First time newbie questions on Coax Sealed box

[AC439]

I have just put together a pair of sealed box full range speaker. I consider this is my first serious speaker project. I have a question about amount of stuffing material to use. I have searched the forum but unable to find info.

Here's some details:

Driver: MTX Oaktron 8" FR paper cone woofer with cloth accordian surround. The tweeter is coax at the center of the woofer. It is about 1/2" diameter with only a 3uF cap in series to block low frequency. I got them from Ebay.

Box: Sealed, 3/4" MDF. Inside dimension: 8.5"(w) x 12"(d) x 16" (h). Speakers mounted at the top half of the box equal distance from 3 sides.

Investment: minimal, $20 for both speakers and $15 for MDF and a few dollars for fiber glass.

Initial test: I put them together without any stuffing and drive them with my Sonic T-amp. First impression is they are very loud. Excessive high end reproduction but not bad sound.

Then I put a 4ohms series resistors (trial and error) to each tweeter and they sound more natural but still a little too much high. However, I like the sound better now and I'm comparing them to a pair of Tannoy 611 II floor speakers. I know this is a relative easy adjustment and I just need to try different resistor values to get a balance sound.

Today, I went to Lowes and got some fiber glass insulation material and stuffed all the inside of the box. The thickness of the insulation is 3 inches. After stuffing all the interior sides, there is very little free space left inside, maybe no more than 10%. Immediately sound test shows the efficiency of the woofer lowered (I have to turn the volume louder to get the same level). The bass output seems reduced compare to pre-stuff condition. I believe I put in too much stuffing.

This is probably another trial and error situation but what is the general guideline of the amount of the stuffing used on a sealed box design? 50%? 25%? 75%?

Thanks and please help.
-AC

[AC439]

As I am waiting for comments, I did some more search on internet . Definitely I believe I had over-stuffed the box.

I removed all stuffings and only put one piece of pink fiber glass insulation on the entire back inside panel directly opposite to the speaker. I also thinned down the fiber glass sheet to about 1 inch. There is nothing holding the sheet now. I have tried shaking the box and the sheet didn't fall off so I hope it will stay.

Sound test: The bass and the volume came back. Also, the sound seems to be clearer and warmer.

Again, this was a trial and error and there was absolutely no calculation involved. The remaining work is to adjust the serial resistor for the tweeter.

[rjbond3rd]

Hi AC439, congrats on your build from a fellow newb.

My understanding is that the purpose of stuffing is two-fold. First, it can keep internal reflections from affecting the cone (i.e., reduce audibility of reflections). Second, if you are using an optimal calculated volume, it can make the box "appear" larger so you can construct a smaller-than-optimal box and get the same result. (Apparently it accomplishes this feat by converting the acoustic energy into heat..)

Since you're working experimentally, I think you just add whatever sounds right in order to cut down on internal reflections having an audible effect on the cone.

At some point, you'll be sure to wonder if the box is optimal. If it's too small, presumably it will have a bump in the response (i.e. be non-flat) which would be boomy. If it's too large, it will start rollling off (at 6db/octave) from too high a frequency and might seem bass-shy. Obviously placement in the room is a major consideration as well.

But who knows, maybe you just nailed it... Congrats again.

[chops]

As far as I know, it doesn't convert the acoustic energy into heat.

What it does is slow down the sound waves, which gives the effect of a larger enclosure. However, if you put too much in, then you just start eating up air space and essentially make the enclosure smaller.

It is also used to damp (absorb) reflected sound within the enclosure.

It is trial and error and you do have to tune to taste.

[rjbond3rd]

Sorry for any inaccuracies -- how did I write that sealed rolls off at 6db/octave when it's obviously 12db/octave? Gah! Undo!

The stuffing converts the air-spring's action from adiabatic to isothermal. I'll stop here before I make any more errors.

[chops]

Originally posted by rjbond3rd
Sorry for any inaccuracies -- how did I write that sealed rolls off at 6db/octave when it's obviously 12db/octave? Gah! Undo!

The stuffing converts the air-spring's action from adiabatic to isothermal. I'll stop here before I make any more errors.

I'm not claiming to know everything, because I don't, and I'm usually wrong more than I'm right. LOL

Don't worry about it.

[AC439]

Thanks for comments. I think I need more reading to understand the info.

Anyway, I went ahead and increased the series resistor from 4 ohms to 5.5 ohms (four 22ohms in parallel). The sound gets more balanced and I am liking the speakers even more. Tested them especially with Hourglass of James Taylor. Vocal sounds very smooth and fat. The highs are not ear irritating and comfortable. Classical music from the speakers are very comfortable. Definitely don't feel tired after hours of listening.

I think I'm going to pause the mods and listen to them for a few more weeks before next adjustment.

Consider I spent about $35 in parts and material, this pair sounds wonderful. Again, I have a pair of Tannoy 611 MKII for comparison and the pair I built sounded very very close to the Tannoy except they produce less bass. (Tannoy has twin 8" woofers)

Oh, forgot to mention, I'm playing mp3 from my Dell desktop into the Sonic T-amp which drives the speakers.

-AC

[Scottmoose]

Is that the original / cheap SI T-amp? If so, it's rolled off below ~100Hz, so if you replace some of the caps (run a search for the details of what's required), you should find yourself getting a little more LF.

[AC439]

Originally posted by Scottmoose
Is that the original / cheap SI T-amp? If so, it's rolled off below ~100Hz, so if you replace some of the caps (run a search for the details of what's required), you should find yourself getting a little more LF.

This is correct. I'm well awared of the mods on the input DC blocking caps, speaker connectors etc. But I have been putting the mods aside. Now I have a little spare time for the projects so I am going to revisit the mods on the amps. I use Winamp to play the mp3 and the internet radio so I use its EQ to boost the bass a little at the moment to compensate for the lows. - AC

[AC439]

I'm happy to report here that after I mod the T-Amp, the sound is even better and very musical from the speakers. The bass is here and I think I can declare completion of the project.

I have reported my T-Amp modification in another forum here:-

Another very happy (but lazy) T-amp owner

Thanks for the comments and you guys are great !
-AC