Speaker Impedance Question

[Gaston]

so is the "bong"sound the actual tone of the resonant frequency of the speaker?

 

[ericgibbs]

so is the "bong"sound the actual tone of the resonant frequency of the speaker?

hi Gaston,

Link to speaker terms: **broken link removed**
Lookup 'Damping'

Regards

 

[Blatman Bond]

What Damping Factor is?

They measured damping factor as speaker impedance divided by amplifier output impedance.

Too high damping factor will force the speaker acoustic reflex, especially in bass.
That makes low-negative-feedback applied to most power amplifier design.

And speaker impedance isn't actually at their 4/8/16 ohm? That was true.
For example : fostex fe206 resonance at 40Hz because weight and spring.

 

[Speakerguy]

Yep, speakers are generally rated at a 'nominal' impedance. This is usually quite a bit above their DC resistance or the minimum they may achieve across their bandwidth. At the low end, you have the resonant characteristic of the driver given by the Thiele/Small parameters (which are actually derived from the impedance using an equivalent circuit for the mechanical system, but anyway...). On the high end you have a lossy inductance, but finding a manufacturer that specs anything other than Le @ 1KHz is uncommon (TC Sounds is the only mfg I know that actually specifies Ls, Lp and Rp for the better inductance model). Once you start throwing in another driver and a crossover things can really get wacky if you are not careful with the design. Bad designers can screw up and easily give you load impedances well below nominal impedance and/or impedance with phase angles more than 45 degrees capacitive. Either is not well liked by solid state amps, and I think can have much more detrimental effect on the sound than something like damping factor. But that's probably more than most wanted to know about that....

Re: my earlier snipe, I'm quite active on other forums, so I'm almost always good about reasoned debate, but I do have an ego about audio stuff I admit. I'll try to pop a xanax before I get all up in arms again about an audio discussion. And I do admit I'm quite green in most other areas of electronics, so you guys needn't worry about me getting out of line - I know where my expertise begins and ends (ends pretty quickly after it begins for the most part ) Heck, I don't even know PIC Basic. I've gotta spend the money for a C compiler, 'cause that's all I know.

 

[Roff]

<snip>A resistor in series with the amplifier output would have the effect of lowering the damping factor by increasing the apparent output impedance<snip>

This is the basis for saying that speakers in series may be underdamped, right?

 

[Hero999]

Just a thought but wouldn't it be better to use a transconductace amplifier as an audio amplifier?

The movement of the cone is proportional to the current not the voltage?

Therefore if we simply vary the current and forget about the voltage we should be able to control it more accurately?

 

[Nigel Goodwin]

There have been a great many schemes over the years, but almost all top amplifers use fairly standard techniques - none of the others seen to produce any better results?.

As suggested earlier, many people like valve amplifiers, despite their poor specs and limitations - and they have totally pathetic damping factors.

 

[audioguru]

Yes, speakers in series are underdamped. Many guitar speaker systems have two or four speakers with two speakers in series. but they like the "effect" of the boomy resonant sound, which is needed to give a little bass response when the back of the enclosure is frequently missing.

 

[Speakerguy]

Hero,

That is exactly true, and current drive is something that has been experimented with for a long time. It totally nullifies any back EMF induced or Le induced distortion. Unfortunately, due to the theoretical infinite output impedance, you have no electrical damping (in Thiele/Small parameter terms, the overall system Q is determined solely by Qms). This means that the speaker will resonate severely at its natural frequency (say 50Hz for a 6.5" speaker typically). Typical damping values for electrical Q are 0.2 - 0.5 for quality speakers, while mechanical Q values are 2-10 for the same quality speakers (Q being the inverse of damping). This means that in a voltage drive system Qes dominates (total Q is Qes*Qms / (Qes + Qms)) and in current drive systems Qms dominates.

 

[Blatman Bond]

If I ask another about how how driving loudspeaker at best result, there are no certain answer.

Look at the first-watt project (transconductance), they say it sounds better, but other?
I've tried to used vary damping factor to any 8 ohm speakers, between 50-600 using fast and linear designed amplifier.
And the result, most speaker with normal-box need less than 200 at bass and less than 400 at middle and high.
And I can't find any article research about it.
Low-feedback voltage-amplifier used by many product till now. Why?
And bootsrap amplifier has good acoustic reflex at low frequency.
It seems that many amplifier factory have it for secret and tell everyone wrong thing about negative feedback.
So speaker driven by voltage, current or power? which better? I need more to know something about it.

