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Mic Compression

We previously looked at the phenomenon of speaker compression, which occurs when increases in gain in the the speaker output fail to track the increased gain at the speaker input. This occurs at the edge of the performance envelope for the speaker/amp combination. The approach shown in the link is a very quick way of determining how hard you can push a powered cabinet OR if the performance of the cabinet might have changed due to damage.

The same approach can be used to determine the acoustic overload point (AOP) with microphones. That is, where does the mic stop increasing its output in spite of rising input sound pressure levels.

As a first test, a Dayton Audio EMM6 mic was placed within a 1cm of the front grill of an Alto TS112A. The TS112A has a  12" low frequency speaker and a 1" high frequency driver. The manufacturer claims 125 dBSPL peak at 1m, and 122 dBSPL continuous.

While the rated figure on the speaker is measured at 1m, we're going to measure as close as we can to the 12" driver in order to increase the level seen the mic. Typically, you'll see values measured at 1m increase by 20 to 40 dB as you approach the surface of the speaker's dust cap. The down side of measuring this close is that for higher frequencies we're not capturing the 1" driver's output. And the output from the 12" speaker will be potentially looking peaky in its response because higher frequencies tend to emerge from multiple points on the surface of the cone rather than the single region we're capturing with the mic. But for now, our aim is to simply overwhelm the mic with sound and see where it compresses. 

For the first plot, the EMM6 mic's reported 1 kHz calibration data was used (-41.4 dBV sensitivity). The following settings on the SPKR Chirp Compression plugin were used:

In the above settings, the QA401 will perform a chirp sweep at amplitudes ranging from -20 to -8 dBV, stepping 2 dB per iteration. A 5 mS windowing function will be applied, and 1/12th octave--fairly heavy--smoothing will be applied to the recovered frequency response. Note that 0 dB of gain as been specified--this is achieved using a modified QA470--at these high levels the output from the mic-preamp is huge (see the section below). The mic to speaker distance was specified at 1m. This is the nominal setting and it means no additional correction will be applied to the data. For distances greater than 1m, the plug-in can apply a far-field adjustment of 6 dB per doubling of distance if desired.

Note that there's not much science behind the selected output range. A lot depends on the volume setting of the speaker itself. You'll need to carefully pick a range that gives the response you are looking for.

The measurement is shown below. Remember the mic cal point was specified for 1K, which is the right side of this graph. So, we can make a statement that either the mic or speaker hit its AOP at around 128 dB @ 1 kHz.

Let's repeat the same test again, this time with an M23R. This is an incredibly flat (± 0.5 dB) reference mic that doesn't need a cal file. Above we didn't load the cal file on that mic, we just used the value the manufacturer provided for the 1 kHz sensitivity. That means we can trust the 1 kHz figure, but we should be a bit suspicious for values that aren't 1 kHz (unless the calibration file had been loaded).

The M23R plot is below:

For the above, there are 8 sweeps that were performed (one more than on the EMM6 mic). What is clear from the plot above if that the speaker can be much louder than 130 dB at 700 Hz. So, what we saw with the EMM6 mic indicates the mic was compressing and not the speaker. At this point, though we can see about 138 dB was achieved at 700 Hz. The rolloff you see starting around 1 kHz is because the crossover is starting to direct the higher frequencies to the high-frequency driver.

Now let's move the mic back 1m. If the speaker was compressing, then we'll see a similar patter to above. If the mic was compressing, then we'll see the compression artifacts go away.

The plot of the M23 @ ~1m is shown below:

Notice above that we still see the top two lines bunched up close together. Take note that the X scale has been expanded to the full 200 to 20 kHz. Because we're in the far field (or at least getting close to it) we can see the handover from the low-frequency speaker to the higher frequency driver--that's probably happening around 1.5 to 2 kHz.

Since we still see the top two traces bunched together even though the mic has been moved a ways back--this tells us the compression we're seeing is from the powered speaker and it's safe to say at this point that we've not hit the compression point of the M23R mic yet. For that, we'd need the ability to get closer to the speaker (beyond the metal grill) OR we'd need a louder speaker.

QA470 Modifications

The QA470 has ±12V rails, and can deliver nearly 18 dBV = 8Vrms of signal output. A mic such as the M23R has a sensitivity around -29 dBV @ 94 dBSPL. This means that at 140 dBSPL (46 dBV above 94 dBSPL, or 17 dBV mic output) the QA470 output will be approaching its limit if the gain is modified to 0 dB (versus its normal 20 dB).

In its normal configuration, the QA470 will begin to clip the output as the M23R approaches around 120 dBSPL.

Modification is quick and easy if you are able to work with 0603 parts on your bench. You just need to remove a single part. Contact support for help if this is something you'd like to do.

Summary

With the QA401, it's easy to add a calibrated mic and make quick dBSPL measurements. You can use an inherently accurate tuned mic such as the M23R, or you can use a less accurate mic such as the EMM6 and rely on the calibrate file to flatten the mics response.

An important point to understand about your measurement mic, however, is where the mic overloads. As the tests above showed, it's a pretty easy task to determine where the mic is limiting or the speaker is limiting. And once you understand these limits, you have a powerful tool for future analysis.