DIY Tempest FR

gatemplin

Merry Christmas everyone! I know many here are busy this time of year but I am hoping someone (ie Dr.Spec, Ron-P) can help me out.

I am wondering if I am missing output in the low 20's with my current sub box. I hear people speak of the wave in the PotC when the medallion is in the water or the "ring drop" in the LotR FotR when Sauron's finger is cut off and hits the ground. I think I could have more ouput here. I just want to make sure I am not missing anything.

My sub is a ported Tempest in a net 128L box which is very well braced. The external port is 8" by 53" with no port noise and tuned to 22 Hz. The sub is powered by the 250W AVA250 Adire Audio plate amp. The VCs are wired in parallel (4ohms).

For the test I set the Harman AVR sub out to -6dB and all speakers small (100hz). I ran the test tones only with the sub on and crossover defeated, phase left at 0. I started adjusting the gain so that the sub produced 75 dBs at 100 Hz.

The graph includes extrapolated radio shack spl correction factors. The spl meter was tripod mounted at listening position.


[IMG]http://C:\Graham\Club Polk\graph2.gif[/IMG]


I am wondering if this looks right considering the sub and tuning frequency. Is the port too wide? Tuned too high etc. I have never bottomed it out or even pushed it into audible clipping. It is always clean and unstrained except near the 50hz hump.
Any comments / suggestions would be greatly appreciated.

Edit: cripes, the axis are labelled incorrectly but you get the picture.

goingganzo

that is a realey nasty graph wow the amp might have a bult in bass boost and you might have some realey nasty room gain. i would say move the sub to the center of the room run a test tone and go around the walls of the room and mesure the output. pick 2 or 3 spots and take the mesurements with the sweep there. put the sub where you get the flatest responce.

gatemplin

Thanks for the advice Gonzo. I dont think it is my amp because I picked it because it has very little bass boost. I would think it would be the room or position. I will try other positions but am limited unless I change my room around.

pjdami

I am wondering if I am missing output in the low 20's with my current sub box. I hear people speak of the wave in the PotC when the medallion is in the water or the "ring drop" in the LotR FotR when Sauron's finger is cut off and hits the ground. I think I could have more ouput here. I just want to make sure I am not missing anything.

On the ring drop what kind of data did you get? The actual ring drop and the implosion scene which follows afterwards should be about the same db. The ring drop is in the low 20s and the implosion scene is more like 40 hz. I registered 98 db on my Hsu STF-2 sub on the ring drop and 106 db on the implosion scene. So the STF-2 is "checking out early" per what Dr. Spec advised me.

Do you have your sub and your main speaker calibrated using Avia or the Sound and Vision DVD? I had a thread a couple of days ago where Dr. Spec helped me a lot in this area and reference volume. Might help you if you take a look at it.

Yes, your graph does look strange. If you want you can email me or PM me your raw data and I can send you back the spreadsheet I made which has the interpolated adjustment factors for the RS meter adjustments.

Paul

Dr. Spec

Ugh....that looks really rough. A very sharp roll-off at 40 Hz. The Fb of the enclosure does not mean it will be flat to that frequency. What is the Q of the final alignment? Did you calculate it? This looks like a classic underdamped (high Q) alignment with a huge hump and a nosedive rather than a gradual roll-off characterized by a low Q critically damped alignment.

Edit:

Also, the internal volume is way too low for that length vent and the chosen tune point. As Vb increases, the efficiency at Fb will also increase. Regardless of the final Q of the alignment, using a Vb that low will require EQ to obtain a flat response down to Fb. Size DOES matter when you are talking subs. I'd say you should be more in the 200L range with much shorter dual 3" flare vents.

Doc

gatemplin

Paul,

I dont have the whole system calibrated yet and dont have a calibration disc. Thanks for the offer on the spreadsheet, I'll take you up on that when I get a chance.

Ed,

I like building the boxes to get more performance. This started out as a sealed box, Q of 0.69. I wanted more output down low so I then added three 3" straight ports but there was too much port noise. I went with a large 8" port but you are right, I never calculated the new Q. I knew that Vb was low compared to port size but was not aware it made that much of a difference. Thank you very much for your help. I think I will try a Adire design next. I was quite dissappointed at the output near Fb.

Thanks to all.

Dr. Spec

No problem.

And straight vents will get noisy long before flared ones do. Going from memory but 17 m/s and 26m/s is the difference between the two before audible port compression occurs. Definitely use flared vents on the next iteration, along with a much larger Vb (yes, it makes a huge difference).

gatemplin

Thanks again Doc, that was what I was looking for. I knew there was a problem but didn't know where. I am almost certain that the next box will be the EBS or maybe the Adire Alignment. And this time I will use flared ports. The freq hump is so annoying on my current sub that I use the lowpass crossover at 40 Hz to avoid it. I will seal the box again tomorrow until the new one is complete.

goingganzo

i would go with a 250l box tuned to 18-22hz depending on taste. when the time comes to buld stryke force mkII i will tune it to 20 hz 17 was a little low but it is a great sub.

gatemplin

Ganzo are you going to use the av15 mkII drivers?

I built the Adire EBS alignment design for the Tempest. It is 340L net with two 4" flares tuned to 15.8 Hz. I am waiting on the ports to arrive then I will retest and post my results.

goingganzo

i will be when i do my upgrade. i have 2 of the origonal drivers. actuley 3 if i count the 1 in my car. but i have plans for my old drivers. my sister loves my ht and when she bulds a house wants one so i will take my drivers and buld her a sub with 1 and tahe the other and but it in my car maybe. or maybe get 4 tuluments lol now that would be killer