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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: Re: Time to wrap it up... posted by Sean on December 28, 2003 at 22:16:55:
You refute your own statements and undermine your own arguments in that post several times."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..."
Nothing "disproven". You confirmed what i said i.e. two impedance peaks with the resultant shifts in phase and lack of control in a vented box as compared to one of the same in a sealed box. Thank you.
"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."
Little power in and lots of power out? That sounds like non-linear distortion that most would call "ringing" or "oscillation". And you have two of these points in a vented system and only one in a sealed system?
"Not to mention that the amplifier is actively driving the system."
The amplifier can only drive what it can load into. Increased impedance means a lack of drivability and reduced power transfer. Once again, one increase in impedance with a sealed box and two impedance peaks with a vented box. Which one do you think will offer more control and which will produce more slop ?
"It's not like you push a swing once and then walk away, letting it "die out" on its own."
That is EXACTLY what resonance or "self-oscillation" is. In your own words "And a resonance means little input is required to get lots of output (in any system, not just drivers) - nothing more." One nudge to the "swing" and it takes off like a jet. With a sealed box, you only get one of these "free rides" due to the single impedance peak. Vented box means "twice as much driver non-linearity" due to the twin impedance peaks.
"With a driver you actually grab and push or pull the swing, and never release it."
The "grab" on a driver is directly proportional to power transfer from the amp. If the amp can't transfer power to the driver, it has lost its' grip and is no longer in control. Which design "loses its' grip" more often due to lack of power transfer?
"The amp is constantly providing current to the driver"
While the amp may be delivering current to the driver, it is not doing so linearly at all frequencies. Bare in mind that there is more current flow or "power transfer" at a lower impedance. Hence the lack of power or current flow at or near speaker resonance due to the higher than normal impedances that the amp sees. Reduced current flow equals less control. Two points of reduced current flow for a vented box, one for a sealed box.
"That's sentence 3."
Thanks again for your honesty, helping to prove my points AND keeping track : )
"Sealed boxes have impedance peaks, and they can have Q<0.7 responses (meaning non-peaked output)."
A low Q design will NOT suffer from uncontrolled oscillation due to being critically damped at resonance AND it benefits from improved transient response. Hard to beat.
"Would that mean the system (sealed box) is responding in a nonlinear manner, and have distortion?"
That is part of the design and not a side effect of introducing another uncontrolled, undamped resonance / colouration. In fact, such an approach goes out of its' way to control such things.
"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!"
See my above notes in the post directly above this. On top of that, the Shiva is neither "optimized" in a sealed or a vented design as it is a "hermaphrodite" i.e. "universal" driver. As such, it does neither as well as a driver that is fully optimized for one specific type of use. Like many other "universal" devices, one size does not fit all and what it does fit, it may not do as good as an item designed for one specific purpose.
"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."
Easier said than done. On top of that, this is probably why we keep seeing / hearing products that don't meet factory spec and sound like crap getting reviewed in Stereophile. Too many "speaker designers" putting "garbage in" and the consumers are getting the "garbage out". Having said that, the computers can only get you so far and it is up to fine tuning by ear, eye and test equipment from there. Then again, this would entail quality control with the resultant increase in time and labor and we all know that "high end companies" can't afford to do that with the "very reasonable" prices that they charge audiophiles.
"There is no such thing as the perfect box for all situations. There IS the perfect box for a given situation."
Nope. There is nothing you can do to get rid of all of the standing waves within a cabinet using the "conventional" dynamic drivers that we currently use to reproduce low frequencies and are stuck with. There are only boxes that reduce specific drawbacks to the point of negligibility. That specific point of negligibility would be a matter of perspective and up to personal preference / interpretation. Sean
>PS... For those that aren't speaker designers or technically minded, all speaker engineers try to avoid operating a driver at or anywhere near resonance in order to increase / maintain linearity. As mentioned, drivers become prone to distortion / self-oscillation at or near resonance. This is why we have crossover networks in speakers i.e. to steer the sound away from one driver that is going into a non-linear mode of operation and shift it to another driver that is better suited for linear operation in that specific frequency range. Obviously, how well or smoothly this is achieved depends on the drivers chosen to blend together at that point, the crossover design used and the skills of the speaker designer.
Since we can't avoid the low frequency resonance of a woofer / sub-woofer, all we can do is cope with the resonance and try to tame it as best possible. This is exactly what a low Q sealed design attempts to do i.e. flatten the peak at output while maintaining good to excellent transient response. On the other hand, using a vented design introduces yet another point of resonance, adding further non-linearities and reducing transient capabilities. If this sounds contrary to ALL the goals that the speaker designer has tried to achieve throughout the rest of the speakers' operating range, it is. None the less, it is exactly what audiophiles have been led to believe "sounds good" due to marketing hype, poor listening skills and lack of education on the subject. This is NOT to put "audiophiles" down but more to condemn the "glossy mags". Obviously, they have NOT done their job at educating those that have trusted them to best represent their interests in this field and hobby. Sean
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- By the way... - Sean 00:00:44 12/29/03 (0)
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