SPL = 20 Log (P/pr) where P is the sound pressure and pr is a reference pressure. Let pr be the reference, 4Pi pressure. Thus SPL(4PI) = 20 log (1) = 0dB. If we keep the electrical power input to the source constant the sound pressure doubles form 4Pi to 2Pi so we go from P to 2P, and SPL = 20 log (2P/pr) = 20 Log(2) = +6dB.
Now, intensity I = P^2 (neglecting a couple of constants). Thus since the sound pressure doubles the intensity goes up by a factor of 4.
Total radiated power is the intrgral of the intensity (or p^2) over a surface enclosing the source,
For a 4pi environment the surface is that of a sphere:
For 2Pi space it is that of a hemisphere.
Obviously the surface area of a hemisphere is 1/2 that of a full sphere and we all recognize that intensity squared (and SPL) go up by 6 dB from 4Pi to 2Pi. Thus while the SPL goes up by 6dB the radiated power only goes up by 3dB.
Given that the electical power input to the system is constant we see an increase in efficiency of +3dB compared to 4Pi radiation. It is well know that the 2Pi efficiency is 3dB greater than the 4Pi efficiency.
Of course the sufrace bounding the hemisphere (floor) would likely allow some radiation through it and typically this "leakage" would increase with lower frequency. Thus we migh not have the total 2Pi gain to deal with at very low frequency. This would make the dipole roll off of a near floor woofer slightly steeper at very low frequency since it would effectively behave as a returning towards 4Pi space at very low frequency.
Now, intensity I = P^2 (neglecting a couple of constants). Thus since the sound pressure doubles the intensity goes up by a factor of 4.
Total radiated power is the intrgral of the intensity (or p^2) over a surface enclosing the source,
An externally hosted image should be here but it was not working when we last tested it.
For a 4pi environment the surface is that of a sphere:
An externally hosted image should be here but it was not working when we last tested it.
For 2Pi space it is that of a hemisphere.
An externally hosted image should be here but it was not working when we last tested it.
Obviously the surface area of a hemisphere is 1/2 that of a full sphere and we all recognize that intensity squared (and SPL) go up by 6 dB from 4Pi to 2Pi. Thus while the SPL goes up by 6dB the radiated power only goes up by 3dB.
Given that the electical power input to the system is constant we see an increase in efficiency of +3dB compared to 4Pi radiation. It is well know that the 2Pi efficiency is 3dB greater than the 4Pi efficiency.
Of course the sufrace bounding the hemisphere (floor) would likely allow some radiation through it and typically this "leakage" would increase with lower frequency. Thus we migh not have the total 2Pi gain to deal with at very low frequency. This would make the dipole roll off of a near floor woofer slightly steeper at very low frequency since it would effectively behave as a returning towards 4Pi space at very low frequency.
Re: total power doubled going from 4Pi to 2Pi?
Hi Kurt,
It took quite some time for me to grasp this concept when John originally posted his papers on his site. (I don't think a lot of other folks are grasping it either...
But the key is that the efficiency changes between 2 and 4pi space.
KCHANG said:
Hi Kurt,
It took quite some time for me to grasp this concept when John originally posted his papers on his site. (I don't think a lot of other folks are grasping it either...
But the key is that the efficiency changes between 2 and 4pi space.
RAAL: Alligator damping, and some interesting comments:
http://www.audioasylum.com/cgi/m.pl?forum=bottlehead&n=124045&highlight=raal&r=&session=
My pet peeve numero uno is edgy fake sounding string instruments, so this caught my attention.
http://www.audioasylum.com/cgi/m.pl?forum=bottlehead&n=124045&highlight=raal&r=&session=
My pet peeve numero uno is edgy fake sounding string instruments, so this caught my attention.
