invented in 1962. The electret material used in this type of microphone is made
by embedding a permanent charge in a material called a dielectric.A charge is
embedded in a dielectric by aligning the charges in the material — sort of like
how you make a magnet by aligning the atoms in a piece of iron.
There is a preamplifier in an electret microphone, to which you provide a
supply voltage. That’s why the projects in this book that use electret micro-
phones have a connection through a resistor running between the plus (+)
lead of the microphone and the +V bus to power the preamplifier. (The resis-
tor reduces the voltage at the + lead of the microphone to the desired supply
voltage.)
Size counts
When you order electret microphones, pay attention to the diameter and
thickness because some can be hard to handle and solder. For most of our
projects in this book, we use microphones with a diameter of about^3 ⁄ 8 " and a
thickness of about^2 ⁄ 10 ". A microphone cartridge with a diameter of about^1 ⁄ 4 "
and a thickness of about^1 ⁄ 10 " turns out to be much harder to handle and solder
to than a microphone cartridge of about^3 ⁄ 8 " and a thickness of about^2 ⁄ 10 ". (Check
out Chapter 6, where we bit the bullet and used a small microphone cartridge
because that project needed some of the capabilities we couldn’t find in a
larger microphone cartridge.)
Many microphone cartridge sizes are specified in millimeters. To help you
translate this, typical diameters of microphone casings are 6 mm (about^1 ⁄ 4 ")
and 9.7 mm (about^3 ⁄ 8 ").
Measuring sensitivity
Sensitivity is another issue that you should pay attention to with microphone
cartridges. Sensitivity is measured in decibels (dB) — and just to confuse
you, this measurement is given as a negative number. A microphone cartridge
with a sensitivity of –40 dB, for example, is more sensitive(provides higher
voltage at a given level of sound) than a microphone cartridge with a sensitiv-
ity of –60.
For example, for the project in Chapter 6 (which has to pick up very faint
sounds as part of a parabolic microphone), you need a highly sensitive
microphone cartridge. We use one with a sensitivity of –35 dB. In Chapter 14,
in which you talk directly into the microphone to record a message, we use a
less-sensitive microphone cartridge, rated at –64 dB.
Connecting your microphone cartridge to your project
To connect electret microphone cartridges to your project, you can get elec-
tret microphone cartridges with solder pads or with leads that you can insert
into a breadboard. We use both in our projects.
Chapter 3: Assembling Your Electronics Arsenal 49