Jim + Auditory-list
While it is true that speakers are "designed" to be driven by the voltage, our detailed research has shown that it is the
current that controls the pressure (i.e., force on the cone). If you want the details I already sent the links, but I'll send them once
more, just in case you missed it.
The theory of how the current drives the force is given by our ASA presentation from last month, at the special session on historic transducers:
* Kim and Allen (2014) Historic transducers: Balanced Armature Receiver (BAR), ASA meeting INDY, IN, Oct 30, 2014 pdf
The full theory is in a Hearing Research article:
* Kim, Noori and Allen, J.B. (2013). "Two-port network Analysis and Modeling of a Balanced Armature Receiver, "Hearing Research, special MEMRO 2013 issue (pdf)
Ms Kim's (i.e., Noori's) PhD thesis has the most complete story, and is available directly from her (noorimail@xxxxxxxxx).
Because it is the current that determines the pressure, via the electrical input impedance, and because this impedance can be (i.e., is) reactive (have inductance), it can be tricky to figure out what is going on. One must know (i.e., measure) the load impedance, or at least the current. Also the distortion is mostly determined by the nonlinear flux vs current (i.e., B-H curve), as described by Hunt, for example.
Hunt mostly quotes Mott and Minor's BSTJ Jan 1951 article, adding little new material to their story.
In the interesting case of the Etymotic ER-3 (insert headphone), they place a 10 ohm resistor across the input terminals.
So if you try to beat this with a series resistor, you will not be successful in changing anything, because all the current is going into that 10 ohm resistor.
I opened up one of my ER-3 and cut out the resistor, and then I could see what was really going on. It worked better too.
When that ER-3 went back for repairs once, they were shocked to see the resistor was missing. I had to admit I cut it out, and wanted it left that way.
In general, adding a series resistor to your circuit, is working blind. You will likely get a random result, and random is never better.
Thus I dont advise it unless you are willing to measure the input impedance and the resulting frequency response of the headphone. While there
are some interesting things you can do with a series impedance, a series resistor is not one of them.
On 12/16/2014 06:12 PM, James W. Beauchamp wrote:
I agree with Bob. Most loudspeakers are designed to be driven by an ideal voltage source. Under this criterion they attempt to get the flattest possible response overall. This is not necessarily the most efficient response. In fact, some loudspeaker designs, like the Small-Thiele closed-box non-vented design, are very inefficient. Jim Bob Masta wrote:From: Bob Masta <audio@xxxxxxxxxxx> Date: Sun, 14 Dec 2014 09:15:19 -0500 To: AUDITORY@xxxxxxxxxxxxxxx Subject: Re: USB sound cards On 13 Dec 2014 at 21:59, Richard F. Lyon wrote:On Sat, Dec 13, 2014 at 5:52 AM, Bob Masta <audio@xxxxxxxxxxx> wrote:It's the other way around: Adding resistance in the driving circuit gives poorer damping. "Damping Factor" for a power amplifier is the reciprocal of output impedance.Bob, I wasn't aware of that definition. I was thinking of http://en.wikipedia.org/wiki/Damping_ratio rather than http://en.wikipedia.org/wiki/Damping_factorOops, you are correct... I should have said it is *proportional* to the reciprocal.Is there an understanding of why high "damping factor" would be good? Jont's findings suggest otherwise (I believe he's saying the current is typically more relevant than the volage).The second reference you cited covers it under "Explanation". Basically, a conventional electrodynamic speaker is both a motor and a generator. Imagine that the speaker receives a momentary voltage pulse, after which is is instantly disconnected from the source. The speaker would ring at its resonant frequency, damped only by friction. The generator would be creating a voltage, but no current, so no load to add damping. However, if instead of disconnection the leads were *shorted* after the pulse, the generator would be driving all its current into the zero-ohm load, giving a maximum damping effect. Conventional loudspeakers are designed to be driven by voltage sources, not current sources. The current may be more "relevant" (in the sense of force generation), but not for getting a flat frequency response from a conventional speaker design. (Although there have been occasional attempts at current drive, the ones I recall required special dedicated amplifiers.) Best regards, Bob Masta D A Q A R T A Data AcQuisition And Real-Time Analysis www.daqarta.com Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card!