Distortion - Understanding the Specs

Every component adds some level of distortion along the audio chain, changing the original signal along the way. The simple truth is that regardless of what you spend you will never hear exactly what was heard in the recording studio or concert hall. Of course another simple truth is that the differences may be so slight you will never notice them, and it stands to reason that the higher in quality your components are the closer you will get to the original.

 

Generally, mass market audio devices will tend to add a little coloration to the music to compensate for things they don’t do well. Once you get into the realm of higher end audio components you will begin to hear real differences and a closer reproduction of the original sound.

 

Distortion: refers to any signal that appears in a reproduced sound that was not in the original recording or program source.

 

Audio reproduction is affected by distortion, and every component in the chain adds some level of it. The higher the quality of the device in the signalAudio Chain chain, the less distortion or deviation from the original. An old adage among audio enthusiasts is that a system is only as good as its weakest link, so if you spend tons of money on your amplifiers and speakers but you are streaming a lo-res file for example, the sound you get out of your system will only be as good as your streamed lo-res files.

 

So since distortion kind of determines how good our listening experience can be, let’s look at the three types of distortion that can affect an audio system.

 

Harmonic Distortion 

A harmonic is a signal that is created from a fundamental signal. Amplifier circuits tend to be the biggest culprit in the creation of harmonics in your audio chain. An amplifier may create an even order harmonic of a fundamental signal. For example, if an amplifier circuit creates a harmonic at 100 Hz, there will be additional unwanted signals at 200, 400, 800, 1600, 3200, 6400, 12800 Hz. Each harmonic gets lower in amplitude, so the harmonic at 200Hz will cause you far more grief than the harmonic at 800Hz and so on. Each instrument, especially acoustic instruments, also create their own harmonics, so oftentimes the artificial harmonic from the amplifier circuit is masked by the actual music itself.

 

Harmonic distortion can also be filtered out, as in the case of a Class D amplifier that is being used to drive a subwoofer or other LF driver. In that case, since the designer only cares about fundamental frequencies below 200Hz (for example), everything above 200Hz is filtered out (lo-pass filter) so we never hear the distortion.

 

Clipping

Clipping can also produce a tremendous amount of harmonic distortion. In the extreme, distortion caused by clipping can completely destroy the listening experience and even damage equipment (especially speakers). Clipping is caused by overloaded components, or weak components that are being asked to do more than they were designed to do. A low power amplifier driving a high-quality pair of speakers will often clip. The danger in this example is the energy that is not being used by the speaker to create sound is transferred into heat energy. Build up enough heat energy in a speaker coil and the windings will open and you’ve got yourself a dead speaker.  

 

Harmonic distortion (listed as Total Harmonic Distortion (THD)) in component specifications is expressed as a percentage of the original signal, and is typically produced in electronic components.

 

 

Intermodulation Distortion 

IM is produced when two or more tones or frequencies are produced at the same time, which is almost always the case in a musical passage. A non-linear characteristic of some component in the chain or circuit creates sum and difference frequencies which are harmonically and musically unrelated to the original tones. Because they are unrelated to the original, unlike harmonic distortion, IM is far more noticeable and objectionable.  

 

Intermodulation Distortion (IM) is also expressed as a percentage of the original signal, and is also typically produced in electronic components.

 

 

Transient Distortion 

Transient Distortion occurs when a component in the chain cannot respond quickly enough to a rapid change in the signal. This is a particular problem in loudspeakers where insufficient damping or some other design flaw creates a mechanical resonance. Even though TD is most noticeable at the loudspeaker, the loudspeaker actually depends on the amplifier to provide damping. The damping factor is the ratio of speaker impedance to the internal resistance (not impedance) in the amplifier output. A damping factor of at least 20 is your best bet.

 

Transient Intermodulation Distortion is produced in an under-designed amplifier when a time delay in the circuit’s feedback loop causes a brief overload in some other circuit element.  An example of this would be when a percussive element in a passage, such as a cymbal, becomes distorted while the strings or other instruments sound normal.

 

When you’re scoping out new equipment for your audio system, these are the three culprits of a bad listening experience. This is where buying quality components makes a huge difference in how your music sounds. You’ll spend a little more money for the engineering and design of quality components that don’t introduce noticeable amounts of distortion, but your ears and quality of life or worth it.

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Comments

  • Russ Quick
    .
    Thank you KEF for the explanation of the different varieties of distortion. While the average listener / Audiophile can't hear these individual distortions, the sum of them can be heard and thanks to KEF, can be diagnosed to build or purchase a quality system. Thanks again, Russ

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