Understanding the Distortion Spec of Cordless Loudspeakers

In this expose, I will highlight the term “total harmonic distortion”, often also titled “THD” which is often used in order to describe the performance of outdoor wireless speakers.

It is often hard to choose a suitable set of wireless surround sound speaker models given the great amount of models. Aside from looks, you will often be faced with having to examine several of the technical specifications. Several of these are probably rather easy to comprehend for instance “output power” or “frequency response”. However, a specification that is not as easily understood is the term regarding how much distortion the loudspeaker has.

THD is expressed either in percent or in decibel and indicates how much the signal that the speaker outputs deviates from the music signal that is input into the speaker. If a loudspeaker specifies a distortion of 10% to provide an example then one tenth of the energy radiated by the speaker is distortion. A distortion of 10% can also be stated as -20 dB. 1% distortion is equal to -40dB.

Harmonic distortion inside a wireless loudspeaker is really the result of several components, such as the power amplifier that is built into the loudspeaker to drive the speaker element. Amplifier distortion generally depends on the amp output power and is now and again given for several output power levels.

Distortion ratings for different output power levels are generally given for a few power levels or as a chart listing distortion versus output power. Both of these techniques allow to better evaluate the quality of the amplifier.Besides, please note that distortion commonly is measured for a certain test tone frequency. Commonly a 1 kHz sine wave tone is utilized during the measurement. Distortion, though, is usually dependent on the signal frequency. Many amplifiers will show increasing distortion with rising frequency. In particular digital class-D amplifiers are going to show rather high distortion at frequencies higher than 5 kHz.

Another element causing distortion is the speaker element that generally works with a diaphragm which carries a coil that is suspended in a magnetic field. The magnetic field is excited by the music signal. The change in magnetic flux, however, is not perfectly in sync with the music signal as a result of core losses and other factors. Also, the kind of suspension of the diaphragm is going to create nonlinear movement. As such there is going to be speaker element distortion which furthermore depends on the amount of output power with which the speaker is driven.

As such both the amp in addition to the loudspeaker element itself contribute to distortion. Moreover, there are other factors that also contribute to distortion. The total level of distortion is the total of all of these factors. The loudspeaker enclose will shake to some extent and as a result add to the distortion.

The overall distortion of the loudspeaker is usually determined by a measurement that comprises a low-distortion audio generator plus a microphone which is connected to an audio analyzer. The audio analyzer is going to calculate the level of higher harmonics and compare these with the main signal to calculate the distortion. Then again, pure sine signals barely give an accurate picture of the distortion of the wireless speaker with real-world signals. A better distortion analysis is the so-called intermodulation distortion analysis in which a test tone that consists of a number of sine waves is used. Next the number of harmonics at different frequencies is calculated.

Wireless loudspeakers will also have some level of distortion during the audio transmission. The level of distortion will depend on the sort of wireless transmission method and also the quality of components. Typically 900 MHz FM transmitters have among the highest amount of distortion. Better products will utilize digital transmission and transmit at 2.4 GHz or 5.8 GHz to minimize audio distortion.


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