I don't think we needs the plans. It was an exponential horn. Dimensions given, from well to poorly known, are:
- driver = Eminence Kappa 15
- mouth size = 8' by 8' (note: would neatly fit into a shipping container)
- length about 17'
- throat size (based on the info I quote here - but I left off the vent)
"I used ~1:1 compression ratio, including the vent area as part of the driver area. I suspect it'd be a problem if the throat is more choked down. Anyway, we went prepared to block the vents and adjust the comp chamber volume if needed, but it wasn't"
- Rear chamber guesstimated from the photo
As I model it, it is excursion limited to ~10 watts, 117dB.
...so it comes quite close to the OP's target (120dB, 20Hz). Playing with Hornresp shows:
To get >120dB: use more drivers and/or displacement.
keeping it simple, using one hearty 24" driver on the same horn, I model it as hitting 135dB with 1000watts (reaching half of Xmax)
Data-Bass
To get to 20Hz: stretch the horn out to 7.5m (25'). That would create some LF ripple, if the mouth size remained the same.
Adding wings to ~double the mouth width would ~halve the dips in the FR. In a shipping container build, this would be achieved simply by opening the doors.
Question - is there a neat way to snip screenshots from Hornresp? I used Gimp, but I think there must be an easier way that I've forgotten.
Disingenuous. The next phrase was "Or even seen a picture of one?", which is in the link I provided.
Bias or not, it is handy to have theory confirmed with personal, practical statements like "this monster horn most definitely had directional bass. It could barely be heard from the back side."
Also: the OP could ask Greg B for more info.
This is a problem with abstract theorising: one can easily forget obvious practical issues. The horn was built on site. It had to be built on site.
First: the people who helped Greg B build that horn were presumably among the ~8,000 people congregating in the desert.
Second: A large horn with only 3 sides (no bottom) is not an object you can easily pick up and move around. Consider that -
a) the destination was a desert, a "desolate surreal trackless plain"
b) you'd need a truck to get the horn there
c) you'd need a small crane to get the horn onto the truck
https://en.wikipedia.org/wiki/Black_Rock_Desert
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The half-space 20-Hz horn mouth is under 12 feet square. These are dinky horns compared to what I was originally planning to build (i.e. full-space horns). I’m a happy about this. I feel like my project was just cut in half!
Question: Should I drop the tune down to 18 Hz – just because I can??? The lowest known musical note is around 14 Hz (pipe organ), right? Am I thinking crazy now, or should I just stick with my original 20 Hz goal?
I’m still a bit uneasy about the rather firm recommendation that I make just one bass horn (aka a mono bottom end). Is this really what’s best? Will stereo bass horns really sound bad comparably? How bad? Again, I’m perfectly fine sitting in the sweet-spot. I feel like I’m cheating if I build just one bass horn.
I’ve read that the human ears are non-directional below about 80 Hz. I’d like to take the Pepsi challenge and hear this for myself. Should I own a pair of stereo bass horns, I can shift the tone between right and left horns, as I’m sweeping down the audio spectrum. Theoretically, below 80 Hz, I shouldn’t be able to tell which horn the sound is coming from, right? This test would confirm whether or not I needed to build a set of stereo horns - or if a single horn would’ve made the cut. .. .
A better question would be – has anyone else ever attempted such a test?
Question: Should I drop the tune down to 18 Hz – just because I can??? The lowest known musical note is around 14 Hz (pipe organ), right? Am I thinking crazy now, or should I just stick with my original 20 Hz goal?
I’m still a bit uneasy about the rather firm recommendation that I make just one bass horn (aka a mono bottom end). Is this really what’s best? Will stereo bass horns really sound bad comparably? How bad? Again, I’m perfectly fine sitting in the sweet-spot. I feel like I’m cheating if I build just one bass horn.
