Have you ever wondered what some of the technical jargon means which vendors use to show the performance of wireless speakers? In this expose, I will highlight a commonly used specification: THD or “total harmonic distortion”.

It is often tough to choose a suitable set of wireless surround sound speaker models due to the huge number of products. Aside from looks, you will often be faced with having to consider a few of the technical specifications. A few of these are probably fairly simple to comprehend for instance “output power” or “frequency response”. Though, a specification that is not as easily understood is the spec relating to how much distortion the speaker has.

THD is expressed either in percent or in decibel and shows how much the signal which the loudspeaker outputs differs from the audio signal that is input into the loudspeaker. A -20 dB or 10% distortion means that one 10th of the radiated audio is a result of distortion whilst -40 dB or 1% would mean that one percent of the energy are harmonic products of the original signal.

A cordless loudspeaker really has a number of components that add to harmonic distortion. One of these is the built-in power amp. This audio amplifier is driving the speaker element. The amplifier itself will have a specific level of distortion. Normally the distortion of the amp will be bigger the more output power it supplies to the speaker. Normally makers will publish amplifier distortion depending on a certain level of output power, generally a lot less than the rated highest amp output power.

Generally distortion is measured with a 1 kHz test tone. Distortion, though, is generally dependent on the signal frequency. Most amplifiers are going to have rising distortion with increasing frequency. In particular digital class-D amplifiers are going to have quite high distortion at frequencies higher than 5 kHz.

The next contributing factor is the speaker element itself. Many speakers employ a diaphragm type driver that is driven by a voicecoil which is suspended in a magnetic field. The coil will track the magnetic field that is controlled by the music signal to move the diaphragm. Yet, this movement is not completely linear. As such the result is distortion brought about by the loudspeaker element. Many suppliers are going to show harmonic distortion based on the power level because normally the higher the speaker is driven the higher the amount of distortion.

The total speaker distortion is as a result brought on by the amp as well as the speaker element and in addition by a series of further contributing factors. Depending on the material used to manufacture the loudspeaker enclosure, there will be vibrations or enclosure resonances. These normally depend on the sound pressure level, the box shape, the enclosure fabric along with audio frequency. Thus extra audio distortion will be brought about by the enclosure itself.

The overall distortion of the loudspeaker is generally determined by a measurement that involves a low-distortion audio generator plus a microphone that is connected to an audio analyzer. The audio analyzer will calculate the amount of higher harmonics and compare these with the main signal to calculate the distortion. Another distortion measurement method that provides a better analysis of the loudspeaker performance with real-world signals is referred to as interharmonic distortion analysis. This technique outputs two signals at different frequencies and measures the amount of harmonics at other frequencies.

Furthermore, please understand that the majority of wireless loudspeakers are going to experience signal distortion during the audio transmission itself. This is for the most part the case for transmitters which utilize analog or FM type transmission. Digital audio transmission on the other hand offers the smallest level of audio distortion. The signal is digitized within the transmitter and not affected during the transmission itself. Digital cordless audio transmitters are available at 2.4 GHz and 5.8 GHz.

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