Speaker excursion what causes it ? - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 155 Old 02-13-2017, 10:17 AM - Thread Starter
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Speaker excursion what causes it ?

Greetings,

The driving force that sets the diaphragm in motion is derived from the following equation:

F=BL x I

BL = (product called the force factor)

B = magnet flux density

L = wire length in the magnet field

I = current flowing through the voice coil

Sine engineers are more accustomed to identifying electrical signals as voltage rather than currents, this seems to be IMO the driving force behind the prevailing logic or commonly held belief (myth) that speaker excursion is a product of voltage based upon this false premise i.e. “speakers only respond to voltage, not current”

Note that voltage is not found anywhere in the equation above, and current being the prime diving force:

(Newton's formula F = mA)

F= force

m = mass

A = acceleration

such that, the radiated pressure follows the instantaneous acceleration and NOT instantaneous displacement, as many mistakenly imagine.

Reference:
http://www.edn.com/design/consumer/4...-voltage-drive
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post #2 of 155 Old 02-13-2017, 10:48 AM
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Current is present in the voice coil, of course. But what determines the extent of the cone excursion is the voltage swing. There is a reason why the sensitivity of speakers is SPL output with a given voltage input. It may be referenced to watts, using the nominal impedance of the speaker, so as a for instance the sensitivity of an 8 ohm speaker may be rated as dB/1meter at 2.83v, or dB/1meter at 1 watt. Nowhere will you ever find it quantified as dB/1 meter at .354 amperes.

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post #3 of 155 Old 02-13-2017, 10:52 AM
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You left out one (several actually) important equation:
I=U/R


F=BL x U/R

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post #4 of 155 Old 02-13-2017, 11:12 AM
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Quote:
Originally Posted by Bill Fitzmaurice View Post
Current is present in the voice coil, of course. But what determines the extent of the cone excursion is the voltage swing. There is a reason why the sensitivity of speakers is SPL output with a given voltage input. It may be referenced to watts, using the nominal impedance of the speaker, so as a for instance the sensitivity of an 8 ohm speaker may be rated as dB/1meter at 2.83v, or dB/1meter at 1 watt. Nowhere will you ever find it quantified as dB/1 meter at .354 amperes.
What determines the extent of a cones excursion is the given amount of Current within a supplied Voltage, not just the Voltage.

2.82 Volts is a theoretical means to produce extrapolated data, when actual measurement isn't possible, not practical, or not as flattering. Estimates based on theoretical Voltage produce results, that are typically 3dB higher then measured realities, with regards to sensitivity specs.

In other words, 2.83 Volts at @ 8 ohms only = a Theoretical Watt and therefore the presences of precisely .354 amperes.

This ratio is locked, therefore it becomes a matter of semantics, as to which values you look at. .354 amps x 8 ohms = 2.83 volts, 2.83 volts / 8 ohms = .354 amps, and so on. So one could state the sensitivity is 'x' at .354 amps into an 8 ohm load, and it would have the precise meaning and numeric outcome. Theory is great, but it doesn't trump actual measurement, which again, produces low scores than the exercise described above.
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Quote:
Originally Posted by Tomas2 View Post
Greetings,

The driving force that sets the diaphragm in motion is derived from the following equation:

F=BL x I

BL = (product called the force factor)

B = magnet flux density

L = wire length in the magnet field

I = current flowing through the voice coil

Sine engineers are more accustomed to identifying electrical signals as voltage rather than currents, this seems to be IMO the driving force behind the prevailing logic or commonly held belief (myth) that speaker excursion is a product of voltage based upon this false premise i.e. “speakers only respond to voltage, not current”

Note that voltage is not found anywhere in the equation above, and current being the prime diving force:

(Newton's formula F = mA)

F= force

m = mass

A = acceleration

such that, the radiated pressure follows the instantaneous acceleration and NOT instantaneous displacement, as many mistakenly imagine.

Reference:
http://www.edn.com/design/consumer/4...-voltage-drive
It is both that cause a speaker to move in and out. Where one is so is the other, they can not be separated one from the other.

It is actually Wattage that moves the speaker, there must be an adequate amount of it, but it takes both Voltage and Current, to produce work: ExI = W. Both the Voltage and Current envelops need to be in phase with one another to produce actual work - the two together, working in precise harmony.

