We’ve likely all heard the sound from a microphone amplified through loudspeakers. This happens often at all sorts of events and performances. A speaker or singer will project their voice into a microphone and their voice will be sent though speakers at a much higher volume. How is this audio amplified?
Does a microphone amplify sound? A microphone is a transducer that converts sound (mechanical wave energy) into audio (electrical energy). Mics do not amplify sound, per se, though some mics do amplify the audio signal from their capsules before the signal is outputted. All mics require further amplification beyond their own.
In this article, we’ll take a deeper look into how a microphone amplifies its signal, if at all. We’ll also look at the typical gain stages in order get a mic signal to actually produce sound out of a loudspeaker.
Difference Between Sound And Audio
Before we get into the meat of this article, let’s break down the difference between sound and audio.
What is sound? Sound is a vibration that oscillates within a medium and changes the pressure along a wave within that medium (gas, liquid or solid). It is a form of mechanical wave energy. Audible sound waves have frequencies in the range of 20 Hz – 20,000 Hz.
What is audio? Audio is the electrical representation of sound. Analog audio is sound represented as an AC voltage while digital audio is sound represented in binary numbers (sample rate and bit depth). Sound can be converted to audio via microphones and audio can be converted to sound via speakers.
Microphones Convert Sound To Audio
As mentioned in the opening paragraphs, microphones are transducers that convert sound into audio.
Regardless of the microphone type, a microphone’s role is to take the sound waves at its diaphragm and produce a coinciding audio signal.
Microphones have diaphragms, which are thin, movable membranes in the mic capsule. These diaphragms are very sensitive and move in reaction to the changing pressure at their surfaces. This changing pressure is largely caused by sound waves.
As the diaphragm moves, a coinciding AC voltage is produced. This AC voltage is essentially the microphone signal. Dynamic microphones (moving-coil and dynamic) produce this signal via electromagnetic induction while condenser microphones produce audio via electrostatic principles.
Depending on the microphone type and design, this signal will be processed further through other components before being outputted from the mic.
For deeper explanations as to how microphones work, check out the following My New Microphone articles:
• How Do Microphones Work? (A Helpful Illustrated Guide)
• What Is A Microphone? (Mic Types, Examples, And Pictures)
How Do Microphones Amplify Audio?
Let me start answering this question by stating that microphones output mic level signals. Mic level signals are relatively low AC voltages, ranging roughly between 1 and 100 millivolts (-60 dBV to -40 dBV).
Regardless of whether the microphone provides any amplification or not, a mic signal will require further amplification. Microphone preamplifiers are used to bring mic level signals up to line level for use in professional audio equipment. Power amplifiers are then sued to amplify line level signals in order to properly driver loudspeakers.
With that preamble out of the way, some microphones do provide amplification.
What is amplification? Amplification is the process of increasing the strength or power of a signal. Amps can be either natural or artificial devices. With microphones, these amps are artificial and focus on boosting the electrical signal of the microphones.
To remain true to the term “amplifier,” I’ll mention here than some microphone components are not true amplifiers, though they are designed to improve the signal strength. I’ll make distinctions below.
Rather than list the potential amplifiers that may be found in a microphone, let’s discuss any component that causes an increase in signal strength:
- Impedance converter (Field-effect transistor)
- Vacuum tube
- Step-up transformer
- Printed circuit board with amplifiers
Let’s discuss each of these in more detail:
Impedance converters are not true amplifiers. Rather, they take a lower level input signal and use it to modulate a higher level output signal.
Impedance converters are necessary components that find themselves immediately after the capsules of condenser microphone capsules. Condenser capsules output signals with incredibly high impedance values. In order for these signals to travel without significant degradation, it’s crucial they pass through an impedance converter.
A microphone’s impedance converter is essentially a field-effect transistor (FET). Perhaps the most common FET type in mics is the junction-gate field-effect transistor (JFET).
- G: gate.
- S: source.
- D: drain.
Microphone impedance converters receives the low-level high-impedance signal from the mic capsule at its gate.
This AC signal at the gate is used to modulate the current (and voltage) between the source and the drain.
We can think of the FET impedance converter as having an input (gate) and output (source-drain). The input signal is low-level high-impedance and controls a stronger output signal with lower impedance.
In this way, the impedance converter acts as an “amplifier” of the microphone signal (though it isn’t a true amplifier).
For more information on microphone impedance converters and transistors, check out my article Do All Microphones Have Transformers And Transistors? (+ Mic Examples).
Vacuum tubes are not true amplifiers. Rather, they take a lower level input signal and use it to modulate a higher level output signal.
Before transistor technology made its way into microphones (in the 1960s), vacuum tubes were required to convert mic signal impedance and provide a boost in signal strength.
For a more detailed account of microphone history, check out my article Mic History: Who Invented Each Type Of Microphone And When?
Vacuum tubes were, and still are, used in microphones and particularly in condenser microphones. They are put immediately after the capsules of these microphones to lower signal impedance and boost the signal.
An added benefit of vacuum tubes is the famous “tube sound” that many audiophiles, musicians, and audio engineers cherish.
- H: heater.
- K: cathode.
- A: anode.
- G: grid
A vacuum tube essentially works like this: a cathode is heated up (either directly or indirectly by a heater). As the cathode heats up, it emits electrons, which are attracted to the positively charged anode plate.
