Yeah, let's keep things in perspective. 1 watt into a horn tweeter is going to give ear-bleeding SPL in a home setting. If nothing else, padding it down will reduce any audible amp hiss.
well your missing the issue here. For the horn tweeter to only see 1 watt, and have the midbass output the same level, the padding device, i.e. the voltage divider, needs to "throw away" a bunch of power. I don't feel like doing calculations again to figure out what this would be, but let's say for the sake of argument that 80+% of the power is often being wasted as heat in the voltage divider. Remember in my above example that the tweeter was seeing 12 watts when the midbass saw 200. This particular calculation was based on my estimates and calculations of what my current main speaker's, the Gedlee Abbey's, would need to achieve 115db's or so.
Pj, I don't think I missed anything. I know how voltage dividers and varying driver sensitivities work and I was trying to agree with you. 🙂
Where is this coming from?
A supposition that we start with a 114 dB compression driver and resistively pad it down to match up with a 100 dB woofer? Or 104 dB to 90 dB?
That ain't what happens....
A supposition that we start with a 114 dB compression driver and resistively pad it down to match up with a 100 dB woofer? Or 104 dB to 90 dB?
That ain't what happens....
noah katz said:
Fine example, either way.
My point is that with a CD waveguide, we get there with HF compensation, not purely resistive loading, and 10 - 15 dB is common.
With a horn that does not require comp like an exponential, for example, straight attenuation is required to achieve balance with a woofer, but where the sensitivity differential was high, vintage designs used autotransformers to accomplish it. Nowadays, we'd biamp to "beat the heat."
"with a CD waveguide, we get there with HF compensation, not purely resistive loading, "
good point
good point
Crossover components - power dissipation requirements
I wrote a little study of crossovers used in speakers like these about 10 years ago. I used Spice to model the circuits and illustrate some concepts. The last dozen pages or so show the voltage across each component in the circuit, in order to know what size capacitors, coils and resistors need to be used in each location.
The tweeter pading resistors need to be rated appropriately, and don't forget the woofer Zobel resistor
See pages 62 - 74 of the Speaker Crossover document on the Pi Speakers website.
I wrote a little study of crossovers used in speakers like these about 10 years ago. I used Spice to model the circuits and illustrate some concepts. The last dozen pages or so show the voltage across each component in the circuit, in order to know what size capacitors, coils and resistors need to be used in each location.
The tweeter pading resistors need to be rated appropriately, and don't forget the woofer Zobel resistor
See pages 62 - 74 of the Speaker Crossover document on the Pi Speakers website.
Wayne:
Several members have observed that with asymmetric waveguides of modest dimensions, vertical pattern control occurs at a higher frequency than horizontal, and thus, directivity is not well matched with the woofer in that transitional region if the horizontal has priority as the design control.
Given that the crossover also occurs in this region, it seems apparent that the vertical nulls, given appropriate attention, might be appropriately aligned so as to enhance vertical pattern control, and thus normalize the transition. You have previously used the word "punctuate," but I'm suggesting that "augment" might be applicable under this circumstance....
Several members have observed that with asymmetric waveguides of modest dimensions, vertical pattern control occurs at a higher frequency than horizontal, and thus, directivity is not well matched with the woofer in that transitional region if the horizontal has priority as the design control.
Given that the crossover also occurs in this region, it seems apparent that the vertical nulls, given appropriate attention, might be appropriately aligned so as to enhance vertical pattern control, and thus normalize the transition. You have previously used the word "punctuate," but I'm suggesting that "augment" might be applicable under this circumstance....
Matching directivity in the vertical and the horizontal planes
Yes, it is well understood that the mouth dimensions set the lower limit of pattern control. So a horn that is wider than tall will lose pattern control in the vertical before it will in the horizontal. Some people call this pattern flip, but I prefer to describe what is actually happening because the "flip" only occurs at low frequency. At high frequency, the pattern approximately matches the wall angle.
