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.
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.
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:
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.
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
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.
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.