RMC
02-22-2017, 09:13 PM
The vented box (often called bass-reflex or tuned port box) has some advantages over the closed box (referred to as acoustic suspension). An important advantage is the reduced woofer cone motion at and on both sides of the box tuning frequency. This results in lower driver distortion over most of the low frequencies, since the vent moves a lot of the air at these frequencies (reduced cone travel).
For a given box volume, finding and using the right ducted port (diameter/length/thickness) can be a pain in the neck (if required vent is too big or small or too long to resonate at desired frequency), specially in small vented enclosures. Sure, sometimes you may use multiple vents but that option doesn't solve every problem plus they require more material, more available space and more work...
At some point something may have to give a little to make the thing fly! The "art" is finding an acceptable compromise, for a given driver or existing box, among the following: VB (box volume), F3 (3db down point in low frequencies), FB (box tuning frequency) and vent Mach (velocity of air in the vent, to prevent noise). You may have to run different scenarios in speaker building software, as I do playing around with these parameters. But knowing ahead of time some basic rules regarding vents, will lead you in the right direction (i.e. guide your search for a solution) and save you some time:
. Increasing vent diameter (Dv +) = increasing box tuning frequency (Fb +), also lowers vent Mach
. Reducing vent diameter (Dv -) = reducing box tuning frequency (Fb -), also increases vent Mach
. Increasing vent length (Lv +) = reducing box tuning frequency (Fb -)
. Reducing vent length (Lv -) = increasing box tuning frequency (Fb +)
Note the direct relation between the first two items, and the opposite relation for the last two. Now based on the above, let's complicate things a little bit like in real life. For a fixed or constant Fb:
. If Dv + then Lv + , if you increase Dv then you have to increase Lv in order to maintain same Fb
. If Dv - then Lv - , if you reduce Dv then you have to reduce Lv in order to maintain same Fb
. If Lv + then Dv + , if you increase Lv then you have to increase Dv in order to maintain same Fb
. If Lv - then Dv - , if you reduce Lv then you have to reduce Dv in order to maintain same Fb
Maximum vent diameter is often dictated by practical considerations such as space available on the cabinet's front panel, acceptable vent Mach, vent tubes on the market, etc. On the other hand, vent length is often limited by obstacles nearby in the box: rear panel, braces, damping material, etc. Though a vent tube with an elbow will also work fine. Vent Mach number may turn from red to green if the 300 watt driver you purchased (for other characteristics than power) will be used with a 100 or 200 watt amp. Plug that number instead in the software to maybe see yourself out of the ditch...
At the computer design phase, you may also want to try a small increase in box volume and/or a small change to Fb to help you find or fit an acceptable ducted port in the enclosure, with minor effect on low-end frequency response. I don't lose sleep over a < 1db bump in response for example.
Box tuning frequency (Fb) may also be a helpful or desirable parameter to play a little with when designing a sound reinforcement speaker for example: for a dance music speaker, designing with a built-in bump (amplitude) in low frequencies gives punch or impact to bass sounds that get people moving, with no extra cost in terms of watts from power amplifier. The bump may be "removed" with EQ when not required, thereby saving some watts in the low end. Naturally, the other way of achieving this bump when needed (supposing you designed for flat response) is with EQ, but at a cost for more watts required and more strain on the woofer.
See if you can logically make Fb correspond to a setting on your graphic equalizer (like 40HZ or 63HZ where you can add -as required- some amplitude for impact or punch to the bass), knowing that in this region of frequencies most of the air moving is done by the vent therefore minimizing the strain on your woofer (cone travel).
I'm doing a small 2-way sound reinforcement project now with a 10" Pro woofer/1" Pro dome in horn tweeter in a box of 36.25 L. net (1.28 cu. ft.), F3 at 58HZ, with an Fb at 62HZ and my EQ has sliders at 63 HZ ! Though powerful at 300+ watts these woofers have limited cone travel (Xmax 4.75 mm), so this is where Fb/vent help will be welcome when used as stand alone units, or I can use them in a bi-amp setup with JBL 15" 2205H bass cabinets I've also made.