The link worked for a while, but now I can't get to it even from AudioAsylum's search facility. Try this direct link:
http://www.audioasylum.com/forums/bottlehead/messages/12/124045.html
http://www.audioasylum.com/forums/bottlehead/messages/12/124045.html
John,
Thanks for sharing. It makes sense. But can I still use a short cut by using gated far field measurements with a 5ms window then design the XO EQ based on them, so that I can forget all the complicated calculations, provided that I only want my speakers to sound good in my own room at the specific positions? What are the problems you can see with such an approach? I am interested in getting an anwser to the question because I think the approach may actually get the speakers sound more accurate IN MY ROOM comparing to using any other methods, not to say the design process can be significantly simplified.
Regards,
Bill
Thanks for sharing. It makes sense. But can I still use a short cut by using gated far field measurements with a 5ms window then design the XO EQ based on them, so that I can forget all the complicated calculations, provided that I only want my speakers to sound good in my own room at the specific positions? What are the problems you can see with such an approach? I am interested in getting an anwser to the question because I think the approach may actually get the speakers sound more accurate IN MY ROOM comparing to using any other methods, not to say the design process can be significantly simplified.
Regards,
Bill
4Pi to 2Pi
John,
Thanks for the explanation. I'd like to explore this topic a bit more, however, if you don't mind. You did not address my question about conservation of energy directly, but your statement "If we keep the electrical power input to the source constant the sound pressure doubles form 4Pi to 2Pi so we go from P to 2P, and SPL = 20 log (2P/pr) = 20 Log(2) = +6dB." implies that there is a different coupling to air when the radiation space changes in terms of solid angle.
One conceptual problem I have about this statement is that it will predict an efficiency greater than 100% if we narrow the radiation solid angle enough. For example, if we change the radiation solid angle from 4Pi to 0.4Pi, according to the argument, the pressure should go up 10 times, accordingly the intensity will go up 100 times. Since the space is only 1/10 of 4Pi, the total power will be 10 times higher than that into the 4Pi space. If the original electrical-to-accustic efficiency is 11%, this reduction of the radiation space will mean that 110% of the electrical energy will turn into acoustic power. That does not sound right. As I said, perhaps I am missing something here, and I'd appreciate your clarification.
Cheers,
Kurt
John,
Thanks for the explanation. I'd like to explore this topic a bit more, however, if you don't mind. You did not address my question about conservation of energy directly, but your statement "If we keep the electrical power input to the source constant the sound pressure doubles form 4Pi to 2Pi so we go from P to 2P, and SPL = 20 log (2P/pr) = 20 Log(2) = +6dB." implies that there is a different coupling to air when the radiation space changes in terms of solid angle.
One conceptual problem I have about this statement is that it will predict an efficiency greater than 100% if we narrow the radiation solid angle enough. For example, if we change the radiation solid angle from 4Pi to 0.4Pi, according to the argument, the pressure should go up 10 times, accordingly the intensity will go up 100 times. Since the space is only 1/10 of 4Pi, the total power will be 10 times higher than that into the 4Pi space. If the original electrical-to-accustic efficiency is 11%, this reduction of the radiation space will mean that 110% of the electrical energy will turn into acoustic power. That does not sound right. As I said, perhaps I am missing something here, and I'd appreciate your clarification.
Cheers,
Kurt
Re: 4Pi to 2Pi
Correct so far.
This is where you mix things up. Having intensity increased 10 times doesn't equal increasing the energy. You just concentrated the energy into one direction , the total acoustic energy energy is still the same (11% of what you put in).
Think about candel in a dark room. The number of photon it emits is the same whether it is seeing 4 Pi space or if you place a mirror behind and a lens in front of it. With your mirror/lens you can now focus the light, so any particular corner of the room can be made much brighter. What you neglect is that other corners don't get any light now. Therefore law of energy conservation stands.
KCHANG said:
Correct so far.
This is where you mix things up. Having intensity increased 10 times doesn't equal increasing the energy. You just concentrated the energy into one direction , the total acoustic energy energy is still the same (11% of what you put in).
Think about candel in a dark room. The number of photon it emits is the same whether it is seeing 4 Pi space or if you place a mirror behind and a lens in front of it. With your mirror/lens you can now focus the light, so any particular corner of the room can be made much brighter. What you neglect is that other corners don't get any light now. Therefore law of energy conservation stands.
