Are you looking to get a pair of wireless surround sound speakers? You might be confused by all of the technical jargon used by the manufacturers to describe the performance of their products. I will clarify one commonly used term which, however, ist often misunderstood: "total harmonic distortion" or "THD".
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 short, "harmonic distortion" describes how much the audio signal is being degraded as a result of the speaker or in other words how much the signal differs from the original signal. There are two common ways to express harmonic distortion, either in percent (%) or in decibel (dB). If a speaker specifies a distortion of 10% for example then one tenth of the energy radiated by the speaker is distortion. A distortion of 10% can also be expressed as -20 dB. 1% distortion equals -40dB.
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.
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.
Total distortion is best determined by measurement. A signal generator is used which provides a highly linear sine tone to the speaker. The sound is picked up by a measurement microphone. The microphone signal is then analyzed by an audio analyzer. The audio analyzer will compute the amount of higher harmonics or distortion. Intermodulation distortion analysis is another method which gives better picture of the speaker distortion performance with real-world signals by using a test signal with two harmonics and measuring how many harmonics at different frequencies are produced by the speaker.
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.
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 short, "harmonic distortion" describes how much the audio signal is being degraded as a result of the speaker or in other words how much the signal differs from the original signal. There are two common ways to express harmonic distortion, either in percent (%) or in decibel (dB). If a speaker specifies a distortion of 10% for example then one tenth of the energy radiated by the speaker is distortion. A distortion of 10% can also be expressed as -20 dB. 1% distortion equals -40dB.
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.
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.
Total distortion is best determined by measurement. A signal generator is used which provides a highly linear sine tone to the speaker. The sound is picked up by a measurement microphone. The microphone signal is then analyzed by an audio analyzer. The audio analyzer will compute the amount of higher harmonics or distortion. Intermodulation distortion analysis is another method which gives better picture of the speaker distortion performance with real-world signals by using a test signal with two harmonics and measuring how many harmonics at different frequencies are produced by the speaker.
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 web page.
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