The issue really is that the mouth exit starts to act like a diffraction slot at low frequencies. A tweeter horn isn't used at low frequencies though, so one can easily design a horn that doesn't lose pattern control in the passband. The thing that complicates the matter is the typical asymmetrical horn that's wider than tall (without a diffraction device) will lose pattern control in the vertical before it does in the horizontal, and if you make it tall enough to have pattern control in the vertical, you lose the benefit of close center-to-center spacing.
To me, it's like a lot of other things in loudspeaker design, you have competing priorities. Do we really want to use a horn large enough to maintains vertical pattern control if that means vertical spacing has to be large enough that vertical nulls cut into the pattern? That makes no sense, it defeats the purpose. Or do we want to sacrifice pattern control entirely, in favor of tight CTC spacing to widen the nulls, something like a Smith horn? That doesn't seem like a good idea to me either, it's really a diffraction slit and that isn't good.
In my way of thinking, the best solution is one that balances each of these priorities. It is to use a horn that provides constant directivity in the horizontal, matching the adjacent subsystem in the crossover region. It has vertical directivity that becomes controlled as low as possible but also has mouth dimensions that make null spacing outside the vertical pattern at HF. The nulls sort of punctuate the pattern on the low end. That's the design approach I've used, and I think it's what makes most sense.
Yes, it is well understood that the mouth dimensions set the lower limit of pattern control. So a horn that is wider than tall will lose pattern control in the vertical before it will in the horizontal. Some people call this pattern flip, but I prefer to describe what is actually happening because the "flip" only occurs at low frequency. At high frequency, the pattern approximately matches the wall angle.
The issue really is that the mouth exit starts to act like a diffraction slot at low frequencies. A tweeter horn isn't used at low frequencies though, so one can easily design a horn that doesn't lose pattern control in the passband. The thing that complicates the matter is the typical asymmetrical horn that's wider than tall (without a diffraction device) will lose pattern control in the vertical before it does in the horizontal, and if you make it tall enough to have pattern control in the vertical, you lose the benefit of close center-to-center spacing.
To me, it's like a lot of other things in loudspeaker design, you have competing priorities. Do we really want to use a horn large enough to maintains vertical pattern control if that means vertical spacing has to be large enough that vertical nulls cut into the pattern? That makes no sense, it defeats the purpose. Or do we want to sacrifice pattern control entirely, in favor of tight CTC spacing to widen the nulls, something like a Smith horn? That doesn't seem like a good idea to me either, it's really a diffraction slit and that isn't good.
In my way of thinking, the best solution is one that balances each of these priorities. It is to use a horn that provides constant directivity in the horizontal, matching the adjacent subsystem in the crossover region. It has vertical directivity that becomes controlled as low as possible but also has mouth dimensions that make null spacing outside the vertical pattern at HF. The nulls sort of punctuate the pattern on the low end. That's the design approach I've used, and I think it's what makes most sense.
Hi all, Wayne
It is fortunate that in the home, one generally sits close to directly on axis, such things as Pattern flip and lobbing directivity are much less of a problem compared to larger spaces where the “audience” occupies a significant angle of coverage and the absorption is so much less / reverberant field so much larger.
In commercial sound, a practical limit I have found for a conventional shape horn would be about 1.6 to 1 angle ratio. Going beyond this makes the pattern flip pretty pronounced.
The greater the asymmetry, the larger the span of pattern flip is.
For example, if one had a simple 90 X 45 horn that was 15 inches across, one finds it starts to loses pattern at about 750Hz in the horizontal plane.
In the Vertical plane, the control is lost at about 3KHz as a result of being half the height and half the angle.
It would be difficult /impossible to cross into that horn at or above the onset of pattern flip and / or without having lobes at crossover as well, the acoustic separation is impossibly large. The best compromise would be to have these lobes outside the listening area at the expense of selectively driving room reverberant level at those frequencies, which would be undesirable in larger rooms.
If one made the horn even more asymmetric, the separation of the two pattern loss frequency increases further, for instance if one has a 30 by 90 horn, the separation is nearly a decade higher than the horizontal plane.
