AudioXpress October 2023 Loudspeaker SPL Response

There’s a very interesting and potentially important article in the October 2023 issue of AudioXpress. The author is Reinhold Lutz. As most know, speakers are often measured today using a near field measurement for the low-end, and a far-field measurement for the high end. In the near-field measurement, the mic is placed very close to the dust cap (10mm or less). In the far-field measurement, the mic is placed roughly 3 times the distance of the largest speaker dimension.

In the near field, room reflections are rendered insignificant because they are so small relative to the signal picked up by the mic. In the far field, the room reflections are significant. The mic is far from the signal source, and the reflections are substantial enough they can impact the measured signal. So, you combat this by a using a large enough room AND time gating. Note that the time gating requirement is what prevents you from using the far-field technique at low frequencies. That is, if you want to measure a 50 Hz tone, you need a gate that is at least 1/50=20 mS to capture the tone. But that suggests a massive room.

And then the challenge becomes “stitching” the two measurements together.

The Lutz article proposes a different approach. And the summary I’ve taken away from the article is thus: You use two matched microphones, spaced 5 cm apart. The signal from the mics are subtracted, which eliminates the room effects–because the mics are so close, they will see the same room reflections. And so subtracting the matched mic signals will null the room reflections. The “near” mic is placed 5 cm from the speaker. The “far” mic is thus 10 cm from the speaker. Both mics will capture the direct sound from the speaker. But, due to the inverse square law, the far mic amplitude will be about 1/4 that if the near mic. And so, this is a 6 dB difference.

A prototype has been built as shown below:

The prototype has two well matched MEMS mics (ICS40619). A small processor is used to turn the mics on an off. Press the button and the mics run for 20 minutes. For every button press, you add 20 minutes of run time. Run time from the CR2032 battery should be hundreds of hours. The battery is 235 mAH, and the CPU, LDO and two mics are under 2uA. When not running, the off current can get down to 200nA and so it can live in a drawer for a long time.

First Look

Placed above an 8" speaker, the near and far mic response can be seen in a normal spectrum plot. The yellow is near, the red is far. The difference between the signals is 7.1 dB. But remember, this will include room reflections.

Room response is best viewed in the time domain, with near subtracted from far. In the plot below, the mics were moved throughout the room to look at how the various reflected signals were picked up. The signal seen by each mic was substantially the same. This is a very important point–because it means when the mics are placed at the speaker, the reflections there will be substantially the same too.

This is the key point: Speaker amplitudes picked up by the mics will be very different due to inverse square law (roughly 6 dB). But the room reflections picked up by the mics will be nearly the same.

Putting the pieces together

In the Lutz article, the math were performed in the analog realm, and a dial was adjusted to null. In the digital realm, we can maybe refine things a bit further.

Let M1 be the near mic, and M2 be the far mic. The far mic (M2) amplitude will be 0.25 that of M1. The reflections (R) will be M2-0.25*M1.

The true speaker response (TSP) is:

TSP = M1 - R = M1 - (M2- 0.25*M1).

The above hasn’t been tried, it’s just speculation at this point.

Comb Filtering Effects

The mics are well matched for amplitude and phase. However, as the frequencies of interest increase, eventually the 5 cm mic spacing will be 180 degrees out of phase and instead of the signals cancelling, they will add. This problem will be present for both reflected and direct signals. The first null frequency will occur around 1.72 kHz for the 5 cm spacing. So, as the frequency increases above 300-500 Hz, the comb filtering begins to become more pronounced and must be compensated for. But luckily, as the frequency gets higher still, time gating with a far field measurement can be used to eliminate the room reflections.


The technique described by Lutz seems important and significant. And with today’s well-matched MEMS mics, potentially very low cost as well. To date, far field measurements can be made in reasonably sized rooms. But low-frequency measurements below 100 Hz or so have been difficult due to the long gating times required to capture the frequencies involved. Lutz’ technique may have changed that.


