Have you ever asked what some of the technical language means that suppliers take to spell out the quality of wireless loudspeakers? In this essay, I will clarify one regularly used spec: THD or “total harmonic distortion”.
It is often tricky to select a suitable set of wireless speakers made by Amphony Inc. given the huge number of products. Aside from looks, you will often be faced with having to examine several of the technical specs. THD is generally not as easily understood as some other commonly utilized specs including “signal-to-noise ratio” or “frequency response”.
THD is expressed either in percent or in decibel and indicates how much the signal which the loudspeaker outputs deviates from the audio signal which is fed into the speaker. The percentage shown as THD shows which amount of energy which is radiated by the loudspeaker are higher harmonics versus the original signal. 10% would mean that one 10th is distortion. 1% would mean one 100th and so forth. 10% equals -20 dB whilst 1% equals -40 dB.
Though, be cautious since there are actually several elements which cause harmonic distortion. Wireless loudspeakers as well as any sort of active speaker or active subwoofer all have built-in power amplifiers to drive the speaker element. The audio amplifier itself will have a certain level of distortion. Usually the distortion of the amp will be larger the more output power it supplies to the loudspeaker. Normally manufacturers are going to publish amplifier distortion based on a specific amount of power, generally a lot less than the rated highest amplifier output power. Distortion specifications for various output power levels are usually shown for several power levels or as a chart showing distortion versus output power. Both of these methods allow to better evaluate the quality of the amplifier.Harmonic distortion measurements are typically conducted by feeding a test signal into the loudspeaker. This tone is a pure sine wave signal with minimum distortion. The frequency of this test tone is usually 1 kHz. Still, amplifier distortion will generally increase with rising frequency, especially in digital class-D products. One more component causing distortion is the speaker element that usually works with a diaphragm that carries a coil which is suspended in a magnetic field. The magnetic field is excited by the music signal. The change in magnetic flux, however, is not completely in sync with the music signal as a result of core losses as well as other factors. In addition, the kind of suspension of the diaphragm will cause nonlinear movement. This results in the signal being distorted by the loudspeaker element itself. What’s more, the bigger to power level with which the loudspeaker is driven, the larger the distortion. Often speaker vendors are going to display distortion for small to moderate output power levels only.
The overall distortion of the speaker thus is the sum total of the amplifier distortion as well as the speaker element distortion. Furthermore, there are other contributing factors. Depending on the material used to manufacture the loudspeaker enclosure, there will be vibrations or enclosure resonances. Those normally depend on the sound pressure level, the enclosure shape, the enclosure material along with audio frequency. Consequently extra audio distortion will be brought on by the box itself.
Total distortion is best determined through measurement. A signal generator is used which provides a highly linear sine tone to the loudspeaker. The audio is picked up by a measurement microphone. The microphone signal is then analyzed by an audio analyzer. The audio analyzer is going to compute the level of higher harmonics or distortion. Another distortion measurement method that offers a better analysis of the speaker performance with real-world signals is called interharmonic distortion analysis. This method outputs two signals at different frequencies and measures the level of harmonics at other frequencies.
Also, please understand that many wireless speakers are going to experience signal distortion during the audio transmission itself. This is mostly the case for transmitters which use analog or FM type transmission. More advanced products employ digital audio transmission. Usually these transmitters work at 2.4 GHz or 5.8 GHz.