454Casull said:
Yes, of course. With all else fixed sensitivity would have to rise with increased efficiency.
In English language documentation, the term "efficiency" was used for a long time when "sensitivity" was meant. That seems to be changing for the better these days. In French the term "Rendement" is used for SPL at a given distance. I like that term.
Do other languages confuse the terms?
Hi,
With the temperature coefficient of either copper or aluminium, how close to melting point will either voice coil have to be to have doubled their resistance at room temperature?
What happens to the insulation at these high temperatures?
I think the mechanism for an effective doubling of driver impedance (Re + acoustic impedance) has little to do with temperature.
When horns are played over a sensible range of SPLs the result will be very low if any measureable power compression.
surely this is untrue.
With the temperature coefficient of either copper or aluminium, how close to melting point will either voice coil have to be to have doubled their resistance at room temperature?
What happens to the insulation at these high temperatures?
I think the mechanism for an effective doubling of driver impedance (Re + acoustic impedance) has little to do with temperature.
When horns are played over a sensible range of SPLs the result will be very low if any measureable power compression.
GM,
"OK Noah, please explain to me the difference between a compression driver dissipating half of its input power as heat to attain a theoretical 50% acoustic efficiency and a point source driver that's got 3 dB of thermal power compression due to VC heating."
The difference is that 50% of the compression driver's input is on average a fraction of a W, so virtually no compression.
A direct radiator is dissipating 99% of a much higher input power to produce the same output.
"I mean the compression driver's VC has got to heat up until its Re is doubled just to overcome the horn's acoustic resistance, so what is it if it's not thermal power compression? "
Sorry, but this makes no sense at all to me.
First, acoustic resistance is a good thing; it's what allows the efficient transduction of diaphragm motion into acoustic output.
I don't understand why you connect acoustic and electrical resistance. The former is necessary for there to be any acoustic output; the latter is just an undesirable and unnecessary reality.
A superconducting VC with zero resistance would be ideal.
As a point of interest, Tom Danley says that power compression starts to become an issue at 1/10th toi 1/8th a driver's rated power.
I think compression drivers are typically 40 - 80 W, vs. a direct radiator maybe 100 W, but the compression drivers are about 20 dB more efficient.
"OK Noah, please explain to me the difference between a compression driver dissipating half of its input power as heat to attain a theoretical 50% acoustic efficiency and a point source driver that's got 3 dB of thermal power compression due to VC heating."
The difference is that 50% of the compression driver's input is on average a fraction of a W, so virtually no compression.
A direct radiator is dissipating 99% of a much higher input power to produce the same output.
"I mean the compression driver's VC has got to heat up until its Re is doubled just to overcome the horn's acoustic resistance, so what is it if it's not thermal power compression? "
Sorry, but this makes no sense at all to me.
First, acoustic resistance is a good thing; it's what allows the efficient transduction of diaphragm motion into acoustic output.
I don't understand why you connect acoustic and electrical resistance. The former is necessary for there to be any acoustic output; the latter is just an undesirable and unnecessary reality.
A superconducting VC with zero resistance would be ideal.
As a point of interest, Tom Danley says that power compression starts to become an issue at 1/10th toi 1/8th a driver's rated power.
I think compression drivers are typically 40 - 80 W, vs. a direct radiator maybe 100 W, but the compression drivers are about 20 dB more efficient.
efficiency can hardly be debated, being the total acoustic energy put out by the transducer vs the energy it consumes. But in my head, sensitivity only makes sense when a location is given, relative to axis and distance from transducer... hence they're not the same...
EDIT: As if a laser beam was filtered through a spreading lens, it's total energy output (efficiency) would remain the same (except lens losses) but the sensitivity on axis would be greatly reduced...
EDIT: As if a laser beam was filtered through a spreading lens, it's total energy output (efficiency) would remain the same (except lens losses) but the sensitivity on axis would be greatly reduced...