Bratislav,
If you follow John's argument, you'll see that the total (i.e., integrated) power increases 10 times if the solid angle is reduced by a factor of 10. If you have 11% to begin with, the reduction of the radiation space will result in 10 X11% = 110%.
By the way, I am not saying that is the right answer. I am questioning it.
Cheers,
Kurt
If you follow John's argument, you'll see that the total (i.e., integrated) power increases 10 times if the solid angle is reduced by a factor of 10. If you have 11% to begin with, the reduction of the radiation space will result in 10 X11% = 110%.
By the way, I am not saying that is the right answer. I am questioning it.
Cheers,
Kurt
OK, I should have read more carefully what you questioned.
But answer is the same - imagine a perfect monopole speaker (that is not affected at all by changes in geomerty around it - it creates a perfect spherical wavefront for all frequencies). In free space its efficiency is say 10% (closer to 1% in reality for efficient systems but whatever). That energy gets radiated everywhere, and total acoustic power is say W. Now place it next to the solid wall - energy still goes everywhere, but it gets reflected off the wall and in effect doubles (2*W). Hence power radiated in this room, on this side of the wall is in effect double that of what it was before.
Analogy with mirrors helps : imagine that wall as a mirror - instead of one speaker, you will see two.
But answer is the same - imagine a perfect monopole speaker (that is not affected at all by changes in geomerty around it - it creates a perfect spherical wavefront for all frequencies). In free space its efficiency is say 10% (closer to 1% in reality for efficient systems but whatever). That energy gets radiated everywhere, and total acoustic power is say W. Now place it next to the solid wall - energy still goes everywhere, but it gets reflected off the wall and in effect doubles (2*W). Hence power radiated in this room, on this side of the wall is in effect double that of what it was before.
Analogy with mirrors helps : imagine that wall as a mirror - instead of one speaker, you will see two.
Bratislav,
I appreciate your stating your view about the total power staying the same. That's why I raised the question of energy conservation in my earlier post. I suspected that John's conclusion of total power doubling going from 4Pi to 2Pi is incorrect. Also, there is no need for the power doubling to explain the 3dB power increase. if we assume the total power is the same but is now distributed into only 2Pi instead of 4Pi, then the power radiated into per solid angle is doubled, which explains the 3dB increase in power. In other words, it is the "power density" that is doubled.
What I am trying to understand is: where did John's derivation go wrong, and which assumption in that derivation is not valid?
Actually, this is kind of off the topic for Lynn's thread, so I'll stop probing. Let's go back to more popular topics, such as which drivers should be used.
Cheers,
Kurt
I appreciate your stating your view about the total power staying the same. That's why I raised the question of energy conservation in my earlier post. I suspected that John's conclusion of total power doubling going from 4Pi to 2Pi is incorrect. Also, there is no need for the power doubling to explain the 3dB power increase. if we assume the total power is the same but is now distributed into only 2Pi instead of 4Pi, then the power radiated into per solid angle is doubled, which explains the 3dB increase in power. In other words, it is the "power density" that is doubled.
What I am trying to understand is: where did John's derivation go wrong, and which assumption in that derivation is not valid?
Actually, this is kind of off the topic for Lynn's thread, so I'll stop probing. Let's go back to more popular topics, such as which drivers should be used.
Cheers,
Kurt
What I have presented is pretty much the standard stuff in acoustic texts and is correct. The only argument is whether the sound pressure actually behaves as indicated, doubling when the acoustic space is reduced by 1/2. That is reasonably correct for a reduction in space form 4Pi to 2Pi. As we know we see a 6dB increase in SPL above the baffle step. That the efficiency increases is also a direct consequence of the doubling of the pressure. It's a well know result.
The problem is that this relationship of doubling the pressure each time you cur the space in 1/2 doesn't apply at infinitum. As an example, consider the usual equation for efficiency of a driver. It states that efficiency is proportional to Sd^2, increasing by 6dB each time Sd is doubled. So if we start with a driver with 1% efficiency doubling Sd 7 times would yield an efficiency of 128%. Obviously this can't happen.