 

[Speakerguy]

Placing speakers in series or parallel shouldn't affect their damping at all. A simple simulation with a Thiele/Small parameter model will verify this.

Here is a good link. You enter the main 3 T/S params and let it calculate an optimum box volume, and it will give you a box size and a frequency response for it.

https://www.linearteam.dk/default.aspx?pageid=newdriver

Leave the params as they are, hit "Optimum" under either sealed or vented box, and it will give you a box size and frequency response for the system.

Now increase the # drivers to whatever you want. Hit "Optimum" again, and it will scale the box size appropriately. The graph will be identical to the first, no matter how many drivers you use. It doesn't even give you a series or parallel connection choice, as that will only affect the system sensitivity, not the inherent frequency response.

 

[audioguru]

Hi Speakerguy,
There are many speaker box design programs available. Most are for only a single speaker. The program isn't smart enough to see a high impedance resonating speaker in series with the speakers in a two series speakers system.

The impedance of a speaker is much higher at resonance than its nominal rating. The damping is destroyed when such a high impedance is in series with each speaker.

 

[Blatman Bond]

Placing speaker in series need more technique, where one and other are easy to influence. And rarely used for two different characteristic speakers.

Why many people like valve amplifier and says their natural sound quality.
I think the speaker need to driven by something between voltage and power.
To let the speaker move as its characteristics.

 

[Hero999]

speakerguy79,
I have another idea.

How about adding a separate feedback coil in the middle of the speaker? Providing it's magnetically shieilded from the outer volce coil which whll be the case if there's another circular hole for it to reside in, in the speaker magnet. Won't the voltage in the feedback coil mirror the movement of the cone rather than the input voltage? Therefore it could be used to provide aditional negitive feedback to the amplifier making it sound nearly perfect.

 

[Speakerguy]

Hi Audioguru,

You're talking to someone who owns both Praxis and LspCAD v6 Though I haven't used them in ages. I'll do a simulation tonight and post the results to verify my claims.

Hero,

That's been done too, at least for voltage drive systems. Google Rythmik Audio for an example. They use a sensing coil wound on the voice coil former to provide a way to measure cone position and turn it into a servo system. Accelerometers work too, the ACH-01 is a popular part that is available at Digikey (Velodyne makes accelerometer based subs). I've never seen a current drive setup with a feedback system around it though I don't think. Sounds like an interesting idea.

 

[Blatman Bond]

Yes it will provide the movement feedback, but if we force the speaker movement as we need, that mean no way for the disturbance (include bass reflex). It might be perfect for speaker with heavily damped box. But we design box for better bass response.

 

[ericgibbs]

hi speakerguy,
Have you ever constructed an home made speaker 'wobbulator'?

I bet 'agu' has.

 

[Hero999]

Google Rythmik Audio for an example. They use a sensing coil wound on the voice coil former to provide a way to measure cone position and turn it into a servo system.

I've read the website but from what I gather the feedback coil is wound on top of the voice coil. What about the effects of magnetic coupling between the two coils?

I assume that they can be cancelled out if you know the mutual inductance but the cancelling won't be perfect especially as different drivers might have a slightly different mutual inductance.

Accelerometers work too, the ACH-01 is a popular part that is available at Digikey (Velodyne makes accelerometer based subs). I've never seen a current drive setup with a feedback system around it though I don't think. Sounds like an interesting idea.

I don't think it will make any difference whether current or voltage control is used since the feedback will counteract any imperfections in either system.

Even with the servo system, it still isn't perfect, the room acoustics and driver cases will all play their role in creating distortion, I suppose a huge horn speaker in a huge concert hall might give that all illusive <1dB THD all the way down to 20Hz.

 

[audioguru]

Have you ever constructed an home made speaker 'wobbulator'?

I bet 'agu' has.

Do you mean a Leslie rotating speaker? No, I never played an electric organ.
I also never played a guitar.

I played the piano when I was very young and played the trombone in my high school band. If I continued with studying music then I would have developed "perfect pitch". I was very close to having perfect pitch and could identify some frequencies by their sound.
When I was young I could hear all the ultrasonic burglar alarms in town.

 

[Nigel Goodwin]

When I was young I could hear all the ultrasonic burglar alarms in town.

Did they have ultrasonics when you were young?

Did they even have burglars?