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Using a MEMS Microphone as a Reference Microphone by Phil Prosser MEMS (micro-electromechanical system) microphones have advantages over electret mics, such as operating at ultrasonic frequencies. They also have good frequency response characteristics, so you can use them as reference microphones, as described in this article. W e received some Knowles SPU0410LR5H MEMS microphone elements from a kind reader named Richard Stone. They were sent to determine their suitability for use as calibrated microphones. That was prompted by our Calibrated Measurement Mic project (August 2023 issue; siliconchip.au/ Article/15903) that used inexpensive electret capsule microphones (ECMs). It used compensation and calibration to provide a flat frequency response, allowing those microphones to be used as measurement devices, eg, to plot the frequency response of a loudspeaker. The MEMS microphones we received are tiny (3.76 × 2.95mm) and connect to a PCB via under-chip pads. They also require a hole in the PCB that’s used as the aperture for the microphone, so they must be soldered to a PCB designed explicitly for them. Soldering them would be tricky for most of our readers. They are surprisingly inexpensive at only around $1 each (less in quantity). Happily, it turns out that you can buy these microphones already assembled to a board from TeensyBat: siliconchip.au/link/abt5 That is just one example; there are quite a few suppliers of similar ‘carrier boards’. The ones we tested came mounted on 7mm circular PCBs. The Knowles MEMS microphone needs a 1.5-3.6V DC power supply and provides an AC output. As a result, they can be connected to our Calibrated Microphone board but some minor modifications are required. These involve adding a 3.3kW series resistor and 3.3V zener across the microphone power supply to obtain a suitable voltage, as shown in the revised circuit diagram, Fig.1. To do this on the SMD version of the PCB, you have to cut the track between capacitor C6 (10μF) and resistor R4 (100kW), which is small but not too fiddly. This is shown in Fig.2, along with the added 3.3kW resistor and microphone wiring. If using an SMD resistor, it can be soldered across the pads spanning the cut location, although adding a miniature through-hole resistor, as shown, is easier. The equivalent changes for the through-hole version of the PCB are shown in Fig.3. In both cases, the rear of the 7mm round microphone PCB mentioned above is illustrated for the wiring. However, you might prefer to route the wires from the other Fig.1: the changes required to the original Calibrated Microphone preamp circuit are minimal. R8, R14 and the four compensation components are not fitted, a 3.3kW resistor replaces the track between pin 1 of CON2 and the 10μF capacitor, and a 3.3V zener across pins 1 and 3 of CON2 limits the microphone’s supply voltage to a safe level. siliconchip.com.au Australia's electronics magazine April 2024  79 Fig.2: this shows how to assemble the SMD version of the PCB and wire it up to the MEMS microphone. The through-hole 3.3kW resistor shown could be replaced with an SMD resistor across the cut section of track (soldered on top of the leads of the other components). Your microphone board might differ from the one shown here, so be careful to wire it up correctly. Fig.3: as with the SMD version, several components are left off the through-hole version of the PCB, one track is cut and a resistor and zener diode are added. Note how the striped end of the extra zener diode goes to the positive (supply) terminal of CON2. side to keep the area with the sensing hole clear. The pads labelled “G” are ground, “O” is the output and “+” is the positive supply. Note that while both of our boards have mounting locations for frequency compensation parts (two resistors and two capacitors), we leave them off for this microphone as it does not require compensation. The MEMS microphone connected this way works a treat. The resulting ‘calibration curve’ is shown in Fig.4. The cyan curve is the frequency response of this microphone, while the Dayton EMM-6 reference mic we used for the original project is in red. The calibration data we have for the Dayton unit only runs from 20-20000Hz, so I cut the measurements off there. Note that the speaker used