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Discussion Starter #1
I was recently playing around with creating some tones and I noticed that my single driver 8" subwoofer in a ported box was able to create very low tones while my eight 12" drivers in my fronts powered by pro dj amps didn't seem to be able to create any tones under 30 hz. They move plenty of air and create plenty of rumble, but nothing that your ear recognizes as a tone. They are each paired up in 4 sealed fiberglass speaker boxes with about 3-4 cu ft per pair.

I've heard that the size of the box in relation to the size of the drivers matters immensely, and my best guess at this point is that the drivers are too big for the box. Could someone explain to me the physics behind this? Why would a smaller driver give me a lower tone? Conceptually it seems like the ability to move the air should create the tone and eight 12" drivers, even if in an untuned box should be able to move enough air for some pretty low tones.

Thanks
 

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Re: physics behind size of box

There is an optimim size of cabinet for every driver according to its T/S parameters. Too big or too small a cabinet will hinder the drivers performance. As for low end output, a sub with a lower Fs will produce more low end output then a sub with a higher Fs, all things being equal. Also a ported design will produce more low end output then a sealed design.
 

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Re: physics behind size of box

Isn't the idea behind an IB system that the bigger the box the lower the tone?
 

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Re: physics behind size of box

IB is a type of cabinet, so to speak. The best output you'll get are from subs that have been designed for that specific application. An example is the Exodus Audio DLP-15. Its parameters dictate that it won't model well in a ported or small sealed box, but it models well in a huge sealed cabinet, which is what an IB is.
 

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Re: physics behind size of box

The 12" are sealed. The main purpose of these speakers are midrange, so I didn't port them, but it seems like with 8 of them they should still be able to put out some low tones even without a port.
 

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Re: physics behind size of box

If the box is too small the internal pressures will keep it from ahchieving low output without a lot of eq and power. Also, as Mike pointed out, the Fs of a driver has an impact on its extension.
 

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Re: physics behind size of box

I was recently playing around with creating some tones and I noticed that my single driver 8" subwoofer in a ported box was able to create very low tones while my eight 12" drivers in my fronts powered by pro dj amps didn't seem to be able to create any tones under 30 hz. They move plenty of air and create plenty of rumble, but nothing that your ear recognizes as a tone.
Something that occured to me as I re-read this paragraph is that it's possible that your 8" sub is distorting at these higher output levels, introducing harmonic distrotions that give the perception of loud deep bass that really isn't there. Just as possible is that your 12's are located in your room in such a spot that low frequencies are literally sucked out by the room.
 

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Re: physics behind size of box

Something that occured to me as I re-read this paragraph is that it's possible that your 8" sub is distorting at these higher output levels, introducing harmonic distrotions that give the perception of loud deep bass that really isn't there. Just as possible is that your 12's are located in your room in such a spot that low frequencies are literally sucked out by the room.
Thats the first thing i thought of. you dont hear those really low freqs as they are below human hearing. So if its a clean input and output at say 20hz you shouldent hear anything at all, just feel the pressurization of the room, stuff shaking, and some weird feelings. If the signal is distorted or the output is distorted, be it over-excursion, port noise, or something else, it may sound like a low freq is being produced but in reality its not supposed to sound like that.

Try different combos of the 12" subs and see if it sounds better to you as room acoustics could be playing with you.

But like others have stated, the drivers you may have used might not be so good for low end performance, especially when put in a sealed box.

What 12" drivers did you use?
 

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Re: physics behind size of box

The drivers are Boss Chaos Special Edition (SE124D)

I'll try to record the tone of the sub and see if it's creating a true tone tonight.
 

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Re: physics behind size of box

Humans can hear below 20hz, it's just that our sensitivity decreases exponentially below ~100hz, so a clean fundamental in the infrasonic range has to be played extremely loud to actually hear the tone.

As for the physics of box size, the larger the enclosure the less the internal air resistance to the cone movement. In a small enclosure, for the cone to travel, it needs to compress the air in the enclosure, and this requires more power, so there is decreased sensitivity. Additionally, since exponentially greater amounts of cone travel are needed to maintain level output as frequencies lower, EQ is needed in the electronics to exponentially increase the power levels as frequencies lower in a small enclosure. In an IB, there really is no resistance to cone movement at all. In something like a LLT, there is very little.
 

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Re: physics behind size of box

The rear volume is a capacitive load, which is why there is a rolloff associated with the rear volume. In an IB (or a large cabinet), the pole is just pushed way lower in frequency.

Adding a vent is like adding an inductor, which causes an LC resonance (the tuning frequency) and explains the even steeper rolloff (another pole).

Thinking of things in terms of DC pressure might make for a good reality check, but is totally irrelevant for AC signals (which all of music falls into). Its the reactance that dominates the behavior.
 

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Re: physics behind size of box

Thinking of things in terms of DC pressure might make for a good reality check, but is totally irrelevant for AC signals (which all of music falls into). Its the reactance that dominates the behavior.
I'm not quite sure I understand what you are trying to say. Low frequencies and small boxes are not a good mix without lots of power and eq. I understand the inductor, capacitor analogies, but in a sealed enclosure I fail to see the difference. The reactance, or air spring, happens much higher, and outside of the range we are looking at here (unless I am missing something :huh:).
 

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The Fc (sealed enclosure resonance) is usually right around the F3 for sealed alignments. I think WinISD might call it Fsc for sealed boxes. If you push the Fc too high, you'll get peaking because the Q is narrow. Push it too low and you get no gain because the Q is too wide. In both cases the main portion of gain happens at the Fc.

Anyways, the Fc is there because of the cabinet capacitance....so I would argue that the air spring is most active at the bottom of the passband.


To get back to the original question...the size of the box really determines the efficiency of the system for a given bandwidth. So basically, there are two approaches a speaker engineer can take. The first step is to pick the bandwidth (how low it will go). The second choice can be either the size of the cabinet or the efficiency of the system.

If the cabinet size is chosen (for aesthetics perhaps), then the efficiency of the system is fixed. Any driver yielding the same bandwidth in the same enclosure is going to have the exact same sensitivity. This will then usually drive the motor selection depending on how loud the system needs to go (louder will require more power and more excursion).

If the efficiency is chosen, then the size of the cabinet will be fixed. Any speaker with the same bandwidth and the same efficiency will have the same sized enclosure. This is usually the path DIY'ers will take after making a driver selection...basically tuning the enclosure to optimize the performance of the driver.
 

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Elite Shackster
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Anyways, the Fc is there because of the cabinet capacitance....so I would argue that the air spring is most active at the bottom of the passband.
I guess this would depend on what you would consider the bottom of the passband. For most sealed home theater designs this is below 30hz. A properly designed ported system will take advantage of the air spring for sensitivity up high while letting the port make up for its loss as frequency diminishes, thus below 30hz the air in the box is no longer acting in a reactive manner. Of course box size can be changed, as you point out, to affect this but you lose the effectiveness of the boost in sensitivity.
 
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