Impedance Analyzer

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Impedance Analyzer

One of the fundamentals of loudspeaker design is the tuning frequency of your enclosure.  Whether it's sealed, ported, bandpass, or any other kind of box configuration, it is absolutely essential that your box is tuned to the frequency you want it at.  So I have made an attempt to explain clearly and easily how to build a device that will allow you to measure the frequency of resonance of your speaker system.  I got the design out of an old issue of Speaker Builder Magazine, but I can't remember which issue!  The impedance analyzer can be used to measure raw driver impedances, enclosed speaker impedances and will help with calculating Thiele/Small parameters as well.

Parts You Will Need

  • 6 binding post terminals (red & black preferred)
  • Rugged toggle switch - SPDT (Single Pull Double Throw)  Just make sure it's got 3 little prong-things to solder to
  • 100 ohm 10 watt resistor (wire wound will work fine)
  • 8 ohm 20 watt resistor
  • 1" x 2" x 4" plastic case
  • Some wire (18 gauge is fine), solder and a soldering iron

All of the above parts can be bought at Radio Shack for under $10.00.  The 8 ohm resistors they sell are actually non-inductive which is even better.  They don't have non-inductive 100 ohm resistors, but unless you're measuring impedances at very high frequencies, the small amount of unductance should be negligable.

Putting it Together

The first thing I did was drill all the holes for the the binding posts and the switch.  Then I mounted them in the enclosure.  Lastly I soldered everything together and made some cool labels to put on the front.  In the event of confusion,  the calibrate mode is always with the 8 ohm resistor as the load.  Most switches when in the "forward" position actually contact the "back" tabs so be careful to note what contacts to where when the switch is in either position.  

Other Things You Will Need

Now that you've got the impedance analyzer all finished, it's time to learn how to use it.  You will need along with your brand new impedance analyzer the following items:

  • Frequency Generator

  • Frequency Counter

  • Amplifier

  • Digital Voltmeter

  • Your speaker

  • Some wires

Connect the frequency generator to your amplifier.  Make sure that you use no active equalization of the signal.  You want a completely flat response going to your speaker.  Connect the frequency counter to the frequency generator.  Connect the speaker output of one channel on your amplifier to the input labeled 'amp in' on your impedance analyzer.  Connect your digital meter the the meter location.  Finally connect your speaker to the speaker out location.

Measuring the Impedance of a Speaker

Turn everything on and set the frequency generator to its lowest range.  You'll want to do a sweep from about 10 hz to 100 Hz.  Set the meter to a usable voltage scale, somewhere around 400mV.  This part is sort of confusing so pay attention.  To calibrate your meter so that it reads "impedance", flip the switch on the IA (Impedance Analyzer) to "calibrate".   You'll want to set the amplifier volume output so that the voltage reading on your meter is equal to the actual resistance of your 8 ohm resistor.  Let's say it was 8.1 ohms.  Turn the amplifier volume up until you read 0.81 volts on your meter.  Or better is to use a lower scale and read 8.1 mV.  Don't try and get 8.1 volts!  Remember there is a 100 ohm resistor is series with your whole circuit.  You would need 108 Volts from your amp to get 8.1 V across that resistor.  Use the lowest scale you can to get the most accurate readings.  Now that the meter is calibrated to measure 8.1 ohms (or rather, volts), sweep the frequency generator up and down and see how much variance in voltage there is.  There should not be a whole lot of change, but it may vary a little bit, depending on the quality of your frequency generator and how flat it is.  That is why you have the switch. To change easily between measure mode and calibrate mode as you sweep throughout many different frequencies.  Now flip the switch to "measure".  This loads the amp with your speaker's impedance so the current will remain the same but the voltage drop across your speaker will change, hence you will see the voltage on your meter change as well. Whatever the voltage says, that is your impedance at that particular frequency.  For example,  6.78 mVolts equates to 6.78 ohms. Note it should change continuously with frequency.  Especially at low frequencies.  You may even notice a peak in the voltage (i.e., impedance).  These peaks are very important.  If the speaker is in free air (no enclosure) then the peak will be at the speaker's resonant frequency or Fs.  This is how you find Fs of a driver.  The peak impedance, where you measure the highest voltage across your meter.  If your meter peaks at 15.67 mV with a 32 Hz sine wave for example, then the Fs of your speaker is 32 Hz and it's impedance at Fs is equal to 15.67 ohms.  If the driver is mounted in a sealed enclosure, the impedance peak will be at the fB of the enclosure and not the Fs of the driver.  This is how you measure the fB of a sealed system.

Measuring the Fb of a Ported or PR Speaker System

To find the Fb of a tuned ported box you need to do as sweep across the 10 - 100 Hz range.  You should notice TWO peaks on your meter.  Record the one peak as FL and the other peak as FH.  Then you must cover the port with an airtight seal, making your speaker essentially into a sealed enclosure.  For PR enclosures you will need to remove the PR and seal the opening with a board and make sure the seal is tight.  Use weather stripping if necessary.  This time measure the peak again, there should only be ONE peak this time.  Record this frequency as FC.  Now use the formula:    FB = square root of (FL2 + FH2 -  FC2) . This should give you a very accurate measure of you actual box tuning frequency.  

Don't forget to switch over to "calibrate" mode often while doing impedance checking because the voltage may change a little.  It will throw off your impedance tests by enough to be significant.  My thanks to Speaker Builder Magazine for printing the article about the Impedance Analyzer and to Radio Shack for actually carrying the parts I needed.  Also thanks to David B. Weems for his formulas in The Great Sound Speaker Manual.




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Front View

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Side View

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Internal View

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This page last updated on September 27, 2016.

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