Quick start guide to measuring for crossover design.
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Holm Impulse is a free and very easy to use measurement program.
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Step 1: Setup speakers, mic & treatment
If measuring in a room, place the speaker in an elevated position in the middle of the room. Choose the biggest room with the highest ceiling that you can manage. Elevate as high as you can without going above the vertical mid point of the room.
Add acoustic treatment to absorb the floor reflection. If the next nearest reflection point is the ceiling, then no further treatment is needed. Gating will stop measuring before the reflection arrives from the ceiling.
Elevate the speakers so the mid point is around 1.8m. Position as far away from significant reflections to the sides. Outdoors is preferable since a bigger reflection free zone is possible.
Place the mix in line with the mid point of the speaker, in between tweeter and mid. Set at 1m horizontal distance.
Step 2: Connections
1. Run a cable from your PC sound card to your preamp input on your sound system. You will probably need an adaptor. Most PCs have a mini jack - you can buy cable that has mini jack > RCA or use an adaptor.
2. Connect mic to your mic preamp.
3. Connect a cable from preamp output to your sound card.
When taking measurements, you will need to adjust mic preamp gain as well as the volume on your sound system and the PC.
Step 3: Holm Impulse setup
The progam has 3 tabs:
1. Device and signal
Check that the inputs and outputs are those which you are using. Input = mic. Output = signal to speaker.
Calibration - import your mic calibration file
2. Data analysis
Default settings should be fine for now.
You are now ready to measure.
Step 4: Take your first measurement
1. Set the gain on your mic preamp, PC and sound system.
2. Click measure
You will hear a measurement signal, and a chart will display.
3. Click options (next to measure)
Raw response measures the room as well - we don't want this for working on a crossover.
Impulse time-window (gating) limits the measurement to the direct sound without any room effect. Choose this option.
Smoothing - enter 12 - this gives 1/12 octave smoothing. Easier to read but it doesn't make the result appear artificially smooth.
4. Set the gate time
Below the impulse response window, click "auto zoom."
Drag the gating with the mouse (red dot) to exclude the first reflection. Normally the first reflection point would be the floor, but acoustic treatment has reduced this reflection enough to use a longer gate. The first reflection in this case is from the ceiling. It pays to do a reality check.
Gate distance = first reflection distance - mic distance
Notice how the response changes as you shift the gate. If you are unsure if your floor treatment is good enough, you can check by shifting the gate window. Note how the response is affected. The ideal is of course to gate out all reflections, but sometimes this can be limiting. You can get a useful measurement with the help of some acoustic treament. Blankets, doonas, foam mattresses and cushions should be adequate.
Step 5: overlay individual driver responses
On the left you can name and overlay 3 different measurements. For crossover work, you should measure each driver individually with and without crossover filters and then summed together.
Turn off phase for clarity.
The method shown here gives valid results down to about 200 Hz. A different setup is required to measure bass.
You can take many measurements, but only 3 will display at a time. Use the arrows next to each of the three to choose which should be displayed.
Step 6: Check tweeter polarity
If you see a dip around the tweeter crossover point, you probably need to invert the polarity. Turn on phase where you have tweeter and mid displayed, each including their crossovers. Note the phase at the crossover.
The acoustic crossover is 3k where both sides are -6db. However the phase shows where the actual filters have been placed, closer to 2.4k. The acoustic crossover sees a phase shift of 150 degrees, but where the filters are placed, phase shift is 180 degrees. This can be corrected by inverting the phase of the tweeter.
This is why individual and summed plots should be shown - you will see a dip at the crossover if the tweeter isn't inverted in polarity.
Active time alignment
Phase shift at the crossover will not always be 180 degrees which can be fixed simply. The easiest way to fix it is by adjusting delay. This means having an AV receiver (to phase align subs) or a digital active crossover.
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