Audio distortion is one of those topics that has been so perjured in the last couple decades that most engineers have either ignored the specifications on their equipment or just had their eyes glaze over as they were given the data. The real question is how much distortion is too much? And the second question is, does a much lower distortion specification really justify the extra cost or other performance compromises such as power efficiency or power output?
Audio Distortion Criteria
In the early days of electronics and audio reproduction, it was not a case of the perception of distortion, but how much audible distortion could you bear to get maximum loudness before the vocal intelligibility suffered. In these vacuum tube powered days, 5% total audio distortion was the standard value applied to audio power amplifiers well into clipping. I still remember Fender guitar amplifiers and early PA systems providing power rating based on the 5% distortion criteria in the 1960s and early 1970s. But as audio high-fidelity (hi-fi) became a craze in this time, manufacturers began using the new solid-state electronics and shooting for near total elimination of audio. Thus, began the specsmanship battles in the 1970s and 1980s of “how low can you go” in rating audio distortion.
The industry smartly used two testable methods of determining audio distortion. The first and most popular is the percent of Total Harmonic Dis-tortion (THD), with ratings typically made near maximum power output of the amplifier. The second distortion test is the Inter-Modulation Distortion (IMD) rating. IMD measurements were made using two audio sinusoidal sources (e.g. 400 Hz and 1 kHz) just below full power capabil-ity, and noting the relative intensities of product (sums and differences) of spectral distortions.
For example, a sum distortion spike of 400 Hz and 1 kHz test tones would be 1400 Hz, and the difference would be at 600 Hz. The IMD ratings were more truthful at determining how much smearing of random audio signals a piece of audio gear produced. Where as THD measurements tended to be truth tellers of the real audio power output limits because designers could hit any distortion number depending how much on the verge of clipping they chose. See Figures 1 and 2 showing spectrum plots for the two distortion measurements.Then there is the real truth on THD ratings, as they are an instantaneous ratio of clean test tones out to distortion harmonics. Most audio power amplifiers today can easily hit 0.1% THD or way better when operating around 80% of power output.
But the same test at 1 or 10 watts might show THD numbers way worse. This is because lower-power outputs do not reduce the audio harmonics in the same proportion. So, the reality is that most live audio gear is run more in the lower-power regions than near max output headroom, where most of the rating measurements are made. Today, it is very common to see mixers, signal processing and power amps with THD ratings in the 0.01% to 0.001% areas because that is much more impressive from a marketing perspective.
Humans and Audio Distortion
Another question is “How well can you hear audio distortion?” The answer is not the same for everyone, but we have general rules of thumb as audio gear designers. If you were critically listening to a sinusoidal test tone and could gradually introduce clipping to the point you could hear the harmonics with the fundamental tone, most people can discern distortion around 0.1% Total Harmonic Distortion. But, if you had random music or speech to listen to, your distortion perception diminishes to around 1% THD. I have personally done these tests and the amounts are correct.
So, now as designers of audio gear, how much of that 0.1% or 1% THD budget do you want to eat up on any piece of gear like amplifier, mixing consoles or other signal processing? The real answer is that you shoot for reasonable design practices based on historical results and pretty much take what the final measurements give you. If you get near the 0.1% THD mark, you may take alternative design approaches that clean up the signal performance. After all, there are competitive designs out there, and you do not want to be the worst piece of gear in the signal path.
The Reality Show
But, and it is a big but, overall audio distortion does not come in itty-bitty amounts along the signal chain. The really big offenders in the audio sig-nal path are generally not the electronics if the engineer is doing a good job in gain structure. The truly big offenders are the “transducers” in the audio signal path. Theses transducers are the microphones and speakers that do the sound pressure to electrical, and electrical to sound pressure conversions. And these transducers are where most of the audio distortion compromises are made, and force the remaining electronics to function with as little distortion as possible.
Obviously, microphones can be pushed by excessive sound pressure levels into distortion. And in many cases today, a screaming vocalist is actually seeking audible distortion to reinforce his/her emotional communication. But other than excessive SPL input, most microphones can be low-distortion contributors when applied properly. And today, we have abundant choices in mic transducer technology to focus on the appropriate audio sound generators; like condenser mics for quiet female vocalists or large diaphragm dynamic mics for loud kick drums.
The real distortion problem child is our mains speaker choices. I am sure most of us have heard distorted HF drivers, buzzy mids and wobbly sub-woofers. Even the best speaker transducers out on the live sound market may have a percent or more of audio distortion when pushed at average capabilities. Can you do something about it? Not much, considering every brand of speaker and the many models available to solve various sound reinforcement applications. The obvious answer is headroom, headroom, headroom! In other words, given no other impediment like space for extra speakers, the cost to purchase extra speakers and the extra amplifiers to power extra speakers; having plenty of speakers pushed well below their peak SPL capabilities is the lowest-distortion solution.
But in the real world, we do have limits in our speaker choices, and we do wish to use every last bit of speaker capability to save overall costs to perform the gigs asked of us. So, occasional audio distortion in live sound reinforcement will be our companion and our reality check. But purchasing quality speakers may be your best way of delivering the cleanest, lowest distortion sound reinforcement.
You can find Mark at marka@fohonline.com.