The problem is that the assumptions in these relationships break down. So when we start to get to extremes we need to go back and look at the problem differently.
The problem is that this relationship of doubling the pressure each time you cur the space in 1/2 doesn't apply at infinitum. As an example, consider the usual equation for efficiency of a driver. It states that efficiency is proportional to Sd^2, increasing by 6dB each time Sd is doubled. So if we start with a driver with 1% efficiency doubling Sd 7 times would yield an efficiency of 128%. Obviously this can't happen.
The problem is that the assumptions in these relationships break down. So when we start to get to extremes we need to go back and look at the problem differently.
Hi
Its like dancing the waltz, isn't it ?
JohnK thanks for the IM plot at post #2895
http://www.diyaudio.com/forums/showthread.php?postid=1378055#post1378055
Its also useful for tweeters – do you have a excel calculation sheet for this as well ?
When calculating for the Seas Excel Millenium for example, the X=1cm @ 50Hz would translate to X=0,5mm @ 1000Hz which is well within the "linear" range of that tweeter giving roughly 100dB in SPL with its 7cm2 of Sd.
Not an unusual value at normal listening levels.
At ~ 8KHz - a range of 3 octaves up – we would get an IM of around -40dB ( = 1%), correct?
Greetings
Michael
john k... said:
Now there's a topic that keep running in circles.
Its like dancing the waltz, isn't it ?
JohnK thanks for the IM plot at post #2895
http://www.diyaudio.com/forums/showthread.php?postid=1378055#post1378055
Its also useful for tweeters – do you have a excel calculation sheet for this as well ?
When calculating for the Seas Excel Millenium for example, the X=1cm @ 50Hz would translate to X=0,5mm @ 1000Hz which is well within the "linear" range of that tweeter giving roughly 100dB in SPL with its 7cm2 of Sd.
Not an unusual value at normal listening levels.
At ~ 8KHz - a range of 3 octaves up – we would get an IM of around -40dB ( = 1%), correct?
Greetings
Michael
mige0 said:Hi
Its like dancing the waltz, isn't it ?
JohnK thanks for the IM plot at post #2895
http://www.diyaudio.com/forums/showthread.php?postid=1378055#post1378055
When calculating for the Seas Excel Millenium for example, the X=1cm @ 50Hz would translate to X=0,5mm @ 1000Hz which is well within the "linear" range of that tweeter giving roughly 100dB in SPL with its 7cm2 of Sd.
Not an unusual value at normal listening levels.
At ~ 8KHz - a range of 3 octaves up – we would get an IM of around -40dB ( = 1%), correct?
Greetings
Michael
You are correct for the excursion, but the frequency ratio would be 8k/1k = 8. At a frequency ratio of 8 the IM would be about -28dB, or closer to 4%. Of course, this assumes pistonic behavior. Also it should be noted that Xmax is just a physical value; (voice coil length - gap height)/2. It has very little to do with whether the driver is linear or not at that excursion.
something similar ?
hi Lynn
your system might lead in this direction :
http://www.audioasylum.com/forums/hug/messages/12/129890.html
hi Lynn
your system might lead in this direction :
http://www.audioasylum.com/forums/hug/messages/12/129890.html
Yes, I noticed that that post too. What I'm most interested in is a LeCleac'h profile horn/waveguide for the two contemplated midranges: 1.4" compression driver, and 6" studio-monitor cone driver. Since I'm not using a basshorn with its intrinsically limited bandwidth, I have the luxury of a considerably smaller mid horn crossing over anywhere from 600~800 Hz.
I really, truly, dislike horn coloration, and cannot ignore it, no matter how hard I try. Karna dislikes it even more, if that was possible - she doesn't even consider any system with that type of coloration to be musical. I can listen around the coloration for a while, admiring the other virtues of the system, but it spoils the long-term listening pleasure.