I’ve read that the human ears are non-directional below about 80 Hz. I’d like to take the Pepsi challenge and hear this for myself. Should I own a pair of stereo bass horns, I can shift the tone between right and left horns, as I’m sweeping down the audio spectrum. Theoretically, below 80 Hz, I shouldn’t be able to tell which horn the sound is coming from, right? This test would confirm whether or not I needed to build a set of stereo horns - or if a single horn would’ve made the cut. .. .
A better question would be – has anyone else ever attempted such a test?
Bass being directional: some theorists say no. Some people who have actually built stereo subs say yes. Couldn't you test this for yourself, with something like the system you posted earlier (your wife's guitar rig)? If you could set the bass to crossover at 80Hz, then flip the bass between mono and stereo, you'd know for sure.
Again, to reference an actual, built example (with concrete bass horns):
"Eso" did his (30-65Hz) in stereo, and says they are better than mono. He also says they cost him about $400 in materials.
Try chucking his username and the term "directional" or "concrete" into a forum search, to get results like this:
Directionality in sub horns - eso - High Efficiency Speaker Asylum
and this:
Re: do you mean? - eso - High Efficiency Speaker Asylum
This is the system:
Audio Asylum - Inmate Systems
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Your call and if you include sub-harmonics, then it'll need to be flat to < 10 Hz........ Personally, my subs are tuned to ~14 Hz for the few pipe organ symphony CDs that have any 16 Hz.
Well, FWIW, until fairly recently, pretty much all consumer recordings were summed mono below 150 Hz and movies are mono below 120 Hz, so it boils down to how high the horn will play and what the polar response is relative to the listening area [LP] as it's collapsing with increasing frequency above its gain BW.
The way I look at it; consider the sheer size of a [mid] bass frequency sound 'bubble' [cir = ~1130 ft/f] and the fact that your head's sound 'bubble' size is going to be up in the lower treble BW along with our hearing acuity rapidly rolls off below ~200 Hz, any 'stereo' one hears/perceives will be some sort of noise with enough higher frequency components, level mismatch, etc., for one to notice, at least that's all me and the others I knew back in our youth when hearing tests, comparisons, were pretty much our only affordable option.
Note that when stereo consoles came along, they typically had a single bass driver in the middle with a dual coil VC to sum mono and can't recall anyone ever noticing unless they actually looked 'under the hood'.
A fun test is to level match two speakers, sum them mono, then stand exactly between them to see how 'balanced' you and/or the speaker's responses are.
GM
Well, FWIW, until fairly recently, pretty much all consumer recordings were summed mono below 150 Hz and movies are mono below 120 Hz, so it boils down to how high the horn
The way I look at it; consider the sheer size of a [mid] bass frequency sound 'bubble' [cir = ~1130 ft/f] and the fact that your head's sound 'bubble' size is going to be up in the lower treble BW along with our hearing acuity rapidly rolls off below ~200 Hz, any 'stereo' one hears/perceives will be some sort of noise with enough higher frequency components, level mismatch, etc., for one to notice, at least that's all me and the others I knew back in our youth when hearing tests, comparisons, were pretty much our only affordable option.
Note that when stereo consoles came along, they typically had a single bass driver in the middle with a dual coil VC to sum mono and can't recall anyone ever noticing unless they actually looked 'under the hood'.
A fun test is to level match two speakers, sum them mono, then stand exactly between them to see how 'balanced' you and/or the speaker's responses are.
GM
Having lived with a 130Hz mixed bass system for decades, including, in one house, with the sub right behind my head, I can't recall ever thinking some of the music is coming from the sub. But you need a clean sub (like a Klipschorn... which also eats some of the higher frequency distortion products along the twisty path) and a steep crossover.
Likewise all the present widespread enthusiasm for subs spread around the room would be stupid if there were any sense of localization from them.
The concern about power seems misplaced since a true horn, uniquely*, multiplies efficiency along with all the other virtues (which derive from increased efficiency). An impedance transformer is surely needed if you are making sound by shaking heavy cones to propagate sound in light air.
A more important puzzle is time alignment of drivers. A look at hollowboy's remarkable system shows how he did it.
