Speakers convert the analog audio signal from an audio amplifier into sound waves that can be heard by the human ear. Speakers can be as simple as a passive buzzer, or as refined as a pair of HiFi audio speakers.

In this article, we will look at how speakers work, how to pick the right speakers for your amplifier, and how to calculate the power and current flowing through a speaker.

The Parts of a Speaker

Shown below is a cross-section of a speaker:

How Speakers Work

The audio signal is sent through the voice coil. This creates a magnetic field that changes with the polarity of the audio signal. The voice coil is positioned in a fixed permanent magnetic field created by the pole piece and the top pole plate. This causes the voice coil to vibrate up and down according to the audio signal.

The vibrating voice coil causes the diaphragm to vibrate, which produces sound waves in the air in front of the cone.

There are several different types of speakers, classified by the range of audio frequencies they can work with:

Speaker Specifications

The speaker should have a label that specifies the frequency range, impedance, power rating.

  • Frequency range: the range of audio frequencies the speaker is able to reproduce.
  • Impedance: Since the voice coil is inductive, speakers have impedance. The impedance you choose will depend on your amplifier. The user manual for your amplifier should tell you which speaker impedance to use.
  • Power rating: Only RMS power (in watts) should be trusted. A 100w RMS speaker should be able to handle a 100w sine wave continuously

How to Find the Right Speakers for your Amplifier

Speaker impedance is the resistance of the speaker to audio signals from the amplifier. A lower speaker impedance implies that the speaker should be used with higher power amplifiers. A higher speaker impedance implies that a lower power amplifier should be used.

When speakers are connected in series, the impedances are added. For example, with two 4Ω speakers connected in series, the total impedance is 8Ω.

When connecting two speakers in parallel, the total impedance is divided by two. So with two 4Ω speakers connected in parallel, the total impedance is 2Ω.

Tube amplifiers demand the correct speaker impedance while solid-state amplifiers generally don’t care about matching impedances.

How to Calculate Amplifier Power

Consider an amplifier with an output swing of 21V RMS and 8Ω speakers connected. To calculate the output power of the amplifier with these speakers, use this formula:

P = V2 / R


P: Output power (W)

V: RMS voltage of amplifier (V)

R: Impedance of the speaker (Ω)

With a 21V RMS output voltage and an 8Ω speaker, the power is:

P = 21V2 / 8Ω

P = 55W

This assumes the power supply has the ability to deliver this power.

How to Calculate Speaker Current

Now, how much current is going into the speaker? This can be calculated using Ohms law:

I = V / R

So in this example we have

I = 21V / 8Ω = 2.6A

Hope this article has helped you to understand how to choose the right speakers for your amplifier! Be sure to leave a comment below if you have any questions.