university site that supports the formula of the resonance in pipes, I'll see if I can find it.
ie Wavelength= 4(L+0.4d) L being the length of the pipe and D the diameter.
That formula is very similar to that used to calculate the electrical length of a 1/4 radiator, or 1/2 a dipole.
Cheers,
Geoff
In radio antenna work, 1/4 and 1/2 wave sections of cable are used as impedance matching devices, filters and phase inverters, exactly what the acoustic models do. I have done many for RF.
The first 1/4 wave (acoustic) line I am aware of was about 1875, the first acoustic frequency meter.
The electical equivalent consisted of 2 parallel wires and a shorting bridge. IIRC called Lecher wires, used for measuring frequency, about 1890. www.argo2012.de/info/englisch/lecherengl.htm
Hi Martin,
That link was not the one I intended. I was after a university site that supports the formula of the resonance in pipes, I'll see if I can find it.
ie Wavelength= 4(L+0.4d) L being the length of the pipe and D the diameter.
That formula is very similar to that used to calculate the electrical length of a 1/4 radiator, or 1/2 a dipole.
Cheers,
Geoff
End effect, how could I forget that? 10mm tubing at 150 Mhz. 97% of true length.
http://www.didjshop.com/physicsDidj02.html
http://www.didjshop.com/physicsDidj02.html
Yes, my Dad is a life-long radioham (ex RAF radio-operator too) and we do much the same in his battery of different ariels. Nothing wrong with the idea itself; it makes a great deal of sense for many systems, and I for one, like them a lot. It's just that Bailey's basic assumtions and methodology (like the moving fibres for example) were fundamentally incorrect and led TL design down the wrong path for about 3 decades. No wonder they remained a black art for so long. Martin's MathCad worksheets include end effect BTW so the actual tuning frequency is fractionally below physical line length, depending on length & taper-ratio. The stuffing lowers the speed of sound fractionally, but not a great deal, and as the resonance is so heavily damped it shouldn't make any difference.
Augsperger used the electrical modelling in his work, which emerged at roughly the same time as Martin's did. Except as he was famous of course, it was published. Twas ever thus. 1875? Interesting. Some of the work I'm currently exploring on acoustics dates from 1915 - mid 1930s (sound mirrors). Pionnering stuff that's been forgotten since then. Shame.
Regards
Scott
Augsperger used the electrical modelling in his work, which emerged at roughly the same time as Martin's did. Except as he was famous of course, it was published. Twas ever thus. 1875? Interesting. Some of the work I'm currently exploring on acoustics dates from 1915 - mid 1930s (sound mirrors). Pionnering stuff that's been forgotten since then. Shame.
Regards
Scott
I wouldn't put all the blame on Bailey. There were a few designs from Australia - they may never had made the trip to the UK. The magazines gave them rave reviews, but sensitivity was woeful, and didn't like a lot of power, so hard to compensate. (who was taking who to lunch?)
So my view on that style of design was one of "ok for background music"
As I have said in other posts, my short experience with TLs has led me to realise they are not the lossy devices I once thought they were.
I wish I had an Altec 515B to try in one. I don't think my 411-8As are suitable.
And it looks like the velocity in a straight pipe is the the same as that in free air, the pipe is accusticly longer. Mathematically, creates the same physical length, a bit shorter.
Geoff
So my view on that style of design was one of "ok for background music"
As I have said in other posts, my short experience with TLs has led me to realise they are not the lossy devices I once thought they were.
I wish I had an Altec 515B to try in one. I don't think my 411-8As are suitable.
And it looks like the velocity in a straight pipe is the the same as that in free air, the pipe is accusticly longer. Mathematically, creates the same physical length, a bit shorter.
Geoff
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.