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resonant chamber speakers

Speakers that resonate in a desktop-size resonance box
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I have some computer speakers that I listen to my podcasts on. Sometimes with ambient noise in the room, I have to turn them up.

I've noticed that even when they are at normal volume, I can hear the program perfectly in the other room ( I don't have a doorway to my room -- it's down some stairs ). So it seems to me that my room acts as a resonant chamber for the speakers.

My thought was to have the speakers pointing into a resonating box, like an acoustic guitar. This box would resonate the sound to an amount good for the room, without having to turn up too much volume.

lawpoop, Dec 13 2008

Steinway Music System http://www.wired.co...pr_steinway_model_c
While most speakers are sealed boxes, Steinway's engineers left these open, replicating the resonance of live instruments [xaviergisz, Dec 15 2008]

SoundBug http://en.wikipedia.org/wiki/SoundBug
Not sure if this is relevant:
Soundbug is a small device that can turn nearly any flat surface into a soundboard, which allows you to listen to media devices without traditional speakers [xaviergisz, Dec 16 2008]


       Isn't that what the cabinet included with all speakers, bar cheap tiny screen-side speakers, actually do? Bass speakers in pro sound systems have huge resonating chambers - calculated to the nth degree and sometimes with labyrinth like complexity.
dannystaple, Dec 13 2008

       As far as I know, it's only the bass that gets the little box. The mid-range speakers are exposed to the open air, so to speak. I'm looking at my pair right now, and there is not box.
lawpoop, Dec 13 2008

       You may be confusing resonating with efficient radiating. Resonating in the body of, e.g., an acoustic guitar contributes to its tone, but not (greatly) to its loadness. A theoretical ideal radiator would have a large area but no mass, and therefore no resonance. A banjo approaches this with its lightweight moving skin and heavy stationary body; hence its sound is loud but of short duration because the energy in a vibrating string is efficiently transferred to the air. Good speakers also have very lightweight cones that move relative to a heavy body.   

       I suspect that the floor was set vibrating, which transferred the sound into the other room. Try placing your speakers against a sheet of thin plywood or a drum skin or a thinnish tabletop; this might give a better effect than a resonating box.   

       My understanding is that the chamber of a speaker is not so much a resonator as a way of increasing the coupling between the air and the speaker element, and thus increasing the effectiveness of a given sized cone. The padding inside is precisely to reduce unwanted resonance at the natural modes of the speaker's air space and solid parts. If you sweep a loud sine wave through some cheap speakers you can clearly pick these resonant frequencies, and they add only suck to the sound.
spidermother, Dec 16 2008

       [Spider] What is a floorset? I don't have any bass or anything on the floor; I just have two cheap speakers, about the size of a palm, that sound great coming from another room. They're sitting on a cheap particleboard desk.
lawpoop, Dec 16 2008

       To produce loud sounds, a small speaker must produce large motions in a small quantity of air. For sound to be heard over a large area, there must be smaller motions in a much larger quantity of air. If all one has is a small speaker element in the middle of a large room, a lot of energy will be wasted on the transition between (large motion, small quantity) and (small motion, large quantity). If another step is added (medium motion, medium quantity), the combined losses in the transition from (L,S) to (M,M) and from (M,M) to (S,L) will be less severe than the single-step loss going directly from (L,S) to (S,L). Adding further steps or, better yet, making a slow gradual transition, will can reduce losses further.   

       A key concept which is incidentally useful in electronics as well as acoustics is that of impedance. Imagine that you have a collection of uniform weights on a frictionless rail, all connected via uniform springs. If you jerk one end of the collection, the motion will be passed on down the line. Each weight in turn will perform almost the same motion as the one you moved directly and will then be motionless, since all of its energy will have been passed on to the next weight in the line. That is, until the wave reaches the end.   

       If the last weight isn't attached to anything, it will accept the shove from its predecessor, but it won't have a successor to stop it. Instead, it will continue past the equilibrium point and then tug on its predecessor. That weight will in turn tug on its former predecessor, and so on, until the wave has reached the starting point. Under that scenario, if the wave compressed the springs while it was going out, it will stretch them on the return.   

