Improving Low Frequency Accuracy in Reverberant Spaces.

Without having a reliable SPL and phase response in the 20Hz-400Hz range no serious design can even be attempted for the crossover network filters. In theory all we have to do is to realize our measurements in a ..concert hall. Its large dimensions will 'move' all reflections away to time instants of 60msec or more. Anechoic segment will increase to 60msec or more providing a critical frequency of less than 32Hz !

Moving away reflections in time is also accompanied by increased attenuation of their amplitude. We must not forget that soundwave amplitude decreases rapidly with distance travelled away from the sound source. In open space (what we call spherical space or 4-pi space) amplitude is proportional to 1/r2 (r is the distance travelled) while in a typical room this law is modified to 1/r approximately.

But we can't have a concert hall available every time we want to measure a speaker component or a complete system.

-What about moving the microphone closer to the DUT ?

Let us study such a case. The following figure describes a typical room with a height of 2.8m. We place the loudspeaker to be measured (mounted on its enclosure of course) in half this height at 1.4m. in this way floor and ceiling reflection will arrive at the microphone position at the same time which is the best scenario for our anechoic segment. We place our microphone at a distance of 1m for conventional reasons (let us not argue that choice for the moment).

As  stated above our anechoic segment is about 6msec and our lowest reliable frequency is about 330Hz. Now let us move the microphone closer to our DUT driver. Let us place it just 20cm away from the driver's dustcap.

What we get is only an 80Hz improvement of the lowest reliable frequency as explained in the figure above. In addition primary sound gets louder. What we mean is that this setup increases the SPL of the direct sound wave while the floor-ceiling reflection maintains its previous SPL. In the IR plot the reflection will seem weaker than the direct response at 0.6msec. Weaker reflections corrupt the FFT results much less. However the problem remains.

-So what can we do to improve drastically low frequency measurement accuracy ?

 The idea is not to broaden our anechoic segment in such a way as to drive our measurement down to 10Hz. The idea that has survived such arguments and technical discussions is to get another measurement, a dedicated one, that records the huge intensity of the direct sound wave emitted in the vicinity of the dustcap. These near-field measurements include reflections but at a level which is unimportant when compared to the SPL of the driver's sound wave. What is interesting is that these measurements have restrictions at high frequencies. So what we actually do is to use the curve of a near field measurement by splicing it to the curve acquired by a conventional measurement at 1m. Excellent descriptions of such a procedure are given throughout the internet by audio enthousiasts, amateurs or engineers. More on this in our design examples. (picture drawn from MH-audio)

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