Hi GM, Noah, all
What Marshal’s math is saying when it offers efficiency or sensitivity is saying do you want a narrower bandwidth horn that is of a high efficiency - or – do you want a wider bandwidth, lower efficiency alignment?
For a horn that is “full size”, operation can be described easily.
The driver is simple at the low end, it is a sealed box speaker with Fb placed somewhat above the low cutoff. If you look at the impedance curve, you see the most important parts.
At resonance, you see a big peak which is the frequency where the spring forces exactly cancel out the mass reactance leaving the (insignificant) acoustic load and mechanical losses to set the peak value.
This is then a parallel tank circuit shunted by the resistance of the radiation and mechanical losses (Qm)
To either side, one has a decreasing impedance caused by either the mass - or - compliance dominating the moving system. These appear as an L for compliance or spring force and C for mass in the tank circuit.
What your seeing in the impedance is a plot of the drivers mobility or ease of movement, as you go to either side of resonance, either the spring force or moving mass decreases the radiators velocity.
Velocity is what the horn wants to be driven with.
The idea with a horn is you place an acoustic load on the moving system such that the impedance peak is dropped to about Rdc X2 (for a 50% efficient speaker).
The horns efficiency bandwidth (if the horn were purely resistive) would extend rougly from where the sealed box impedance curve reaches Rdc X2 at the low and high end.
A driver with a stronger motor, produces a bigger wider impedance curve and so it also has a wider efficiency bandwidth. A greater load produces greater BW but less efficiency.
At the low end, a real horn is always too small AND it is not purely resistive, it becomes more reactive the lower you go (to a point). This reactance has the effect of a mass that increases (to a point) with decreasing frequency.
By having the compliance volume and driver mass just right, the increasing effective mass can partially cancel out the increasing spring force. In the case of the sealed box one can picture that “IF” one were able to make the drivers mass increase as you went down, that the “resonance” point would be much wider, meaning the radiator is available to move, over a greater bw.
This “Reactance annulling” has some range of tuning, if one adjusts the “t”, one effects the rate of change of the mass reactance at the low cutoff. If you picture the change in shape, it is clear a “t” of .1 looks a lot more like a port initially and has more mass than an exponential horn.
So, a horn that is 50% efficiency has an impedance which rises to 2 times the Rdc of the driver.
When you measure a horn, you use a Voltage, it is normal for a horn to have a usable measured response FAR wider than just the efficient part. On the other hand, Bass horns like the Lab sub are designed to only be used IN the efficiency range.
One can have a horn that is over 50% by having the resistive impedance rise to greater than 2X Rdc but normally this is not done as it leaves a narrow efficient bandwidth.
Around 60% is a reasonable upper limit for a narrow bw horn.
The on axis response of a horn, at the high end is also effected by directivity of the horn.
This can easily increase the on axis SPL by 10dB over an omni directional radiation of the same power.
Also, at the high end there is a 2 pole acoustic low pass filter between the driver radiator and throat, made of the trapped front volume and mass of the air in the throat.
If these portions are sized properly, this filter can both extend the high frequency cutoff and roll off the output above that which reduces distortion.
Anyway, some thoughts about horns, one of my favorite things.
Best,
Tom Danley
Noah, Gregory, I recorded some Fireworks on the 4th with a microphone thing I am working on.
This will tax your system, put it on a CD, do not make an mp3 it kills it.
Turn up slowly, try it on good headphones first.
http://www.danleysoundlabs.com/knowledge baSE.htm
What Marshal’s math is saying when it offers efficiency or sensitivity is saying do you want a narrower bandwidth horn that is of a high efficiency - or – do you want a wider bandwidth, lower efficiency alignment?
For a horn that is “full size”, operation can be described easily.
The driver is simple at the low end, it is a sealed box speaker with Fb placed somewhat above the low cutoff. If you look at the impedance curve, you see the most important parts.
At resonance, you see a big peak which is the frequency where the spring forces exactly cancel out the mass reactance leaving the (insignificant) acoustic load and mechanical losses to set the peak value.
