Little help with overdamped ported RS225 alignment

[morbo]

I have just finished the cabinets for my budget 2way speaker using a TB 871 and Dayton RS225 per side. My bass alignment is meant to be overdamped, tuned to about 23-25hz in a 70-75L box. To this end purchased one of these flared ports:

http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=269-932

The price and aesthetics were right, and the generous flare should help with vent velocity right? Well maybe I jumped the gun a bit. These are labelled as 3"ID, which they are in the center. Modelling them in BAssBoxpro as a 11.5" dual flared 3" port gave me a tuning in the low 20s. However after measuring the woofer in-box with the port installed, I am getting peak port output somewhere in the 30-35hz range (33 I think). Seems like this port should be modelled as closer to a 4" ID. This is not ideal, as these speakers are meant to be used for low-level (apartment) home theater use, and after testing them in this application, I can see that there is a lot of information below the tuning frequency, and on some scenes the cones are flopping around like crazy. To their credit the RS225s seem to be near-bulletproof, but I am concerned that this will be an issue for the eventual owners of these speakers.

Does anyone have any practical ideas for how to lower the tuning frequency, bearing in mind that I've already made the huge 5 3/4" cutout for this port, and the heavy flare at the back end? My enclosures are about 16" deep internally, so I have 3-4 more inches at most behind the port to work with. So far the best I have thought of is to extend the port by gluing bristol board or somesuch to it.

[cjd]

Can you do an impedance measure? That'll pinpoint more accurately than anything the exact tuning frequency. Otherwise, to lower frequency you need to add length. If that doesn't cut it, you may need to make a plug for the hole and go with something different. Which isn't what you want to hear I know, but you could fabricate something that fits the hole and looks good if you've got a finish you can't easily fix.

C

[morbo]

cjd - I should have Z measurement capability within a week, just waiting for someone to make me some cables to serve as a 'jig' for justMLS (I am hopeless with the electronics/soldering part of this hobby). In the meantime, is this something I could measure with frequency sweeps and a multimeter?

[ThomasW]

To check the port tuning all you need is a digital multi-meter, an amp, a tone generator, and a 100 ohm 20 watt (or similar) power resistor.

Put the resistor in series with the + lead going from the amp to the woofer. Wire the negative lead as usual.

Now send 50Hz sine wave into the speaker at low volume. Then with the meter set on "AC" measure the +/- terminals of the woofer. Set the output from the amp so the meter reads between 1&2 volts.

Now lower the frequency in steps 5Hz at a time. Each time write down the reading from the meter. As you go down in frequency there will be a point where the voltage bottoms, then starts back up again with the next lower frequency. The lowest meter reading is the port tuning frequency.

So for example

50Hz-1.2V
45Hz 1.3V
40HZ 1.2V
35Hz-1.1V
30Hz-1.0V
25Hz-.75V
20Hz-.5V
15Hz-.75V
10Hz-1.1V

In this example the port tuning is 20Hz. That's where the meter reading is the lowest voltage.

Obviously you can use steps smaller than 5Hz for greater accuracy.

If you need a tone generator and have a computer close to the amp. You can use the output of the soundcard to make the tones. Click the link NCH freeware tone generator to download the old version of the program. The old version has no expiration date. The new version available from the NCH website has the ability to record test tones direct to your hard drive, but it expires in 30 days

[morbo]

thanks Thomas, I have the multimeter and NCH tone generator (agreed its great software!), I'll have a look in my parts bin for the resistor and hopefully run the sweeps tonight.

[morbo]

I finally got the right resistor to measure this... according to voltage, the tuning frequency is 27hz, which is wierd because my measurement showed the port output 'hump' at 30-35hz. I may leave it as is, as the over excursion issues seems diminished now that the drivers/port are properly mounted, perhaps because there are no air leaks anymore. I may still try to lower the tuning frequency, I tried taping in a 3" elbow which lowered the tune to 23hz, and the excursion seemed a bit better controlled on most material that way.

[Curt C]

I can see that there is a lot of information below the tuning frequency, and on some scenes the cones are flopping around like crazy.

You might consider building a passive hi-pass filter if you can accommodate that into your setup. A 12 dB/octave HP filter at 15-20 Hz will go a long way to keep those unloaded cone gyrations under control, and has almost no insertion loss.

Here's a link to the necessary calculations:

http://www.t-linespeakers.org/tech/filters/passiveHLxo.html

C

[morbo]

Curt, that is a great idea! I would love to do it, but in this case I think both end users will be going

source (optical/coax) -> Panny XR50 reciever -> speakers

so I'm not sure where I could insert it in the signal chain... I sure do wish that receivers still had defeatable rumble filters built in - I have an old Yamaha with a 50hz rumble filter that has come in handy on a number of occasions.

[JohnL]

Have you designed a notch filter for the RS225 yet? Fiddling with notch filters isn't my speciality...

[morbo]

JohnL - I am crossing pretty low (600hz) 2nd order, so I am hoping I won't need a notch filter. I have used the KxProject DSP crossover tools to emulate this, and I don't hear the break up on the woofer at all so far. That said I haven't taken proper measurements yet, or done any work on the passive XO. So I suppose I may need to do the notch, if so I'll probably let LSPCad figure out the values.

It should be said that the bass/lower mids from these woofers is just amazing! Especially considering the price point. I'm very pleased with them in this alignment, still may try to extend the port some, but they are sounding great right now.

[JohnL]

Cool, let me know how it ends up. I'm using two in a sealed box crossing at about 1200, so I think I'm gonna need a trap. I may have to figure out how to do this myself. I've always used pretty well behaved drivers in the past; however you're right, they sound very good.

John