In this article, I will explain the term "total harmonic distortion", often also called "THD" which is frequently used to describe the performance of wireless outdoor speakers.
Wireless speakers come in all different shapes and sizes. Deciding on the right model can often be tricky. I am going to shed some light on a commonly used term which is used to describe the technical performance of the speaker: "total harmonic distortion" or "THD". Total harmonic distortion is usually not very well understood. However, this terms is still important in terms of determining the quality of a particular model. Other terms, such as "output power" or "frequency response" are usually easier understood.
In a nutshell, THD shows the difference between the sound that is produced by the speaker versus the audio signal with which the speaker is driven. The most common ways to express distortion are percent and decibel. These two conventions can be translated into one another. The percentage shown as THD describes which amount of energy that is radiated by the speaker are higher harmonics versus the original signal. 10% would mean that one tenth is distortion. 1% would mean one hundredth etc. 10% equals -20 dB while 1% equals -40 dB.
A wireless speaker actually has several components which contribute to harmonic distortion. One of these is the built-in power amplifier. This amplifier is driving the speaker element. Usually the higher the amplifier is driven the higher the amount of amplifier distortion. For this reason, some manufacturers will list amplifier distortion depending on amplifier power.
Harmonic distortion measurements are usually made by feeding a test tone into the speaker. This tone is a pure sine wave signal with minimum distortion. The frequency of this test tone is usually 1 kHz. Distortion, however, is usually dependent on the signal frequency. Most amplifiers will show increasing distortion with increasing frequency. In particular digital class-D amplifiers will have fairly high distortion at frequencies above 5 kHz.
The second contributing factor is the loudspeaker element itself. Most speakers use a diaphragm type driver which is driven by a coil that is suspended in a magnetic field. The coil will follow the magnetic field which is controlled by the audio signal to move the diaphragm. However, this movement is not perfectly linear. This results in the signal being distorted by the speaker element itself. Also, the larger to power level with which the speaker is driven, the larger the distortion. Often speaker manufacturers will show distortion for small to moderate power levels only.
As such both the amplifier as well as the speaker element itself contribute to distortion. In addition, there are other factors which also contribute to distortion. The total amount of distortion is the sum of all of these factors. Depending on the material used to build the speaker enclosure, there will be vibrations or box resonances. These usually depend on the sound pressure level, the box shape, the enclosure material and audio frequency. Therefore additional sound distortion will be caused by the box itself.
The overall distortion of the speaker is usually determined by a measurement which includes a low-distortion audio generator and a microphone which is connected to an audio analyzer. The audio analyzer will determine the amount of higher harmonics and compare these with the main signal to compute the distortion. However, pure sine signals hardly give an accurate picture of the distortion of the wireless speaker with real-world signals. A better distortion analysis is the so-called intermodulation distortion analysis where a test tone which consists of several sine waves is used. Then the number of harmonics at other frequencies is measured.
Also, please note that many wireless speakers will experience signal distortion during the audio transmission itself. This is mostly the case for transmitters that use analog or FM type transmission. Better models will use digital transmission and transmit at 2.4 GHz or 5.8 GHz to minimize signal distortion.
Wireless speakers come in all different shapes and sizes. Deciding on the right model can often be tricky. I am going to shed some light on a commonly used term which is used to describe the technical performance of the speaker: "total harmonic distortion" or "THD". Total harmonic distortion is usually not very well understood. However, this terms is still important in terms of determining the quality of a particular model. Other terms, such as "output power" or "frequency response" are usually easier understood.
In a nutshell, THD shows the difference between the sound that is produced by the speaker versus the audio signal with which the speaker is driven. The most common ways to express distortion are percent and decibel. These two conventions can be translated into one another. The percentage shown as THD describes which amount of energy that is radiated by the speaker are higher harmonics versus the original signal. 10% would mean that one tenth is distortion. 1% would mean one hundredth etc. 10% equals -20 dB while 1% equals -40 dB.
A wireless speaker actually has several components which contribute to harmonic distortion. One of these is the built-in power amplifier. This amplifier is driving the speaker element. Usually the higher the amplifier is driven the higher the amount of amplifier distortion. For this reason, some manufacturers will list amplifier distortion depending on amplifier power.
Harmonic distortion measurements are usually made by feeding a test tone into the speaker. This tone is a pure sine wave signal with minimum distortion. The frequency of this test tone is usually 1 kHz. Distortion, however, is usually dependent on the signal frequency. Most amplifiers will show increasing distortion with increasing frequency. In particular digital class-D amplifiers will have fairly high distortion at frequencies above 5 kHz.
The second contributing factor is the loudspeaker element itself. Most speakers use a diaphragm type driver which is driven by a coil that is suspended in a magnetic field. The coil will follow the magnetic field which is controlled by the audio signal to move the diaphragm. However, this movement is not perfectly linear. This results in the signal being distorted by the speaker element itself. Also, the larger to power level with which the speaker is driven, the larger the distortion. Often speaker manufacturers will show distortion for small to moderate power levels only.
As such both the amplifier as well as the speaker element itself contribute to distortion. In addition, there are other factors which also contribute to distortion. The total amount of distortion is the sum of all of these factors. Depending on the material used to build the speaker enclosure, there will be vibrations or box resonances. These usually depend on the sound pressure level, the box shape, the enclosure material and audio frequency. Therefore additional sound distortion will be caused by the box itself.
The overall distortion of the speaker is usually determined by a measurement which includes a low-distortion audio generator and a microphone which is connected to an audio analyzer. The audio analyzer will determine the amount of higher harmonics and compare these with the main signal to compute the distortion. However, pure sine signals hardly give an accurate picture of the distortion of the wireless speaker with real-world signals. A better distortion analysis is the so-called intermodulation distortion analysis where a test tone which consists of several sine waves is used. Then the number of harmonics at other frequencies is measured.
Also, please note that many wireless speakers will experience signal distortion during the audio transmission itself. This is mostly the case for transmitters that use analog or FM type transmission. Better models will use digital transmission and transmit at 2.4 GHz or 5.8 GHz to minimize signal distortion.
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You can find further details about wireless speaker system products as well as wireless surround sound products at Amphony's website.
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