for this test was rolling off in its response at low frequencies, so the measurements are noisy down low. The measured response is entirely consistent with published data. The MEMS microphone’s output level is much higher than the Dayton microphone, and per the data sheet, the SPL (sound pressure level) limit is not that high, so you will be limited in making near-field measurements or dealing with high SPLs. In terms of calibration, if you only want to measure up to 10kHz, you can probably ignore the calibration file or Fig.4: the raw frequency response of the Knowles MEMS microphone (blue) compared to the reference Dayton EMM-6 (red). The Knowles response is very close to what’s stated in their data sheet. The thinner, dashed red curve is the Dayton curve shifted up to make it easier to compare to the Knowles curve. 80 Silicon Chip Australia's electronics magazine siliconchip.com.au Scope 1 (left): the MEMS microphone picks up 22kHz sound waves just fine. According to the data sheet, it will work up to at least 80kHz. The sensitivity drops off above about 25kHz, but it will definitely still pick up signals above that. Photo 1: this MEMS microphone has a footprint under 4 × 3mm and picks up sound via the small ‘acoustic port’ hole in the base. You can see how the pad arrangement makes it tricky to solder; the only practical method is reflow (IR or hot air). make one by taking data from the published curves. In my opinion, the critical frequency response areas are in your crossover zones, typically in the 100-5000Hz region, making these microphones an interesting option if you are OK fiddling with tiny ICs. Richard was interested in using them to measure the output of ultrasonic parking sensors. The only ultrasonic source I knew I had was an old-school remote from the 1960s, in which the ‘buttons’ make springloaded hammers tap brass rods. The resulting ultrasonic signals were picked up by the TV set. It was an unusual arrangement! I used this circuit to measure the output of that remote control, with the result shown in Scope 1. The two buttons generate high frequencies at relatively high levels; the one shown in Scope 1 is at 22kHz. That is above the range of human hearing, although it might freak out your dog or cat! The bursts are short, so if you could hear them, it would be as a click. So, as far as I can see, these are a real option for ultrasonic measurements. They are also pretty good for use as a basic calibrated microphone over the SC audible frequency range. Parts List – MEMS Reference Microphone SMD version Through-hole version 1 double-sided PCB coded 01108231, 64 × 13mm 1 Knowles SPU0410LR5H MEMS microphone on carrier PCB Semiconductors 2 BC860 45V 100mA PNP transistors, SOT-23 (Q1, Q2) 1 BC849C 30V 100mA NPN transistor, SOT-23 (Q3) 3 6.8V ¼W zener diodes, SOT-23 (ZD1-ZD3) [BZX84C6V8] 1 3.3V 0.6-1W axial zener diode (ZD4) [1N4728] Capacitors (M2012/0805 50V X7R, unless otherwise noted) 1 100μF 50V radial electrolytic (max 8mm diameter) 1 100μF 10V low-ESR radial electrolytic 1 10μF 16V X5R 3 1μF 50V non-polarised SMD electrolytics, 4mm diameter [Altronics R9600] 2 2.2nF 5% NP0/C0G 2 1nF 5% NP0/C0G 2 470pF 5% NP0/C0G Resistors (all SMD M2012/0805 size 1%, unless noted) 2 150kW 1 100kW 1 39kW 1 5.6kW 1 2.2kW 1 1kW 1 330W 2 47W 1 3.3kW (through-hole or SMD, 1/4W 1%) 1 double-sided PCB coded 01108232, 99 × 13mm 1 Knowles SPU0410LR5H MEMS microphone on carrier PCB Semiconductors 2 BC560 45V 100mA PNP transistors, TO-92 (Q1, Q2) 1 BC549C 30V 100mA NPN transistor, TO-92 (Q3) 3 6.8V 400mW or 1W axial zener diodes (ZD1-ZD3) [1N754] 1 3.3V 0.6-1W axial zener diode (ZD4) [1N4728] Capacitors 1 100μF 50V radial electrolytic (maximum 8mm diameter) 1 100μF 10V low-ESR radial electrolytic 1 10μF 35V radial electrolytic 3 1μF 63V/100V MKT 2 2.2nF 63V/100V MKT 2 1nF 63V/100V MKT 2 470pF 50V C0G/NP0 ceramic Resistors (all axial 1/4W 1%) 2 150kW 1 100kW 1 39kW 1 5.6kW 1 3.3kW 1 2.2kW 1 1kW 1 330W 2 47W This is an updated version of the parts list from the August 2023 issue. In short, the changes were the addition of the SPU0410LR5H MEMS microphone, 3.3V zener diode, 3.3kW resistor; and the removal of one each of the 10kW and 2.2kW resistors. The case parts are not included; see the August 2023 issue for those. siliconchip.com.au Australia's electronics magazine April 2024  81