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In Reply to: RE: Downfire posted by Zene on January 08, 2012 at 19:53:44
I have built over a 100 of my downfiring Edgarhorn 35Hz seismic horn subs over the last decades. They work best on a concrete floor. Wood floors are invisible at those low frequencies. Have you heard a car sub out side the car? All you can hear is a boom- boom. You can down fire a horn on a wood floor if you isolate the bottom of the enclosure off the floor with spikes or wheels and load the bottom with mass like sand or cement paver blocks. I also build a stepped pyramid reflector in the bottom to guide the waves out and also to work as a mass load.
For horns bottom loading also makes the horn sub a near field speaker that integrates well with upper horns. A forward firing sub only works in a large room because you have to be one wave length away in the far field zone for it to sound good. At 100 hz a wavelength is about 10' which is a good listening distance for mid bass horns so a forward firing horn wouldn't integrate well. But a bottom firing sub would.
From my experiments bottom firing works well up to 500 hz. Above 500Hz the waves become disorganized and attenuated. My SLIMLINE horns use bottom loading for the midbass.
For subs you can also fold the path length. One MIT Prof told me that it is just like water in a fire hose- the water goes where ever the hose goes.
Follow Ups:
One downside of downfiring audio signals above 100 hz is that one can start to localize that they are coming from a different place than the midrange driver - the higher the crossover the worse the problem. I had 18 inch drivers in open baffle and they were crossed over using two crossovers, one at about 100 hz (2nd order) and another at 170 hz (2nd order). When bass was being played, you could tell that as the bass player was moving up and down the scale that the sound was moving up and down between the two drivers. It seemed that the bass player was jumping up and down when playing! Sometimes we must live with compromises, but just know that this likely would be the case if you downfire in the mid-bass range.
Retsel
I downfire the horn loaded woofer in my David floor stander cab, crossed over to the MTM section at 200Hz, and there are no directionality issues. If the sub and tops aren't within the same vertical footprint it can be an issue. But not always. This issue is brought up on page 26 of this long strange trip:
http://www.avsforum.com/avs-vb/showthread.php?t=1353217
My observation is supported by surveys which suggests that localization of the woofer, separate from the midrange, occurs at about 80 hz. For some, they can localize the woofer at a frequency lower than that while others localize the woofer at a crossover frequency higher than 80 hz. My crossover which occurs in the 100 to 200 hz range was high enough to definitely locate the woofer separate from the midrange.
As I design my next open baffle speakers, I will try two things. The first thing would be to crossover the midrange at a lower frequency. I have Lowthers on open baffle principlely based on the Basszilla design, and Dick Olsher designed the crossover to high pass the Lowthers at 150 hz. I think that this is conservative, and because of the woofer localization which I observed, I will try crossing over the Lowther at 80 hz instead. While Lowthers don't have much x-max, the 15 ohm Lowthers which I own do have about 3 mm of x-max so they likely will be able to handle the lower crossover.
The other thing that I could do is move the woofer up on the baffle. Right now to obtain as much reinforcement as possible, I have the drivers as close to the floor as possible. But I could move the drivers up to be much closer to the midrange driver.
Retsel
Bill, would you tell Retsel that horns are a different animal. Ok, I'll tell him, horns are a different animal. Just kidding Retsel, I'm sure you are aware of these problems with horns, but a lot of people with no experience with them need to realize the exponential (pun intended) rate at which they grow as frequencies go down. A very famous engineer designed an UB waveguide/horn for me and it had a 36" x 36" mouth and was only good to 300 Hz. To be built later. If I were to limit my cross to 80 Hz the horns I'm building would be my UB and I would need a cone subwoofer for the bass. The last thing I want in my system is some HT thinking effects sub. I'm mystified as to why everyone is nuts about deep subs for music, except maybe organ and some symphony music. Even Romy said he liked the sound with the subs turned off on a lot of his music. If I get a solid 40 Hz from my new horns I will be happy and not have every thing in the house rattle. My old Khorns kept me entertained for years and they had no deep bass. I did a sweep of several very low individual frequencies around 30 Hz about 100 dB, listened and fixed as many rattles as I could find. I quit about 20. And that didn't include the walls, ceiling, floor, doors and windows moving around like a Funhouse. The only thing that didn't travel was the fireplace (side note, my TT resides on it now.) We're talking weeks just to find the small rattles. The sweep would have to be one cycle at a time to be really effective. Movie watchers in general could care less about the quality of the sound and the more rattles the better. It stirs up the room. If you think this is a side slam to the HT crowd, you'd be right. The mfg's and sales engineers have been endocrinating the effects subs for more profit than anything else and most of us have bought into it. Music needs to be handled a lot more carefully at low frequencies. Those early speaker designers were not stupid. They were a little phoney with their 20 - 20K advertising, but in reality that was at least -10 dB down making -3 dB somewhere around 35 - 40 Hz. The engineers did make the speakers right.
