Regarding ULF and Mid bass - Page 2 - AVS Forum | Home Theater Discussions And Reviews
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post #31 of 41 Old 04-26-2019, 08:00 PM
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Quote:
Originally Posted by BassThatHz View Post
That's not 100% correct.

To have a specific frequency the cone has to cycle at that rate.
Given an amount of excursion, THAT... will determine the cone-speed required.

Even more precisely...
For a given cone-mass and electromagnetic field strength and suspension stiffness and box volume and tuning (and a bunch of other T/S values and box-model formulas), THAT will determine the amount of watts to reach a particular excursion... and thus cone-speed, regardless of the particular frequency being targeted.

At least... I think I said that all correctly No?
I'm in complete agreement with this and I think we are looking for the term 'rise time' to get to the frequency. The cone can't go from rest to the desired frequency until it overcomes inertia. That impulse takes some amount of time to accomplish. Then there is the time it takes for the cone to stop moving after the signal stops. There has to be some overshoot due to inertia.
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post #32 of 41 Old 04-27-2019, 07:42 AM
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There may or may not be overshoot. Q of the enclosure/system will tell you that.

And there really isn't an acceleration "lag" to get to a steady state frequency in the way you imply. Cone motion at a steady frequency is not constant velocity but constant acceleration. If the motor is strong enough to accelerate a cone to produce a given frequency, then it doesn't matter if that's coming from full stop or already moving, the acceleration needed is the same.

Now the second derivative of velocity or first of acceleration is jerk. Jerk may have a correlation with group delay in a driver, not sure and would have to look that up. And I honestly don't know what if any attributes of a motor might alter jerkability independent of acceleration. Jerk isn't a trivial phenomenon... in engines valve float and especially valvetrain vibration is more dependent on jerk of the cam profile than on acceleration. So it does matter... I'm just not sure how much it matters in loudspeaker design.

Last edited by Bigus; 04-27-2019 at 08:01 AM.
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post #33 of 41 Old 04-27-2019, 08:05 AM
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Quote:
Originally Posted by SpinMonster View Post
There has to be some overshoot due to inertia.
Both overshoot and undershoot due to inertia.
You just need a bigger motor to overcome that, but that bigger motor has more mass. (i.e. the rocket equation.)

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Originally Posted by sassuki View Post
Sounds like what you’re explaining is also why a double bass array smooths in-room response.
but with even more point-sources in the array I suppose...

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Near a large subwoofer (or port) perhaps there is mass particle flow greater than necessary to support the SPL generated. A localized phenomenon that dissipates quickly as wavefront propogates. Maybe. I believe that is one of the proposed mechanism of nearfield tactility.
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Originally Posted by Trimlock View Post
I think what is often confused with particle acceleration is people assume the driver is throwing particles all over the room.
Vortices can cause port wind to flow across the entire room (or great distances).

A net positive flow, as bigus was eluding to, which WOULD increase tactility via kinetic energy (i.e. the "wind" hitting stuff).

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Originally Posted by Trimlock View Post
Where it is actually how fast the particles compress and decompress. This also explains the benefits of being sea level.
Just to be clear. That would be the frequency, not the SPL.
The localized density determines the pressure (SPL).
i.e. how "much" was compressed, not the "rate" at which it was compressed.

The speed of sound changes at sea-level and the negative-maximum SPL (~191db or whatnot). But I believe that would be the only benefit, so not much of a benefit for a normal rig. But more on this below...

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The wave travels at constant velocity; the particles (molecules) move back and forth.
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There is a localized particle velocity that is coupled to compression and rarefaction in propogation of a pressure wavefront.
I according to this Michigan University article:
https://pages.mtu.edu/~suits/SpeedofSound.html

Static pressure, temperature, humidity and molecular mass
effect the the speed of sound and maximum/average particle velocity.

I thought viscosity too, but maybe not

Since those values don't often change that means they are basically constants. As I theorized might be the case. Unless I'm missing something here?