A horn like the Smith horn Wayne mentioned would be like the T-35, best used mounted vertically because the pattern flip is so severe.
For the speakers at work, made for the best possible sound quality in large spaces, the goal is to have constant directivity with no lobes or perturbations down to the frequency the mouth size and wall angle governs.
They avoid lobes and interference, by placing the sources acoustically close enough together at crossover, that they combine coherently into one acoustic source and by using filters which accommodate the physical depth spacing, eliminate the normal crossover phase shift, the result being a large number of drivers can measure /act as “one” driver and horn that covers a wide bandwidth.
For the home builder, I would urge you not to go past or much past the 1.6 to 1 ratio and for a given mouth width, the horns usable frequency (directivity speaking) goes down as you make the vertical angle closer to the horizontal angle..
Also, stand where your speaker is going to be with a protractor. The goal if a stereo image a concern is partly to have the largest direct field possible.
Look into your room and by eyeball, ask your self what angles covers the entire listening area or more BUT minimizes the energy projected at the walls and ceiling / floor?.
Does it make sense to make the horn mouth the same size but use a narrower symmetric angle (no pattern flip, lowest possible directivity loss frequency for a given angle and size) instead of wide and asymmetric?
The sort of “flip side” of this approach is that symmetry can bite you, if one concentrates all of the “issues” precisely and exactly about one axis, they often add somewhere out front in an undesirable way.
For that reason, perfectly round horns often have unexpected baggage as well.
Best,
Tom Danley
It is fortunate that in the home, one generally sits close to directly on axis, such things as Pattern flip and lobbing directivity are much less of a problem compared to larger spaces where the “audience” occupies a significant angle of coverage and the absorption is so much less / reverberant field so much larger.
In commercial sound, a practical limit I have found for a conventional shape horn would be about 1.6 to 1 angle ratio. Going beyond this makes the pattern flip pretty pronounced.
The greater the asymmetry, the larger the span of pattern flip is.
For example, if one had a simple 90 X 45 horn that was 15 inches across, one finds it starts to loses pattern at about 750Hz in the horizontal plane.
In the Vertical plane, the control is lost at about 3KHz as a result of being half the height and half the angle.
It would be difficult /impossible to cross into that horn at or above the onset of pattern flip and / or without having lobes at crossover as well, the acoustic separation is impossibly large. The best compromise would be to have these lobes outside the listening area at the expense of selectively driving room reverberant level at those frequencies, which would be undesirable in larger rooms.
If one made the horn even more asymmetric, the separation of the two pattern loss frequency increases further, for instance if one has a 30 by 90 horn, the separation is nearly a decade higher than the horizontal plane.
A horn like the Smith horn Wayne mentioned would be like the T-35, best used mounted vertically because the pattern flip is so severe.
For the speakers at work, made for the best possible sound quality in large spaces, the goal is to have constant directivity with no lobes or perturbations down to the frequency the mouth size and wall angle governs.
They avoid lobes and interference, by placing the sources acoustically close enough together at crossover, that they combine coherently into one acoustic source and by using filters which accommodate the physical depth spacing, eliminate the normal crossover phase shift, the result being a large number of drivers can measure /act as “one” driver and horn that covers a wide bandwidth.
For the home builder, I would urge you not to go past or much past the 1.6 to 1 ratio and for a given mouth width, the horns usable frequency (directivity speaking) goes down as you make the vertical angle closer to the horizontal angle..
Also, stand where your speaker is going to be with a protractor. The goal if a stereo image a concern is partly to have the largest direct field possible.
Look into your room and by eyeball, ask your self what angles covers the entire listening area or more BUT minimizes the energy projected at the walls and ceiling / floor?.
Does it make sense to make the horn mouth the same size but use a narrower symmetric angle (no pattern flip, lowest possible directivity loss frequency for a given angle and size) instead of wide and asymmetric?
