Have you ever wondered what some of the technical jargon means that manufacturers use to specify the performance of wireless speakers? In this article, I will explain one frequently used term: THD or "total harmonic distortion".
It is often difficult to pick a suitable set of wireless speakers given the large amount of products. Aside from the looks, you will often be confronted with having to study some of the technical specifications. Some of these are probably relatively easy to understand such as "output power" or "frequency response". However, a term which is not as easily understood is the term relating to how much distortion the speaker has.
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.
Distortion specs for different power levels are usually given for several power levels or as a diagram showing distortion versus output power. Both of these methods allow to better evaluate the quality of the amp.Normally distortion is measured with a 1 kHz test tone. However, amplifier distortion will usually increase with increasing frequency, especially in digital class-D models.
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.
The overall distortion of the speaker thus is the sum of the amplifier distortion and the speaker element distortion. In addition, there are other contributing factors. The enclosure of the speaker will vibrate to some extend depending on the sound pressure level. These vibrations will also be non-linear in nature and contribute to distortion.
To find out the overall distortion of a loudspeaker, a signal generator is used which provides an ultra-linear signal to the speaker as well as a measurement microphone which is connected to an audio analyzer to determine the amount of harmonics emitted by the speaker. Another distortion measurement technique which gives a better analysis of the speaker performance with real-world signals is called interharmonic distortion. This method outputs two signals at different frequencies and measures the amount of harmonics at different frequencies.
Wireless speakers will also have some amount of distortion during the audio transmission. The amount of distortion will depend on the type of wireless transmission method as well as the quality of components. Usually 900 MHz FM transmitters have among the highest amount of distortion. Digital audio transmission on the other hand offers the least amount of audio distortion. The signal is digitized inside the transmitter and not affected during the transmission itself. Digital wireless audio transmitters are available at 2.4 GHz and 5.8 GHz.
It is often difficult to pick a suitable set of wireless speakers given the large amount of products. Aside from the looks, you will often be confronted with having to study some of the technical specifications. Some of these are probably relatively easy to understand such as "output power" or "frequency response". However, a term which is not as easily understood is the term relating to how much distortion the speaker has.
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.
Distortion specs for different power levels are usually given for several power levels or as a diagram showing distortion versus output power. Both of these methods allow to better evaluate the quality of the amp.Normally distortion is measured with a 1 kHz test tone. However, amplifier distortion will usually increase with increasing frequency, especially in digital class-D models.
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.
The overall distortion of the speaker thus is the sum of the amplifier distortion and the speaker element distortion. In addition, there are other contributing factors. The enclosure of the speaker will vibrate to some extend depending on the sound pressure level. These vibrations will also be non-linear in nature and contribute to distortion.
To find out the overall distortion of a loudspeaker, a signal generator is used which provides an ultra-linear signal to the speaker as well as a measurement microphone which is connected to an audio analyzer to determine the amount of harmonics emitted by the speaker. Another distortion measurement technique which gives a better analysis of the speaker performance with real-world signals is called interharmonic distortion. This method outputs two signals at different frequencies and measures the amount of harmonics at different frequencies.
Wireless speakers will also have some amount of distortion during the audio transmission. The amount of distortion will depend on the type of wireless transmission method as well as the quality of components. Usually 900 MHz FM transmitters have among the highest amount of distortion. Digital audio transmission on the other hand offers the least amount of audio distortion. The signal is digitized inside the transmitter and not affected during the transmission itself. Digital wireless audio transmitters are available at 2.4 GHz and 5.8 GHz.
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You can get further details about wireless speakers as well as wireless surround sound products at Amphony's web page.
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