Distortion Types: Harmonic vs. Intermodulation
By 2017, the conversation around distortion in car audio had evolved beyond a single number on a specification sheet. With advanced measurement tools and DSP-driven systems becoming standard, distortion was no longer treated as a general flaw; it was understood as a set of distinct behaviours, each with its own cause and audible effect.
Two types in particular became central to both engineering and listening: harmonic distortion and intermodulation distortion.
Harmonic Distortion: The Predictable Addition
Harmonic distortion occurs when a speaker adds extra frequencies that are mathematically related to the original signal. These are called harmonics, and they follow a predictable pattern. In moderate amounts, harmonic distortion is often less offensive to the ear. In fact, certain types, particularly lower-order harmonics, can even be perceived as warmth or richness. This is why harmonic distortion, while measurable, is not always immediately objectionable. However, as levels increase:
- Detail begins to blur
- Tonal accuracy suffers
- The original signal becomes masked
From an engineering standpoint, harmonic distortion is relatively straightforward to measure and analyse. It was the primary focus of early audio testing, including the methods used in the late 1970s when our journey began.
Intermodulation Distortion: The Unpredictable Problem
Intermodulation distortion (IMD) is more complex and more problematic. Instead of adding predictable harmonics, IMD occurs when multiple frequencies interact and create entirely new, unrelated frequencies. These frequencies were never part of the original recording. What makes IMD particularly damaging is that it:
- Does not follow a musical pattern
- Introduces dissonant artifacts
- Masks detail in a way the ear finds unnatural
What you hear:
- Vocals losing clarity
- Instruments blending unnaturally
- A general sense of “confusion” in complex passages
Unlike harmonic distortion, IMD becomes more noticeable as music becomes more complex, which is to say, most real-world listening.
Why This Matters in a Car
Car audio systems operate under constant stress:
- Multiple frequencies are playing simultaneously
- High output levels required to overcome road noise
- Limited speaker placement and enclosure volume
These conditions make intermodulation distortion particularly relevant. A system may measure well in harmonic distortion at a single tone, yet still sound unclear when playing music because IMD is not being controlled.This is why, by 2017, serious system design had moved toward multi-tone testing and real-world signal analysis, rather than relying solely on traditional THD figures.
Where Distortion Comes From
Both types of distortion originate from similar physical limitations:
- Motor nonlinearity (uneven magnetic force)
- Suspension stiffness at high excursion
- Thermal compression under load
- Inductance variation in the voice coil
The difference lies in how these nonlinearities interact with the signal. A single tone reveals harmonic distortion. Multiple tones reveal intermodulation distortion. And music, of course, is never a single tone.
Our Approach at Austin Audio
From the beginning, our focus has been on eliminating measurable distortion in real conditions, not just in controlled tests. What began in 1978 as a response to audible distortion inside a vehicle has evolved into a multi-layered engineering process, one that now includes:
- Driver design optimised for linear excursion
- Precision motor symmetry
- DSP calibration to manage system interaction
For example, in our modern systems, we do not rely solely on low THD figures. We evaluate how components behave under complex signals, ensuring that intermodulation effects are minimised alongside harmonic distortion.
Because in practice, a system that performs well on paper but struggles with real music is not performing well at all.