Richard
For a given box volume, finding and using the right ducted port (diameter/length/thickness) can be a pain in the neck (if required vent is too big or small or too long to resonate at desired frequency), specially in small vented enclosures. Sure, sometimes you may use multiple vents but that option doesn't solve every problem plus they require more material, more available space and more work...
At some point something may have to give a little to make the thing fly! The "art" is finding an acceptable compromise, for a given driver or existing box, among the following: VB (box volume), F3 (3db down point in low frequencies), FB (box tuning frequency) and vent Mach (velocity of air in the vent, to prevent noise). You may have to run different scenarios in speaker building software, as I do playing around with these parameters. But knowing ahead of time some basic rules regarding vents, will lead you in the right direction (i.e. guide your search for a solution) and save you some time:
. Increasing vent diameter (Dv +) = increasing box tuning frequency (Fb +), also lowers vent Mach
. Reducing vent diameter (Dv -) = reducing box tuning frequency (Fb -), also increases vent Mach
. Increasing vent length (Lv +) = reducing box tuning frequency (Fb -)
. Reducing vent length (Lv -) = increasing box tuning frequency (Fb +)
Note the direct relation between the first two items, and the opposite relation for the last two. Now based on the above, let's complicate things a little bit like in real life. For a fixed or constant Fb:
. If Dv + then Lv + , if you increase Dv then you have to increase Lv in order to maintain same Fb
. If Dv - then Lv - , if you reduce Dv then you have to reduce Lv in order to maintain same Fb
. If Lv + then Dv + , if you increase Lv then you have to increase Dv in order to maintain same Fb
. If Lv - then Dv - , if you reduce Lv then you have to reduce Dv in order to maintain same Fb
Maximum vent diameter is often dictated by practical considerations such as space available on the cabinet's front panel, acceptable vent Mach, vent tubes on the market, etc. On the other hand, vent length is often limited by obstacles nearby in the box: rear panel, braces, damping material, etc. Though a vent tube with an elbow will also work fine. Vent Mach number may turn from red to green if the 300 watt driver you purchased (for other characteristics than power) will be used with a 100 or 200 watt amp. Plug that number instead in the software to maybe see yourself out of the ditch...
At the computer design phase, you may also want to try a small increase in box volume and/or a small change to Fb to help you find or fit an acceptable ducted port in the enclosure, with minor effect on low-end frequency response. I don't lose sleep over a < 1db bump in response for example.
Box tuning frequency (Fb) may also be a helpful or desirable parameter to play a little with when designing a sound reinforcement speaker for example: for a dance music speaker, designing with a built-in bump (amplitude) in low frequencies gives punch or impact to bass sounds that get people moving, with no extra cost in terms of watts from power amplifier. The bump may be "removed" with EQ when not required, thereby saving some watts in the low end. Naturally, the other way of achieving this bump when needed (supposing you designed for flat response) is with EQ, but at a cost for more watts required and more strain on the woofer.
See if you can logically make Fb correspond to a setting on your graphic equalizer (like 40HZ or 63HZ where you can add -as required- some amplitude for impact or punch to the bass), knowing that in this region of frequencies most of the air moving is done by the vent therefore minimizing the strain on your woofer (cone travel).
I'm doing a small 2-way sound reinforcement project now with a 10" Pro woofer/1" Pro dome in horn tweeter in a box of 36.25 L. net (1.28 cu. ft.), F3 at 58HZ, with an Fb at 62HZ and my EQ has sliders at 63 HZ ! Though powerful at 300+ watts these woofers have limited cone travel (Xmax 4.75 mm), so this is where Fb/vent help will be welcome when used as stand alone units, or I can use them in a bi-amp setup with JBL 15" 2205H bass cabinets I've also made.
Richard