The sort of “flip side” of this approach is that symmetry can bite you, if one concentrates all of the “issues” precisely and exactly about one axis, they often add somewhere out front in an undesirable way.
For that reason, perfectly round horns often have unexpected baggage as well.
Best,
Tom Danley
Tom Danley said:
One thing I have found, and I believe many others agree on, is that a 90 degree horizontal pattern is useful for home hifi. When speakers with wide CD coverage are situated so their forward axis are crossed in front of the listener, not only is room coverage uniform but the Haas effect increases the listening area where stereo imaging is good. The thing is, for this to work, you really need fairly wide horizontal coverage and it has to be spectrally balanced, i.e. CD.
Now that may not be your bag, but for those of us that want 90 degree horizontal coverage, the next question remains, how much vertical coverage do we want. Do we want to use a symmetrical horn, or do we want to use an asymmetrical horn with narrower vertical coverage?
I have long proposed that narrower vertical coverage reduces ceiling slap at HF and it also allows tighter CTC spacing, providing a taller forward lobe with greater separation between nulls.
Tom Danley said:
Wayne Parham said:
As Tom points out, making the vertical pattern too narrow may actually have the opposite effect and give more ceiling splash, not less, in the region where the ear is most sensitive to such things.
And you can't talk about C-C spacing without talking about XO frequency. Tom points out quite correctly that making the vertical dimension of the horn bigger lets you cross lower so you aren't necessarily making the null separation smaller.
Well, sure, you can do the design wrong, making something that doesn't match directivity in the horizontal planes by having too high a crossover point. I don't think any of us would suggest that.
Crossover optimization for DI-matched two-way speakers
This is how I find a crossover that satisfies all the conditions required to simultaneously match the woofer and tweeter sensitivities, provide top-octave equalization for the compression driver's mass rolloff, match the woofer and tweeter horizontal patterns and also give a nice tall forward lobe with nulls set wide above and below the speaker. That's a lot to do at the same time, but they're all important.
I used to meticulously calculate phase angles and baffle positions to set the position of the forward lobe and vertical nulls. In fact, the first few drafts of the Speaker Crossover document mentioned above in an earlier post, used to have several pages on the subject of finding the acoustic positions and electrical phase to set the position of the forward lobe. I described it as the "window" of constructive summing, with the nulls being set outside, formed by destructive interference from path length differences. I went through a process of calculating electrical phase angles and combining them with estimated acoustic phase and position to determine where the nulls and (major and minor) lobes were. But I later removed those pages because it was too wordy and complicated, and seemed better discussed in another venue. That's what we're doing here now.
You can use this process whether you're building a speaker with an axisymmetrical horn/waveguide or an asymmetrical horn/waveguide. The difference between them is manifested solely in the vertical (size of the forward lobe) but the process for crossover optimization is the same.
There are a handful of loudspeaker manufacturers and hobbyists groups that make speakers with this general design philosophy. Most of them fall into one of two groups. There are those that use round or square horns with 90° axisymmetrical CD coverage. Then there are those that use rectangular, oval or elliptical horns with 90° horizontal and approximately 40° vertical coverage. Let's look at each of them, to see what they get from the bargain.
The 90° axisymmetrical (round or square) horn is nice at first glance because it can be made very pure. Polars look great. If this kind of horn were used in isolation, alone in freespace, it would be a no-brainer. It would be the hands-down winner.
Problem is that's not the case. For one thing, it is combined on a baffle with another driver or drivers. The vertical spacing causes a forward lobe to form that is relatively pure, provided the crossover is appropriate for the drivers and their depth from the baffle (respective distances to the listener). Above and below this forward lobe, nulls form which are basically cancellation notches. The response at these angles is terrible, with a huge dip in the crossover region. Outside the null angle, response starts a roller coaster ride of peaks and valleys, depending on the angle and frequency. So basically, there is nothing useful from sound energy outside the null angle.
