An Explanation of the Total Harmonic Distortion of Wireless Speakers

In this article, I will explain the term "total harmonic distortion", often also called "THD" which is frequently used to describe the performance of wireless speakers.


Looking for the perfect model from the huge amount of products, you may have a hard time understanding some of the technical language and terms that you will find in the specifications of today's wireless speakers. THD is usually not as easily understood as some other commonly used terms such as "signal-to-noise ratio" or "frequency response".


THD is expressed either in percent or in decibel and shows how much the signal which the speaker outputs differes from the audio signal which is fed into the speaker. A -20 dB or 10% distortion means that one tenth of the radiated sound is a result of distortion while -40 dB or 1% would mean that one percent of the energy are harmonic products of the original signal.


Harmonic distortion inside a wireless speaker is actually the result of several components, including the power amplifier which is built into the speaker to drive the speaker element. Amplifier distortion usually depends on the amplifier output power and is sometimes specified for several power levels.


Distortion specs for different power levels are usually given for several power levels or as a diagram showing distortion versus output power. Both of these methods allow to better evaluate the quality of the amp.Normally distortion is measured with a 1 kHz test tone. However, amplifier distortion will usually increase with increasing frequency, especially in digital class-D models.


Another element causing distortion is the speaker element which typically works with a diaphragm that carries a coil which is suspended in a magnetic field. The coil is moving in accordance to the change in the magnetic field which is excited by the audio signal but does not correspond 100% with the signal due to core losses and other factors. As such the result is distortion caused by the speaker element. Many manufacturers will specify harmonic distortion depending on the power level because usually the higher the speaker is driven the higher the amount of distortion.


The overall speaker distortion is thus caused by the amplifier as well as the speaker element and in addition by a number of additional contributing factors. The speaker enclose will vibrate to some extend and thus contribute to the distortion.


To find out the overall distortion of a loudspeaker, a signal generator is used which provides an ultra-linear signal to the speaker as well as a measurement microphone which is connected to an audio analyzer to determine the amount of harmonics emitted by the speaker. Another distortion measurement technique which gives a better analysis of the speaker performance with real-world signals is called interharmonic distortion. This method outputs two signals at different frequencies and measures the amount of harmonics at different frequencies.


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