However, Current is prime, as there can be no Voltage without it, in the first place, making it the prime mover.
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post #6 of 155 Old 02-13-2017, 11:25 AM - Thread Starter
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Quote:
Originally Posted by Bill Fitzmaurice View Post
But what determines the extent of the cone excursion is the voltage swing.
Greetings Bill,


After much research, I've come to the logical conclusion, that the speaker driver itself, in the end obeys only current, not caring what the voltage across the terminals happens to be. I understand in advance this statement may seem counter intuitive, but my premise is based upon the essence (AC) being the driving "force" of excursion in the first place.

If this premise was misguided, then voltage would need to be included in the equations I provided above.

Typical "voltage drive" amplifiers IMO are simply attempting to behave as a current source but suffer from vary impedance demands made upon them. This also correlates to the apparent superiority of current drive amplifiers.

For me anyway, all this hinges/relates to the legacy of trying to quantify electrical signal in terms of "voltage" only.

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post #7 of 155 Old 02-13-2017, 11:38 AM - Thread Starter
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Quote:
Originally Posted by AMO BRO View Post
It is actually Wattage that moves the speaker,
Agreed,

P = I^2 x R

and voltage drop across a given load, in this case being impedance.

A "voltage drive" amplifier is not a constant voltage source.
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post #8 of 155 Old 02-13-2017, 11:40 AM
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Originally Posted by Tomas2 View Post
I've come to the logical conclusion, that the speaker driver itself, in the end obeys only current, not caring what the voltage across the terminals happens to be.
Explain how you can provide constant current into a load that is not constant impedance. If you can't provide constant current into a load that is not constant impedance excursion, and therefore output, would vary with frequency.
Quote:
2.82 Volts is a theoretical means to produce extrapolated data
There's nothing theoretical about it. When you measure the sensitivity of a speaker it's done with a constant voltage signal, because while current and power vary with the load impedance, voltage swing does not.

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post #9 of 155 Old 02-13-2017, 11:42 AM
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Originally Posted by Tomas2 View Post

A "voltage drive" amplifier is not a constant voltage source.
Agreed
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post #10 of 155 Old 02-13-2017, 11:50 AM - Thread Starter
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Quote:
Originally Posted by Bill Fitzmaurice View Post
There's nothing theoretical about it. When you measure the sensitivity of a speaker it's done with a constant voltage signal, because while current and power vary with the load impedance, voltage swing does not.
With varying impedance, a voltage drive amplifier, try as it may, but ALL fail to maintain a "constant voltage" to some degree...some more than others
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post #11 of 155 Old 02-13-2017, 11:54 AM
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Originally Posted by Bill Fitzmaurice View Post
There's nothing theoretical about it. When you measure the sensitivity of a speaker it's done with a constant voltage signal, because while current and power vary with the load impedance, voltage swing does not.
It's all theory sir! The machines that are used for measurement, are running mathematical extrapolations, based on merely two knowns. Namely, Voltage (because its easier than current to measure) and resistance (because it too is easy to measure). The rest is all very much programed theory.

And when actually 'measured', as in at 1m/1w the signal agitation is pink noise, which doesn't output a constant voltage. The sensitivity score is then selected, as being the frequency (or pass band, depending on standard being used) exhibiting the highest SPL @ 1m/1w, or 1w/1m, however, you prefer to express it.

The speaker doesn't have a uniform / linear sensitivity to all frequencies throughout its pass band, that is quite impossible. The spec published is the highest.

They should publish the actual sweep, so we would all have a more informed understanding (but many don't take actual measurements any more, they theorize what the performance is likely to be, and when they do, the scores are always bolstered / higher to the linear tune of 3dB vs real-world measurements), however, one could look to a frequency response sweep (if available), then locate the highest peak, and reasonably assume that is where the Sensitivity spec is based.

Regardless of being measured or purely theoretical, current resides within every volt in a locked relationship, based on load, no exceptions, not even in here!
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post #12 of 155 Old 02-13-2017, 12:07 PM
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It's all theory sir! The machines that are used for measurement, are running mathematical extrapolations.
Obviously you've never run a manual sweep with a sine wave generator across the audio bandwidth, noting that the voltage stays rock steady at 2.83v, or whatever voltage level you use. I've done so hundreds of times, long before the existence of PC based measurement tools, as did literally generations of acoustical engineers before me.
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post #13 of 155 Old 02-13-2017, 12:13 PM - Thread Starter
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..However, Current is prime, as there can be no Voltage without it, in the first place, making it the prime mover.
Well said, this is the premise I'm attempting convey !

