| 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. |
Nearfield measurement