The particle motion being 1.2 to 1.7x the speed of sound apparently... THAT I wasn't expecting to hear, but I suppose there is no upper-limit on that velocity (other than light-speed), for example: a nuke will push a "mile+ of air" in like ~1ms...

The article also briefly touches on atmospheric-attenuation of sound energy,
that would be in addition to the distance-law of wave propagation reducing the SPL over distance.

It doesn't mentioned boundary reinforcement of waves or interference nullification of overlapping waves though.

I'm not sure if either velocity has any correlation to SPL or Hz, other than: "particles must move" (gases/liquids)... which they do randomly regardless (i.e. static pressure / atmospheric pressure)
No?

Last edited by BassThatHz; 04-27-2019 at 08:42 AM.
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post #34 of 41 Old 04-27-2019, 08:23 AM
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Quote:
Originally Posted by Bigus View Post
And there really isn't an acceleration "lag" to get to a steady state frequency in the way you imply. Cone motion at a steady frequency is not constant velocity but constant acceleration. If the motor is strong enough to accelerate a cone to produce a given frequency, then it doesn't matter if that's coming from full stop or already moving, the acceleration needed is the same.
Ok, but that is a big *IF*

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Originally Posted by Bigus View Post
Now the second derivative of velocity or first of acceleration is jerk. Jerk may have a correlation with group delay in a driver, not sure and would have to look that up. And I honestly don't know what if any attributes of a motor might alter jerkability independent of acceleration. Jerk isn't a trivial phenomenon... in engines valve float and especially valvetrain vibration is more dependent on jerk of the cam profile than on acceleration. So it does matter... I'm just not sure how much it matters in loudspeaker design.
As more of the coil leaves the gap, that would have an impact.

Also, as the magnet heats up, possibly as well.

Also, as the watts increase, the counter electromotive-forces increase (especially for the area remaining in the gap, mostly...)

Bottoming and clipping too.

All will have an impact on cone motion and thus it's ability to track the signal accurately.

Square-waves are notoriously difficult to reproduce because the cone has to move at infinite acceleration and then come to a full stop, rise and repeat. Basically it has to move like a UFO, it is the inertia that causes the problems for both the jet and pilot, it kills them if the acceleration gets too high and also makes the maneuvers near-impossible.

Last edited by BassThatHz; 04-27-2019 at 08:37 AM.
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post #35 of 41 Old 04-27-2019, 08:40 AM
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Particle velocity is dependent on SPL and acoustic impedance of the medium (low velocity for high SPL in low impedance media like a metal rod, the opposite in high impedance media like air).

It isn't fixed, it varies with SPL. I'm just not sure about the local conditions near driver cone. Unsuspecting there is a component of mass air flow (like the fan example) that isn't found in the far field but I am not sure of that.
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post #36 of 41 Old 04-27-2019, 08:57 AM
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Quote:
Originally Posted by BassThatHz View Post
Moving-mass and BL over the first 5mm's is very important for efficient mid-bass.
Also, the PA-460 likely has more cone-area because the surround is smaller and thinner and lighter.
The coil is smaller and lighter.

That's why the UM is ~88db/w/m and the PA is ~98db/w/m.

Higher mass means that it is slower to start and slower to stop, like a 18-wheeler vs a Lambo.

Sinewaves and sweeps aren't a good representation of impulse response.

I think you would find that if REW used an actual impulse for the IR chart the result would be a lot different than the derived value it currently calculates.

Ignoring test tones. Music and Movies aren't sinewaves nor sweeps. It's somewhere of in-between the two extremes. It's not an impulse but not a sweep either. It's variable-frequency and variable-amplitude, one doesn't know either until it arrives and the cone is asked to reproduce it.

