I have been dreaming of building a theater/listening room where the room itself is the horn. For the sake of simplicity of construction it would be a slowly expanding conical horn with rectangular cross section, where the walls and ceiling would expand out from the sound source at whatever flare rate is appropriate for loading the drivers down to 20 hz, and terminating wherever a good sized listening area is achieved. The entire rear wall would be a very deep acoustic absorber; contemplating the use of wood chips on the order of 6 or 7 feet in depth, contained behind wire mesh, a layer of foam, and fabric.
Thoughts?
Thoughts?
My thought is it sounds more like a nightmare than a dream :^).
At any rate, you would want an exponential rather than conical horn expansion.
A straight sided low frequency horn approximating an exponential expansion is no more difficult to build than a conical, and a "slowly expanding conical horn" actually requires expansion in two directions- if two walls are parallel, the expansion is parabolic.
At any rate, you would want an exponential rather than conical horn expansion.
A straight sided low frequency horn approximating an exponential expansion is no more difficult to build than a conical, and a "slowly expanding conical horn" actually requires expansion in two directions- if two walls are parallel, the expansion is parabolic.
You mean a nightmare acoustically, or in construction? It would expand in all directions, effectively; the roof would expand relative to the floor and the walls relative to each other. I want to avoid a segmented horn because of artifacts that would be introduced at intersects between segments. Just as the horn mouth introduces reflections so to I imagine to be true of each intersect. It would be significantly less complicated to build a straight sided horn. I build and fix things for a living.
Primarily nightmarish in terms of construction time and expense vs any acoustical benefit.
What "artifacts" are you concerned about?
Folded horn bends generally are not problematic for low frequencies.
Compared to what?
"Just as the horn mouth introduces reflections so too I imagine to be true of each intersect." By artifact I meant distortion.
I imagine the higher I want to reach with the bass horn the less perturbations are tolerable, in terms of distortion.
Compared to a segmented horn, with several angle changes.
Best rear wall absorber (anechoic) is packed pyramidal structures about 10 long to 1 wide (eg 1 metre to 10 cm) and made from polystyrene or similar. Both absorbs sound energy on the first bounce and reflects it deeper for full absorption. Wood chip is just a home for spiders and other vermin and a source of dust and volatile resins that you would not want anywhere near your records/CDs and the players.
I am married and may wind up doing an in room horn also.
I just had to build the wife a master bedroom and other spaces with all of the things she wanted.
I just had to build the wife a master bedroom and other spaces with all of the things she wanted.
I have seen rooms like that, but only in photographs. So it has been done, although not exactly as you describe. The reports were very good.
The closest I've some was my lava cave listening room. Not a horn, but a long, hypo-echoic chamber. Response down to ~24 Hz with 12" woofers.
If you look around the web, you should find examples of what you want to do. If I see any, I'll post them here.
The closest I've some was my lava cave listening room. Not a horn, but a long, hypo-echoic chamber. Response down to ~24 Hz with 12" woofers.
If you look around the web, you should find examples of what you want to do. If I see any, I'll post them here.
The closest I've found was the royal device subwoofer, not quite what I was looking for but a similar concept of the room being part of the horn. That project is what got me excited and started my mind down this path. The Matterhorn was the only other one, but I'll bet it has lots of acoustical problems. Hard to describe how much I love good clean bass, and my concept is the only way I can think of to eliminate a lot of the variables that interfere with the pursuit of the perfect theater/listening room.
Please do share any reports you have about similar projects. Good to know I'm not the only person who considered this approach.
Please do share any reports you have about similar projects. Good to know I'm not the only person who considered this approach.
Horns usually work by propagating sound waves from the mouth of the horn into free space outdoors or approx free space in a room, where the room volume is much greater then the horn volume. So that allows the pressure wave in the horn to continue to expand out of the horn. Horns with poor launch of the wave out of the mouth get reflections back in. What you are you dreaming of looks like it could defy physics.
Changing the mouth impedance from free air to any absorption is likely to radically change the fundamental physics of the horn's function. The mouth impedance is connected to the throat impedance. Its one system. Also given the absorption cant be 100% there will reflections with standing waves and nodes set up.
I could be wrong. I dont do horns myself. I suggest getting expert advice before starting a major construction build costing what, tens of thousands?
Modelling would be less painful then a full build
1. software would be interesting
2. Physical modelling can be done with a large horn. Use a frequency to scale. Measure with the mic outside and then inside the open horn with the mic in the approx listening position. Then measure with the horn mouth blocked off.
Heres just a sim of a parabolic horn into free space and then into absorption. Note with the mouth impedance added the fundamental wave propagation within the horn is entirely lost. The sim indicates if you were sitting in that horn there would be no bass to be heard. Just higher frequency noise.