Ben
*maybe including playing into sulphur hexafluoride is a second "unique" approach; I mention this because of OP's welding knowledge
Likewise all the present widespread enthusiasm for subs spread around the room would be stupid if there were any sense of localization from them.
The concern about power seems misplaced since a true horn, uniquely*, multiplies efficiency along with all the other virtues (which derive from increased efficiency). An impedance transformer is surely needed if you are making sound by shaking heavy cones to propagate sound in light air.
A more important puzzle is time alignment of drivers. A look at hollowboy's remarkable system shows how he did it.
Ben
*maybe including playing into sulphur hexafluoride is a second "unique" approach; I mention this because of OP's welding knowledge
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I reported similar findings for a bass only speaker crossed with LR4 @ 150Hz.
The two satellites seemed to be the source of a music/audio. None appeared from the side located bass speaker.
I think if you measure the inductance of the 4/0 welding cable you will find it does not vary much from any 4/0 cable... may not make a big diff for a sub, but it will kill any DF hopes you may have or need.
Subs behind your listening position? Easy to hear.
To work right a sub needs to be so-called "time aligned" with the rest of the system.
Perhaps my hearing/perception is sensitive to this, but I find that once you hear a system set up this way, listen to certain types of source that "reveal" differences of this sort, when things are "wrong" it's not "right".
I find a difference between one and two subs one that is not easy to overlook.
Pop music with summed LF notwithstanding. There are enough two mic recordings and other recordings with LF not summed so that I want the ability to reproduce it faithfully.
Ymmv...
And I am also wondering amongst those who have built and used LF horns of various configurations, if you find the subjective and perceived sound quality to be all the same?
Also, in live venues, does the bass "develop" out in the room the same way between various horn configs and ported enclosures??
_-_-
Subs behind your listening position? Easy to hear.
To work right a sub needs to be so-called "time aligned" with the rest of the system.
Perhaps my hearing/perception is sensitive to this, but I find that once you hear a system set up this way, listen to certain types of source that "reveal" differences of this sort, when things are "wrong" it's not "right".
I find a difference between one and two subs one that is not easy to overlook.
Pop music with summed LF notwithstanding. There are enough two mic recordings and other recordings with LF not summed so that I want the ability to reproduce it faithfully.
Ymmv...
And I am also wondering amongst those who have built and used LF horns of various configurations, if you find the subjective and perceived sound quality to be all the same?
Also, in live venues, does the bass "develop" out in the room the same way between various horn configs and ported enclosures??
_-_-
Good example of stereo bass, often sited:
https://www.youtube.com/watch?v=lt-udg9zQSE
Of course the lower part of the bass is pretty much summed by a room, but it is stereo in the recording. Also the higher part of it shows up in the main speakers as GM suggests upthread.
I wonder if this would be summed in the outdoor setting the OP is pondering, or would it sound stereo? hmmm...
https://www.youtube.com/watch?v=lt-udg9zQSE
Of course the lower part of the bass is pretty much summed by a room, but it is stereo in the recording. Also the higher part of it shows up in the main speakers as GM suggests upthread.
I wonder if this would be summed in the outdoor setting the OP is pondering, or would it sound stereo? hmmm...
Originally Posted by weltersys
1) In the case of the Keystone Sub, the excursion of the TH is less than the BR with the same low corner for a given voltage at most frequencies.
Although specific discussion of the Keystone design is off topic for this thread, but since you insist on continuing to write information that does not apply to the Keystone alignment, I'll correct you again, but please continue the discussion in the Keystone thread if you feel compelled to.
1) My tests compared a ported box of 9.24 cubic feet gross external (8.5 cubic feet + the externally mounted B&C18SW115-4, it's net volume of .37 cubic feet was mounted outside the BR cabinet, adding .74 cubic feet volume) volume to the 15.52 gross cubic foot Keystone sub (driver mounted internal), a 41.5% larger enclosure. The BR tuning was painstakingly increased a bit at a time by reducing the port length until it matched the TH Fb of 37 Hz.
