Have you ever wondered what some of the technical jargon means which manufacturers use to spell out the quality of wireless outdoor speakers? In this commentary, I am going to explain a commonly used term: THD or “total harmonic distortion”. It is often tricky to select a suitable set of cordless speakers given the huge amount of models. Aside from looks, you will often be faced with having to examine some of the technical specs. Total harmonic distortion is typically not that well understood. Nonetheless, this term is still vital in terms of determining the quality of a specific model. Different specs, just like “output power” or “frequency response” are normally easier understood.
Briefly, “harmonic distortion” describes how much the music signal is being degraded because of the speaker or in other words how much the signal differs from the original signal. There are 2 widespread ways in order to state harmonic distortion, either in percent (%) or in decibel (dB). A -20 dB or 10% distortion means that one tenth of the radiated audio is a result of distortion as -40 dB or 1% would mean that one percent of the energy are harmonic products of the original signal. Harmonic distortion inside a cordless speaker is really the consequence of several elements, including the power amp which is built into the speaker to drive the loudspeaker element. The amp itself will have a certain amount of distortion. Generally the distortion of the amplifier will be bigger the more output power it supplies to the speaker. Usually vendors are going to specify amplifier distortion depending on a certain amount of power, usually a lot less than the rated maximum amp output power.
Distortion ratings for different power levels are usually shown for several output power levels or as a chart showing distortion versus output power. Both of these methods allow to better evaluate the performance of the amp.Furthermore, please note that distortion normally is measured for a certain test tone frequency. Commonly a 1 kHz sine wave tone is used during the measurement. Distortion, however, is normally dependent on the signal frequency. Many amplifiers will exhibit rising distortion with rising frequency. Particularly digital class-D amps will have fairly high distortion at frequencies above 5 kHz.
The next contributing factor is the loudspeaker element itself. The majority of loudspeakers employ a diaphragm kind driver that is driven by a coil that is suspended in a magnetic field. The magnetic field is excited by the audio signal. The variation in magnetic flux, however, is not completely in sync with the music signal because of core losses as well as other factors. Also, the kind of suspension of the diaphragm will bring about nonlinear motion. As such the result is distortion caused by the loudspeaker element. The majority of makers are going to display harmonic distortion depending on the power level because generally the higher the loudspeaker is driven the higher the amount of distortion.
The overall distortion of the loudspeaker therefore is the total of the amplifier distortion in addition to the speaker element distortion. Furthermore, there are other contributing factors. Depending on the material used to manufacture the speaker housing, there will be vibrations or box resonances. Those typically depend on the sound pressure level, the box shape, the housing material in addition to audio frequency. For that reason additional audio distortion is going to be brought about by the box itself.
The overall distortion of the loudspeaker is generally determined by a measurement which comprises a low-distortion audio generator plus a microphone that 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. However, pure sine signals barely give an accurate indication of the distortion of the cordless loudspeaker with real-world signals. A better distortion analysis is the so-called intermodulation distortion analysis in which a test tone which includes several sine waves is used. Then the number of harmonics at different frequencies is measured.
Wireless speakers will also have some amount of distortion during the audio transmission. The amount of distortion is going to depend on the sort of cordless transmission technique in addition to the quality of components. Generally 900 MHz FM transmitters have among the highest level of distortion. Digital audio transmission on the other hand provides the lowest amount of signal distortion. The audio is digitized inside the transmitter and not affected during the transmission itself. Digital wireless audio transmitters are available at 2.4 GHz as well as 5.8 GHz.