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Forgive me for being contrary, but I believe it would act like an infinite horn. If absorption was nearly complete how could it act any other way? Where would all the energy go? I believe the simulation is flawed. Are you indicating that the cone wouldn't move and all that energy would be converted to heat in the voice coil? My horn would have the volume of a truncated horn and room combined. The rest of the room being a continuation of the horn would somehow prevent it from working? Let's say... the cone moves, causes a pressure wave in the air that travels down the horn(room), it is intercepted by the absorber which converts it to heat, an acoustical heater if you will... Now let's say you happen to be in the path of that pressure wave, what's to prevent your ear drums from vibrating? I believe the absorber would act as the resistance, the same as the dissipation of acoustical energy to heat in free air. As far as the driver is concerned what would be the difference?
The sim is just a sim. Its not set up to model your design. Its a possible indicator of what could go wrong if the absorption impedance is too high.
Just to repeat: I could be well wrong. I dont do horns.
The absorption of your horn mouth/rear wall looks like its going to be critical to get the mouth impedance approaching free air for your concept to work. I dont know what the impedance of wood chips six feet deep would be. Six feet is impressive. Do you have neighbours within a mile? If not can you have the rear wall open with giant folding wall-doors that extend the horn on a nice day?
So are you going to model first or are you so confident in your concept that you will launch ahead and do a full structural build?
Whats your budget?
Just to repeat: I could be well wrong. I dont do horns.
The absorption of your horn mouth/rear wall looks like its going to be critical to get the mouth impedance approaching free air for your concept to work. I dont know what the impedance of wood chips six feet deep would be. Six feet is impressive. Do you have neighbours within a mile? If not can you have the rear wall open with giant folding wall-doors that extend the horn on a nice day?
So are you going to model first or are you so confident in your concept that you will launch ahead and do a full structural build?
Whats your budget?
I would pass on the 6 or 7 feet of wood chips -- the weight alone could not be restrained with anything sufficiently acoustically transparent.
Maybe Owens Corning 703 or 705 rigid fiberglass, cut into appropriate triangles and pyramids. You'd only need a little over a foot in depth to have a VERY high coefficient of absorption.
Cheers
edit: Forgot to mention -- there's a member in the U S northwest building (or by now has built) a bass horn from concrete. Have been meaning to look that up, as it has been a while now since I last saw the thread.
Maybe Owens Corning 703 or 705 rigid fiberglass, cut into appropriate triangles and pyramids. You'd only need a little over a foot in depth to have a VERY high coefficient of absorption.
Cheers
edit: Forgot to mention -- there's a member in the U S northwest building (or by now has built) a bass horn from concrete. Have been meaning to look that up, as it has been a while now since I last saw the thread.
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I'm definitely going to model it first. Probably will build a scale model first and take measurements. But the purpose of modeling would be more to get the dimensions and flare rate appropriate to the desired frequency response. I have zero doubt that this thing will work and produce a prodigious amount of clean bass. I am most worried about whether I can create a good enough absorber. In my mind a nearly perfect absorber would act the same as free air. If anyone has a concept as to why this would not be so please share your thoughts.
Budget? ~1500 usd for roof, <500 for walls made of adobe, <200 for poured adobe floor with linseed oil treatment, about 1000 for 15 yards of wood chips and other materials to contain it including other room treatments(half that if I get the wood chips for free), 500 for two 15's, ~500 for mid bass/lower midrange drivers/horns, 200-300 for middle/upper midrange/lower treble drivers/horns, 100 for super tweeters, <1000 for projector/screen, ~1000 for crossovers/amplifiers/etc, ~1000 for other stuff. Probably around 7000 if I splurge a little on electronics and drivers, if I am conservative, I think the whole project could come in at about 5000. To many audiophiles this will sound ridiculous, but this is the power of doing things yourself.
I need a very high coefficient of absorption at very low frequency, this will require a lot of mass and volume.
The wood chips would be contained with wire mesh and lumber. I build and fix things for a living, this will not be a challenge.
Are you referring to the royal device subwoofer? If not I would love a link.
Hmm, so you basically want a huge compression horn shaped plane wave tube [PWT] with a 20 - ?? Hz BW using ?? number of drivers, right?
Nice. Adobe walls and a floor with linseed are beautiful too.
With good planning you can be in a win- win situation. If against all expectation you can make a massive conical horn with an incredible rear wall absorber you might just get a massive win with the sound and a new construction career. If it doesn't win you have a beautiful listening room with great acoustics given a sloping ceiling and angled walls means a lack of strong nodes with a more conventional sub