7 Reasons Not To Check Your Mixes In The Car!

Beware The Car Check
By Philipp Paul Klose

We’re often advised to check mixes on a car stereo. But how useful is the fabled ‘car test’ in reality?

The idea of checking your mixes on your car’s stereo system must be one of the most often‑repeated music‑production tips. But have you ever stopped to think why it’s suggested — or even whether it’s genuinely useful? Although lots of music is consumed in cars (and looks set to be for years to come), and the change of listening environment might helpfully put you in a different frame of mind, cars don’t generally offer a great listening environment for all sorts of reasons. In fact, I’ll go as far as to say that you cannot really trust what you hear, particularly in modern cars. And what’s the point in doing a listening test if you don’t know how to interpret the results?

In this article, I’ll explain what you need to know about in‑car audio so that you have an idea of if/how to adapt your audio for in‑car listening, and what you should keep in mind if using car audio systems for mix checks.

Nonlinear In-car Audio Systems

The first thing to note is that audio systems in cars are far from linear: almost all have their bass frequencies boosted. This is partly due to unavoidable physics and partly by design. Small mixing rooms have room resonances (‘modes’), where reflected signals make it hard to judge bass notes because they either sum to make a frequency massively over‑pronounced or cancel out to a greater or lesser degree. Obviously, cars are smaller ‘rooms’ than even (most) home studios, so have significant resonances from the deepest bass notes right up to around 350‑400 Hz. In the extremes, there may be up to ±12dB of variation from the average level at a given frequency.

Unlike studio monitors, which are typically designed for a flat frequency response, the overall frequency response of the typical car audio system is deliberately skewed, losing high end by about 1.5dB/octave in many cases (Figure 1). The goal is to be easy on the ears in prolonged driving sessions, and the tilted curve is achieved by acoustic engineers through a combination of the modal bass boost of the car, the contributions of the midrange and tweeter speakers, and the ‘correction’ of their outputs using DSP filtering. But not all troughs and peaks in the frequency response can be compensated for; DSP can do a lot but it cannot fix everything.

Figure 1. The frequency response of a 2022 Audi A6 (in black). The light blue line is the average, while green is the ±3dB border. The HF response falls by 1.5dB/octave. Note the troughs too, especially at 180Hz.

When driving, the noise produced by the car through its engine, exhaust, tyres and flowing wind noise is also significant, and it doesn’t contain much high‑frequency content. So the perceived balance when travelling is different from that when the car is stationary, and the same can be said when stationary of when an engine is idling or the engine completely switched off. If your mix sounds bloated, bass heavy and even a bit dull in the car, then, especially if that’s the case when the car is stationary, this could well be due to the frequency response of the car. It’s very important that you consider this before you rush to reorganise your mix’s low end after listening in your car!

Figure 2. The purple line is an average of 10 tested in‑car audio systems. The average is not linear, tilted, and bass boosted. Green is for comparison -- a very linear Neumann KH310 studio monitor. Dark blue is driving noise at 100kph of a 2022 Audi A6; the noise inside the car is mainly in the low frequencies.

Is Your Car’s Amp...

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Published December 2022

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