In this editorial, I will highlight the spec “total harmonic distortion”, often also named “THD” which is commonly used to describe the performance of wireless loudspeakers.
Looking for the perfect model from the enormous amount of models, you might have a hard time understanding several of the technical jargon and terms which you will find in the specs of the latest wireless outdoor speakers sold by Amphony. THD is generally not as easily understood as several other frequently used specs such as “signal-to-noise ratio” or “frequency response”. In brief, THD describes the difference between the audio which is produced by the loudspeaker versus the audio signal with which the loudspeaker is driven. The most widespread methods to state distortion are percent in addition to decibel. These two conventions can be converted into one another. The percentage given as THD shows which amount of energy that is radiated by the speaker are higher harmonics versus the original signal. 10% would mean that one 10th is distortion. 1% would mean one 100th and so on. 10% equals -20 dB as 1% equals -40 dB. However, be careful since there are actually several elements that bring about harmonic distortion. Cordless loudspeakers and any kind of active loudspeaker or active subwoofer all have built-in power amps to drive the loudspeaker element. The amplifier itself will have a specific amount of distortion. Generally the distortion of the amplifier will be larger the more output power it supplies to the speaker. In general vendors will publish amplifier distortion based on a specific level of output power, normally a lot less than the rated highest amp output power.
Distortion ratings for various output power levels are generally provided for a number of power levels or as a diagram showing distortion versus output power. Both of these techniques allow to better evaluate the performance of the amplifier.Generally distortion is measured with a 1 kHz test tone. Nevertheless, amplifier distortion will generally increase with increasing frequency, especially in digital class-D types. Distortion is furthermore induced by the loudspeaker driver itself. The majority of speakers use a driver that carries a coil. This coil is positioned in a magnetic field. The coil is going to follow the magnetic field which is controlled by the music signal to move the diaphragm. Nonetheless, this movement is not completely linear. As such the result is distortion brought about by the speaker element. A lot of vendors will display harmonic distortion based on the power level because typically the higher the loudspeaker is driven the bigger the level of distortion.
The total distortion of the speaker therefore is the total of the amp distortion and the loudspeaker element distortion. In addition, there are other contributing factors. Depending on the material used to construct the loudspeaker enclosure, there will be vibrations or enclosure resonances. Those usually depend on the sound pressure level, the enclosure shape, the enclosure fabric and audio frequency. For that reason extra sound distortion will be caused by the box itself.
The total distortion of the speaker is normally determined by a measurement which consists of a low-distortion audio generator as well as a microphone which is attached to an audio analyzer. The audio analyzer will determine the level of higher harmonics and compare these with the main signal in order to compute the distortion. An additional distortion measurement method which provides a better analysis of the speaker quality with real-world signals is known as interharmonic distortion analysis. This technique outputs two signals at different frequencies and measures the level of harmonics at different frequencies. Furthermore, please note that many cordless speakers 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. Better types are going to utilize digital transmission and transmit at 2.4 GHz or 5.8 GHz to reduce audio distortion.