       If instead the spring from the last weight was fixed to an immovable object, then when the last weight pushed on it, the immovable object would push back twice as hard as one of the weights would have. The result would then be that the last weight would push back its former predecessor, etc. until the wave reached the starting point. Under that scenario, if the wave compressed springs as it went out, it would compress them on the return.   

       If there are any non-uniformities in the weights, a wave that hits a too-heavy or too-light weight will be partially reflected (the phase would depend upon whether the weight was, in fact, too heavy or too light). Although it's harder to visualize, a similar situation will occur if some of the springs are harder or softer than others.   

       The goal of good speaker design is to control reflections. They can't be eliminated entirely, but minimizing them will reduce energy loss and improve spectral performance.
supercat, Dec 17 2008

       Questions for the experts:   

       1. Why are speaker cones conical?   

       2. What is the effect of changing the cone angle?
Texticle, Dec 17 2008

       [supercat] I just read that LEDs have the same issues; the changes in refractive index (analogous to impedance mismatch) from semiconductor to capsule to air cause a lot of the light to be reflected back and wasted.   

       A nice demonstration might be to place the last ball of a Newton's cradle against a very heavy lump of steel. My prediction is that the same number of balls as are released will bounce back at the same end, as very little energy will pass into the heavy block.   

       Another way of looking at it is that the physical size of the speaker must approach the wavelength of the sound being produced to give efficiency. I like the analogy of moving a coin back and forth in water. You can make tiny close-spaced ripples well enough, but to make 1m long waves with appreciable energy you would be better off with a garbage can lid. I've heard of a cellar being converted into a huge subwoofer (a.k.a. fully sick subwoofa mate). Makes sense, when you consider that the wavelengths involved are tens of metres.   

       [texticle] I thought the cone shape was simply because if you tried to jiggle a thin flat disk by its centre it would distort rather than move as a unit. Could be wrong. Cheap cardboard speakers, eg computer internal speakers, seem to be more conical than expensive ones. Better materials would allow a flatter shape.   

       //what is a floorset// Don't know. By "the floor was set vibrating" I meant "the floor was made to vibrate".
spidermother, Dec 17 2008

       1) Better stiffness per unit mass than most other shapes.   

       2) For a fixed circumference and cone material and mass, changing the cone angle between 180 degrees/flat and (say) 30 degrees will give varying stiffness. Optimum stiffness for a given mass and material will tend to be somewhere between the angular extremes.   

       Again with other characteristics fixed, a flat cone will tend to be floppier / subject to lower frequency resonances than a conical shape. Ideally, the cone should be resonance-free, or at least have well-damped resonances above the useful driver frequency range.   

       Tweeter drivers are often dome-shaped, and woofers conical with a central dome covering the voice coil, both intended to make very stiff drivers for a given mass.   

       Ideally, a loudspeaker should produce flat frequency response (fixed acoustical output level for uniform electrical input), and low distortion.   

       I think the original post may be referring to some form of "horn loading" i.e. acoustical impedance transformation (as described by [supercat]) caused by the shape of the rooms / hallway, rather than "resonance" which would tend to give non-uniform frequency response. Try listening to sounds through paper towel roll tubes coupled to your ears. The sounds will be louder at particular frequencies, much softer at others (related to the tube length.)   

       Also note that low frequencies will tend to be enhanced by locating speakers near the corners of a room (with the walls acting somewhat like a megaphone.) At low frequencies (< 200Hz in most listenting rooms), the room and its resonances also have a large effect on the overall frequency response.   

       A similar approach was used in Edison's wax cylinder reproducer, and in old-style victrolas, which used conical horns to match the small motions of the stylus to the large volume of air in the room.   

       Incidentally, boxes and their enclosed spaces are only one of several ways to prevent bass-frequency cancellation; in-wall mounted speakers, or large flat panels have much the same effect.
csea, Dec 17 2008


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