duty , typically in reference to a constant load .
THD ( Total Harmonic Distortion ) As we ’ ve seen previously , it ’ s the harmonic content of the waveform that shapes the timbre ( tonal character ) of the sound . An accurate amplification of a signal is linear , meaning the amplified waveform is directly proportional to the source waveform . When plotted on an X-Y graph , where X = time and Y = amplitude , all points on the source waveform are multiplied by the same number to achieve the amplified rendition .
THD is essentially the addition of harmonic content during the amplifying process . It is expressed as a percentage comparing the energy of the harmonic additions to the energy of the original waveform ( power of the harmonics [ P1 ] ÷ power of the original waveform [ P2 ]). Imagine delivering a pure sine wave to the power amplifier input , then comparing the actual amplified waveform with the theoretically linearly amplified waveform . From our previous look at phase relationships , we know that combining a waveform with an identical waveform 180 degrees out of phase results in silence . When the original waveform is combined with the amplified waveform at the same fundamental amplitude , then combined with the original waveform 180 degrees out of phase , the resulting waveform should contain any distortions that occurred within the circuit , but none of the original signal . Simplistically , it is this resulting power that is compared to the original sine wave to achieve the THD rating .
The lower the THD , the better . It is commonly believed that the human ear can ’ t detect distortions less than about two percent but taking into account the cumulative nature of audio signals and their anomalies , a low THD rating is important .
Total harmonic distortion less than one percent is considered acceptable . Bear in mind that the THD rating should be specified in reference to operating sensitivity , impedance , and frequency . A typical THD rating might be displayed as THD < 0.05 % @ 4 ohms , + 4 dBm , 20 Hz – 20 kHz .
IMD ( Intermodulation Distortion )
Intermodulation distortion ( IM or IMD ), usually the result of poor amplifier design , is typically less subtle than harmonic distortion . Whereas harmonic distortion is essentially a rebalancing of the naturally existing harmonic content , IMD adds previously nonexistent content . These distortions are typically the sum and difference of frequencies that are present in the original audio waveform .
Because it is not harmonically related to the original waveform , intermodulation distortion is typically more noticeable — and generally perceived as more harsh and grating — than THD . IMD produces higher levels of ear fatigue in comparison to a similar amount of THD .
SPEAKER SENSITIVITY The sensitivity rating quantifies the speaker ’ s ability to output energy when a specified voltage and frequency band are supplied at its input . Typical sensitivity ratings assume the source , delivered to the speaker or cabinet input , is a 1-kHz tone at one watt ( 2.83 volts @ 8 ohms ). It also assumes that a calibrated microphone is placed one meter from — and directly in
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