As you can see in the test results using 5 Hz increments below (posted in the OP of the Keystone Sub thread) not only does the BR excursion exceed the larger TH at most frequencies, it has a much wider range of frequencies just above Fb with large excursion compared to the TH. The BR also "unloads" below Fb with far more excursion than the TH, reducing the usable range below FB. The tests were conducted at only 38 volts, the driver's excursion is well within the very linear BL range. It is also apparent that the other drivers used in the TH have similar excursion response, with the exception of the 4015LF, which required the exit dimension reduced, which in turn lowered FB by a few Hz, but the "step down" mode considerably reduced it's excursion below Fb.
2) If you want a simulation with more than a rough approximation of the Keystone design you will need to use Akaback, as the Keystone exit can not be modeled in Hornresp. Djim "fudged" Hornresp inputs enough to get a simulation that roughly approximates the frequency response, but the "fudged" inputs do not reflect actual cross sectional areas of the Keystone "as built".
As far as your claim that "given an equal amount of space the (dual) ported box will win, even with some hefty power compression factored in", it is not quite true in this case, as the 15.5 cubic foot Keystone has equivalent output to a pair of BR cabinets totaling 18.48 cubic feet, before port compression is factored in. You can also read about the actual measured BR port compression in the Keystone thread.
The measurements, and listening tests over years of use in various indoor and outdoor venues have proved to me that the Keystone sub not only saves space, weight and cost compared to a pair of BR with roughly the same output, but in the real world of sagging voltage AC power sources, the 50% reduction in amperage drawn translates to less voltage drop, resulting in more peak power available from the amplifiers, thus considerably more delivered SPL than BR cabinets.
Although a Keystone sub won't equal what Eric's giant concrete horn will per driver, when cost is considered, multiples with a path length appropriate for the desired FB (whether 29, 20, or 16Hz) could beat the concrete horn in every conceivable metric (other than the "concrete aesthetic"), including longevity, if built using an external sheath of LP SmartSide panel and two layers of grill fabric.
Art
1) In the case of the Keystone Sub, the excursion of the TH is less than the BR with the same low corner for a given voltage at most frequencies.
JAG,
Although specific discussion of the Keystone design is off topic for this thread, but since you insist on continuing to write information that does not apply to the Keystone alignment, I'll correct you again, but please continue the discussion in the Keystone thread if you feel compelled to.
1) My tests compared a ported box of 9.24 cubic feet gross external (8.5 cubic feet + the externally mounted B&C18SW115-4, it's net volume of .37 cubic feet was mounted outside the BR cabinet, adding .74 cubic feet volume) volume to the 15.52 gross cubic foot Keystone sub (driver mounted internal), a 41.5% larger enclosure. The BR tuning was painstakingly increased a bit at a time by reducing the port length until it matched the TH Fb of 37 Hz.
As you can see in the test results using 5 Hz increments below (posted in the OP of the Keystone Sub thread) not only does the BR excursion exceed the larger TH at most frequencies, it has a much wider range of frequencies just above Fb with large excursion compared to the TH. The BR also "unloads" below Fb with far more excursion than the TH, reducing the usable range below FB. The tests were conducted at only 38 volts, the driver's excursion is well within the very linear BL range. It is also apparent that the other drivers used in the TH have similar excursion response, with the exception of the 4015LF, which required the exit dimension reduced, which in turn lowered FB by a few Hz, but the "step down" mode considerably reduced it's excursion below Fb.
2) If you want a simulation with more than a rough approximation of the Keystone design you will need to use Akaback, as the Keystone exit can not be modeled in Hornresp. Djim "fudged" Hornresp inputs enough to get a simulation that roughly approximates the frequency response, but the "fudged" inputs do not reflect actual cross sectional areas of the Keystone "as built".
