Have you ever asked what some of the technical language means that vendors use in order to show the performance of wireless loudspeakers? In this editorial, I will highlight a frequently used specification: THD or “total harmonic distortion”.
Wireless loudspeakers are available in all different shapes and sizes. Selecting the right model may often be tough. I am going to shed some light on a commonly utilized term that is utilized in order to publish the technical performance of the speaker: “total harmonic distortion” or “THD”. THD is usually not as easily understood as a few other commonly utilized specs including “signal-to-noise ratio” or “frequency response”.
In brief, THD shows the difference between the audio that is produced by the speaker versus the audio signal with which the speaker is driven. The most widespread ways to show distortion are percent in addition to decibel. These two conventions can be translated into one another. The percentage given as THD shows which amount of energy which is radiated by the speaker are higher harmonics versus the original signal. 10% would mean that one 10th is distortion. 1% would mean one hundredth et cetera. 10% is equal to -20 dB while 1% equals -40 dB. On the other hand, be cautious as there are in fact several elements which bring about harmonic distortion. Wireless loudspeakers as well as any kind of active loudspeaker or active subwoofer all have built-in power amplifiers in order to drive the speaker element. The power amplifier itself is going to have a specific amount of distortion. Generally the distortion of the amp will be bigger the more output power it supplies to the loudspeaker. Customarily vendors will specify amplifier distortion based on a specific amount of power, by and large a lot less than the rated highest amplifier output power.
Having amp distortion specs for a few output power levels provides a better picture of the amplifier distortion performance.Harmonic distortion measurements are typically done via feeding a test tone into the speaker. This signal is a pure sine wave signal with minimum distortion. The frequency of this test tone is normally 1 kHz. Distortion, though, is usually dependent on the audio frequency. Most amplifiers will show rising distortion with rising frequency. Particularly digital class-D amplifiers will show fairly high distortion at frequencies above 5 kHz.
The second contributing factor is the loudspeaker element itself. The majority of speakers employ a diaphragm kind driver which is driven by a coil that is suspended in a magnetic field. The voicecoil will follow the magnetic field that is controlled by the music signal in order to move the diaphragm. Yet, this movement is not completely linear. As such the result is distortion caused by the speaker element. Most vendors will exhibit harmonic distortion depending on the audio level as typically the higher the speaker is driven the bigger the amount of distortion.
The total distortion of the speaker thus is the sum of the amp distortion as well as the loudspeaker element distortion. On top of that, there are different contributing factors. The enclosure of the speaker will vibrate to some extent depending on the sound pressure level. These vibrations are going to also be non-linear in nature and add to distortion.
In order to figure out the overall distortion of a loudspeaker, a signal generator is utilized which supplies an ultra-linear signal to the speaker and a measurement microphone which is attached to an audio analyzer in order to calculate the level of harmonics emitted by the loudspeaker. Then again, 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 where a test tone which includes a number of sine waves is used. Then the number of harmonics at other frequencies is calculated.
Furthermore, please understand that most wireless speakers introduced at this link will experience audio distortion during the audio transmission itself. This is mostly the case for transmitters which utilize analog or FM type transmission. Digital audio transmission on the other hand provides the smallest level of signal distortion. The audio is digitized in 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.