That’s a very interesting technique. Alternatively using two closely spaced microphone has been used for sound intensity measurements and as a form of probe microphone to only pick up close source sounds.
Using Mems mikes is really nice since they are very uniform from unit to unit. Are those top port or bottom port?
Sound intensity seems like a closely related subject but not easy to understand. It suggests that you could measure the sound from a driver in a room ignoring even adjacent sound source. This is the best description I have found: Sound Intensity Measurements Captures only Source Noise or Sound Mems mikes may make it more accessable. A matched pair of B&K’s for a sound intensity probe is many thousands $…

1 Like

I would definitely bite on testing this. The Speakertester Pro used 2 mics for a long time. And this was part of the logic behind it. That was an interesting article. I like your implementation.

Now to get this into the hands of people that want to use your equipment!


You are unquestionable NOT in the far field of a large cone at 5 cm. If you’re not far field, you’re not at -6 dB /Double distance. What this creates is what Crown sold for many year under the trademark “Differoid mic” It’s essentially a figure-8 bidirectional mic ( full cancellation for all frequencies from any direction of equal distance) It has a forward lobe and a reverse lobe, and you’re using the proximity effect, which all directional mics have, that is the result of unequal levels at the + and - capsules for nearby sources. Why not just use a good old KSM44 or any bidirectional studio mic. It will do the equivalent job of isolating the room and be equally impossible to calibrate over an extended frequency range.


1 Like

Hi @daleshirk, you are correct on this. But I think the technique the author outlined doesn’t require a precise fall off. On their box, they use a dial to null. In the digital domain it could be smarter. In the math I showed above, the assumption was 0.25 for M2 amplitude (far mic) versus M1 (near mic). That 0.25 figure could be anything and probably varies a lot based on the speaker. But I think the key the M1 and M2 mics are seeing effectively the same regarding room reflections. And that’s why I built the board. I wanted to move it around the room and see for myself. I have two Earthworks M23R mics that I could have used, but doing with MEMS brings a ton of benefits. And dual matched mics, powered from a coin cell, is nice.

Yes, @1audio, in the same issue Jan Didden put the box to the test and noted the same: Testing could happen with higher ambient levels of noise and it allowed them to drop the testing levels to lower SPLs.

@Kravchenko_Audio, if another build of boards are made, I will make sure to have some to share.

1 Like

I thought post #9 from today was an interesting way ti investigate this technique.

Yes it is. The don’t include the time delay, which means the comb filtering isn’t happening but for low frequencies it’s a good place to start. I had started with some sims too, but about 15 minutes into it I figured I could have a board released with two mems mics faster. That’s why version 1.193 of the SW release had the L-R added to the oscilloscope–so I could subtract the mic signals from each other quickly.

That was the important learning from me from the article and building the mic board: the room effects are pretty consistent at low frequencies.

If you need mic comparisons I have two good sets of mics. Neutrik and AWA.


Noted, @Kravchenko_Audio! I have two matched M23R Earthworks mics (+/- 0.5 dB). But it’s tough to beat the phase matching of MEMS. And while the M23R’s are $600 each, the ICS40619 MEMS mics nominally have comparable sensitivity matching for just $2 each.

So I have limited knowledge about MEMS mics. So this is mostly questioning.

This is going to be nearfield testing?

And the mic is not exactly a high SPL mic, nor a low distortion mic.

A quick look at Knowles and Infineon has found considerably better microphones. Better distortion at high SPL and better more applicable frequency response curves.

Not sure about the ins and the outs. But in mic terms these are a damn site better that the TDK that you mentioned.


Hi @Kravchenko_Audio, yes, these are all somewhat better. But they are all also bottom ported. I wanted a top-ported mems mic because the soldering will have zero impact on the performance whereas bottom ported requires a very consistent application of solder paste for repeatability. Since these are prototypes, I’d prefer to remove the solder paste coverage from the equation.

I kind of deduced this. And I looked hard for such animals in MEMS mics. I can’t seem to find any!