As far as your claim that "given an equal amount of space the (dual) ported box will win, even with some hefty power compression factored in", it is not quite true in this case, as the 15.5 cubic foot Keystone has equivalent output to a pair of BR cabinets totaling 18.48 cubic feet, before port compression is factored in. You can also read about the actual measured BR port compression in the Keystone thread.
The measurements, and listening tests over years of use in various indoor and outdoor venues have proved to me that the Keystone sub not only saves space, weight and cost compared to a pair of BR with roughly the same output, but in the real world of sagging voltage AC power sources, the 50% reduction in amperage drawn translates to less voltage drop, resulting in more peak power available from the amplifiers, thus considerably more delivered SPL than BR cabinets.
Although a Keystone sub won't equal what Eric's giant concrete horn will per driver, when cost is considered, multiples with a path length appropriate for the desired FB (whether 29, 20, or 16Hz) could beat the concrete horn in every conceivable metric (other than the "concrete aesthetic"), including longevity, if built using an external sheath of LP SmartSide panel and two layers of grill fabric.
Art
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If you want an accurate sim you do. Your sim is not even close to accurate, it's like comparing a golf cart to a motorbike. Sure, the are both transportation vehicles but there are very significant differences.
Already the problems start. He said very specifically that he thought it was silly to carefully calculate the expansion. "Exponential" has a very specific meaning and flare rate - since he just built something that looked vaguely exponential and didn't document the actual flare, we have no way to simulate it.
You might think it doesn't make a big difference but it does - it affect everything, including the volume of the horn enclosure. And when coupled with all the other little fudge factors in your sim, the end result isn't even close.
Again, not specific, and it does make a difference. "About 17 feet" could mean anywhere from 14 feet to 21 feet. He's showing that he's not overly concerned with the details, so why would we trust the dimensions of the stuff he didn't even bother to measure?
In that thread he specifically says "Length was about 15 feet. Mouth area was about 68 square feet." So either you got the dimensions wrong or he is posting different details in different places. This stuff matters. A lot.
This is perhaps the biggest problem. You left the duct that connects the rear chamber to the horn path completely out of your sim. That duct changes EVERYTHING.
And that's not the only problem here.
His actual statement was "The compression chamber was vented into the throat of the horn, which no doubt helped the low end. I don't think you can sim this with hornresp - maybe with akabak."
But when you look at the picture, it looks like the rear chamber is ported to the outside, not into the throat. Maybe it's vented to the outside AND into the horn throat. Either way, these details are VERY important and far from clear.
And a final problem - he said "I used ~1:1 compression ratio, including the vent area as part of the driver area."
Assuming it's end loaded and not offset (which is a pretty good guess I think but not necessarily true) with the square segment construction he used, it would be IMPOSSIBLE to make the compression ratio anywhere near 1:1, it would have to be a lot lower, maybe even 1:2. Here's a visual to help out with this issue.
An externally hosted image should be here but it was not working when we last tested it.
The black square is a cross sectional area of the horn throat area where the driver is mounted. The big black circle is the outside diameter of the driver. The red circle is the actual Sd - a bit smaller than the outside measurement of the driver. The little black circle is the duct that connects the rear chamber to the horn throat.
Even if it's built to tight tolerances like shown here the square area of the throat is WAY more than the area of the Sd (red) and also way more than the area of Sd + duct.
Therefore, it's almost impossible to conceive that his estimation of 1:1 compression ratio is even close to accurate.
If it's not accurate, your whole throat area in the sim is inaccurate, and the implications of that are HUGE. Change your S1 to 1600 sq cm (which is definitely within the realm of possibility, probably a lot closer than 800) and see what happens to your sim.
I'd say your guess at the rear chamber volume and dimensions are way off. It's not a good idea to guess anyway, this is why you need plans.
Your sim is so inaccurate in so many ways that it's rendered completely and absolutely invalid. It has no resemblance at all to what that guy built.
He wasn't very detail oriented when designing it and clearly you aren't that worried about detail when trying to reverse engineer it.
Using that type of driver is far from keeping it simple, unless you used the "large voice coil" checkbox in Hornresp. If you didn't the sim has nothing to do with reality for yet another reason.
