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Speaker Asylum General speaker questions for audio and home theater. |
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Speaker Asylum General speaker questions for audio and home theater. |
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In Reply to: sealed vs ported posted by Sean on December 28, 2003 at 19:09:48:
Sean posted:There are always ways to get there and maintain the quality of signal. That is, IF one is willing to do their homework and spend the money to achieve those goals. After all, if integrity of signal and specific design / performance goals truly are important, one probably wouldn't be willing to accept anything less or compromise those goals.OK, so with an unlimited budget you can get there. Fair enough. Now how about doing it at a price that most people are willing to pay? Not many can afford thousands for subs, and many even find $700+ a bit steep. Now how will you get decent output and extension in your sub? Oh, and keep the box size down, too, because lots of people don't want 4+ cubic foot boxes in their rooms.
Sure, if you have no limits it's easy. But that's not really applicable to the vast majority of even the audiophile community, let alone the public at large. At least in my experience, but what do I know...
Sean posted:Sealed enclosures are always the simplest. They typically have far less math involved, lower parts count, greater chance of success, less chance for unaccounted side-effects, etc... If properly implimented with all factors considered, such a design will "probably" solve the problem.
That's a pretty big "probably"...;) There's this little thing called Hoffman's Iron Law, and when combined with a budget, sometimes you have to do other things than a sealed box. And sometimes even if you can get there with a sealed box, you're better off looking at a vented box since it has some definite advantages over the sealed approach.
Sean posted:Other than that, i see that you didn't refute any of the above information, so i'll have to assume that the laws of physics still apply and that my theories / teachings abide within them.
Umm, no. There's so much there, and much of it misguided, that it'll take a few pages to straighten out. However, I'd recommend rather than me writing it all down, that you look at a good book like Fundamentals of Acoustics by Kinsler, Frey, Coppens, and Sanders, especially the part about driver design and operation in cabinets. But here's one part to discuss, right off the top...
Sean posted:Just the opposite is true of a vented design. That is, you have a peak(s) at resonance, not a dip. This is because the driver and / or vent is self-oscillating and is NOT responding to the input of the drive signal in a linear fashion. If a speaker produces a peak ( or a dip for that matter ) ANYWHERE in its' passband, it is responding in a non-linear manner. That is a distortion and it is directly related to a lack of linearity i.e. increased output with decreased drive.
Actually you have a peak at resonance in ANY driver system - sealed or vented. In a vented box you have two resonances with different frequencies, and the system ends up with an impedance MINIMUM at the system tuning frequency. In a sealed box you have a peak at the single system resonance, and that also is the system tuning frequency. But they both have a peak, with the vented box having a minimum impedance at system tuning frequency, and the sealed having a maximum impedance at the system tuning frequency. So that's sentences one and two disproven...
And a resonance means little input is required to get lots of output (in any system, not just drivers) - nothing more. Don't put power in, you don't get power out. Not to mention that the amplifier is actively driving the system. It's not like you push a swing once and then walk away, letting it "die out" on its own. With a driver you actually grab and push or pull the swing, and never release it. The amp is constantly providing current to the driver, and that means there's a proportional force on the driver at all times (BLi is the force, where i is the current, BL is the motor force factor). That's sentence 3.
As far as a peak in response, can you show than an impedance peak translates to a frequency response peak? If not, then it's not an acoustical problem now, is it? Sealed boxes have impedance peaks, and they can have Q<0.7 responses (meaning non-peaked output). Would that mean the system (sealed box) is responding in a nonlinear manner, and have distortion? There's sentences 4 and 5.
And about that impedance peak thing especially about sealed boxes... For an example, take a Shiva, and put it in a sealed 85 liter box. Impedance peaks at 66 Ohms at 35 Hz. Now add a 3" diameter by 12.75" long port to tune to 20 Hz. There are two peaks, the upper one at 37 Hz at 58 Ohms, and the lower one at 11 Hz at 54 Ohms. So the sealed box has a higher impedance peak. According to your theories, the sealed box would have worse transient response!
Oh yeah, the sealed box will have a 40 Hz F3, and reach a peak output of 93 dB linear (16.5mm one way excursion) at 20 Hz (120W). With half the power (60W), the vented box has the same 20 Hz output, but the F3 is 26 Hz. Oh yeah, above 14 Hz, the vented box has less excursion, too. Meaning of course less distortion.
So we see that the vented box has a lower impedance peak, more output for less power input, and less cone excursion for lower THD. All in the same box size, with the same driver.
In this comparison, I'd say the vented box acquits itself quite nicely...
Lastly, you keep insisting that's it's difficult to design a vented box... Modern tools like LspCAD, SoundEasy, LEAP, and others make it actually quite simple. Again, this shouldn't be difficult, especially for a professional. If they can't get it right, nor if they can't afford decent tools, then I'd be quite suspect about the other aspects of their designs. I'd assume this isn't too much of a stretch.
Rather than continuing this endless debate, I'll just wrap up with a single statement:
There is no such thing as the perfect box for all situations. There IS the perfect box for a given situation.
Dan Wiggins
Adire Audio
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Follow Ups
- Time to wrap it up... - Dan Wiggins 21:51:19 12/28/03 (2)
- Re: Time to wrap it up... - Sean 22:16:55 12/28/03 (1)
- By the way... - Sean 00:00:44 12/29/03 (0)
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