Have you ever wondered what some of the technical language means that vendors take in order to show the performance of wireless outdoor speakers shown at www.amphony.com? In this article, I am going to clarify a commonly used specification: THD or “total harmonic distortion”. It is often hard to pick a suitable pair of wireless speakers given the huge amount of products. Aside from looks, you will often be confronted with having to examine a few of the technical specifications. THD is usually not as easily understood as some other frequently utilized 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 that the speaker outputs differs from the audio signal that is input into the speaker. The percentage given as THD describes which amount of energy which is radiated by the speaker are higher harmonics versus the original audio. 10% would mean that one 10th is distortion. 1% would mean one hundredth et cetera. 10% is equal to -20 dB whilst 1% equals -40 dB.
Harmonic distortion in a wireless loudspeaker is really the consequence of a few components, such as the power amplifier which is built into the speaker in order to drive the loudspeaker element. The audio amplifier itself is going to have a specific level of distortion. Normally the distortion of the amplifier will be higher the more output power it supplies to the speaker. Generally producers will show amp distortion based on a specific amount of output power, by and large a lot less than the rated highest amplifier output power. Distortion ratings for various output power levels are usually provided for a few output power levels or as a diagram listing distortion versus output power. Both of these methods allow to better evaluate the quality of the amplifier.Furthermore, please note that distortion typically is measured for a specific test tone frequency. Commonly a 1 kHz sine wave tone is used during the measurement. This allows evaluating equipment from different vendors. Though, distortion normally varies with different frequencies. Many digital amps will show growing distortion amid higher frequency which cannot easily be discovered by glancing at the spec sheet. The next contributing factor is the loudspeaker element itself. The majority of loudspeakers use a diaphragm type driver that is driven by 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 entirely in sync with the music signal due to core losses and other factors. In addition, the type of suspension of the diaphragm will create nonlinear motion. As such the result is distortion brought about by the speaker element. Most suppliers are going to publish harmonic distortion based on the power level as normally the higher the speaker is driven the higher the level of distortion. The overall distortion of the speaker thus is the total of the amplifier distortion in addition to the speaker element distortion. On top of that, there are different contributing factors. The loudspeaker enclose is going to shake to some extent and thus add to the distortion.
In order to find out the total distortion of a speaker, a signal generator is used that provides an ultra-linear signal to the loudspeaker and also a measurement microphone which is connected to an audio analyzer to determine the amount of harmonics emitted by the speaker. On the other hand, pure sine signals hardly give an accurate indication of the distortion of the cordless speaker with real-world signals. A better distortion analysis is the so-called intermodulation distortion analysis where a test tone which consists of a number of sine waves is used. Then the level of harmonics at other frequencies is calculated.
In addition, please note that many wireless speakers are going to experience signal distortion during the audio transmission itself. This is mostly the case for transmitters that use analog or FM type transmission. Digital audio transmission on the other hand has the smallest amount of signal distortion. The music is digitized within the transmitter and not affected during the transmission itself. Digital wireless audio transmitters are available at 2.4 GHz and 5.8 GHz.