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post #14 of 155 Old 02-13-2017, 12:16 PM
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Originally Posted by Bill Fitzmaurice View Post
Obviously you've never run a manual sweep with a sine wave generator across the audio bandwidth, noting that the voltage stays rock steady at 2.83v, or whatever voltage level you use. I've done so hundreds of times, long before the existence of PC based measurement tools, as did literally generations of acoustical engineers before me.
Sure have. They produce a fake impression toward real-world efficiencies. Run a pink noise sweep and from 20Hz-to 20KHz, and you get anything but a linear sensitivity outcome. Take the power above one watt, and they get even more non-linear, because the current passing through the VC is changing...

Your way, produces higher sensitivities score, the better way reveals more likely sensitivity's.

Which would you rather have in hand, a spec based on a metric that is nothing like actual music injection or application or one that at least strives to present a more realistic spec.

Either way, both are just estimates, i prefer the more realistic one.

PS I'm down for a credential joist, but lets just keep it to mathematical facts for now.
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post #15 of 155 Old 02-13-2017, 12:48 PM
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Anyone else find it odd that two members who both just joined happen to misuse commas extensively in the same manner as another member who makes eerily similar wordy and mostly semantic arguments?

Maybe its just me.
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Originally Posted by Bigus View Post
Anyone else find it odd that two members who both just joined happen to misuse commas extensively in the same manner as another member who makes eerily similar wordy and mostly semantic arguments?

Maybe its just me.
Well, they may be new members or what not, but what they state is absolutely correct. An electromotor system like the ones used in common dynamic drivers obey F=BLI. It is not semantics, it is fundamental electrophysics. Laws of nature, if you will. The force generated is proportional to the current, NOT voltage.

Most speaker driver designers knows this (of course) and they also know that what will drive the complete speaker in the end is a voltage amplifier so they go through a great deal of effort to try make the response of the driver smooth over the "translation" between voltage and the current actually needed (in the design window).
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post #17 of 155 Old 02-13-2017, 01:52 PM
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I'm not making any argument against the math presented. AFAIK, the reason we reference everything to voltage is because our amplifiers try to maintain voltage source behavior, largely because current source amplification is difficult to do. Feel free to correct me.

And of course that wasn't my point.
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post #18 of 155 Old 02-13-2017, 02:17 PM
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PS I'm down for a credential joist, but lets just keep it to mathematical facts for now.
Really? Credentials? How about starting with a real name?
There's nothing that you can teach me, and obviously nothing that you aren't cocksure that anyone can teach you, so this relationship ends now. I've better things to do.
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post #19 of 155 Old 02-13-2017, 02:24 PM
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Quote:
Originally Posted by Bigus View Post
Anyone else find it odd that two members who both just joined happen to misuse commas extensively in the same manner as another member who makes eerily similar wordy and mostly semantic arguments?

Maybe its just me.
I think it's just you because one joined the forum yesterday and the other joined a year ago.
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However, Current is prime, as there can be no Voltage without it, in the first place, making it the prime mover.
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There can be no Voltage, without the presence of Current. It is impossible. Current is the prime mover, in Power, or actual work. If something moves, it is firstly because of the presence of sufficient Current to act upon it.
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Well, I was referencing the two members posting in this thread, the one you quoted and tomas2, who both post eerily similar to the other you quoted. They both just joined. But thanks for the quotes... it illustrates my point!
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I think it's just you because one joined the forum yesterday...
Shortly after a similar thread was unceremoniously shut down, with multiple deletions and in all likelihood sanctions. Coincidence?

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I'm not making any argument against the math presented. AFAIK, the reason we reference everything to voltage is because our amplifiers try to maintain voltage source behavior, largely because current source amplification is difficult to do. Feel free to correct me.

And of course that wasn't my point.
Bigus, (just for reference, your handle gets autocorrected to "bogus", so if you see me referencing you as bogus it's not because I think you are (far from!) but because of evil autocorrect ).

