I recently read the recommended enclosure tuning. Although the formula was correct, the reason for creating ported over sealed was somewhat confusing. If you port a box, it doesn't give either more low end extension or more spl, it does both. It can give more low end extension and spl over a sealed design because you are tuning it to a specific frequency. No matter where you tune the enclosure, it will narrow the bandwidth but increase spl. Below the resonant frequency, the woofer will be completely uncontrolled and above it, you will suffer a severe drop off in output. This isn't always bad as subwoofers are usually cut off above 70hz and any wise person would use a subsonic filter set between 15 and 20hz.
The article also stated that when tuned to a specific resonant freq. the port vibrates in phase and the woofer cone is virtually motionless and all of the sound comes from the port, hahaha, anyone who has ever designed a ported enclosure knows that the air moves at the same rate from the surface of the woofer cone as it does from the port opening. If the cone remained motionless, the air wouldnt come out of the port! The term in phase means that both the air from in front and behind the woofer is exiting the enclosure at the same speed!
One more addition, you tune the port to the enclosure, not the sub. The surface area of the port will vary with box size, but roughly, for optimum efficiency, you will use about 15 to 16 inches squared for each cubic foot of box volume and at 16 inch port length, the box will generally be tuned near 40 hz. Changing port length changes the tuning freq. longer equals lower freq. tuning, shorter equals higher freq. tuning.
Replies (1)
swez on 10/29/2006 08:31:05
Agreed, ported/vented enclosures have some benefits and drawbacks over sealed designs in both SPL and low bass extention abilities. We may sacrifice some power handling of the sub when port tuning is very low and enclosure get very large.
Also, as we get near port tuning frequency of a given design, the cone does move less and more SPL output is noted at the port and not as much output from direct cone motion. This is due to acoustical wave coupling from the cone and port, in a give size Vb of an enclosure. (Vb is enclosure size available, minus the port/vent and woofer displacements)
I believe I read the same article you are referencing. That article was pretty good, but is very general in scope and nature. It's a good overview for sure, but the devil is in the details of a specific enclosure design and sub pairings.
It also seemed to me, they referenced a general rule of thumb for determining port displacements as being more like 12 in^3 for a typical 1 ft^3 box. That depends on whether we are talking about flared ports VS high SPL vents. either way, the key elements are still Vb, port diameter, port/vent length and calculated port/vent area. Here, we want low air velocity at the port to minimize port noise issues.
Swez
PS Welcome to CK!
The article also stated that when tuned to a specific resonant freq. the port vibrates in phase and the woofer cone is virtually motionless and all of the sound comes from the port, hahaha, anyone who has ever designed a ported enclosure knows that the air moves at the same rate from the surface of the woofer cone as it does from the port opening. If the cone remained motionless, the air wouldnt come out of the port! The term in phase means that both the air from in front and behind the woofer is exiting the enclosure at the same speed!
One more addition, you tune the port to the enclosure, not the sub. The surface area of the port will vary with box size, but roughly, for optimum efficiency, you will use about 15 to 16 inches squared for each cubic foot of box volume and at 16 inch port length, the box will generally be tuned near 40 hz. Changing port length changes the tuning freq. longer equals lower freq. tuning, shorter equals higher freq. tuning.
Replies (1)
swez on 10/29/2006 08:31:05
Agreed, ported/vented enclosures have some benefits and drawbacks over sealed designs in both SPL and low bass extention abilities. We may sacrifice some power handling of the sub when port tuning is very low and enclosure get very large.
Also, as we get near port tuning frequency of a given design, the cone does move less and more SPL output is noted at the port and not as much output from direct cone motion. This is due to acoustical wave coupling from the cone and port, in a give size Vb of an enclosure. (Vb is enclosure size available, minus the port/vent and woofer displacements)
I believe I read the same article you are referencing. That article was pretty good, but is very general in scope and nature. It's a good overview for sure, but the devil is in the details of a specific enclosure design and sub pairings.
It also seemed to me, they referenced a general rule of thumb for determining port displacements as being more like 12 in^3 for a typical 1 ft^3 box. That depends on whether we are talking about flared ports VS high SPL vents. either way, the key elements are still Vb, port diameter, port/vent length and calculated port/vent area. Here, we want low air velocity at the port to minimize port noise issues.
Swez
PS Welcome to CK!
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