need help tuning PR's and box! pleeze!

[speekergeek]

Ok here it is, i made a box with internal dimensions 18" x 18" x 22.5", i forget the cu. footage offhand but it is almost 4 i think. anyways, i have a dayton DVC 15" driver and 2 old mtx terminators 15" speakers as the PR's. I have already added over a kilogram to the speakers and the freq response is nowhere near the expected values. winisd said i should be 3db down at around 28 hertz with 1000 grams on the PR's. i have more than that on the PR's (not much more) and the response is crazy. it is 101db at63hz, then it is 96db @ 44hz and it keeps going down from there. should i shell out the big bucks and pay for the dayton PR's? should i stuff the box with some dacron or fiberglass? I already spent almost 200 bucks on the box alone so i can't scrap it. With the mtx's, i crank up the volume and it makes wierd noises, i just think it is because i didn't put all the screws in, but i know it doesn't have all the excursion the dayton has. pleeze help, i am up to my neck in this project, wallet is running empty. I will delay this project as long as it takes i guess. the box without the speakers weighs in at 100lbs by the way/ i am proud of it. I will make a thread about the project at some point. Thanks




Mike

[kelticwizard]

The MTX Terminators you use as Drone Cones, (Passive Radiators), have weight on them to begin with.

I hope your weights are removable.

Have you tried making the Drone Cones lighter?

Reason I am asking is that if you tune either a vented box or a Passive Radiator box too low, it will begin to take on the frequency response of a Closed Box, which means less output in the bass range.

That sounds like what is happening here.

Try putting 600 g on the Passive Radiators and see what your output is there.

An 18" by 18" by 22.5" box is 4.2 Ft³, or 119.3 liters, if any Metric folks want to join in.

[kelticwizard]

I just looked up the mmd, (moving mass) of some MTX 15 inch subs, and that particular model had 125 grams on it.

If your MTX's have similar weight, then you are starting out with 250 grams on the Passive Radiators already.

Add over 1000 g and it looks like you are going too low.

Incidentally, do you know how to measure speaker impedance? You should have lowest impedance at the frequency you wish to tune the box to, that goes for a Passive Radiator or a vented box. Measuring impedance-cheap and easy to do-will tell you what your box is tuned to. All it requires is a cheap multimeter that measure AC voltage-which most people have handy, (or can borrow).

[Ron E]

First thing, how did you measure the speakers and what environment are they in? This makes a big difference, as LF measurements are difficult.

The easiest way to determine the amount of mass you need is to calculate the port length needed for a port with the area of your PR.

in a ~120L box, tuned to ~20Hz, you would need a port ~10.4 meters long.

The mass of air in this would be 1.8kg., so you will need about 650-700 grams on each PR, after subtracting off the existing mass of the cones.

[speekergeek]

I started off by having no weight on the PR's, then i took a freq response with my radio shack spl meter, i did it outside with a test disc i made on my own about 4 inches from the cone. i used the correction values i found on the internet. then i took another freq response after adding 9 ounces each, the freq response seemed a little more extended, only by a few hz though. so i added another 9 ounces on it and took another freq response, another few hz added to the graph i made. so i went out and bought a 16 oz weight and added that too, so now the weight ADDED to the cones is over 2 LBS each. i took another freq response, and only a few more hz were added.
I have the equipment to take a impedence chart but i didn't even think of that. I am still dumbfounded. so it is looking like the most accurate wa to find out if the box is tuned correctly would be an impedence graph instead of a freq response then.
BTW the weights are kinda removable, the speakers are not the greatest so if i mess up the cone tearing off weights it is no big deal.

[Ron E]

Close miked measurements will not get you a frequency response curve for the whole system. You will get a response for the woofer only, without the contribution from the PR's. A close miked response _will_ tell you where the tuning frequency is, because there will be a notch (a very sharp dip with little or no output) in the woofer close miked SPL at the tuning frequency. This notch is much more accurate indication of tuning than the impedance curve.

Looking back at the parameters of the woofer, a 120L enclosure is rather small for that woofer, although box design also depends on how you will connect the woofer to the amp. Are the coils in series or parallel or ?

I suggest you measure the parameters and model with something a little more powerful than WinISD.

[speekergeek]

why didn't i think of that! genius, pure genius! i could measure the response of the PR's and superimpose it on the graph of the driver's response. Eureka! The voice coils are paralell. I was trying for as small a box as possible due to WAF. I dismantled my 9 cu ft. sonotube subwoofer for this little endeavour, i am going to keep it because you never know when you need a 9 cu. ft. tube of cardboard. what software would you suggest that is more powerful than winisd, that is affordable or free as my budget is TIGHT! just lost my job, well kinda, outta the military finally.




mike

[speekergeek]

Okay i measured the freq response of one of the PR's and it pretty much filled in most of the area where the driver was going Kaput, but the PR is putting out about 13 db less than the driver, is there a way to measure bothe the PR's, or maybe add them together? they are on different sides of the box. and how do i add 80 db plus 80db? would it be 83db? i know it isn't 160, i am not very good at math.


mike

[kelticwizard]

First, you have two Drone Cones and one is 13 dB down from the woofer. That relationship should change with frequency.

Second, the two Drone Cones put out 6 dB more than each individually. So the Drone Cones are 7 dB down. If the highest the Drone Cones get compared to the woofer is that, then something is amiss. At the resonance frequency, the output of the Drone Cones should be considerably higher than the woofer.

The guess here is, you still probably are tuned too low.

[kelticwizard]

Speekergeek:

I might have a problem with your selection of woofer.

I looked up the woofer at the Parts Express website. It said nothing of a symmetric magnetic field, extended pole piece, Faraday ring, shorting ring, or reduced second order distortion because of magnetic linearities.

A woofer in a Vented or Passive Radiator system, when driven just moderately hard, will actually leave it's midpoint and go all the way to end of it's travel, reducing output and producing awful distortion-actually clipping the waveform in half!

Here is an explanation:
forward diaphragm displacement--have you heard of this?

Unless a manufacturer tells you that it has a symmetrical magnetic field, or shorting ring, it probably does not have it on the woofer.

However, I think there is a chance that this woofer might have the symmetrical field. That is because this woofer sure looks like Parts Express' version of the Adire Tempest. The construction is similar, the Thiele-Small parameters are similar, even the language the two companies use to describe the woofer is similar.

Interestingly, even Adire does not speak of a symmetrical field except in the Tempest White Paper, where it is mentioned in the pole piece section. Most companies with symmetrical fields tell you right off.

The price of this woofer is only $15 less than a Tempest. Unless it has a symmetrical field, I would trade it in for a Tempest-much better value.

I have used Parts Express, and they are a good outfit with good products. I have a hunch they may have built in a symmetrical magnetic field, and forgot to mention it. Hard to believe that they would make such a long throw woofer without using one.

But I would definitely call their toll free help line tomorrow to find out. Ask about a symmetrical magnetic field, an extended, (not vented), pole piece, or a shorting ring. Make sure you get someone who is not "winging it" on the other end-someone who knows what you are talking about.

Otherwise, I would not use this woofer. It is expensive, and for the money you should get one with a symmetrical magnetic field. Otherwise, you will get high distortion at the lowest bass ranges.