SSA Car Audio Forum: Impedance Rise - SSA Car Audio Forum

I've been meaning to write this up and post it here for sometime... thankfully Aaron pushed me a little.

A couple of quick comments: I've tried to make this as straight forward and digestible as possible; I've explained these things to the best of my knowledge... bear in mind, I am no Electrical Engineer.

What follows is the result of monthss of research and hours of playing / testing my own equipment.

Lastly - I'll leave this locked until Denim gets a look at it.

-Nick

At your service – I hope you enjoy this thread.

Everyone has heard of the mystical term impedance rise, but more & more I read and hear people saying “I’m not worried about that yo…”
Well, sad to say, but that person will not see what they expect from their Acme 9000.
Why? Impedance rise.

Now, I do not understand all of the physics behind this… But I do understand the end result and a portion of the process. Hopefully, someone will take something from this post and make use of it

First, and foremost: Immediately following is how you go about mapping a given loaded enclosure for impedance shift (rise or fall). If you are brave, keep reading for a little background on Impedance shift (both thermal & EMF related).

This procedure also can be used to find the real world tuning of any ported enclosure, in a given install.
More importantly, in my opinion, the second method gives the example of the best way to go about seeing what you amplifier ‘puts out’. With regards to this, any given amplifier will produce different results in different install.

With either method, You are going to need a few basic things:

*At least one Voltmeter (VAC) –or- DMM.
*A supply of test tones. Preferably 20-80hz - NOT a sweep
*An audio system: Signal source, amplifier, drivers, enclosure
*A calculator of spreadsheet program.
*Pen & Paper

Throw in a clamp meter –or- a resistor (Radio Shack) and you’re in business.


-Nick

Resistor method:

You are going to need the items mentioned above, but, in addition:

An additional VOM/DMM would be helpful.
* A 10 ohm resistor. Preferably non-inductive and able to handle 10 watts or better.

That’s it, the resistor should cost no more then $5, and you probably have a DMM or two by now. You’re going to be amazed what you find out for a $5 investment.

To map any enclosure, you need to measure the impedance. Unfortunately, Impedance changes as the driver’s coils travel. Thankfully, Ohm taught us a few different ways to find impedance. Enter the math:

Quote

Confused? Open up Microsoft Excel. Copy & Paste:

Quote

As for the physical how to:
Make a voltage divider circuit:

->A 10ohm resistor in series across the speaker wire / terminals
->Connect the speaker wire to an amp that is connected your source unit
->Secure your VOM’s leads into a.)The amplifer’s output terminals b.)The resistors terminals (or ends) c.) Set both to VAC
->Power up and play your test tones AT A LOW VOLUME.
->Record the readings from each VOM (or switch leads quickly from amplifier terminals to resistor terminals quickly)
->Power down and plug your readings into the spreadsheet

annnnd You’re done.



A couple of important points:

*Use LOW volume. The resistor probably cannot handle very much wattage. You smoke it, bad things could happen.
*Though tedious, you should try to use 1hz increments. 5hz at the most. A lot can change in your driver over a 5hz band.
*The average DMM sample twice per second (iirc), so you will need each tone to be at least 5 seconds long – this’ll give you a chance to glance at both and write them down. At each frequency note the voltage at the amp terminals and voltage at the resistor.
*I would put the results into Excel then plot Rs and frequency... make a line chart in your worksheet, it’ll be so much easier to grasp once you have a visual.
*I dunno how much the listening area (car interior or room shape) come into play… If you are testing in-car, open the windows to simulate your normal listening space. Unless You are only concerned about the sealed interior (a la dBdrag).

There are other ways to do this. Though, for me, they are the same difficulty level (low). It’s just a matter of choice / preference, but I thought that I’d lay out both options.

Now, I suppose the following method would be better for any competitor.

You are going to need the items mentioned in post #1, but, in addition You’ll need a decent Clamp Meter, to measure amperage. These can be had on Ebay for $30-50, and make a great investment.

