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.
Looking for the perfect model from the huge amount of products, you may have a hard time understanding some of the technical language and terms that you will find in the specifications of today's wireless speakers. THD is usually not as easily understood as some other commonly used terms such as "signal-to-noise ratio" or "frequency response".
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.
Harmonic distortion inside a wireless speaker is actually the result of several components, including the power amplifier which is built into the speaker to drive the speaker element. Amplifier distortion usually depends on the amplifier output power and is sometimes specified for several power levels.
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.
Another element causing distortion is the speaker element which typically works with a diaphragm that carries a coil which is suspended in a magnetic field. The coil is moving in accordance to the change in the magnetic field which is excited by the audio signal but does not correspond 100% with the signal due to core losses and other factors. As such the result is distortion caused by the speaker element. Many manufacturers will specify harmonic distortion depending on the power level because usually the higher the speaker is driven the higher the amount of distortion.
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.
Looking for the perfect model from the huge amount of products, you may have a hard time understanding some of the technical language and terms that you will find in the specifications of today's wireless speakers. THD is usually not as easily understood as some other commonly used terms such as "signal-to-noise ratio" or "frequency response".
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.
Harmonic distortion inside a wireless speaker is actually the result of several components, including the power amplifier which is built into the speaker to drive the speaker element. Amplifier distortion usually depends on the amplifier output power and is sometimes specified for several power levels.
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.
Another element causing distortion is the speaker element which typically works with a diaphragm that carries a coil which is suspended in a magnetic field. The coil is moving in accordance to the change in the magnetic field which is excited by the audio signal but does not correspond 100% with the signal due to core losses and other factors. As such the result is distortion caused by the speaker element. Many manufacturers will specify harmonic distortion depending on the power level because usually the higher the speaker is driven the higher the amount of distortion.
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 get further details about wireless speaker system products as well as wireless surround sound products from Amphony's website.
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