That's not how barn door "wings" work. They provide a bit more boundary loading but it definitely isn't going to smooth out the ripples in the frequency response.
No it's not handy. I've seen people say without any doubt that the world is flat, different people say they were abducted by aliens, still other people say that we only use 10 percent of our brains. Just do a quick search for "myths disproved by science" and you will see that a lot of people have a lot of crazy misperceptions about basic everyday things around them.
This guy clearly didn't have a clue how to design or evaluate a horn. I think we've spent enough time on this oddly undocumented project.
He said he measured 20 hz. I said he probably didn't actually measure anything, if he did he would have kept some record of the measurements. Everything you say about this leans more toward the possibility that he never measured the thing. What are you trying to prove here?
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1) Since you are planning to "go big", might as well build for 16 Hz response, those low frequencies are not limited to pipe organs, there are loads of synth pedal tones going that low, for example "The Longships", an Enya bonus track surprised me yesterday with some floor rattling bottom.
2) "Best" has many metrics to choose from. Stereo horns can potentially sound better in the sweet spot, but worse on either side of center. Assuming two horns equal to the size of a single using double the drivers and power, the 6 dB headroom afforded allows more SPL with less distortion.
3) Experimenting is the only way to determine your hearing response.
Because of noise induced hearing loss, and unusual low frequency hearing, I can hear 16 Hz easier than 4kHz, while the "average" person would perceive the 4kHz tone some 60 dB louder. Discussions have revealed that there are many individuals that are incapable of hearing below 20 Hz, though they can "feel" it. I know from my own experience that I can hear clean tones on headphones down to below 8 Hz, and the headphones show very low distortion, I am not hearing the harmonics and "thinking" I hear the fundamental.
So forget theory- I can put on blindfolds, spin around, and still point to a distant idling tractor that measures 60 dB down above 80 Hz in an open air situation.
At your listening position, the shop to the right would seriously affect that test, it's reflection will be fairly strong compared to the horn's initial wave, so LF "imaging" will be off. As many have pointed out, most intelligent life forms mix LF content center, the reflection will be doubled with two subs, giving a rockabilly slap-back double kick drum effect. Cool, eh?
Since you are probably already using Peltor hearing muffs, do yourself a favor and buy a pair of GK Ultraphones (Tell Gordy Art sent you) so you can hear music as it was recorded, with a 30 dB reduction of background noise (30 dB reduction means the noise sounds 1/6th as loud) letting you listen at non-damaging levels while still hearing fine detail even in deafening environments.
Then check out your hearing with this test- it only goes to 30 Hz but you can download lower tones to get a "feel" whether you hear them or not. You may also find as many do that LF sound below 30 Hz tends to make you feel uneasy, or nauseated if the level is extreme. I can't sit at certain tables in some eateries because the air ducts have resonance around 15-20 Hz that make me feel ill after a short period of time.
http://newt.phys.unsw.edu.au/jw/hearing.html
Then you can take a listen to these tests, and determine what level of distortion at various frequencies you can detect:
http://www.klippel-listeningtest.de/lt/
Have fun,
Good Luck!
Art
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There are pipe organs with a 8 hz pipe. And there is "ambience" at low frequencies. So if you want truly full range reproduction there's no lower limit to the frequency response. But there is practical vs ideal to consider.
You already have everything you need to test this. Take your wife's stereo system outside, place the bass bins 18 meters apart and walk around the yard listening and measuring the frequency response at different locations.
You absolutely do not need stereo separation in the subwoofer frequencies and you absolutely do not need more spl than one horn can provide, so there are no benefits whatsoever to doing two sub horns. There are significant lobing problems though.
As others have mentioned, you will hear a difference, but it's not at sub frequencies, it's harmonics and distortion at higher frequencies. Again, you already have everything you need to test the theories, just haul your wife's system outside and do some tests.