Another thing we don't want is wide vertical spacing between drivers. The wider the spacing, the closer the vertical nulls. If the vertical spacing between drivers is too wide, then the forward lobe is squished to a thin layer, basically straight in front of the speakers. Too narrow a null angle makes the speakers phasey and unlistenable, because any movement in your chair puts you in a null.
That's a problem with round horns. They're just too tall and they make the vertical distance between sound sources too great. Closer center-to-center spacing is made possible with rectangular, oval or elliptical horns.
Another problem with 90° round horns is even if you could design a speaker with them that didn't have nulls (you can't), you still would not want the reflections from the floor and ceiling. Most houses have eight foot ceilings, and HF reflections from this surface are unnatural sounding. Probably the most objectionable reflection is ceiling slap. So the last thing you want is tall vertical coverage from a tweeter horn.
Those problems make round horns unworkable to me. Like I said, if they were used in isolation, outdoors, they'd be perfect. No other horn shape would compare. But when combined with other sound sources, the design becomes nonviable, in my opinion because of the narrow angle of the vertical nulls and the level of HF output at large vertical angles.
That brings me to certain kinds of horns and waveguides with asymmetrical patterns having approximately 90°x40° coverage. These have side walls that are shaped exactly the same as the round horns I was just talking about, but the vertical dimension is shorter. The side walls set the horizontal pattern to be constant 90°. The vertical wall angle is made narrower so that HF pattern is not so tall. This also allows closer center-to center spacing with other drivers, because the horn/waveguide is not as tall. The throat transition is smoothly made, matching the driver exit angle to the asymptote flare angle. In that respect, and in horizontal coverage, the asymmetrical horn/waveguide is the same as the axisymmetrical horn/waveguide. It is only the vertical pattern that is different.
The good news about the shorter vertical mouth dimension is it allows closer center-to-center spacing. Its wall angle sets the pattern at the upper range of the passband, making coverage not very tall and limiting ceiling reflections. That's good too. But the bad news is the shorter vertical mouth prevents it from controlling the pattern at the lower end of the passband. The frequency where pattern control is lost is set by the specific features of the horn or waveguide, but most of the ones I've used and/or investigated start to lose pattern control about 3kHz. Some lose it fairly rapidly, others more slowly. But all have collapsing directivity as frequency rises, because that's what it means to lose pattern control at low frequency. The pattern widens down low, which means it collapses up high.
Vertical directivity collapses through the crossover region up to 3kHz or so, because its size prevents it from maintaining control any lower. In the crossover region between about 1kHz and 2kHz, the vertical nulls actually sort of punctuate the pattern, doing more to notch out the edges of the pattern than the horn, itself. No horn of these dimensions can do much below 3kHz in terms of vertical pattern control because the mouth height is too small. But vertical nulls set the pattern below 3kHz more than the horn/waveguide does anyway. Above 3kHz, the horn/waveguide begins to become more controlled, setting the pattern usually around 40° or 50°, which is plenty of vertical coverage. That's just what we want for home hifi, with our eight foot ceilings.
This is how I find a crossover that satisfies all the conditions required to simultaneously match the woofer and tweeter sensitivities, provide top-octave equalization for the compression driver's mass rolloff, match the woofer and tweeter horizontal patterns and also give a nice tall forward lobe with nulls set wide above and below the speaker. That's a lot to do at the same time, but they're all important.
I used to meticulously calculate phase angles and baffle positions to set the position of the forward lobe and vertical nulls. In fact, the first few drafts of the Speaker Crossover document mentioned above in an earlier post, used to have several pages on the subject of finding the acoustic positions and electrical phase to set the position of the forward lobe. I described it as the "window" of constructive summing, with the nulls being set outside, formed by destructive interference from path length differences. I went through a process of calculating electrical phase angles and combining them with estimated acoustic phase and position to determine where the nulls and (major and minor) lobes were. But I later removed those pages because it was too wordy and complicated, and seemed better discussed in another venue. That's what we're doing here now.
You can use this process whether you're building a speaker with an axisymmetrical horn/waveguide or an asymmetrical horn/waveguide. The difference between them is manifested solely in the vertical (size of the forward lobe) but the process for crossover optimization is the same.
