Measuring horns (nearfield)

[tiroth]

I can't seem to find anything authoritative on acoustic measurement of horns, so I apologize if this has been covered already: is a nearfield measurement accurate for a front loaded horn? I would imagine that the microphone is placed in the plane of the horn's mouth?

 

[BillFitzmaurice]

Is it accurate how? It is if you're trying to measure the nearfield response. The usual method is outdoors from 2 or 4 meters away, extrapolating to arrive at 1w/1m.

 

[tiroth]

From Claudio Negro (thanks!)

The near field measure is used to overcome the effects of low frequency standing waves caused by diffractions and room reflections; at this low frequencies the driver diaphragm acts like a rigid piston and the near field measure is directly proportional to the far field one and is not affected by the environment in which the driver is. This technique is not full range, it's upper frequency limit is determined by the effective cone diameter : Fmax = 10950/D where D is in centimeter or Fmax = 4311/D with D in inches. To know D just look at the driver specification sheet, or calculate it from the Sd using the formula D=2*(Sd/3.14)½ or measure the diameter with a ruler not considering the foam suspension. So in the Focal case, D=15 so Fmax = 730 Hz that means that we can use the near field response till this frequency.

Since this special case measurement usually assumes that the measurement mike is virtually touching the driver, I'm not certain it is accurate to measure a horn. That is what I am asking...

 

[BillFitzmaurice]

If you measure outdoors at a distance those problems are overcome. If you can't then you measure at, or as close as is possible to, the systems radiating plane, which in the case of a horn is the horn mouth.

 

[tiroth]

I appreciate your suggestion Bill, but I'd like to know if this is actually part of the special case of the nearfield measurement. It sounds to me that you don't think so.

 

[BillFitzmaurice]

It is, but it's not the preferred method for measuring horns. The reason is that with the mic in the mouth you're going to be measuring phase induced anomalies caused by reflections off the horn sides, which are considerable because of the size of the mouth relative to the frequencies passed; this doesn't present much of a problem when measuring direct radiators. Horn responses need some distance from the source for the response to settle out, preferably at least 4 times the distance across the mouth. Measuring at the horn mouth will show a lot of peaks and dips that don't show up when measuring from a distance.

 

[tiroth]

Thanks very much! Exactly what I was looking for.