Welcome Spkrboy

 

What model 12"? What is the recommended enclosure for 1 12? Sharing air space or seperate air spaces? Depending upon what you built you may need to cut space, not add polyfill. Help us help you

Fej

 

2 woofers sharing the same space in a half cylinder shape in the wheel well of a S2000. woofer specs: dual 4-ohm free air resonance:30hz Qts: 0.526 Qes: 0.614 Qms: 7.698 Efficiency: 89.58dB

using poly-fil from Michael's. Listening mostly to rock music. cossover point somewhere around 80hz. Amp is a RF Punch P8002. listening level, almost cranked up since I have a convertible.

is that enough info and what were u saying about basic testing equipment? I'm sure my electronics savvy friend could do it if i can figure out what I need.

 

Need the Vas, but so far, with a EPB that high, these are not well-suited for a sealed box in a small car.

A quick rule-of-thumb for determining what type of enclosure a driver should go in is by dividing the Fs by the Qes. This is called the Efficiency Bandwidth Product. If the result is <50, the box should be sealed. If it's over 100, the driver will usually work better in a vented box. In between, the driver might work well in either.

I'd have to start another thread to get the testing info out, but suffice to say, you can find starter directions in Vance Dickason's "The Loudspeaker Design Cookbook." Joe D'Appolito's "Testing Loudspeakers" is a more advanced text, but worth the read. For a quick reference, go here for the Wallin Jig:

http://mysite.verizon.net/tammie_eri...jig2/jig2.html

 

vas is 56L. by the way Fs/Qes is 48.86, less than 50! sorry i had a typo above. Qes is 0.614

 

From what you're saying it sounds like these drivers will be fine in the space provided, assuming you can seal it up and keep the resonances to a minimum. A Qcb of 0.85 would be had at 2.4 feet for the pair of drivers. A Qcb of 1.0 would need 1.6 feet.

Because you're in a convertible, it's unlikely you'd hear a significant difference between the two boxes. Not to mention that increasing average SPL gradually negates the effects of stuffing anyway. If I were you, I'd find a way to stiffen up the metal panels that will make up the "box" and apply something like Black Hole to reduce standing waves and the like. The stuff is expensive, but well worth the money if you have enough invested in the rest of the system.

If you have the capability, you should have the driver tested to verify the parameters before getting too involved in this. Once you have the actual numbers, you can calculate how big the enclosure space "appears" to be by looking at the impedance curve of the installed driver pair. The precise amount of space is unimportant, only how it makes the driver behave.

 

I'm actually using the poor man's dynamat, i.e. the aluminum/asphalt tape they sell at home depot that works just as well. I'm lining pretty much the whole trunk, the entire doors, and the transmission tunnel. Since I listen to rock music, and you said in the previous page I would want a Qcb of .7-1.0, I think I'll try a little stuffing and see how it goes. I bought 40oz so I'll have plenty to work with.

by the way here's what the box will look like:



and two rings will sit on top of it at an angle, which will give me some more volume. going to cover the top with carbon fiber:

 

Asphalt melts, falls off, and gets everywhere. I hope you don't run in to problems with that stuff.

 

Spkrboy apparently has a lot of experience here but I can see that he's left you all with your heads spinning. I am going to try to make an analogy for all the "car guys" here.

A speaker and it's suspension acts like a spring. And like a spring or even a guitar string, it has a resonant frequency. That's the frequency it wants to naturally oscillate to. The free air resonance of a speaker can be determined by plotting the impedance curve. All you need to do this is an oscillator, a voltmeter and a resistor. Then you can find the exact frequency of the resonance. Typically a speaker's frequency response falls off very rapidly below this point (Fs).

A speaker produces just as much sound from the rear as it does from the front. A speaker box is designed for two things. The primary one is to keep the sound from the rear from combining with the sound from the front and cancelling each other out, because they are 180 degrees out of phase with each other. Did you ever notice how the bass disappears from a speaker as you pull it out of a box? The air inside a box also acts like a spring. Bigger box, bigger spring. And just like a spring, it has a resonant frequency. The filler in a box sort of acts like a shock absorber, lightly damping the spring action, changing resonance. And just like a shock on a car, it can be too stiff and completely disrupt the action of the spring. You can also use enough of the stuff that it will actually consume box space and therefore make the spring smaller.

Now, the idea is to make the spring in the box resonate at a frequency just slightly below the resonant frequency of the speaker, thereby enhancing the output of the speaker right where it starts to roll off. This usually extends the low frequency ability of the speaker by another half octave or so.

And of course, just as Spkrboy said, you can't measure this stuff with a simple RTA. That's why they build huge anechoic chambers for test measurements. Or use more sophisticated digital equipment like FFT to get pinpoint accuracy. An RTA will get a different result every time you use it, particularly in such a small complicated environment as a car interior. Which further begs the question, why worry about these tiny details for a car? A studio, yes. A concert PA, yes. A home, yes. A car, well....... suffice it to say I'm happy with the stock system in my car. When I want to listen to great sound, I go home or to the studio. You just can't get that in a car, no matter how much time and money you spend.

