Tuning dilema with new sub

[Brownee]

Hi all, I just wonder if someone can give me some help please.

I am in the process of building a sonosub from a Rythmik servo kit.

The designer calls of a 3 c/f enclosure tuned to 20-22 Hz. I have done all the calculations using various software for the port length ( about 632mm for a 100mm ID pipe) and have come to the tuning phase of the build. I have used two methods to check the FB of the enclosure. 1. Measuring the impedance curve with speaker workshop and the other running a sweep and using a SPL meter to determine the highest port output. Both have come up with a higher actual tuning than the calculated port length (it measures 26 Hz). Can anyone tell me if this is normal to have so much variance from the calculations or point me to some things I can check?

Thanks

[auplater]

Several things come to mind...

did you use the published specs for the speaker, or actual measured specs (Qts, Fs, etc.) when you calculated the port length?

did you include the port area in the enclosure volume and/or correct for the driver volume?

have you tried adding length to the port to lower the box tuning and then measuring again?

[Collo]

Hi there Brownee,

there are a few factors that may have caused your problem

1 - Did you subract the space taken up by the port and driver to get your "working volume" ? If not, your port will be too short, resulting in a higher tune.

2 - Did you model as "one end flanged" - this changes the end correction applied.

3 - Did you use flared ports? - The "effective length" changes slightly.

Grab your self a copy of "sonosub.exe" which does all this for you:
http://www.users.bigpond.com/bcolliso/sonosub.htm

BTW I'm also in Newcastle - drop me a line

regards
Collo

[owdi]

Have you broken in the driver? The fs can fall by half an octave while the suspension loosens up. Try removing the driver from the enclosure, and measure it's impedance. You'll probably find fs is much higher than in the specifications.

Dan

[AKN]

Hi,

Driver parameters is not going to alter box tuning frequency, it is strictly determined by effective box volume and port.
It think Collo is right on for an possible explanation.

[auplater]

Originally posted by 4fun
Hi,

Driver parameters is not going to alter box tuning frequency, it is strictly determined by effective box volume and port.
It think Collo is right on for an possible explanation.

that's true... I agree, guess I wasn't clear by saying you shouldn't include port area if you did...sounds like a miscalc of the box volume.

I was wondering where the 20 - 22 hz tuning came from re: driver specs...which is why I asked.. So tune lower by increasing port length

[Brownee]

Thanks all for the suggestions, but I think I have most of them covered.

Thanks Anders that’s what my research has led me to. As I understand it, tuning is a direct relationship between port diameter/length and the effective volume of the enclosure so it shouldn’t matter what driver I use.


Hi Collo, yep, I got your great little program. (I’m very impressed I must say). I used it to determine the rough sizes for the build and then done the calc manually as it is not a standard build. You actually made the port for me a while back and have only now got around to building the sub. I pretty sure I have taken all your points into consideration. I will run through what I did and maybe you can pick the mistake.

I don’t have the driver parameters for the driver as it is a kit servo design from here
http://www.rythmikaudio.com/servo_product.htm

Actually the T/S parameters are listed but not needed as the designer specifies that for the ported kit you need 3 cu ft tuned to 20-22 Hz.

The design calls for 3cf which is 85 ltr. The tube I am using is 405mm ID. I don’t know the displacement of the driver but allowed 3 ltr as other 12” drivers seem to be around that. I have also allowed for the amp box that is built into the side of the tube and the internal braces. It is difficult to calculate but after a bit of math and a bit of estimate I allowed 5 ltr which I think is generous. I allowed 4.5 ltr for the 100 ID port as only 500mm of the port is in the enclosure. I have let the port extend outside the enclosure to provide clearance to the driver and will cover the overhang with an end cap assembly to make it look nice.
So for tube length calculations I would use 85( spec vol.) +3( driver) +5( ampbox & bracing) +4.5(port vol)= 97.5 ltr to work out the length of the enclosure. So using some simple math(that I have checked numerous times) comes up with a internal tube length of 760mm. I am reasonably sure that volume is correct or even a tad larger.
It is easy for me to adjust the length of the port to get the correct tuning but it is just that it will be quite a bit longer than the calculations suggest it should be. I might measure the sub impedance again tonight to make sure the I am doing correctly.

I can post some photos if you would like.

Thanks again
Brownee

[Collo]

I'm glad to hear you've finally gotten the thing built

Some photos and / or a dimensioned drawing would be good.

If you're allowing the port to extend beyond the endcap, you should model it in WinISD as "Two free ends"

Re-checking your measurements would be a good idea. Something doesn't add up!