Dynamic Microphone

Up to this point, the posts in this blog have addressed in sequence my “journey” of setting up a simple home music recording studio, starting from an empty space and progressing all the way to a finished audio file.  It’s been a two-year journey that has covered a large number of topics !  Obviously, there are many more topics worth investigating and discussing  –   filling in the gaps, so to speak.  I’ll start with a brief overview of microphones in this post and in the next few posts. 

I haven’t used microphones in my home music studio yet, since I’ve focused solely on my Yamaha digital keyboard as the sound source.  If I wish to record another instrument, such as voice, violin, acoustic guitar, flute, etc., I’ll need to incorporate microphones in my signal chain.
 
A microphone is a transducer, converting sound wave energy to electrical energy.  The sound wave is a longitudinal pressure wave (compressions and rarefactions of the air molecules) that causes a thin diaphragm in the microphone to vibrate.  There are basically two different mechanisms of electromagnetic physics that can convert the mechanical vibration of the diaphragm into an electrical signal:  (1) the electromotive force (EMF) (which is actually a voltage) created by the movement of electric charge in a magnetic field, and (2) the voltage across a charged two-conductor electric capacitor created by the movement of one of the conductors.  Varying magnetic EMF is the operating principle of “dynamic” microphones, and varying capacitor voltage is the operating principle of “condenser” microphones.  Let’s take a look at one type of dynamic microphone below, simply called a dynamic microphone.  Another type of dynamic microphone, called the ribbon microphone, and the condenser microphone will be discussed in subsequent posts.
 
 Dynamic Microphone
 
The microphone (mic) shown in the photo at the top of this post is a Shure SM58 dynamic mic.  The Shure SM58 has been one of the most popular microphones in the music industry for decades, particularly for live vocal performances.
 
The “inner workings” of a dynamic mic are shown in the figure below.

adapted from C.R. Nave http://hyperphysics.phy-astr.gsu.edu/hbase/Audio/mic.html

The sound pressure waves (1.) strike the diaphragm cone  (2.) and cause it to vibrate.  The cone is attached to a metal wire coil (3.) that can move over a cylindrical magnet pole (4.).  As the coil moves, the mobile charge carriers in the wire feel a force, the Lorentz magnetic force, creating an EMF voltage across the terminals of the coil.  This voltage is the source of the electrical signal in the microphone output circuit (5.).
 
Let’s look more closely at the EMF voltage generated by this moving wire coil.  The Lorentz magnetic force that acts on the mobile charge carriers in a wire is shown in the figure below.

adapted from C.R. Nave http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/genwir2.html#c1

Here’s my simple “derivation” of the EMF voltage generated by the moving wire coil in the dynamic microphone:

The pressure of the sound wave causes the cone to vibrate about its rest position. Consequently, the coil of wire moves with positive and negative velocities, and the generated voltage in (5) is proportional to the velocity.  This voltage waveform therefore “images” the pressure variations of the input sound wave and constitutes the output signal of the microphone.
 
The important characteristics of a microphone include:
 
1.  Polar response pattern
2.  Frequency response
3.  Output signal level
4.  Output resistance (impedance)
 
Dynamic mics usually have a “cardioid” polar response pattern, as shown here:

Cardioid Pattern by Galak76

With a cardioid pattern, the mic picks up sound well in a forward-looking direction while rejecting sound from its rear.  This pattern is good for isolating sound sources on a recording.
 
Because of the fairly heavy diaphragm and coil, movement of the coil is ‘sluggish’ and restricted, which in turns decreases its high-frequency and transient response.  A typical frequency response curve for a dynamic mic is given here:

Shure SM58 Microphone Frequency Response

Typical microphone output levels vary in the range  1 – 10 mV and their output impedances range from  50 Ohm  to  600 Ohm .  For the Shure SM58,  the spec sheet gives an output level of  1.6 mV and an output impedance of  300 Ohm.
 
Dynamic mics have a number of key advantages that include,

•    Rugged and inexpensive
•    Don’t require external power (phantom power)
•    Moderate frequency response

 
The sound produced from a dynamic mic is characterized as mellow and well rounded. 
 
There is another kind of  “dynamic”  microphone, called a ribbon mic, that operates by sound waves vibrating a thin, delicate metallic ribbon in a magnetic field.  We’ll take a closer look at the ribbon microphone in the next post.

---