This post is about measuring the very small difference in distance between a listening location and the speaker drivers.
Why?
All speaker driver's have what's called an acoustic center, a place from which the sound emanates. This place is located towards the listener or away from the listener based on the speaker type, size and general geometry. Consider a 2-way speaker with a dome tweeter and 10" woofer.
The acoustic center of the tweeter may be in front of the baffle, while the woofer is probably several inches behind the baffle. This means that for frequencies where they overlap the woofer's signal will lag the tweeter significantly, causing a phase misalignment, which then will cause additive (constructive) and subtractive (destructive) interference. We'll show more on this below.
A crossover designer needs to know this physical difference in order to accurately match the phase of the high and low pass filters. Getting this right often requires inverting a driver's polarity or adding or removing a pole in one of the filters. This is a major reason why we turn to crossover simulators like like VituixCAD or XSim but for them to work correctly we must know the relative distances between two drivers.
Here we present a simple method of determining the relative distance. For the example below we'll take actual data from a speaker I built, but we'll remove the crossover entirely. This design uses an AMT tweeter with 7" mid-woofer:
Safety Tip:
It's always good to keep in mind the delicacy of most tweeters, so you want to either use signals which have no bass or put a large (47uF is a good value) capacitor in series with the tweeter before testing. If you do, include it in your simulation before proceeding. Obviously, this testing does not require the measurements to be very loud, just loud enough so that at 1m you have a clean signal that's above the noise floor. Usually you can do this well below 2.83V (1W at 8 Ohms).
Step by Step
The first part of the process is simple: Measure the individual drivers. Save the results as FRD files. In the chart below these are the blue and yellow lines (offset for clarity).
Second, measure the combined output of both drivers. Save that and load it as a reference line. See the green line, below. It looks rather ragged right? This is what happens with two drivers and poor phase matching. The simulator's idea of the output (in black) and the reality (in green) are not matching up.
At this point you should have 3 FRD files. One for each driver and a third for the combined output. For each driver, load the appropriate individual FRD, as done in the schematic, above though you can't tell from the picture. The combined input you'll load to the FR plot as a reference line.
I'll now show you what this all looks like.
In the chart below I show the XSim simulation with no woofer delay in
black and the actual in green. The green one is what I
measured driving both drivers at the same time without a XO. We start the simulation starts assuming there is no delay but the
reality is that there is an unknown amount of delay and therefore the
simulation and reality won't yet match.
The last part is to incrementally add delay to the woofer in the simulation, and as you do so the black line will become the green line:
Doing this is like setting the focus on an old-school camera. You turn the focus dial until the image just passes perfect and then you turn it back a smidge and you have achieved ideal focus. In this case you increase the simulated woofer distance until the simulation just passes the ideal, then turn it back and you now know the effective distance from the woofer to the tweeter.
I hope this helps demonstrate how these measurements add up. Worth pointing out that the sonic signature in the overlap area (~2-7kHz) isn't really part of either driver but it's own unique result. Also worth noting that the ugliness of the green line is completely eliminated once the actual crossover is put into place. You don't have to worry about this being something you need to fix, demonstrated by the final result with the crossovers in place is below:
As you can see, there's nothing of the green line or black line left once we are done with the crossover design.