The acceleration and velocity values of the cone during music are variable.
Imagine an Indy 500 race where the goal isn't to cross the line first but rather to start and stop and re-start and re-stop as accurately as possible (best tracking), regardless of the asked magnitude of the acceleration or duration, and the driver not knowing what instruction comes next, or when.
Full blast into Drive and full blast into Breaking and Reverse. The 18-wheeler is at a massive disadvantage. [IMG class=inlineimg]/forum/images/smilies/biggrin.gif[/IMG]

This is easily demonstrable by waving your arm. Now increase the excursion to 3ft while maintaining the same speed, and then increase the speed 10x while maintaining the same excursion (or NOT reducing the excursion). Nearly impossible for a human. Too much mass and not enough dilithium crystals captain!!!


^ The cone may be asked, multiple times, to switch directions back and forth, before even crossing the resting-position, on any given side, at any given time!
It's being asked to play multiple different frequencies, at multiple different amplitudes, all at once, and without "coloring outside the lines" in real-time (and without knowing what comes next, at any given moment!)

It's amazing that it can achieve only 1% distortion under those conditions, or even 10% THD!!!
OMG OMG OMG!!! I’ve been seeking a response like this for years! I see so much emphasis on test tones and I just didn’t believe that all flat responses were equal and this confirms my suspicions.

Thank you for the detailed response. This gives me hope

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post #37 of 41 Old 04-27-2019, 09:10 PM
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I'm sorry but a lot of that reads like the "faster bass" talking points that used to be so common and still crop up from time to time. If a driver can produce the frequency of interest at full amplitude, the driver is "fast enough" for the task. Sure, there are other factors like Q that may shed light on ringing/damping behavior, but for the most part these car analogies "run off the track" shortly after the starting line.
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post #38 of 41 Old 04-29-2019, 12:50 PM
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Quote:
Originally Posted by Bigus View Post
There may or may not be overshoot. Q of the enclosure/system will tell you that.

Right


Quote:
Originally Posted by Bigus View Post
Cone motion at a steady frequency is not constant velocity but constant acceleration.

If acceleration were constant it wouldn't take the cone long to leave the room

Nothing (displacement, velocity, acceleration, jerk) is constant in an AC waveform.

Noah
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post #39 of 41 Old 04-29-2019, 01:12 PM
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Quote:
Originally Posted by noah katz View Post
Right





If acceleration were constant it wouldn't take the cone long to leave the room

Nothing (displacement, velocity, acceleration, jerk) is constant in an AC waveform.
You are right of course. What I meant to convey, and typed before thinking, is that the cone is continually accelerating. But obviously not constant in magnitude.

But my comments were specifically addressing the initiation of motion, not steady state sinusoidal motion. The magnitude of acceleration required to start moving from full stop does not exceed what is necessary to maintain output at that amplitude. I was curious whether jerk during transition from no motion to sinusoidal motion might exceed maximum jerk found during steady sinusoidal motion. I don't know... obviously didn't spend that much time thinking about it.
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post #40 of 41 Old 04-29-2019, 07:24 PM
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Quote:
Originally Posted by Bigus View Post
If the motor is strong enough to accelerate a cone to produce a given frequency, then it doesn't matter if that's coming from full stop or already moving, the acceleration needed is the same.
And how do you describe the situation where there is a weak motor? Heavy cones without strong motor force don't overcome....what? What differs if 2 cones with the same MMS have different motor force driving them?

I'm not arguing, I'm asking, what is different? Some attribute that difference to why pro-sound subs have more impact particularly in the mid bass. I'm simply asking for the language you use to describe what happens with weak motors.
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post #41 of 41 Old 04-29-2019, 08:24 PM
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For a given displacement, higher frequency requires higher velocity and acceleration. And for a given frequency, higher displacement requires higher velocity and acceleration. To play higher loud requires a strong motor and/or lighter cone that is easier to accelerate.

Relatively speaking, holding cone and suspension equal, the low frequencies don't require as much motor strength. Even if the cone is fairly heavy, so long as suspension is not too stiff and box large enough, the motor doesn't have to be such a beast. For proof, look at IB subs.

That prosound woofers tend to be strong performers in the midbass area is no surprise. Among other things, it is something they were specifically designed to do. There are always tradeoffs. One thing they aren't typically as good at is door jarring 10Hz output.
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