SD + XMAX = SPL ...correct?
03-20-2007, 08:24 PM
#21
Originally Posted by andrewsfm
Apparently motor force has something to do with this too.
The amount of power needed to reach the maximum xmax maybe?
The amount of power needed to reach the maximum xmax maybe?
Is there a TS 'velocity' parameter? I think you'll need it to make your calculation...
Sound is about pressure waves. As the speaker cone moves, it's the pressure waves you hear, not the displaced air. Think 'pebble in a pond' - it's the waves (energy) that move, not the water. Pushing the cone around to create these waves is the motor's job , and it has to be able to move the speaker cone over the whole XMax distance (and back) in a very short period of time to create any meaningful pressure waves. So cone/motor velocity is going to be a big factor: big cone + low velocity = *whif*, big cone + high velocity = !thump!. It won't take much power to reach XMax if the cone velocity is low - you only need to overcome the internal 'drag' of the speaker assembly. High velocity long distance (XMax) movement is where the power goes - into making waves (as well as overcoming internal speaker drag).
Wikipedia's been getting some bad press recently, but the T/S entry doesn't look too bogus, and it's got this to say about Vd (peak displacement volume):
Originally Posted by Wikipedia
Vd
Specified in litres (L). The volume displaced by the cone, equal to the cone area (Sd) multiplied by Xmax. The same Vd value may be obtained by having a small cone with a large Xmax, or a large cone with a small Xmax. Comparing Vd values will give an indication of the maximum output of a driver at low frequencies. High Xmax drivers may be inefficient, since much of the voice coil winding is outside the magnetic gap at any time and therefore contributes little or nothing to cone motion.
Specified in litres (L). The volume displaced by the cone, equal to the cone area (Sd) multiplied by Xmax. The same Vd value may be obtained by having a small cone with a large Xmax, or a large cone with a small Xmax. Comparing Vd values will give an indication of the maximum output of a driver at low frequencies. High Xmax drivers may be inefficient, since much of the voice coil winding is outside the magnetic gap at any time and therefore contributes little or nothing to cone motion.
Originally Posted by Wikipedia
Xmax
Specified in millimeters (mm). In the simplest form, subtract the height of the voice coil winding from the height of the magnetic gap, taking the absolute value and dividing by 2. This technique was suggested by JBL's Mark Gander in a 1981 AES paper, as an indicator of a loudspeaker motor's linear range. Although easily determined, it ignores non-linearities and limitations introduced by the suspension. Subsequently, a combined mechanical/acoustical measure was suggested, in which a driver is progressively driven to high levels at low frequencies, with Xmax determined at 10% THD. This method better represents driver performance.
Specified in millimeters (mm). In the simplest form, subtract the height of the voice coil winding from the height of the magnetic gap, taking the absolute value and dividing by 2. This technique was suggested by JBL's Mark Gander in a 1981 AES paper, as an indicator of a loudspeaker motor's linear range. Although easily determined, it ignores non-linearities and limitations introduced by the suspension. Subsequently, a combined mechanical/acoustical measure was suggested, in which a driver is progressively driven to high levels at low frequencies, with Xmax determined at 10% THD. This method better represents driver performance.
Originally Posted by Wikipedia
Notes about large signal behavior
It is important to note that T/S parameters are linearized small signal values. This is an idealized view of the performance of a loudspeaker, and the values of parameters vary with drive level. Fs generally decreases as power level increases. Bl is generally maximum at rest, and drops as the voice coil approaches Xmax. Re increases as the coil heats and the value will double by 270 °C, the point of rapid driver failure. The result of these level-dependent nonlinearities is some complication in computer modelling, usually predicting higher SPL than can actually be achieved. Sophisticated magnet or coil designs attempt to linearize Bl and reduce the value and modulation of Le.
It is important to note that T/S parameters are linearized small signal values. This is an idealized view of the performance of a loudspeaker, and the values of parameters vary with drive level. Fs generally decreases as power level increases. Bl is generally maximum at rest, and drops as the voice coil approaches Xmax. Re increases as the coil heats and the value will double by 270 °C, the point of rapid driver failure. The result of these level-dependent nonlinearities is some complication in computer modelling, usually predicting higher SPL than can actually be achieved. Sophisticated magnet or coil designs attempt to linearize Bl and reduce the value and modulation of Le.
