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MEMS Calibrated Microphones
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 r e c e i ve d s o m e K n ow l e s
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
that was published in the September
2024 issue. It 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:
https://pemag.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
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.
Practical Electronics | April | 2025
19
Constructional Project
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.
other 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
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.
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Practical Electronics | April | 2025
MEMS Calibrated Microphones
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).
file or 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 ul-
trasonic 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
PE
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, no more than
4mm in diameter
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)
1 100kW
1 39kW
1 5.6kW
2 150kW
1 1kW
1 330W
2 47W
1 2.2kW
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%)
1 100kW
1 39kW
1 5.6kW
2 150kW
1 2.2kW
1 1kW
1 330W
1 3.3kW
2 47W
This is an updated version of the parts list from the September 2024 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 September 2024 issue for those.
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