Using the process above as a reference point – You can do the same thing with a clamp meter & VOM. I’ll go ahead and omit the minor points, as they are listed above…

Physical Process:

By clamping onto the speaker wires (in parallel) you can read the amperage going to the drivers at each given frequency.
By probing the amplifiers output terminals with a VOM (set to VAC) you can read the voltage output at each given frequency.
Again, use test tones

You don’t need a resistor for this: V/A=R

Important points:

*You only need to record amperage (clampmeter) and voltage (VOM/DMM) at each frequency.
*As before, plug each number into the spread sheet, graph enjoy.
*This method is much easier for some folks…. If they have a clamp meter ($30-50 on Ebay).
*You can test at close to full volume
*Be careful of doing this full tilt (as you will be playing the drivers for at least 4 minutes)
*With this method [list] You can check at much higher power levels… i.e.- this method will include heat generated impedance rise
*You can setup and test a little more quickly then resistor based testing
*As a by-product, You can get the real world output of your amplifiers – albeit, depending on the volume at which you are testing.

Both of these methods should work identically (aside from power level effects already mentioned)… again You might be amazed at what you find.

Quote

I do want to point out that Impedance drops as You apply more power. So, while testing at 1/2 volume will Give You a good reference to start tuning the enclosure with - when it come's time to measure acutal power for competition, You NEED to test at full tilt. Impedance lowers as amperage increases.

Your choice.

Now… to move onto the physics…

By now, you’re either heading to your amp with tools in hand –or- wondering where the heck impedance rise comes from.

This is where I’ve gotten to in my studies… so I don’t have all of the answers. Please read on with this in mind. These are my findings & summations. The following is only a jumping off point… I am not an Electrical Engineer.

-Nick

EMF, Motor cooling, and enclosure design. These are the three biggest causes of impedance shift.

*Back EMF is not a complete negative, it provides a measure of checks & balances… but that’s over the head of this thread and Me.
*Motor cooling obviously affects the nature of the coil. The hotter the metal coil, the higher it’s impedance. A well designed motor cooling system (top plate vents, vented pole pieces, pole piece type, etc…) will greatly help the driver’s stability and longevity. I’ll stop here and refer you to the link below.
*Enclosure design will obviously affect a driver in many ways. Port area, port tuning, port coupling, vB, etc… Cone speed has a whole lot to do with the ability of the motor to work efficiently. Again… I’ll refer to a link below

Without further adieu – links.


Great thread involving Loyd (Bigbassman), Kurt (Warbleed), and Ash of Sound Domain - http://forum.sounddo...c;f=11;t=075857

Great site that covers EMF and speaker motors in general - http://sdf1.cc/~neowall/vas/phy.htm

Definition of Back EMF - http://www.sweetwate...hp?find=BackEmf

Thanks Denim.

sheesh, thats an amazing write up Nick
now i just need to get off my lazy ass to install and test my subs

What a waste of time. I already know this much. You suck!

On a serious note, I can't wait for Dave, Mike, or Nick to enlighten us more in the actual EE department. I'd love to get into further deyail on this.

One thing I've been wondering is how these newer amps coming out with their ability to produce a continous amount of the same power into any nominal impedance account for the shifts in impedance. I know the JBL/Crown amps are wired differently for different nominal loads, therefore, I'm led to believe they will not work to full potential with shift involved. The JL and PG amps, I just have no clue on.

Yes Ryan, I know you know this. So do some others... but lot's of folks dont know, or dont know how to account for it.

Yea, I'm curious in the same fashions.

Someone asked me to post this up...

-Nick

If only my DMM werent broken

NjccBflo, on Mar 29 2005, 09:09 PM, said:

I was just kidding. I actually like reading things like this that actually get into detail. It helps to refresh yourself once in a while. Keep the brain functinononuibm mnbo/


Whoops.

you know... thats a damned fine point.
I dunno much about the Crown, with it being so out of reach...

Seperate circuits? It's not like you can make the same board into a variable output and still step up to that much voltage...

Good post, nothing new, but always nice to have there for reference

NjccBflo, on Mar 30 2005, 02:31 AM, said:

thats one reason why I want to try the Crown amps so bad, they were awesome in club's and bars that I worked in, rock solid and just abused the speakers while holding a low temperature the whole time

Nice write up----I myself am in stages of testing differant classes of amplifers and back EMF and impedence rise within the amplifer itself-----I am also trying to get a patent on one of my new ideas----will tell you when/if I get it.
As for the write up-----you covered alot of questions anyone would have about the little known(and mostly understood----or not cared about) subject.-----Jeeesh I'm tired. B)