The reason I'm still considering a horn/waveguide midrange is there are several studies that claim that horn/waveguides with modern profile designs that are axisymmetric and are also 1 foot or shorter in length have no audible horn coloration. So they say. AES studies claim a lot of things that don't agree with my perceptions at all, so this claim may be true, or it might not. Dunno.
The other reason I'm considering a horn/waveguide is my experience with multiple direct-radiator drivers is there is a price to be paid in coherency, particularly in the midrange. Multiple drivers seem to work fine in the bass, where wavelengths are large anyway, but they fall down badly in the midrange, with a phasey, diffuse sound that lacks impact. I spent a lot of time on the Ariel improving the coherency of the sound, and it's a hassle I don't want to do again, especially with a 3-way system, which have much more serious problems with overall coherency.
The AudioKinesis system I heard at the RMAF had very low horn coloration - I don't know if I'd say it was zero or not. The Emerald Physiks had more than I can accept and I found the HF reproduction harsh and raw-sounding, with lots of driver breakup. I've heard Avante-Garde Duos and Trios at many different shows and in several different homes and consider them to have very strong horn coloration (JBL class) and quite mediocre-sounding drivers as well - the cone breakup of the mid driver was quite obvious, and the system cried out for a decent pro midrange. The AG's would sound much better if the mid and HF drivers were better-quality - less breakup, smoother out-of-band response, all the usual desirable things.
In terms of low horn coloration, the AudioKinesis is the lowest I've heard so far - and it had a reasonably small waveguide and a very good compression driver. The superb 10" TAD Alnico midbass certainly didn't hurt, either. My suspicion is the higher the efficiency, the better the drivers have to be - the colorations that remain are more audible than murky-sounding low-efficiency drivers.
My sensitivity to horn coloration is much higher than most audiophiles from what I can tell, so I'm guessing the majority of the readers of this forum would be happy with a high-quality horn mid and horn HF unit.
I like what horns do well - effortless dynamics and vivid tone colors - so I'm looking for folks that could build a LeCleac'h profile waveguide for the 1.4" midrange compression driver and the prosound midrange cone driver. By keeping the dimensions compact, restricting the bandwidth of the mid horn, and choosing a wide-dispersion low-diffraction profile (and I can measure this, not guess), I'm hoping the coloration falls below subjective audibility. I'm keeping my fingers crossed on this one, and hedging my bets with a quartet of direct-radiator mids.
P.S. When I mentioned 120 dB SPL headroom many posts ago, I wasn't referring to the entire frequency band. 120 dB at 30~55 Hz requires a mammoth basshorn or a wall-sized infinite-baffle array, and that's way beyond my goal. "Conventional" soffit-mounted studio-monitor dynamics will be plenty good enough.
I really, truly, dislike horn coloration, and cannot ignore it, no matter how hard I try. Karna dislikes it even more, if that was possible - she doesn't even consider any system with that type of coloration to be musical. I can listen around the coloration for a while, admiring the other virtues of the system, but it spoils the long-term listening pleasure.
The reason I'm still considering a horn/waveguide midrange is there are several studies that claim that horn/waveguides with modern profile designs that are axisymmetric and are also 1 foot or shorter in length have no audible horn coloration. So they say. AES studies claim a lot of things that don't agree with my perceptions at all, so this claim may be true, or it might not. Dunno.
The other reason I'm considering a horn/waveguide is my experience with multiple direct-radiator drivers is there is a price to be paid in coherency, particularly in the midrange. Multiple drivers seem to work fine in the bass, where wavelengths are large anyway, but they fall down badly in the midrange, with a phasey, diffuse sound that lacks impact. I spent a lot of time on the Ariel improving the coherency of the sound, and it's a hassle I don't want to do again, especially with a 3-way system, which have much more serious problems with overall coherency.
The AudioKinesis system I heard at the RMAF had very low horn coloration - I don't know if I'd say it was zero or not. The Emerald Physiks had more than I can accept and I found the HF reproduction harsh and raw-sounding, with lots of driver breakup. I've heard Avante-Garde Duos and Trios at many different shows and in several different homes and consider them to have very strong horn coloration (JBL class) and quite mediocre-sounding drivers as well - the cone breakup of the mid driver was quite obvious, and the system cried out for a decent pro midrange. The AG's would sound much better if the mid and HF drivers were better-quality - less breakup, smoother out-of-band response, all the usual desirable things.