There are a handful of loudspeaker manufacturers and hobbyists groups that make speakers with this general design philosophy. Most of them fall into one of two groups. There are those that use round or square horns with 90° axisymmetrical CD coverage. Then there are those that use rectangular, oval or elliptical horns with 90° horizontal and approximately 40° vertical coverage. Let's look at each of them, to see what they get from the bargain.
The 90° axisymmetrical (round or square) horn is nice at first glance because it can be made very pure. Polars look great. If this kind of horn were used in isolation, alone in freespace, it would be a no-brainer. It would be the hands-down winner.
Problem is that's not the case. For one thing, it is combined on a baffle with another driver or drivers. The vertical spacing causes a forward lobe to form that is relatively pure, provided the crossover is appropriate for the drivers and their depth from the baffle (respective distances to the listener). Above and below this forward lobe, nulls form which are basically cancellation notches. The response at these angles is terrible, with a huge dip in the crossover region. Outside the null angle, response starts a roller coaster ride of peaks and valleys, depending on the angle and frequency. So basically, there is nothing useful from sound energy outside the null angle.
Another thing we don't want is wide vertical spacing between drivers. The wider the spacing, the closer the vertical nulls. If the vertical spacing between drivers is too wide, then the forward lobe is squished to a thin layer, basically straight in front of the speakers. Too narrow a null angle makes the speakers phasey and unlistenable, because any movement in your chair puts you in a null.
That's a problem with round horns. They're just too tall and they make the vertical distance between sound sources too great. Closer center-to-center spacing is made possible with rectangular, oval or elliptical horns.
Another problem with 90° round horns is even if you could design a speaker with them that didn't have nulls (you can't), you still would not want the reflections from the floor and ceiling. Most houses have eight foot ceilings, and HF reflections from this surface are unnatural sounding. Probably the most objectionable reflection is ceiling slap. So the last thing you want is tall vertical coverage from a tweeter horn.
Those problems make round horns unworkable to me. Like I said, if they were used in isolation, outdoors, they'd be perfect. No other horn shape would compare. But when combined with other sound sources, the design becomes nonviable, in my opinion because of the narrow angle of the vertical nulls and the level of HF output at large vertical angles.
That brings me to certain kinds of horns and waveguides with asymmetrical patterns having approximately 90°x40° coverage. These have side walls that are shaped exactly the same as the round horns I was just talking about, but the vertical dimension is shorter. The side walls set the horizontal pattern to be constant 90°. The vertical wall angle is made narrower so that HF pattern is not so tall. This also allows closer center-to center spacing with other drivers, because the horn/waveguide is not as tall. The throat transition is smoothly made, matching the driver exit angle to the asymptote flare angle. In that respect, and in horizontal coverage, the asymmetrical horn/waveguide is the same as the axisymmetrical horn/waveguide. It is only the vertical pattern that is different.
The good news about the shorter vertical mouth dimension is it allows closer center-to-center spacing. Its wall angle sets the pattern at the upper range of the passband, making coverage not very tall and limiting ceiling reflections. That's good too. But the bad news is the shorter vertical mouth prevents it from controlling the pattern at the lower end of the passband. The frequency where pattern control is lost is set by the specific features of the horn or waveguide, but most of the ones I've used and/or investigated start to lose pattern control about 3kHz. Some lose it fairly rapidly, others more slowly. But all have collapsing directivity as frequency rises, because that's what it means to lose pattern control at low frequency. The pattern widens down low, which means it collapses up high.
