November 28, 2010

Measurement techniques - an introduction

There are various methods, depending on the application. Here are some quick suggestions.

For choosing the placement of a subwoofer:
Farfield measurement - place the mic in the listening position and measure with REW. You will see many peaks and dips as the room response is included.

For choosing subwoofer filter settings to integrate with the mains:
Nearfield measurement - place the mic as close as possible to the cone while avoiding contact. The room is excluded and you will see a smooth result.

Crossover design:
Gated response to avoid room reflections - Mic @ 1m with speaker and mic elevated

Types of measurement

1. Farfield Measurement

A farfield measurement is taken at a greater distance than most other measurements and shows the influence of the room. When full range measurements are made, the top end will be generally rolled off because most rooms absorb more of the top end. The bass will usually have large peaks and dips in the response. +/- 20 db swings are not uncommon.

Here is an example:

The dark blue line shows the speaker response, but light blue is a farfield measurement. Notice the roll-off  above 1k and the many peaks and dips. This is the influence of the room.

This type of measurement is mostly used for the bass range. It is useful in consdering the impact of room modes. Farfield measurements are best for choosing the location of speakers and subs. 

2. Gated free field measurement

The above chart also shows a gated measurement at 1m. Before any room reflections arrive, the software gates the data and stops recording. This gives a result similar to an anechoic chamber. The problem is that the bottom end is limited. If early reflections are present, the bandwidth will be limited. If the time window is shortened, the lower measurement limit is increased.

The speaker should be placed so that the first reflection point is as far away as possible.This means elevating the speaker and mic, but not beyond the vertical midpoint of the room. Otherwise, the ceiling will introduce an earlier reflection.

If done outdoors, a fullrange measurement can be done if the speaker is elevated high enough.

3. Nearfield measurement

Nearfield measurements are valid only for bass up to 100 Hz. The room response is swamped due to the mic being very close to the cone - as close as possible without risk of contact.


In this example, a sealed Rythmik servo sub is shown with different crossover settings. The response anomalies at around 160 Hz are as a result of a chamber that was installed for a bandpass experiment. Normally this measurement would be even smoother.

4. Ground plane

Ground plane measurements are useful for bass. A speaker is placed on the ground (typically outdoors) and the mic is usually placed on the ground at 2m away. Unlike a nearfield measurement, ground plane can operate up into the midrange and a location should be chosen that is free from reflections. Gating should still be used, but in a suitable location the useful low frequency limit will not pose a problem. Where reflections pose a problem, a ground plane measurement can be combined with a nearfield.

5. Half space

A half space measurement may be chosen to avoid reflection problems, but it can add problems of it's own. The speaker is mounted in a baffle that is flush with the floor/ground. It's important that it be flush, or baffle diffraction will contaminate the data. If it's not possible to flush mount, acoustic treatement can be used and ideally this should be flush. Otherwise the problems may in fact be more than those this method tries to avoid.

Compared to a conventional gated freefield measurement, this method avoids the first floor reflection and the need to elevate speaker and mic. The gating will only need to avoid reflections from walls.

Adding them together

No practical and accessible method can do it all. An anechoic chamber or a gated freefield measurement high up in the air are the ideal choices, but they don't suit the casual enthusiast with 2 hours on a Sunday afternoon available. The solution is to combine various measurements and use the one that fits.

Further reading

How to measure your subwoofer (without the room)
How to measure your subwoofer (and find out what the room is doing)
How to measure your crossover with Holm Impulse

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