In terms of low horn coloration, the AudioKinesis is the lowest I've heard so far - and it had a reasonably small waveguide and a very good compression driver. The superb 10" TAD Alnico midbass certainly didn't hurt, either. My suspicion is the higher the efficiency, the better the drivers have to be - the colorations that remain are more audible than murky-sounding low-efficiency drivers.
My sensitivity to horn coloration is much higher than most audiophiles from what I can tell, so I'm guessing the majority of the readers of this forum would be happy with a high-quality horn mid and horn HF unit.
I like what horns do well - effortless dynamics and vivid tone colors - so I'm looking for folks that could build a LeCleac'h profile waveguide for the 1.4" midrange compression driver and the prosound midrange cone driver. By keeping the dimensions compact, restricting the bandwidth of the mid horn, and choosing a wide-dispersion low-diffraction profile (and I can measure this, not guess), I'm hoping the coloration falls below subjective audibility. I'm keeping my fingers crossed on this one, and hedging my bets with a quartet of direct-radiator mids.
P.S. When I mentioned 120 dB SPL headroom many posts ago, I wasn't referring to the entire frequency band. 120 dB at 30~55 Hz requires a mammoth basshorn or a wall-sized infinite-baffle array, and that's way beyond my goal. "Conventional" soffit-mounted studio-monitor dynamics will be plenty good enough.
Lynn have you contemplated the Azura horns?Lynn Olson said:
http://www.azurahorn.com/
Also wondered if you'd seen these.
http://audioheritage.org/vbulletin/showthread.php?t=12126
http://audioheritage.org/vbulletin/showthread.php?t=12967
Hi Lynn, John, All
“I really, truly, dislike horn coloration, and cannot ignore it, no matter how hard I try. Karna dislikes it even more, if that was possible - she doesn't even consider any system with that type of coloration to be musical. I can listen around the coloration for a while, admiring the other virtues of the system, but it spoils the long-term listening pleasure.”
Lynn I understand, some of the most hideous sounding speakers I ever heard were horn loaded.
On the other hand “horn coloration” is not something, which necessarily always goes with horns.
I would urge you to look into Bill Woods conical shape horns, conical horns can be essentially a spherical section in their radiation if driven by a point source (an origin too small to have its own directivity).
Also, not all drivers are equal obviously and if you look at most phase plug designs, one sees that many produce a converging wave at the summation point, while to drive a horn, it should be a diverging wave front.
Drivers like the BMS 4550 (for one) and some 18Sound drivers have a structure which produces a compatible diverging wave front at the driver exit.
Earl’s HOM’s are produced when the dimensions are such that there is directivity, while the sound is also required to change direction.
Loading a conical horn, even a huge one (who’s angle is constant) with a proper wavefront or small source, doesn’t have these issues.
On the other hand, larger exponential, Tractrix and other curved wall horns (except for Earl’s which has the slowest change possible) automatically produce some of these effects as the radiation angle narrows with frequency, no matter what driver is used.
Since your obviously in the mood to get something going (and hopefully your leg is better etc) I would suggest an experiment.
Get a bms 4550 driver (it’s a good one) and make a conical horn, no more than say 60 degrees angle, about 12 to 18 inches across and if your liking the sawdust, make the outer 1/3 of the horn, radius into an angle about 1.4 times greater than the main wall angle.
This nulls the narrowing in the pattern that normally happens as one approaches the pattern loss frequency (at the low end of the response)
Unlike most direct radiators, this will not produce flat response yet.
Measure the horn and derive and apply the eq needed to flatten it out.
Do this electronically first as making a passive electrical filter that does this AND accommodates the drivers impedance curve is more difficult.
EQ here is ok as what your fixing is then fixed in magnitude and phase.