The Klipshhorn is not a "clean" speaker by modern standards. It's also not anywhere near an impedance transformer. Only full size horns are impedance transformers with gain across the entire passband.
Your comments are quite odd considering the fact that you don't actually own a horn, yet you do own at least two moving coil "subwoofers" that are as far away from the the things you claim are important as you could possibly get.
Yes, this is getting too far OT and it's my fault for trying to further the conversation in this direction with proof.
But before letting this drop completely, how were you measuring excursion? If it was by eye there is a huge margin for error.
Second, measuring at 5 hz increments is not really enough data points. It's very likely that you didn't capture the PEAK excursion frequency inside the passband - this is the only frequency at matters at all.
Third, matching fb isn't the way to do it, you have to match the low knee. Otherwise the frequency response at the low knee is going to be quite different - in fact the low knee frequency will be different. The low knee is the key to everything - it's why we size boxes the way we do.
Fourth, you didn't even mention which driver was used in the ported box. Obviously that changes everything, you can't compare different drivers for this exercise.
I'm willing to save this discussion for another day, but I'm not wrong. The Keystone is a tapped horn not unlike any other tapped horn, it doesn't mysteriously deviate from the rules that all other tapped horns follow. It absolutely will have more excursion at it's max excursion frequency inside the passband than a smaller ported box with the same driver and low knee.
I don't get this 'lobing' issue. What is so special about bass tones that suddenly 'lobing' starts to play a role? With stereo, the wavelength of all but the lowest frequencies is shorter than the distance between the speakers. So?
Good, clean bass is directional. My speakers go in room to around 30 Hz with very low distortion (.1%@40Hz). If the bass is panned into one speaker, you can definitely hear which one it is. No doubt about it. Afaik bass is mono-ized on all but a few recordings because the capability of most speakers to reproduce real bass is quite limited, and studio's produce for the masses.
Full mutual coupling (+6db) only occurs when the two sound sources are with 1/4 wavelength of each other.
With a 60 foot center to center distance between the horns, they are not within a 1/4 wavelength even at the bottom of the passband, so there's no full mutual coupling and no +6db.
Lobing has to do with the off axis response dictated by the triangulated distance between the two sound sources playing the same frequency and the listener. This happens at bass frequencies and higher frequencies (it happens at all frequencies), the only difference is the distances involved.
When the sound sources are playing the same frequency and within 1/4 wavelength distance at that frequency, the two sources act as a single source so you get full mutual coupling (+6db) and no lobing issues. But when the distance exceeds 1/4 wavelength the sound sources destructively interfere, so you get less than +6db of coupling and lobing issues.
When the frequency is 60 feet center to center and the frequency is 20 hz, the distance between the sound sources is WAY more than 1/4 wavelength, so you get less than +6db of constructive summation and you get lobing issues off axis.
These lobes will present as massive notches in the frequency response at areas that are off axis. The further off axis you go, the lower in frequency these notches will appear.
It's a big problem. It's why speakers are designed with the tweeters and mids pushed as close together as possible (because they are playing the same frequency at ~ the same spl at the crossover frequency), and using driver sizes that facilitate this close center to center distance. The concept and theory is exactly the same at low frequencies, the only difference is the distances involved. Lower frequencies have a longer wavelength so they can be further apart, but once you exceed 1/4 wavelength they do not act as a single sound source, do not fully sum to +6db and they destructively interfere with each other at locations off axis.
No it most certainly absolutely definitely is not unless you have cab dimensions or waveguide dimensions large enough to control the dispersion. Even a full size horn (like the ones being discussed here) have limited control of dispersion at the bottom of the passband. They are quite effective at pattern control at the top of the passband though, while "regular" size subs are not, which gives a good indication of how big a sub needs to be to have any influence over pattern control, directivity, or "directional"ity as you call it.
And since subwoofer frequencies are rarely mixed in stereo and there's no benefit to it even if it is mixed in stereo, there's no need for multiple subs in an outside environment, assuming you can get the full desired spl out of a single unit, which is more than possible in this case.
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