After all that I guess I can answer my own question - there isn't a velocity parameter per say; that information is 'embedded' within the other TS parameters. And maybe I can answer yours too, as it doesn't sound like there's an easy Vd/Xmax route to MaxSPL for a given driver.
On the other hand, someone more knowledgeable may have a very different and/or much better comment!
Cheers,
--
Terry McG
03-20-2007, 09:11 PM
#23
I don't know if velocity has a whole lot to do with it, because a 30 Hz tone can only be delivered at 30 vibrations per second. Anything faster or slower wouldn't be 30 Hz.
I think you're getting into the damping factor which I believe is how fast the sub returns to it's 0 position to be ready for the next note, otherwise it becomes sloppy.
I'm starting to think it all starts with magnet strength which is then limited by the xmax.
I think you're getting into the damping factor which I believe is how fast the sub returns to it's 0 position to be ready for the next note, otherwise it becomes sloppy.
I'm starting to think it all starts with magnet strength which is then limited by the xmax.
03-20-2007, 09:50 PM
#24
I wouldn't worry much about linear xmax in a spl driver..... as far as say gap height or mechanical xmax it may or may not be an issue between two subs...... one may have more but that dosen't necassarilly mean it will get louder....
If you want to find reference spl numbers on paper follow this:
No = 9.64 * 10^(-10) * Fs^3 * Vas / Qes
(% Efficiency = No x 100)
SPL = 112 + 10 * log (No x power in watts)
let me know if you want an example....
Last edited by Haunz; 03-20-2007 at 11:56 PM.
03-20-2007, 10:58 PM
#27
Without bringing up BL curves and so forth, remember that the BL spec is the amount of BL at the drivers resting position.
Xmax, on the other hand, is defined as the distance the cone can travel, one way, while maintaining 70% of it's original BL. Yes, this implies that BL drops off as the cone moves away from resting position (happens in both directions, unfailingly).
So while you may want a large amoutn of BL over a smaller range for SPL usage (especially for ported boxes), a high power sealed box would need a good bit of Xmax to keep the cone under control.
You can think of the BL curve as an engine's torque curve, although with different implementation.
Xmax, on the other hand, is defined as the distance the cone can travel, one way, while maintaining 70% of it's original BL. Yes, this implies that BL drops off as the cone moves away from resting position (happens in both directions, unfailingly).
So while you may want a large amoutn of BL over a smaller range for SPL usage (especially for ported boxes), a high power sealed box would need a good bit of Xmax to keep the cone under control.
You can think of the BL curve as an engine's torque curve, although with different implementation.
03-21-2007, 12:04 AM
#29
BL plays into efficiency so I wouldn't worry about it explicitly... you generally always want the most BL possible, but at a point it comes with trade offs...
things like shorting rings, dual gaps, ect, will help keep the BL curve flat but at the same time they kill efficiency... either by killing effective BL or by wasting power...
As for the above eq you will get infinate baffle spl @ 1m either outside or in an anechoic situation... no box, no cabin gain....
for an example, take a 15" DD9500....
fs = 33Hz
vas = 85L
qes = .31
No = 9.64 x 10^(-10) * 33^3 * 85/.31
= .000000000964 * 35937 * 274
= .0095 (or .95% efficient)
now say we are giving it 1500 watts of sine wave
SPL = 112 + 10 * log (.0095 *1500)
= 123.5 db
again, this would be @ 1m outside or in an anechoic environment with the sub mounted in an infinate baffle... its definatly not the last word on SPL, but its about the only fair compairison you can make on paper between two subs....
things like shorting rings, dual gaps, ect, will help keep the BL curve flat but at the same time they kill efficiency... either by killing effective BL or by wasting power...
As for the above eq you will get infinate baffle spl @ 1m either outside or in an anechoic situation... no box, no cabin gain....
for an example, take a 15" DD9500....
fs = 33Hz
vas = 85L
qes = .31
No = 9.64 x 10^(-10) * 33^3 * 85/.31
= .000000000964 * 35937 * 274
= .0095 (or .95% efficient)
now say we are giving it 1500 watts of sine wave
SPL = 112 + 10 * log (.0095 *1500)
= 123.5 db
again, this would be @ 1m outside or in an anechoic environment with the sub mounted in an infinate baffle... its definatly not the last word on SPL, but its about the only fair compairison you can make on paper between two subs....
Last edited by Haunz; 03-21-2007 at 12:11 AM.