With that said, current source isn't really that difficult to do, but it presents a problem when for the last 100 years or so voltage has been the means to transfer the signal to recreate a recording.

I've touched upon this subject several times before here at AVSF, that dynamic drivers are complex electroacoustical devices that most of the time will perform better when driven by a predominantly current source (using a sensibly designed passive XO rather than being driven in pure active XO and direct drive configurations). The main reason for this is the fundamental law that the force acting upon the membrane is proportional to current, not voltage...). 6+ years ago I participated in a similar topic where I elaborated a bit more on the topic (you'd have to follow my responses in thread to the end...):

Bi-amping, No flame, no theory just real world ear based experience
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post #23 of 155 Old 02-13-2017, 03:04 PM - Thread Starter
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Well, they may be new members or what not, but what they state is absolutely correct. An electromotor system like the ones used in common dynamic drivers obey F=BLI. It is not semantics, it is fundamental electrophysics. Laws of nature, if you will. The force generated is proportional to the current, NOT voltage.
Greetings hevi,

Indeed I'm a new member and it's a privilege to contribute to this forums collective knowledge. I do not personally know any other member here and I wish to thank all who take time to post or simply read the comments that are made.

I also agree this is not a matter of semantics, and I think ultimately will can all converge or align with the math as it presented.

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post #24 of 155 Old 02-13-2017, 03:19 PM
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And the example given then wasn't driven by a current source either. In the nearly 6 years since, have you come up with anything more convincing?
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Bogus... lol, I hate autocorrect. I've thought numerous times about changing the name to something less juvenile, but I don't think AVS allows that and its been this way for a long long time. Oh well.

On occasion I've run across the topic of transconductance amps. Seems interesting from an intellectual perspective, but none of it ever really stuck I suppose. Maybe you could summarize. What is the primary benefit of current drive? I would assume perhaps a more linear frequency response as the frequency dependent impedance is removed from the equation, but with good drivers already being pretty flat impedance-wise and frequency-wise within the useable distortion bandwidth, except at low frequencies, and with the ubiquity of ever expanding eq capabilities, is that really necessary? Are there other benefits such as lower driver distortion, some meaningful change in power handling, etc?
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And the example given then wasn't driven by a current source either. In the nearly 6 years since, have you come up with anything more convincing?
If you please could elaborate the mathematics that lead you up to your wrong conclusion that the example I made was driven by a pure voltage source I would be more than thrilled, or rather baffled, because if you do, you'd be violating more laws of physics than I could ever imagine....
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Bogus... lol, I hate autocorrect. I've thought numerous times about changing the name to something less juvenile, but I don't think AVS allows that and its been this way for a long long time. Oh well.

On occasion I've run across the topic of transconductance amps. Seems interesting from an intellectual perspective, but none of it ever really stuck I suppose. Maybe you could summarize. What is the primary benefit of current drive? I would assume perhaps a more linear frequency response as the frequency dependent impedance is removed from the equation, but with good drivers already being pretty flat impedance-wise and frequency-wise within the useable distortion bandwidth, except at low frequencies, and with the ubiquity of ever expanding eq capabilities, is that really necessary? Are there other benefits such as lower driver distortion, some meaningful change in power handling, etc?
No, I won't summarize. I am not discussing transconductance amps at all. I am discussing reality.The fundamental electroacoustical *FACT* that F=BLI is a fundamental, physical, proven "law of nature" makes the two "newcomers" assertions correct. That is what matters in this thread....
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post #28 of 155 Old 02-13-2017, 03:58 PM
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And there is even a book---
http://www.current-drive.info
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Acoustic Mafia - Hear No Evil
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post #29 of 155 Old 02-13-2017, 05:34 PM
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No, I won't summarize. I am not discussing transconductance amps at all. I am discussing reality.The fundamental electroacoustical *FACT* that F=BLI is a fundamental, physical, proven "law of nature" makes the two "newcomers" assertions correct. That is what matters in this thread....
Wow. I know text can misconvey emotion sometimes, but dang dude. What happened, not get enough sleep last night?
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post #30 of 155 Old 02-13-2017, 05:40 PM
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If you please could elaborate the mathematics that lead you up to your wrong conclusion that the example I made was driven by a pure voltage source
Your reading skills are poor: I said it wasn't driven by a current source. Which it's not.
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