I hope this all helps clear things up and I think Spkrboy will agree with my analogies here and might even be able to add to it.

 

I completely agree with your assessment, and I wish I had your skill at explanation.

I'll have to take issue though with the part about good sound in a car. I've never heard a stock car system worth a damn, including the Lexus/Levinson system. Does that mean I can get top-notch home sound in a car? No, that would be delusional. Can't even come close, and I'll never argue it. Does that mean we have to suffer with inferiority?

 

I'm glad you liked it. Sounds like we're on the same page together.

Well I agree it certainly can be improved. And for those who don't have a decent system at home and/or spend a lot of time traveling in their car, I would concur on spending a little time and money. I work from home now and don't travel much by car so it's just not that important to me. I have world class facilities available to me.

 

[Start Jealousy Rant]

Robvuk and Sprkboy, you guys suck.

Who do you think you are, coming in here and spewing all kinds of knowledge? I mean, do you really expect us to learn something? Geezzzzz...

World class facilities... hrumph....



[End Jealousy Rant]



Seriously guys, that is some great information! I have been an audio nut for over 20 years (as a hobby) and I am always impressed when I meet people that really know their stuff when it comes to audio.

I have Vance Dickason's "The Loudspeaker Design Cookbook" and Joe D'Appolito's "Testing Loudspeakers" books and they are just amazing! It's one thing to understand how the flow of electrons through a coil develops a magnetic field... but to apply it to a speaker (mathematically) to find out the response characteristics is mind boggling!

One question: I always assumed that a smaller box acted as a stiffer spring vs. a larger spring as you mentioned earlier. Any chance you could expand on why this is the case? Maybe I am reading into it too literally... I'm thinking of the whole PV=nRT equation... but I realize that there is much more to it than that.

Thanks in advance!

Mark

Disclaimer: I was kidding above... in case it wasn't completely obvious.

 

Ok i undertand.

Do you have an equation for the amount of filler/filler density for a given subwoofer and given box?

What information would be needed to calculate this?

 

You have to teach yourself how to make simple impedance measurements of the driver. Both in free air and in the desired enclosure. Armed with this data, you can add stuffing to the box until you reach your desired Qcb or response curve. Ultimately then you listen, and adjust some more accordingly. There is no simple calculation for this, only general rule of thumb, and every box/driver combination has to be treated differently.

The parts to make the tools to make these measurements are easily found at Radio Shack. There are loads of instructions on the net regarding the software and use of these tools. There's even freeware to do it.

FastHotRod... when he said the spring was smaller, he meant stiffer. Adding stuffing absorbs some of the energy inside the box, making it "appear" that the box is somewhat larger, and the spring is longer (more compressible).

 

Sorry guys, I was away on the boat for the weekend.

Yes, the smaller box has a HIGHER resonant frequency, just like a smaller or thinner guitar string.

 

Once again Spkrboy beat me to it. The amount of stuffing is generally a trial and error thing. If you have a closed/sealed box, it is less critical than a ported enclosure though. On a ported enclosure, you will usually find that just a covering of 3 sides will do it, whereas a sealed box can be overstuffed to a greater degree.

 

That's what I thought. I can understand 'smaller box = stiffer spring effect = higher frequency roll off' due to simple physics. When I read that a bigger box acted as a bigger spring I thought I was going crazy there for a second.

So what is really taking place here? Is it that a smaller box can't allow for 'expansion' of the wave? Is it a wave length issue at all?

My gut reaction is that the larger box has less mechanical resistance against the cone assembly, allowing for increased cone motion at the same input signal level... but then again the total output would be 'higher' and not just the lower end of the frequency spectrum.

As for damping material... I once heard that the losses in energy actually had to do with thermodynamics; that the air moving though the damping material caused friction, and the sound energy turned to heat energy, creating losses in the box that effectively 'absorbed' the wave, increasing the effective box size overall.

Yes, I am over-thinking this... (I'm an electronics tech, and I'm working towards my BSEE so I'm a curious kinda guy when it comes to electrical stuff.)

Mark

 

The resonant frequency of the box is all that we're concerned with. We don't really care how "stiff" it is. As long as it resonates at the desired frequency and with the desired output, in the case of a ported box. If wavelength was an issue, the box would have to be huge since 100hz is over 10ft. long.

Don't over-think the damping. It's not that important. Just throw a bit in the box and call it a day. It helps dampen higher harmonics that are created between the sides of the box. The energy absorbed at low frequencies is miniscule.

 

Oh and I'm glad you cleared that up. My feelings were hurt.

 

I found a nice rockford fosgate to use for my system and it ended up being blown... very disappointed. oh well i'll get another one. really want to learn how to make a box with just the right Q.