This is then a parallel tank circuit shunted by the resistance of the radiation and mechanical losses (Qm)
To either side, one has a decreasing impedance caused by either the mass - or - compliance dominating the moving system. These appear as an L for compliance or spring force and C for mass in the tank circuit.
What your seeing in the impedance is a plot of the drivers mobility or ease of movement, as you go to either side of resonance, either the spring force or moving mass decreases the radiators velocity.
Velocity is what the horn wants to be driven with.
The idea with a horn is you place an acoustic load on the moving system such that the impedance peak is dropped to about Rdc X2 (for a 50% efficient speaker).
The horns efficiency bandwidth (if the horn were purely resistive) would extend rougly from where the sealed box impedance curve reaches Rdc X2 at the low and high end.
A driver with a stronger motor, produces a bigger wider impedance curve and so it also has a wider efficiency bandwidth. A greater load produces greater BW but less efficiency.
At the low end, a real horn is always too small AND it is not purely resistive, it becomes more reactive the lower you go (to a point). This reactance has the effect of a mass that increases (to a point) with decreasing frequency.
By having the compliance volume and driver mass just right, the increasing effective mass can partially cancel out the increasing spring force. In the case of the sealed box one can picture that “IF” one were able to make the drivers mass increase as you went down, that the “resonance” point would be much wider, meaning the radiator is available to move, over a greater bw.
This “Reactance annulling” has some range of tuning, if one adjusts the “t”, one effects the rate of change of the mass reactance at the low cutoff. If you picture the change in shape, it is clear a “t” of .1 looks a lot more like a port initially and has more mass than an exponential horn.
So, a horn that is 50% efficiency has an impedance which rises to 2 times the Rdc of the driver.
When you measure a horn, you use a Voltage, it is normal for a horn to have a usable measured response FAR wider than just the efficient part. On the other hand, Bass horns like the Lab sub are designed to only be used IN the efficiency range.
One can have a horn that is over 50% by having the resistive impedance rise to greater than 2X Rdc but normally this is not done as it leaves a narrow efficient bandwidth.
Around 60% is a reasonable upper limit for a narrow bw horn.
The on axis response of a horn, at the high end is also effected by directivity of the horn.
This can easily increase the on axis SPL by 10dB over an omni directional radiation of the same power.
Also, at the high end there is a 2 pole acoustic low pass filter between the driver radiator and throat, made of the trapped front volume and mass of the air in the throat.
If these portions are sized properly, this filter can both extend the high frequency cutoff and roll off the output above that which reduces distortion.
Anyway, some thoughts about horns, one of my favorite things.
Best,
Tom Danley
Noah, Gregory, I recorded some Fireworks on the 4th with a microphone thing I am working on.
This will tax your system, put it on a CD, do not make an mp3 it kills it.
Turn up slowly, try it on good headphones first.
http://www.danleysoundlabs.com/knowledge baSE.htm
Re: 100% Impossible!
I think we all can agree to that, and that applies to efficiency. But sensitivity at some point in space on the other hand, can surpass the share sound pressure of what an ideal 100% efficient unidirectional loudspeaker could produce (total input power minus losses radiated into freespace), if the acoustic energy produced, how unefficient it may be, is directed to a very limited area.. Sensitivity and horns becomes a matter of directing all the acoustic energy to the area where the listeners ears is likely to be found. Because of this, a horn can reach very high sensitivity on axis (or within a limited area), without breaking any physical laws of efficiency.
calvert73 said:
I think we all can agree to that, and that applies to efficiency. But sensitivity at some point in space on the other hand, can surpass the share sound pressure of what an ideal 100% efficient unidirectional loudspeaker could produce (total input power minus losses radiated into freespace), if the acoustic energy produced, how unefficient it may be, is directed to a very limited area.. Sensitivity and horns becomes a matter of directing all the acoustic energy to the area where the listeners ears is likely to be found. Because of this, a horn can reach very high sensitivity on axis (or within a limited area), without breaking any physical laws of efficiency.
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