Vertical directivity collapses through the crossover region up to 3kHz or so, because its size prevents it from maintaining control any lower. In the crossover region between about 1kHz and 2kHz, the vertical nulls actually sort of punctuate the pattern, doing more to notch out the edges of the pattern than the horn, itself. No horn of these dimensions can do much below 3kHz in terms of vertical pattern control because the mouth height is too small. But vertical nulls set the pattern below 3kHz more than the horn/waveguide does anyway. Above 3kHz, the horn/waveguide begins to become more controlled, setting the pattern usually around 40° or 50°, which is plenty of vertical coverage. That's just what we want for home hifi, with our eight foot ceilings.
catapult said:
I believe that's part of Wayne's point: It's not possible to make the forward lobe wide enough for the nulls to fall outside the directivity window of an axisymmetric waveguide, unless it's a very narrow one.
catapult said:
Help me, now, at what frequency must we cross the Gedlee Abbey, for example, in order for the vertical nulls to coincide with, or occur outside of, the design vertical window?
Hi guys
Catapult is correct but I guess what I was saying could be clarified .
Also Wayne you said a couple things which I don’t understand, particularly this.
“But all have collapsing directivity as frequency rises, because that's what it means to lose pattern control at low frequency. The pattern widens down low, which means it collapses up high.”
For a constant directivity horn, this is not true at all, to the degree it is “CD”, it’s pattern does not change above pattern loss frequency. A "perfect" CD horn has the same dispersion angle at 20KHz as it has anywhere else above PLF.
This may not matter as much in the home where you sit on or nearly on axis but is very important when the listeners occupy a significant angle of the speaker’s coverage, you want everyone to have the same frequency response.
In the example I used of a 45 by 90 degree horn, I understand your concern about the CC spacing BUT….
Lets say you kept the exact same mouth height and preserved the same CC distance.
If you used a 67 degree horn angle (half way between 45 and 90) in both the vertical and horizontal planes, you would have eliminated pattern flip entirely, you would have lowered the onset of pattern loss an entire octave over the 90X45 horn, both are desirable things.
Also, if you look at room geometry, one normally has 90 degree corners so 90 degree source placed in the corner at 45 degrees, still illuminates a vast area compared to where the people sit. Any toe in angle other than 45 means it is illuminating the walls well in front of the audience (bad juju).
On the other hand, with a narrower horizontal pattern, one has a range of toe in angles which do not spray the walls in front and if the pattern is narrow enough, the speakers can be toed in further so that the center of the beam is aimed at the farthest seat kitty corner. This makes the amplitude related “sweet spot” larger than normal too.
When in doubt, make a drawing and use a protractor to examine what is actually needed in both Vertical and Horizontal planes given the speaker height and location.
What is the largest horn angle which could be used in both planes which causes the wall and floor/ceiling bounces to happen ideally at or behind the depth of the listener position.?
I say largest angle as for a given size, this puts the pattern loss F as low as possible.
If not identical angles how close can you make them?
Please don’t get the idea this stuff is simple, it is a study of tradeoffs and less than obvious relationships.
Catapult is correct but I guess what I was saying could be clarified .
Also Wayne you said a couple things which I don’t understand, particularly this.
“But all have collapsing directivity as frequency rises, because that's what it means to lose pattern control at low frequency. The pattern widens down low, which means it collapses up high.”
For a constant directivity horn, this is not true at all, to the degree it is “CD”, it’s pattern does not change above pattern loss frequency. A "perfect" CD horn has the same dispersion angle at 20KHz as it has anywhere else above PLF.
This may not matter as much in the home where you sit on or nearly on axis but is very important when the listeners occupy a significant angle of the speaker’s coverage, you want everyone to have the same frequency response.
In the example I used of a 45 by 90 degree horn, I understand your concern about the CC spacing BUT….
Lets say you kept the exact same mouth height and preserved the same CC distance.
If you used a 67 degree horn angle (half way between 45 and 90) in both the vertical and horizontal planes, you would have eliminated pattern flip entirely, you would have lowered the onset of pattern loss an entire octave over the 90X45 horn, both are desirable things.
Also, if you look at room geometry, one normally has 90 degree corners so 90 degree source placed in the corner at 45 degrees, still illuminates a vast area compared to where the people sit. Any toe in angle other than 45 means it is illuminating the walls well in front of the audience (bad juju).
