Technology

Intro

I like to think of a sound system as a chain of devices through which the sound travels before making it to your ears. It starts from the recording studio and ends with you. One of the links in this chain is your loudspeaker. In contrary to many of the devices in this chain, there has not been a great breakthrough in speaker technology for a long time. Although, there has been much improvement, there hasn't been a fundamental change -one that improves the bottom line- in how speaker operate internally. This means that there are still many substandard loudspeakers out there, and these days speakers are most frequently the weakest link in the chain.

The two primary aspects of a loudspeaker are its frequency response (FR) and harmonic distortion (HD) performance. Frequency response is the more well-known measure, and there are many resources available to help you understand it. Harmonic distortion, however, is more complicated and gets far less attention than it deserves, even though it is arguably more important than frequency response. Therefore here I am going to focus on harmonic distortion.

What is Harmonic Distortion?

When fed with a sine wave of frequency f, an ideal speaker system should only produce an output of frequency f. However, in real world, speakers produce f, plus frequency components that are integer multiples of f ( 2xf, 3xf, 4xf, 5xf, 6xf. ). This is the result of various non-linearities in the drivers. Each of these frequency components leaves a different signature on the sound.

What does Harmonic Distortion sound like?

Harmonic distortion makes a spaced out acoustic guitar, percussion, and vocals sound fuller and more vibrant. That is why you hear those kind of tracks at stores when they are trying to show off their speakers. However, when the material contains more than a handful of acoustic instruments and vocals, the sounds start masking each other and you start to lose the details. In worse cases all you hear is this mixture of indistinguishable instruments. In general, high levels of HD might sound full and lively at first, but it will cause ear fatigue pretty soon. That's why a 15 minute audition at the store is never enough to judge speakers.

Furthermore, frequently for various reasons, speakers with higher HD tend to sound louder during A/B testing in store. Louder speakers also trick the listener into thinking that they sounds more dynamic and lively; however, the exact opposite could be true.

What causes HD?

Poor HD performance is often the result of manufacturers going for cheap drivers. Good drivers usually have a number of components in their motor structure that help reduce non linearaties which cause HD. Those components are often missing from cheaper drivers that employ a motor design that is simpler and cheaper to manufacture.
Designers can always tweak the design to get an acceptable FR. What makes harmonic distortion different from frequency response is that the designer cannot do anything to improve harmonic distortion once the drivers are chosen.

As a general rule if you are immediately impressed with a pair of speakers, the chances are there is something wrong with them. It sounds counter intuitive, but speakers that impress you immediately usually color the sound and add their own character, and thats not what speakers should do. Speakers should be pure, adding nothing of their own, because when they add something of their own, what they add, masks what is originally in the material.