This driver will be happy with a high pass filter above say 500Hz in the home, use at least a 2nd order filter.
Mate this to a lower range system and give it a listen.
Handling the upper half of the range, flattened out, your ear should not recognize it as a horn.
Kchang (is that amazing little full range driver demo I heard once Kurt?), John, there is an upper limit to mutual coupling which seems to reach an asymptotic point around 25-30% efficiency, while a large speaker array may appear to be higher than this due to forward directivity.
I wish I had some experience with these open baffle bass speakers but to be honest I haven’t ever played with them and for some time I have been focusing on horns and horn drivers.
I would guess the ideal driver for that is pretty different than those for sealed or vented boxes though. The difference would be that less motor strength is needed, as one would want a Qt that is greater than one I would think.
If you can model the “ideal driver”, which I assume John and others can, you might be able to interest Eminence in making a purpose built driver for it. Jerry built up a driver for a DIY project I did some years back so it isn’t unheard of anyway.
Best,
Tom Danley
“I really, truly, dislike horn coloration, and cannot ignore it, no matter how hard I try. Karna dislikes it even more, if that was possible - she doesn't even consider any system with that type of coloration to be musical. I can listen around the coloration for a while, admiring the other virtues of the system, but it spoils the long-term listening pleasure.”
Lynn I understand, some of the most hideous sounding speakers I ever heard were horn loaded.
On the other hand “horn coloration” is not something, which necessarily always goes with horns.
I would urge you to look into Bill Woods conical shape horns, conical horns can be essentially a spherical section in their radiation if driven by a point source (an origin too small to have its own directivity).
Also, not all drivers are equal obviously and if you look at most phase plug designs, one sees that many produce a converging wave at the summation point, while to drive a horn, it should be a diverging wave front.
Drivers like the BMS 4550 (for one) and some 18Sound drivers have a structure which produces a compatible diverging wave front at the driver exit.
Earl’s HOM’s are produced when the dimensions are such that there is directivity, while the sound is also required to change direction.
Loading a conical horn, even a huge one (who’s angle is constant) with a proper wavefront or small source, doesn’t have these issues.
On the other hand, larger exponential, Tractrix and other curved wall horns (except for Earl’s which has the slowest change possible) automatically produce some of these effects as the radiation angle narrows with frequency, no matter what driver is used.
Since your obviously in the mood to get something going (and hopefully your leg is better etc) I would suggest an experiment.
Get a bms 4550 driver (it’s a good one) and make a conical horn, no more than say 60 degrees angle, about 12 to 18 inches across and if your liking the sawdust, make the outer 1/3 of the horn, radius into an angle about 1.4 times greater than the main wall angle.
This nulls the narrowing in the pattern that normally happens as one approaches the pattern loss frequency (at the low end of the response)
Unlike most direct radiators, this will not produce flat response yet.
Measure the horn and derive and apply the eq needed to flatten it out.
Do this electronically first as making a passive electrical filter that does this AND accommodates the drivers impedance curve is more difficult.
EQ here is ok as what your fixing is then fixed in magnitude and phase.
This driver will be happy with a high pass filter above say 500Hz in the home, use at least a 2nd order filter.
Mate this to a lower range system and give it a listen.
Handling the upper half of the range, flattened out, your ear should not recognize it as a horn.
Kchang (is that amazing little full range driver demo I heard once Kurt?), John, there is an upper limit to mutual coupling which seems to reach an asymptotic point around 25-30% efficiency, while a large speaker array may appear to be higher than this due to forward directivity.
I wish I had some experience with these open baffle bass speakers but to be honest I haven’t ever played with them and for some time I have been focusing on horns and horn drivers.
I would guess the ideal driver for that is pretty different than those for sealed or vented boxes though. The difference would be that less motor strength is needed, as one would want a Qt that is greater than one I would think.
If you can model the “ideal driver”, which I assume John and others can, you might be able to interest Eminence in making a purpose built driver for it. Jerry built up a driver for a DIY project I did some years back so it isn’t unheard of anyway.
Best,
Tom Danley
Member
Joined 2003