Reference Speaker when using acoustic timing reference to measure delay

[jtalden]

Quick Impression:
The overlay SPL Chart of the 2 measurements show much stronger SPL support with delay in the 1.2-3.5kHz range. This suggests the XO is about 2 kHz and the timing is very close to ideal. The overlay of the GD (group delay) chart supports this, showing a smooth transition from the W to the horn in that range. Without the delay the there is a fall then rise and fall of the GD in this range suggesting the W sound arrival leads the horn arrival at that frequency. The overlay of the Impulse and Step Response charts also support this conclusion.

From these charts it is pretty clear the timing is very close to ideal with delay.

Side note:
The Impulse and step charts suggest both drivers are operating with negative polarity. This may be the way the drivers are wired, or possibly a reversal within the measuring system setup. It is of no real sound consequence from a practical perspective so long as there is consistency in both channels.

 

[MAD]

Thank you for your input / suggestions and the link! And yes, it seems that both Hi and Mid are reverse polarity. I recognized it when looking at excess phase being constant 180 degrees for both Hi and Mid. I will doublecheck - maybe it’s the controller.

What do you mean with „the XO is about 2 kHz“, my measurement is w/o any XO, just the 2x12“ with delay and Hi in parallel?

 

[jtalden]

In my quick scan of the thread I did not see the XO frequency stipulated so I tried to deduce what it was based on the data. I correctly assumed I would be corrected if I guessed wrong.

In a general sense the acoustic XO range is the frequency range where both drivers are contributing strongly to the total SPL. So in this case of no electrical XO we still have an acoustical XO range. Knowing now there is no electrical XO, I would now say the acoustic range is about 400-3500 Hz as there is still some SPL impact 400-1200 Hz that I originally discounted as too wide for even a shallow electrical XO filter. The delay applied was very effective in aligning the phase through the upper part of the range, but it didn't work as well in the 400-1200 portion. It difficult to align the phase over very wide acoustic XO ranges.​

When an electrical XO is applied the ideal delay timing is likely to change. The extent of the change depends on the filter parameters chosen.

If you were looking for the raw driver characteristic (SPL and Impedance) for input to a passive crossover designer then I am not sure as to what those various programs are looking for their "acoustic center" input.

If you intend to use a DSP XO then the delay should be revaluated with the chosen XO applied. One way to do that is to start with initial rise of the 2 impulses aligned (as you did here) and then adjust the delay in maybe 20° increments of the XO frequency to assure that the SPL support is maximized over the XO range. There are various other method that can be used as well.

Given an mdat with:

• Acoustic timing engaged
• XO engaged
• Mic at ~1.5-2 m on listening position axis.
• Sweep setting 150-20k Hz
• Measure (one speaker):

1. 2-Mids alone
2. Horn alone
3. 2-Mids+Horn (optionally)

With that mdat of 2 (or 3) measurements I can confirm the setting you found, or recommend the change needed for the ideal delay setting. We look for the closest phase tracking of the 2-mids vs the horn driver through the XO range. That assures strong SPL support and a stable frontal lobe, where direct sound arrives at the LP from high and low drivers at the same time at the frequencies in the XO range.

 

[MAD]

Thanks for your offer, I will definitely come back on this soon :-)
I've learned two days ago, that my external little DAC used for measurement (I assumed it' s better quality than the built-in soundcard) has an internal, non-switchable 3dB bass boost and is not really linear above 10kHz :-(
So I've ordered a Steinberg UR22 - it just arrived. I will redo my measurements with the new USB interface, maybe the negative polarity is also resolved then. I've checked that it's not the speaker polarity, the DSP or the power amp, so I think it's the DAC...

 

[MAD]

It was not the DAC, with the UR22 I also have negative polarity, so I switched it in REW. I've done (still with the old DAC, but the measurement is only reasonable above 300Hz, so no issue with the bass boost I hope) some measurements with/without EQ and aligned (roughly) the phase in the XO area (1.2kHz) with an allpass and delay, XO is LR24 - see attached measurements. This is good enough for the party Mid/Hi Box I quickly assembled for carneval, but for the "real" Mid/Hi Box I need to invest more time ;-)

 

[jtalden]

Good Job! The phase tracking looks very good.