I'm in the process of building a 3-way speaker. The ports I'm using are somewhat tapered, which makes it harder to determine the correct length. I know such ports can be modeled in Hornresp, but I don't know how. Also, as Troels notes, the required port length to achieve a desired Fb can be significantly shorter than the calculated length (depending somewhat on the amount of damping material).
My plan of attack for now is to make multiple nearfield measurements (port + driver summed). Starting with the port a bit longer than the calculated length, and shortening it 5mm for each consecutive measurement. This approach seems labourous to me, I hope there's an easier way to determine the right port length based on just one FR & Z measurement.
My plan of attack for now is to make multiple nearfield measurements (port + driver summed). Starting with the port a bit longer than the calculated length, and shortening it 5mm for each consecutive measurement. This approach seems labourous to me, I hope there's an easier way to determine the right port length based on just one FR & Z measurement.
Calculate port length with the smaller inner diameter of the tapered port. Than make the measurements.
Hi Sonse, thanks for your reply. But it would seem to me that I'd cut the port too short if I used the smaller side ID for the length calculation?
What is your purpose? Having the port cut right for a specified tuning? or find the best sounding tuning?
If the former, you just need to trace the impedance to find the tuning.
Ralf
I'm aiming for the lowest f3. Have been using WinISD to roughly model the box for my driver (Dayton SD215A-88 in a 36L box, fb = 31Hz). This is close to the QB3 alignment. The resulting acoustic rolloff is nice and gradual.
I'm starting to think it's impossible to arrive at the optimal tuning and corresponding port length without taking multiple measurements around the calculated 'ballpark' port length. Just too many variables. (actual T/S parameters, damping influences, unknown effects of shortening a tapered port, etc.)
F3 in acoustics doesn't have any special purpose. Best to use F6 (and maybe F10). Aiming for the lowest F3, and putting that speaker in a real room produces a ugly one-note-bass. This is what Zaph said about tuning an enclosure: http://zaphaudio.com/mantras.html
<< For vented boxes, the box size and tuning frequency that I select is rarely what a box modeling program might consider optimum. For folks new to DIY speakerbuilding, the first software tool they often use is a freeware box modeling program like WinISD. The bad news is that what WinISD considers optimum is merely the the alignment that results in the lowest F3 in anechoic conditions. These programs don't consider room response, power handling, group delay or any of a number of other issues when they pop out a "optimum" box size and tuning frequency. So don't email me saying "Why did you do that when WinISD says this is optimum?" The answer is: Because I'm smarter than WinISD.
The vented alignment I typically use doesn't have a name. I'm sure I didn't invent it, but I haven't seen much out there to describe it. Normally, I'll tune lower to get a more gradual rolloff that works better with room response and puts power handing where I need it. It's basically a type of tuning that is a combination between sealed and vented alignments. Note that this is not an Extended Bass Shelf (EBS) tuning. That kind of tuning implies an oversized enclosure volume, with a dip above the tuning frequency. You get the lowest anechoic F3 with that configuration, but in a real room you get an overbearing one-note bass at the tuning frequency, a huge midbass dip, and crappy power handling. >>
But again, if your purpose is to cut the port to provide the tuning you have simulated, then you only need to trace the impedance. The tuning is the frequency of the valley between the two peaks.
If you want to find the best sounding tuning, then you need to listen to the speakers for a long time before making another cut at the ports...
Ralf
Don't even need impedance, just use a frequency generator app to find at what Hz the woofer stops moving.
Someting along the lines of what GM suggests is to put the measurement mic very close to the woofer cone and as far away from the port as possible and measure the frequency response. It should show a dip where the tuning frequency is.
Regards
Charles
Regards
Charles
I understand your point. But I consider room acoustics a mess. Any listening position will have peaks at some frequencies, and troughs at others. In the past I tried fixing the curve at the listening position (by DSP), which didn't really work out, as canceling waves will keep canceling irrespective of the amplitude (also, it completely messes up the response in other listening positions). Also, I'd like speakers to sound good in any room, not just the one it was designed for.
But maybe there is a general rule of thumb for taking a room into account?
I was suggesting to trace the impedance only because for me it is the easiest method. YMMV
Ralf
Yes, aiming for the lowest (anechoic) F3 is the wrong one. A more shallow slope generally interacts better with the room and produces lower F6 and F10 points.
Ralf
Ah, understood. I'll aim for a more shallow slope from now on.
Thanks for the suggestion, but the cabinets have already been made (and I don't feel like building new ones).