Are you in the market for new speakers? There are literally hundreds of speaker models available in the market today. Have you ever wondered what some of the technical jargon means that manufacturers use to specify the performance of good-quality outdoor speakers? In this article, I will explain one frequently used term: THD or "total harmonic 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.
However, be careful that there are actually several components which cause harmonic distortion. Wireless speakers as well as any kind of active speaker or active subwoofer all have built-in power amplifiers to drive the speaker element. The amplifier itself will have a certain amount of distortion. Typically the distortion of the amplifier will be higher the more output power it supplies to the speaker. Normally manufacturers will specify amplifier distortion based on a certain amount of power, usually a lot less than the rated maximum amplifier output power.
Having amplifier distortion specs for several output power levels gives a better indication of the amplifier distortion performance.Also, please note that distortion typically is measured for a specific test tone frequency. Typically a 1 kHz sine wave tone is used during the measurement. This allows comparing equipment from different manufacturers. However, distortion usually varies with different frequencies. Many digital amplifiers will show increasing distortion with higher frequency which cannot easily be discovered by looking at the spec sheet.
The second contributing factor is the loudspeaker element itself. Most speakers use a membrane 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 extent depending on the sound pressure level. These vibrations will also be non-linear in nature and contribute to distortion.
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.
Another factor contributing to distortion is the signal transmission of wireless speakers, especially with models that transmit an analog signal at 900 MHz. More advanced models use digital audio transmission. Typically these transmitters work at 2.4 GHz or 5.8 GHz.
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.
However, be careful that there are actually several components which cause harmonic distortion. Wireless speakers as well as any kind of active speaker or active subwoofer all have built-in power amplifiers to drive the speaker element. The amplifier itself will have a certain amount of distortion. Typically the distortion of the amplifier will be higher the more output power it supplies to the speaker. Normally manufacturers will specify amplifier distortion based on a certain amount of power, usually a lot less than the rated maximum amplifier output power.
Having amplifier distortion specs for several output power levels gives a better indication of the amplifier distortion performance.Also, please note that distortion typically is measured for a specific test tone frequency. Typically a 1 kHz sine wave tone is used during the measurement. This allows comparing equipment from different manufacturers. However, distortion usually varies with different frequencies. Many digital amplifiers will show increasing distortion with higher frequency which cannot easily be discovered by looking at the spec sheet.
The second contributing factor is the loudspeaker element itself. Most speakers use a membrane 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 extent depending on the sound pressure level. These vibrations will also be non-linear in nature and contribute to distortion.
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.
Another factor contributing to distortion is the signal transmission of wireless speakers, especially with models that transmit an analog signal at 900 MHz. More advanced models use digital audio transmission. Typically these transmitters work at 2.4 GHz or 5.8 GHz.
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You can get further details concerning decent wireless speaker models as well as affordable wireless surround sound speakers from Amphony's web page.
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