Speaker Testing

[Stub Mandrel]

I've got another thread about testing microphones.

 

I bought a cheap Behringer reference microphone and it seems to work pretty well using  it with REW to get a reference trace from my cheap Thomann powered speaker (EQ set flat, 4 10 second frequency sweeps at 0.5m in a large room with plenty of soft furnishings and curtains closed).

 

I asked a query there which has gone unanswered that I wonder of the likes of @stevie or @Phil Starr can answer.

 

REW also produces phase plots, and the one for the speaker is below. It shows several phase reversals, mostly well beyond hearing range.

 

One just above 100hz probably relates to the reflex port and is to be expected. 

 

Another is just below 2kHz and is accompanied by a dip in the SPL trace.

 

My question is: Does this make it likely my tweeter is wired back to front, and could reversing the wiring help eliminate the dip?

 

 

Any other observations welcome (yes I know this is not a very flat trace at all, although it sounds fine and works well as a monitor).

[Bill Fitzmaurice]

Phase shifts like that are normal, and don't matter, as you can't hear them. The dip and shift in the crossover region could be polarity related, the only way to know is to reverse the tweeter polarity and test again. It could also be time align related, which can only be fixed by moving the tweeter to get the acoustic centers of the tweeter and woofer the same. The shift at 100 Hz is probably port related, possibly indicating port tuning at 100 Hz. The 200 Hz dip could be from floor bounce. The best way to take measurements is outdoors, well away from boundaries, taking two measurements. The first is ground plane, with the mic a few cm above the ground. The second is with the cab on its back, the mic suspended above. This measurement will have a dip where the wave reflected off the ground meets the front wave at 180 degrees, the result of the baffle being 1/4 wavelength from the ground. The two are spliced to get the final result. You can do it in one step by digging a hole in the ground, putting the speaker in facing up, back filled with the baffle flush to the ground. It's how Roger Russell of McIntosh used to take measurements. 

 

When you take measurements 1/6 octave smoothing is preferred, as that's the limit of what you can hear.  

[Stub Mandrel]

Thanks  Bill lots of useful stuff there.

 

As far as I can tell the transducer in the horn is pretty well aligned with the main driver, so I'll experiment with changing its polarity to see what happens. 

Edited 2 hours ago by Stub Mandrel

[Bill Fitzmaurice]

A bit of semi-interesting stuff is the design of the Altec A-7. The woofer horn had two purposes. One was increased sensitivity in the lower mids, but the other was to time align the acoustic centers of the drivers. The aligning procedure was to wire the HF driver reverse polarity, feed the speaker with a sine wave at the crossover frequency, then slide the HF horn on its sled atop the cab back and forth until the SPL reading was at a minimum. That placed the acoustic centers of the drivers correctly. Swap the polarity on the HF driver back and it was good to go. 

[Stub Mandrel]

Nice lateral thinking.