Why we really do not like Impedance Bridges

Many times, we’ve had installers call us and tell us that we’ve shipped them defective speakers. When we press a little, we often find that they’ve pulled a speaker out of the box, slapped an impedance bridge on it, and got a number that looked way off. They assume that because the number is so far off that the speaker must be bad. This is why all our technical data sheets contain impedance curves. While a speaker may have a nominal impedance of 8 ohms, the curve may bounce all around from 7 ohms up to 60 ohms and beyond. It depends on the frequency that you look at. This is the problem with an impedance bridge- it looks at one frequency.

Let’s look at the example of the HP129a. It is a speaker with a nominal impedance of 8 ohms. Notice in the plot below, though, that the actual impedance bounces all over the place. It is about 7 ohms at 200Hz, 11 ohms at 1 kHz, and so on. So why is this speaker called an 8 ohm speaker? We call it 8 ohms because that is about what the lowest point of the impedance curve is. Turn the tap knob from transformer bypass to the 150w position, and now the impedance curve shifts up. Where to? Well, we have an equation to figure it out.

Z =  V²                         Z = impedance

                                p              v= system voltage (i.e. 70.7v, 100v, etc…)

                                                p=transformer power – tap setting

So we have an equation that will tell us what the transformer does to the nominal impedance. Let’s again look at the HP129a. Let’s say it is being used on a 100v system, and tapped at 150w. Using the equation above, we find that the speaker looks like it has an impedance of 67 ohms. What if a cautious installer pulls the speaker out of its box , turns the tap switch to the 100v, 150w position, and puts an impedance bridge across the input? He doesn’t see 67 ohms. He sees 100 ohms. The speaker must be bad, right? No. The impedance bridge isn’t telling the whole story.

So here's the process that impedance bridge people should use:

            1. Check our stated system impedance (4, 8, or 16 ohms)

2. Look at the actual impedance at the impedance bridge measurement frequency (1 kHz) and see what the impedance actually is

3. Figure out what the impedance at their tap should be

Z =  V²                         Z = impedance

                                p              v= system voltage (70.7v, 100v, etc…)

p= transformer power – tap setting

4. Multiply the predicted impedance from step 3 by the impedance from step 2

5. Divide the result of step 4 by the impedance from step 1

The result you get in step 5 is the impedance that the speaker should show on the impedance bridge.

For example, the HP129a:

1. Stated system impedance is 8 ohms

2. Actual impedance at 1kHz is about 12 ohms

3. On a 100v system, the 150 watt tap tells us to look for an impedance of 67 ohms

4. 67 x 12 = 804

5. 804 / 8 = 100.5 ohms

This matches the measured impedance of 100 ohms for the HP129a on the impedance bridge from earlier. This little math exercise will work for every speaker. This is why we don’t like impedance bridges, though. The results are meaningless unless you do some further processing of the data. Without reference to the impedance curve for that particular speaker, the information gathered with an impedance bridge is irrelevant, and will only make it look like the speaker is no good.


- Jon Hart, King of Engineering, R&D SoundTube Entertainment