Reflex port tuning below Fs?
- Thread starter ivanlukic
- Start date
Not at all.
You don't tune a port below Fs, you tune an enclosure; the overall LF alignment is the summed interaction of it and the driver's electromechanical characteristics. It's quite normal to see box tunings < Fs, nothing unusual about that at all. In fact, I'd guess at about 50% of vented boxes are so tuned.
You don't tune a port below Fs, you tune an enclosure; the overall LF alignment is the summed interaction of it and the driver's electromechanical characteristics. It's quite normal to see box tunings < Fs, nothing unusual about that at all. In fact, I'd guess at about 50% of vented boxes are so tuned.
This works best with drivers that have a Qts at the higher end of the range suitable for reflex tuning. I use drivers with an fs of 42 Hz and Qts of 0.44 in a box that is larger than Vas, tuned to 30 Hz. I would get a louder box with higher power handling if using a slightly smaller volume and higher tuning frequency. But with my tuning I get better low-end extension.
Regards
Charles
Regards
Charles
Another way of looking at it that might be helpful for you..
The driver has its own set of physical characteristics. The stiffness of the surround, the stiffness of the spider. The magnetic "grip" of the motor system. The mass of the cone. All these things interact with the air mass trapped behind the driver when mounted in a box to produce a response.
When you put a port in the box it adds another variable. The port is just a small mass of air that resonates at a single frequency. So asking where you tune that frequency in relation to just one parameter and how will it sound is a lot like asking "how fast will my car go with four cylinders?" you need to understand the system first. See how you go with this:
Thiele / Small parameters explained with real world cases
P.S: There are lots of modelling tools available to help you get a good "alignment" for your driver. Basically, there will be a narrow window of box volume and port tuning that will give you the best compromise for bass extension, transient response and box size. The further you move away from that "alignment" the worse one, or all of those characteristics will be.
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I guess it's not just the air mass within the port(s) that tune the box. For a given box capacity, a wide and short port will yield another tuning frequency than a narrow, but longer port with the same capacity.
To the OP: I suggest you to use WinSIM or another comparable tool to guesstimate the box's and driver's combined performance.
Best regards!
Knowledge is power. Instead of asking a bunch of random questions like "will my driver work in XX box with YYY tuning", get yourself together man and learn to use Box Modeling programs. There you can answer these questions all by yourself, and learn something in the process. It's really not difficult!
Here are some free, simple ones that you can try:
Unibox: UniBox - Unified Box Model for Loudspeaker Design - Kristian Ougaard
The Woofer Box and Circuit Designer: Loudspeaker Design Software
When modeling a vented box, look at the vent velocity at full power, cone excursion, and include the effect of port resonance when possible (in Unibox). The vent velocity should stay below about 17m/s, and cone excursion should not exceed Xmax around the box tuning frequency and slightly below that. At tuning there should be a smooth rolloff without a hump in the response (which will lead to ringing). These are really all the "suitability" criteria that you need to look at.
Here are some free, simple ones that you can try:
Unibox: UniBox - Unified Box Model for Loudspeaker Design - Kristian Ougaard
The Woofer Box and Circuit Designer: Loudspeaker Design Software
When modeling a vented box, look at the vent velocity at full power, cone excursion, and include the effect of port resonance when possible (in Unibox). The vent velocity should stay below about 17m/s, and cone excursion should not exceed Xmax around the box tuning frequency and slightly below that. At tuning there should be a smooth rolloff without a hump in the response (which will lead to ringing). These are really all the "suitability" criteria that you need to look at.
There are some merits in tuning a ported cabinet a bit lower than usual - if port resonance doesn't become an issue of course. Group delay is a bit better and response has gradual drop (lower Q) so it works better in small rooms (and even in some large ones). Curt explained it with few examples using WinISD simulator.
group_delay
Other than what Curt wrote, i'd add that lower tuning (preferably bellow 30Hz) also moves minimum motion notch and maximum port air velocity, lower in frequency. Moving minimum motion notch somewhere where there isn't much music material maybe doesn't look like a big deal but at higher volumes and the right track playing the coil will not move enough to cool itself and by that adding to power compression or burning the voice coil, in the worst case scenario.
group_delay
Other than what Curt wrote, i'd add that lower tuning (preferably bellow 30Hz) also moves minimum motion notch and maximum port air velocity, lower in frequency. Moving minimum motion notch somewhere where there isn't much music material maybe doesn't look like a big deal but at higher volumes and the right track playing the coil will not move enough to cool itself and by that adding to power compression or burning the voice coil, in the worst case scenario.
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I'm on the same case with you. My mini desktop speakers uses 4" SB woofer with fs~ 61Hz. Qts ~0.48 and the box are tuned at around 52Hz.
I think with pretty high Qts (maybe above 0.45). It might be tuned fb lower than fs.