On the other hand, with a narrower horizontal pattern, one has a range of toe in angles which do not spray the walls in front and if the pattern is narrow enough, the speakers can be toed in further so that the center of the beam is aimed at the farthest seat kitty corner. This makes the amplitude related “sweet spot” larger than normal too.
When in doubt, make a drawing and use a protractor to examine what is actually needed in both Vertical and Horizontal planes given the speaker height and location.
What is the largest horn angle which could be used in both planes which causes the wall and floor/ceiling bounces to happen ideally at or behind the depth of the listener position.?
I say largest angle as for a given size, this puts the pattern loss F as low as possible.
If not identical angles how close can you make them?
Please don’t get the idea this stuff is simple, it is a study of tradeoffs and less than obvious relationships.
Tom Danley said:
I'm not sure I agree. Catapult suggested lowering the crossover frequency in order to increase the angle of the vertical nulls. The trouble is the directivity of the direct radiating midwoofer is approximately 90 degrees where its diameter is one wavelength across. That sets the crossover frequency and if you go too high or too low, then horizontal directivity is not matched.
Tom Danley said:
What I am saying is that when a horn loses pattern control, the directivity widens. There is some ripple in response and directivity causing the brief narrowing blip just prior to widening as frequency goes down. But in general, the pattern widens as frequency goes down.
The converse of this is as frequency goes up, directivity collapses. It is wide down low, becomes more narrow as the horn gains control.
This is true for any horn, including CD. The point you are making is a CD horn used within the passband has constant directivity. That's true. And below that, its directivity increases.
I don't think any of us disagrees with that.
The whole reason I mentioned it is that an asymmetrical horn loses pattern control in its narrow dimension before it does in the wide dimension. A 90x40 horn will probably lose directivity control in the vertical at the low end of the passband. Then again, the nulls tend to cut into that widened edge. In my opinion, it's better to have the decreased CTC spaing than the pattern control. I believe I said this, but maybe you didn't read that part.
Tom Danley said:
Yes, and we're talking about constant directivity here too, about spectral balance over a wide listening area. All of the speakers we're discussing in this thread are designed specifically to provide uniform coverage through a wide horizontal angle. We're looking for seat-to-seat uniformity and good imaging. Specifically, the idea is to provide stereo balance in a reasonably wide area by crossing the forward axis slightly in front of the listeners. That way, movement left to right puts you closer to one speaker (but further off-axis from it) and further from the other speaker (but nearer to being on-axis with it). This provides a self-balancing effect and improves imaging over a wider seat-to-seat area. It requires CD on the horizontal axis and relatively wide coverage. I prefer 90 degrees for this setup, as do many others.
Tom Danley said:
Agreed, and many of the horns we're talking about are 90x50 or so. My wood horn is 90x60 at the crossover point and about 90x40 in the top octave. But there are also several horns advertised as having 90x40 coverage, and I've found some that work well in a loudspeaker like this. As I keep saying, the nulls limit the low end more than the horn pattern does anyway.
It's a balance of competing priorities we're talking about here. To increase the pattern width, you have to increase mouth size. That in turn reduces the null angles. Where is the magic number for vertical coverage? I don't think there's a best single figure, but I do think its in the 40-60 degree range.
You can work with this 40-60 degree range and probably find a good solution. I wouldn't go any narrower than that because the horn won't gain pattern control until too high a frequency and you don't need (or want) a narrower pattern anyway. I wouldn't go much taller than that, because the null angles get too narrow and cut into the pattern. It doesn't matter how well the horn controls the vertical pattern if the nulls cut it in half.
Tom Danley said:
Yes, that's the way my cornerhorns are and have been since the 1970's. I think it's the best setup, provided the room has the right corners.
Tom Danley said:
Yes, but when toed in in front of the listenering area, if you have too narrow horizontal coverage, you lose width of the target zone. If you toe-in a 90 degree speaker by 45 degrees, you have the best coverage for home hifi, in my opinion. The setup is described here:
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