Physics Behind Size Of Box - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 10-21-08, 07:53 PM
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Steve

Join Date: Apr 2006
Posts: 2,398
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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Old 10-22-08, 12:55 AM
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Mike Bentz

Join Date: Sep 2006
Location: Chicago
Posts: 438
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.

-Mike Bentz

"It's territorial with the soundboard. So you're mixing and some dude comes by spewing opinions and trying to turn knobs. It's akin to going up to an artist and painting over his unfinished masterpiece. You just want to shove your paint brush up his nose and throw the soundboard out the window!"

Last edited by DrWho; 10-22-08 at 01:04 AM.
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Old 10-22-08, 01:17 AM
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thxgoon

Join Date: Feb 2007
Posts: 1,506
Re: physics behind size of box

Quote:
DrWho wrote: View Post
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 ).
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Old 10-23-08, 09:41 PM
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Mike Bentz

Join Date: Sep 2006
Location: Chicago
Posts: 438
Re: Physics Behind Size Of Box

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.

-Mike Bentz

"It's territorial with the soundboard. So you're mixing and some dude comes by spewing opinions and trying to turn knobs. It's akin to going up to an artist and painting over his unfinished masterpiece. You just want to shove your paint brush up his nose and throw the soundboard out the window!"

Last edited by DrWho; 10-23-08 at 09:58 PM.
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Old 10-24-08, 01:30 AM
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thxgoon

Join Date: Feb 2007
Posts: 1,506
Re: Physics Behind Size Of Box

Quote:
DrWho wrote: View Post
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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