I will certainly keep your comments in mind as I build. All info is helpful. Thanks, Zene
"My observation is supported by surveys which suggests that localization of the woofer, separate from the midrange, occurs at about 80 hz."
It does, but as in all things audio what you can't hear doesn't matter. I wouldn't run a traditional separate sub/satellite with higher than an 80Hz crossover, but there are instances where you can run higher and not tell the difference. One is with the sub and main in the same vertical plane, another is with a wall of subs.
There are, of course, many compromises that we live with when we have audio systems. This may be one that we can choose to live with, particularly when one realizes gains somewhere else. This is one reason why people change speakers and other components from time to time.
I guess when finding the ultimate audio system, we try to find the least number of compromises that we can live with and that matter much less than the good aspects of any system.
Retsel
Dr. Edgar ... thanks much. I always appreciate your comments. I didn't know about DFing being nearfield. That really helps in making sure I continue.
It's really easy to postpone a horn this size indefinitely.
Zene
"Have you heard a car sub out side the car? All you can hear is a boom- boom."The reason is that you aren't hearing the output of the speaker going through the car body, you're hearing the resonant frequency of the car. Inside the car you hear the full spectrum of the sub's output, though obviously at levels no sane person would tolerate.
To answer the OPs question you would incorporate the distance from the nominal mouth opening of the horn to the floor into the design as a segment in your model, taking into account wall or corner placement as well.
Edits: 01/10/12
Bill ... glad you answered cry. I will rework Hornresp to incorporate corner. First pass looked great. Thanks, Zene
What you're doing isn't any different than a corner horn, like a Klipsch. All that differs is the orientation of the horn. If you run it higher than 200Hz you'd want to use a reflector. You still may want one depending on the distance from the last horn bend to the floor; if that's 1/4 wavelength or more within the pass band use a 45 degree reflector.
Bill ... to clarify my lousy math, the last bend to floor is 74" and bandwidth calculated low and highest cross will be 38 Hz & 150 Hz; therefore, I do or do not need baffle? ASSume it won't hurt to have one anyway?
Zene
74 inches is 1/4 wavelength at 45Hz, so you could get a fair amount of cancellation centered at that frequency if you don't use a reflector. You'd want to use a reflector at that last horn bend as well, and for that matter at all of the bends where the distance between them is more than 22 inches for a 150Hz bandwidth.
Thanks again, Bill ... Only one 90 deg bend and I've already planned on using a 45 (give or take) deg plate ala LaScala style. I have no idea why the bent pipe is still used for lower frequencies. Kicker plates at the bottom are going to take some thought as 45 deg reflectors leaves a lot of open flat floor area to reflect sound back up. A pyramid in the middle doesn't seem right because of the two side walls. I think it's show and tell time with cardboard and scissors. Mouth is 36" square and will be about 18" off the floor. Good news, the bottom section will be separate from main horn so I can play with config. Thanks to you and everyone I think it's going to be one helluva UB if nothing else. Zene
"I have no idea why the bent pipe is still used for lower frequencies"
+1, it serves no sonic purpose to do so much below 400Hz. If it's used to lighten the cab by using a thinner material then it's valid.
"45 deg reflectors leaves a lot of open flat floor area to reflect sound back up"
So long as most of the flat area is covered you're OK. You can take Huygen's advise and restrict the pathway through the bend too.
"A pyramid in the middle doesn't seem right because of the two side walls"
With a 150Hz passband I doubt it would matter.
"Good news, the bottom section will be separate from main horn so I can play with config"
Absolutely.
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