Therefore, by heating the vacuum tube, an electrical current is produced within the tube and outputted from the tube.
The signal from the microphone capsule is sent to the grid of the triode tube and controls the flow of electrons from the cathode to the anode.
The grid will accept the low-level high-impedance signal from the capsule and use it to modulate the stronger, lower-impedance current between the anode and cathode.
If we think of a microphone’s triode tube in terms of inputs/outputs, we develop the following:
The grid is the input which receives the capsule’s signal.
The cathode-anode is the output, which outputs a stronger signal that corresponds to the input signal at the grid.
So the triode vacuum tube acts as an “amplifier” of the microphone signal (though it isn’t a true amplifier).
Step-up transformers are not true amplifiers. Rather, they are passive devices that take a lower level input signal and use it to modulate a higher level output signal.
Step-up transformers typically find themselves at the outputs of dynamic microphones.
Through the principles of electromagnetic induction, the step-up transformer takes the AC voltage in one circuit and causes an “stepped-up” or increased AC voltage in a secondary circuit. Both circuits are independent and only share the transformer in common.
Transformers have the added benefit of block DC voltage.
- MC: magnetic core.
- P: primary winding.
- S: secondary winding.
A step-up transformer is made of a central magnetic core and two conductive windings known as the primary winding and the secondary winding.
The primary winding is part of the electrical circuit that carries the diaphragm’s converted audio signal. Since the winding coils around the magnetic core, any AC voltage (mic signal) that runs through it will cause a changing magnetic flux within the core. This is due to electromagnetic induction.
As the magnetic flux of the magnetic core varies, electromagnetic induction causes an AC voltage to be created across the secondary winding, which is also coiled around the core.
So the primary winding is part of the circuit that carries the mic signal of the mic’s capsule. This AC voltage causes a varying magnetic flux in the magnetic core, which, in turn, induces a voltage across the secondary winding. The secondary winding completes a circuit with the microphone output.
The ratio of turns in the coils is equal to the ratio of voltage in the coils. A step-up transformer has fewer turns in the primary coil than in the secondary coil.
As a simple example, if the primary coil has half the amount of turns as the secondary coil, the voltage across the secondary will be twice than of the primary.
If we look at a step-up transformer, we can think of the primary winding as being the input and the secondary winding as being the output. That being said, the transformer only acts as an “amplifier,” though it isn’t a true amplifier.
For more information on microphone transformers, check out my article Do All Microphones Have Transformers And Transistors? (+ Mic Examples).
Printed Circuit Board
Printed circuit boards (PCBs) may contain true amplifiers that actually increase the gain of an input signal and produce a stronger output signal.
Typical Gain Stages From Microphones To Loudspeakers
Whether it’s pseudo-amplification or real amplification, microphones only boost their signals so much. At the output of any microphone, the signal is still at mic level.
In order to get this relatively weak signal to move loudspeaker cones and be heard, we need more amplification beyond the microphone itself.
To simplify these levels with values, I’ve provided the following table:
|Lower-End Signal Strength||Upper-End Signal Strength|
|Mic Level||-60 dBV (1 millivolt)||-20 dBV (100 millivolt)|
|Line Level||-10 dBV (316 millivolts)|
|+4 dBu = 1.78 dBV (1.23 millivolts)
Speaker level has much more voltage that line level, but is highly dependent on the impedance and size of the loudspeaker it is driving.
For more detail on nominal audio signal levels, check out my articles Do Microphones Output Mic, Line, Or Instrument Level Signals? and What Is A Microphone Audio Signal, Electrically Speaking?
Now let’s look at some typical gain stages to get a microphone signal to speaker level:
In the following examples, I’ll bold the devices that provide amplification to the signal.
- Active loudspeaker (mic input)
To learn more about plugging microphones directly into loudspeakers, check out my article How To Plug A Microphone Into A Speaker.
- Microphone Preamplifier (mic input)
- Mixing board or Interface
- Power amp (line input)
- Passive loudspeaker
- Microphone Preamplifier (mic input)
- Mixing board or Interface
- Active loudspeaker (line input)
For more information on microphone gain staging and amplification, check out the following My New Microphone articles:
• What Is Microphone Gain And How Does It Affect Mic Signals?
• Do Microphones Plug Into Amps? (Guitar Amps, Preamps, Power Amps, Etc)
What happens when you put a microphone next to a speaker? If a microphone is sending signal to a loudspeaker and that loudspeaker is projecting sound, placing the mic next to the speaker could cause feedback. The mic signal is sent to the speaker, which sends sound back into the mic and overloads it, causing a terrible hum/squeal known as feedback.
For more information on microphone feedback, check out the following My New Microphone articles:
• What Is Microphone Feedback And How To Eliminate It For Good
• 12 Methods To Prevent & Eliminate Microphone/Audio Feedback
Can a speaker be used as a microphone? Yes, a speaker can be turned into a microphone by simply reversing the signal flow. In doing so, the speaker cone will act basically like a huge moving-coil dynamic diaphragm. As the cone moves, a coinciding audio signal is outputted from the loudspeaker.
For a detailed article on turning a loudspeaker into a microphone, check out my article How To Turn A Loudspeaker Into A Microphone In 2 Easy Steps.