Originally Posted by Vinculum
I'm not getting this.
Just throwing arbitrary numbers around.. If you NEED to move the cone XX mm in the bottom octave to be useful, why would you not want a lower Q (say .577) and do it with 1000 watts instead of 4000 watts in a higher q (.707) and adding more power compression? Is a HP really necessary instead of calculating the proper amount of power to use as a maximum or doing an EQ shelf cut? I dont believe people with smaller IBs (2x18) are using high pass filters yet they have the ultimate large box. You either make it hard to move the cone down low (small box) and squeeze your amps to make it happen, or ease up on the job and watch where you walk.
What am i missing? fill me in!
Speaking of IBs, Isn't it silly how IBs are the greatest thing since sliced bread, yet the same # of drivers in a sealed config is looked upon as lesser. They both move the same amount of air...
Assuming you have high power handling drivers (which you do):
Using your example, you would be giving up 6dB above F3.
Lowering the power plant's potential is the same as utilizing a HP filter in that both are a means to protect the driver from over excursion below F3 in a bigger box. One method carries the price of poorer group delay and transient response by loss of extension. The other reduces headroom above F3 by 6dB in your example, which is actually fairly close to reality in the case of a driver compliment that will handle 4KW peaks.
A sealed box of the proper size will:
1. Protect against over excursion below F3.
2. a) Lower certain non linear distortions because the trapped air becomes the dominant restorative force.
2. b) In a push-pull configuration, reduce (H2 or H3 or both, depending on your school of thought and/or test results) harmonic distortions.
3. Allow use of a power plant that maximizes output potential across the entire spectrum of the sub's capability.
With the L/T, acoustic system Q is also irrelevant. Consider this: If your box is modeled to restrict excursion to 80% of X-max for a given power plant, your Qtc may well be higher than .707. This yields a hump at the knee. When you properly design the L/T to flatten the response, your EQ curve, which is a mirror of response, will apply a cut at the knee before it begins to boost the signal below that point.
2dB free lunch at the bottom end of the music range.
You can then, obviously, shape the input signal to result in any anechoic system Q you prefer, or, as is the case with The Marchand Bassis, dial in an infinite combination of anechoic system Q and F3.
In the case of my own designs, I have pre selected anechoic F3 and Q options that are employed by 3 position switches, similar to Mr. Ding's user controlled F3/Q.
The only caveat is that available Q options is proportional to the amount of boost applied.
I'd also like to mention that the popular phrase that 'Bass is all about moving air' is an incomplete thought. Mutual coupling, placement against a wall, placement in a corner, acoustic coupling with a horn, etc., all increase bass output without increasing displacement.
Yes, it is silly that IB is said to be superior to any other sealed configuration. The major difference is that the IB reverts to the driver suspension as the sole restorative force vs a small sealed sub which replaces the mechanical restorative force with a cushion of trapped air (though not entirely). The IB has a much larger potential to create sympathetic vibrations (which many mistake as being audibly a good thing). Corner loading is predominant with a sealed sub and not an option with IB.
The evidence would indicate that the sealed box can have less non linear distortion, better transient response and offer less sympathetic vibration, which boils down to less colored in-room reproduction.