Interesting commentary about IB subs
Apr 10, 2006 17:07:30 GMT -7
Post by ThomasW on Apr 10, 2006 17:07:30 GMT -7
Doug Purl is a long time member of the DIYspeaker list (formerly the bass-list). He recently posted this in response to a question about the difference in distortion between a manifold and a line array IB.
Doug was most helpful when I designed my first IB and I take great pleasure reading his posts, given his way with words..
"Mounting woofers under the floor in either a crawl space or especially in a basement conduces to an ideal subwoofer. A chimney stack is favored by many because it is an easy structure to build, permits mounting drivers across from each other for vibration cancellation, and allows easy access to the drivers should a fault develop after using your 10KW amp.
Although one can calculate an ideal size for the manifold entering the listening room, I don't think it will matter sonically if it is anywhere in the range approximating the area of the subwoofer cones summed. Those are felt frequencies, and a robust whole-house subwoofer will rattle the building long before any distortion components will become audible.
The building will mask the subwoofer's trivial misbehavior. And it is unlikely that the distortion will get to audible levels anyway, presuming quality drivers. Besides, the ear tolerates a lot of distortion down there, at SPL levels where your ears will say uncle before the system distortion reaches the threshhold of perception.
The manifold at its point of entry functions as a virtual driver. Some fellows like to guild the lily and avail their system of the benefits of having the manifold speak into a reduced angle, such as next to a wall or in a corner. Both cone excursion and amplifier power requirements are eased considerably by such an arrangement. If one wants a large opening into the listening room (really, the entire house will become the listening room for the bottom octaves), it is possible to span three floor joists, giving a width of a bit over 28 inches. The joist straddling the manifold will cause no audible effect, and indeed furniture that sits a few inches above the floor can sit over the manifold and conceal it. You must screen the manifold mouth to prevent creatures from disappearing down there, such as pets, wives, children, and vermin.
As I have tediously mentioned before, it is also possible to build a subwoofer into the attic. The rafters or roof trusses will need cross bracing (collars in carpenter lingo). If the attic is poorly ventilated the drivers will be subjected to temperature and humidity cycling. If you have full-perimeter soffit intake venting and full-ridge exhaust venting along the peak of your roof you will have less temperature rise than in the older fashion of buttoned-up attics and will probably grow old without needing to recone. It is now understood in the construction industry that the name of the game with attics is to keep as much as possible the roofing material at the same temperature as the underside of the roof. That requires two things: good ventilation across the entire underside of the roof and substantial insulation between the living space and the attic. The house proper and the attic system are separate -- or should be -- and just happen to lie adjacent to one another. If your house relies on gable vents and/or a few vents penetrating through the roof, I guarantee you have inadequate attic ventilation.
Has anyone compared the distortion levels
> of the manifold design against that of a flat baffle?
Dick Greiner at U. Wisconsin Madison had students measure his in-wall systems (multiple Altec 15" woofers in very large boxes mounted in the wall abutting a massive flagstone chimney and hearth. One of my son's high-school buddies studied for his EE under Greiner, and he reported the system as effortless and awesome. A true infinite baffle is a huge device suspended in space extending from here back to the Big Bang and out to the next universe and beyond. A practical baffle is never infinite, but it is virtually infinite. By the time a wave got around a real infinite baffle and mixed with its counterpart, you wouldn't care.
A flat baffle in a room wall would act the same as one in the floor.
Doug Purl"
Doug was most helpful when I designed my first IB and I take great pleasure reading his posts, given his way with words..
"Mounting woofers under the floor in either a crawl space or especially in a basement conduces to an ideal subwoofer. A chimney stack is favored by many because it is an easy structure to build, permits mounting drivers across from each other for vibration cancellation, and allows easy access to the drivers should a fault develop after using your 10KW amp.
Although one can calculate an ideal size for the manifold entering the listening room, I don't think it will matter sonically if it is anywhere in the range approximating the area of the subwoofer cones summed. Those are felt frequencies, and a robust whole-house subwoofer will rattle the building long before any distortion components will become audible.
The building will mask the subwoofer's trivial misbehavior. And it is unlikely that the distortion will get to audible levels anyway, presuming quality drivers. Besides, the ear tolerates a lot of distortion down there, at SPL levels where your ears will say uncle before the system distortion reaches the threshhold of perception.
The manifold at its point of entry functions as a virtual driver. Some fellows like to guild the lily and avail their system of the benefits of having the manifold speak into a reduced angle, such as next to a wall or in a corner. Both cone excursion and amplifier power requirements are eased considerably by such an arrangement. If one wants a large opening into the listening room (really, the entire house will become the listening room for the bottom octaves), it is possible to span three floor joists, giving a width of a bit over 28 inches. The joist straddling the manifold will cause no audible effect, and indeed furniture that sits a few inches above the floor can sit over the manifold and conceal it. You must screen the manifold mouth to prevent creatures from disappearing down there, such as pets, wives, children, and vermin.
As I have tediously mentioned before, it is also possible to build a subwoofer into the attic. The rafters or roof trusses will need cross bracing (collars in carpenter lingo). If the attic is poorly ventilated the drivers will be subjected to temperature and humidity cycling. If you have full-perimeter soffit intake venting and full-ridge exhaust venting along the peak of your roof you will have less temperature rise than in the older fashion of buttoned-up attics and will probably grow old without needing to recone. It is now understood in the construction industry that the name of the game with attics is to keep as much as possible the roofing material at the same temperature as the underside of the roof. That requires two things: good ventilation across the entire underside of the roof and substantial insulation between the living space and the attic. The house proper and the attic system are separate -- or should be -- and just happen to lie adjacent to one another. If your house relies on gable vents and/or a few vents penetrating through the roof, I guarantee you have inadequate attic ventilation.
Has anyone compared the distortion levels
> of the manifold design against that of a flat baffle?
Dick Greiner at U. Wisconsin Madison had students measure his in-wall systems (multiple Altec 15" woofers in very large boxes mounted in the wall abutting a massive flagstone chimney and hearth. One of my son's high-school buddies studied for his EE under Greiner, and he reported the system as effortless and awesome. A true infinite baffle is a huge device suspended in space extending from here back to the Big Bang and out to the next universe and beyond. A practical baffle is never infinite, but it is virtually infinite. By the time a wave got around a real infinite baffle and mixed with its counterpart, you wouldn't care.
A flat baffle in a room wall would act the same as one in the floor.
Doug Purl"