I would say that sensitivity is largely determined by ribbon weight, and size
But ofcourse magnet design matter, but from my experience less
With regards to width of ribbon/gap, I wouldnt go beyond 20mm
I have also found that VERY long/high is no advantage
About 4 feet, and lifted from the floor is fine
Listening at the middle, or slightly above is ideal
Maybe due to ribbon sagging/hanging, and loss of control, if bigger
If you really want it big, maybe consider a 50-60mm wide planar with a row of magnets in the middle
But tricky to build
But advantage is that with "south" in the middle, and "north at both sides, you could avoid long return wires
But ofcourse magnet design matter, but from my experience less
With regards to width of ribbon/gap, I wouldnt go beyond 20mm
I have also found that VERY long/high is no advantage
About 4 feet, and lifted from the floor is fine
Listening at the middle, or slightly above is ideal
Maybe due to ribbon sagging/hanging, and loss of control, if bigger
If you really want it big, maybe consider a 50-60mm wide planar with a row of magnets in the middle
But tricky to build
But advantage is that with "south" in the middle, and "north at both sides, you could avoid long return wires
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Sensitivity for a ribbon is very much affected by the ribbon gap, that is the gap between the sides of the ribbon and the magnet and distance between magnets , more so than slight differences in ribbon weight..
The middle will always have the highest output due to it's linesource
summation.
Borat :
I'm not sure what .06 T would give you for efficiency you will also have to weigh in the other factors previously listed ......
Many DIY ribbon tweeters are built with the goal of getting higher effiiency than the common 91-94db/watt 25mm dome, and this requires a gap field of about 0.6-0.8T. This is easy to achieve with NdFeB magnets. For a tweeter, I would design the gap flux to reach 96-98 db/watt effiency to provide some magical musical dynamics. A high efficiency ribbon tweeter avoids the need to play with horns and compression drivers.
A longer, larger area midrange ribbon with efficiency <90 db/watt requires very high currents into very low a resistance, so there is value in keeping the gap field >0.1T.
For large ribbons, it is so difficult to generate high currents that the steel and magnet cost to stay above 90 db/watt is worth the effort.
==============
Bl - The product of magnet field strength and the length in meters of ribbon suspended in the magmatic field
Re - DC resistance of the ribbon, measured in ohms.
Mms - Mass of the diaphragm, including acoustic load, in kilograms.
Sd - Projected area of the driver diaphragm, in square meters.
η0 - The reference efficiency of the driver, in percent
η0 =(5.445×10-4 m²·s/kg * (Bl²*Sd²/Mms²*Re ))*100%
dB(1 watt) = 112.02 + 10*log(η0)
SPL at 1 meter for an input of 2.83 volts is then:
dB(2.83 V) = dB(1 watt) + 10*log(8/Re)
= 112.02 + 10*log(η0) + 10*log(8/Re)
it seems to me that you can always double efficiency by making the ribbon material twice as thin.
i mean you can always picture a ribbon as two ribbons in an iso-barik alignent and then just remove one of them so you get same output from 1/2 power input.
the point of isobaric is to work smaller box but if you're running a dipole its just dead weight.
in other words i think reducing ribbon thickness can go a long way in increasing efficiency without the need for a strong field.
lets do some quick math. a prosound woofer that is 95db efficient will have about 1/3 of its coil inside a field about 0.8T so the average field on the coil is about 0.3T
it will also have about 1/3 of its moving mass being the voice coil so you can say that the average field on the mass is 0.1T
f
so 0.1T gets it up to 95db despite having a moving mass of about 250 grams. the only thing it has going for it is large surface area. but if i keep surface area on the same order of magnitude ( say 4" wide ribbon, 2 meters long ) and field strength only a little lower ( 0.06 vs 0.10 ) but reduce moving mass from 250 gram to ... 10 gram. efficiency should go above 100db/watt.
i mean you can always picture a ribbon as two ribbons in an iso-barik alignent and then just remove one of them so you get same output from 1/2 power input.
the point of isobaric is to work smaller box but if you're running a dipole its just dead weight.
in other words i think reducing ribbon thickness can go a long way in increasing efficiency without the need for a strong field.
lets do some quick math. a prosound woofer that is 95db efficient will have about 1/3 of its coil inside a field about 0.8T so the average field on the coil is about 0.3T
it will also have about 1/3 of its moving mass being the voice coil so you can say that the average field on the mass is 0.1T
f
so 0.1T gets it up to 95db despite having a moving mass of about 250 grams. the only thing it has going for it is large surface area. but if i keep surface area on the same order of magnitude ( say 4" wide ribbon, 2 meters long ) and field strength only a little lower ( 0.06 vs 0.10 ) but reduce moving mass from 250 gram to ... 10 gram. efficiency should go above 100db/watt.
Right
Well, I dont know the amount of gain from a lighter ribbon, but its a VERY important factor, and probably not solely about sensitivity
Transient response may be a more important factor
I suppose theres an ideal size
Long time ago DIYers found that the glue used was about the worst part, in terms of weight
Some have used the thin "japan" paper
The kind used to build wings for model planes
Others have done the lead stribes from baking paper
The ones with paper on one side and thin foil on the other
Held together with medical "angel" tape
Båndsei do that too, and I definately liked what I heard at his place
But theres a lot of return wires involved in a big ribbon
I would choose a slim planar
No "hanging" ribbon, which is an issue with long ribbons
Better control of diaphragm
Probably better sensitivity
Higher impedance
And its possible to do it without any return wires
But big diy ribbon and planars both have the probably greatest advantage of all, no impedance transformer
AND they can be crossed low
And its pretty easy to DIY a fairly good one without too much effort
But it needs support from a woofer, which may be a bigger challenge
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I would never claim that
It was only ONE of several points
Also dont forget that having just a couple more signal strings makes it possible to make them a bit wider, and maintain the same impedance
Which will result in improved power handling
I have made several long ribbons, a long time ago
And I do remember the flapping back and forth
Completely out of control at anything higher than "normal" low SPL
Sure, you can deal with that by using steep aktive filters, or higher xo point
I would like it to go down to 300hz
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A loudspeaker is a system, the diaphragm is simply a constituent of the system. In a traditional ribbon alignment where a magnetic circuit is utilized, the mass of air behind the ribbon is coupled to the diaphragm. The mass of air is often much larger than the mass of the ribbon. We thus must include the total mass of the system (ie mass of ribbon + mass of air) if we wish to reach resolution on its efficiency.
By reducing the mass of the ribbon, you are simply approaching the mass of the air. The differences in mass quickly become negligible and an increase in sensitivity may not necessarily be observed.
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Hello Line source ,
Interesting , as i have never seen a big ribbon having 90 Db/2.83 V/M sensitivity , could you explain more .
Sure, but not possible with corrugated ribbon
Another significant problem with tention, be it ribbon or planar, is the resulting "drumskin-effect"
And the tendency to "rattle"
Which makes the corrugation crutial
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well i understand that. but i think by the time ribbon mass is on the same order of magnitude as air mass efficiency should already be quite good.
Instead of mass, you may consider compliance. Would you not admit that a weaker spring is easier to deflect (less required force) than a stronger spring?
but compliance only becomes a factor at lowest frequencies. it would effect a cutoff frequency more than efficiency.
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