mh-audio.nl - Farfield Measurement

Resource to calculate, building and measuring Hi Fi Loudspeakers and more...


zondag 5 februari 2012 47 users online

Calculate Low CutOff Frequency

Calculate Phase Response

There are two kinds of far-field measurement: gated and not-gated one.

In the gated measurement we set the markers so that we limit the data to be used to calculate the frequency response.
Eliminating reflections from the measurement in this way allows anechoic testing to be conducted in a normal room.

The relationship is a simple one: if you apply a time window of 6 milliseconds (0.006s �not unusual for measurements conducted in a typical domestic room), then the frequency resolution of the measurement will be limited to 1/0.006 = 167 Hz.

The not-gated far-field measurement, instead, uses all the data including the room response, and is useful to see how the speakers will sound in the environment they will be used, helping us understand where to place them.

In the far field measurement the microphone is placed at a distance at which the driver looks like a point source and its sound pressure level is inversely proportional with its distance from the mic (6 dB drop for each doubling of distance).

The industry standard sets this distance to 1 meter to allow integration of the various drivers in a front baffle, that is by 1 meter usually the woofer and tweeter response have added together so a single test gets both (however when testing speakers that uses line sources, like ribbon or array drivers, the mic-speaker distance is increased to 2 meters).

If you are testing drivers separately then you can use any distance that works but be aware that if it's true that a less than 1 meter distance increases the signal to noise ratio, it's also true that a too close distance causes the driver not to act as a point source, thus the measurement cannot be considered far-field.

Is common to use a distance equal to 4-5 times the driver diaphragm diameter, but not exceeding more than 1 meter: it rarely works for anechoic (gated) measurements cause the reflections are very close to the signal and you don't get any low frequency response (unless you have a huge measurement room, where floor-ceiling-sidewall reflections happens very late).

Using 1 meter or more for far-field (not gated) in-room response is just fine.

In the gated measurement it is very important to place the loudspeaker in the room center, so that all the parallel walls are equally far from it.
I use to put the speaker on a stand to center it between the floor and the ceiling. Also put some absorbing mat on the floor between the mic and the speaker, as well as on the microphone stand.

fig. 1

Low frequency cutoff = 343 /( (2 * (x2 + h2)0.5)

Where 343 m.s-1 is the speed of sound. For a room with a ceiling height of 2.4 meter and a measuring distance of 1 meter this gives:

Low frequency cutoff = 343 /( (2 * ((0.50 + 1.20)^0.5) = 132 Hz

So, for this size room you know that any data shown below 132 Hz is not reliable!

Bear in mind that you should put an extra margin on this because there are very few data points collected in the lower octaves of the measurement.

fig. 2
From fig. 2 becomes clear that sound energy starts arrive after roughly speaking 2.7 ms at the mictofoon.

This sound front is followed by response in the form of a subdued ultrasound which is reduced after 6 ms to a very low level.
After 6.3 ms a second, smaller quantity of sound energy arrive. This is the floor reflection.
A third front on approximately 10.5 ms is the reflection against the ceiling.
Finally we see a fourth (negative) front at 12 ms, this is the reflection against a sidewall.

First refection (-dB) at 180°, Second reflection (+dB) at 360°.

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 Nearfield or Ground-plane measurements.

Farfield measurement

Calculate Low CutOff Frequency

Calculate Phase Response