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Thursday, August 17, 2017
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  Nearfield measurement

Calculate Upper Frequency Limit Convert Speaker Measurement Convert Port Measurement




Nearfield measurement on the BassMidrange driver of my Chario T200


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    ( D in centimeter )

or

Fmax = 4311 / D    ( 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.

Example: Visaton AL 200

So in the Visaton case, D = 16.6cm so Fmax = 660 Hz that means that we can use the near-field response till this frequency.

However this is valid for a driver mounted on infinite baffle, while lowers for a driver mounted on a panel, cause of the baffle effects, as reported by C. J. Struck & S. F. Temme.

Just use the baffle dimension instead of the driver diameter, in the seen formulas.
In our case, the baffle dimension is 22 cm therefore Fmax = 10950 / 22 = 498 Hz.

The microphone has to be placed very close to the driver center, but not attached to it.
The maximum distance between the two is given by the multiplication 0.055 * D, as written by D. B. Keele,
so the Visaton center cone will be as close to the mic as possible, <= 0.9 cm.



   Calculate upper frequency limit











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   Convert Port measurement






Measurement Approach Implementation Advantages Disadvantages Limits
Anechoic Chamber Acoustical measurements done within an indoor, (ideally) reflection-free environment Climate-controlled, artificial environment in which to measure amplitude response, noise & distortion, diffraction effect & directional response characteristics Cost; extremely large chamber needed for accurate LF amplitude response, noise & distortion, etc measurement Chamber , device under test (DUT) size; depth, type & configuration of absorptive material used within the chamber
Ground Plane Measurement Measurement done with the DUT & microphone typically placed on the ground, with the emissive radiating surface(s) pointed at the microphone Low cost; ease of implementation, within known limits, can provide accurate measurement data Upper & lower frequency limits. Other than the ground upon which it rests, DUT must be placed well away from any reflective surfaces or objects large enough to influence measured amplitude response Noise pollution, Inclement weather (when done outdoors)
Half-Space or Hemispherical Free Field Measurement Device affixed flush-mount with surface such as baffle, ground surface or clear wall of Hemi-anechoic chamber Depending on implementation and type of data sought, can provide excellent results Cost of indoor Hemi-anechoic chamber; use of baffle invites cancellation?. Out door, in-ground placement requires DUT to be placed well away from any reflective surfaces or objects large enough to influence measurements Approach requires all emissive radiators be on one side of the cabinet
Windowing (gated) Measurement taken and unwanted data windowed out Fast data acquisition & post-processing Requires significant data post-processing and the ability to skillfully interpret the results LF measurement accuracy defined by environmentally determined window length. Poor tolerance for time variance.
Near Field Measurement Measurement done with microphone placed near to, centered on and normal to front emissive surface of each acoustic radiator When implemented correctly, the near-field amplitude response provides for an accurate facsimile of the far field response Multiple emissive surfaces require multiple measurements along with subsequent post processing Upper frequency limit determined by size of DUT.


Adapted from: Audioholics.com ( by Gene DellaSala )



Click here to read about Farfield or Ground-plane measurements.