Loudspeaker non-linearity

Why low volume testing isn't really testing

Some bassists believe that with experience you can tell how a cab will perform at a gig based on listening to it at lower SPL. To a degree this is true because a well engineered cab should exhibit fairly linear performance when working comfortably within its limits. However (and this is a big however), with the modern penchant for both downsizing your bass cabs but using greater power to drive them, it is often the case that a bass cab is being pushed out of its comfort zone and into the zone of increasing non-linearity in performance.

The non-linearities of a loudspeaker

So how does this non-linearity affect things? Well to put it as simply as possible the cab exhibits increasing distortion with increasing SPL which means the tone you hear at lower volumes is very different to the tone at higher volumes. What causes this distortion?

1. Cone over-excursion

As the cone is asked to move beyond Xmax the speaker motor system loses proper control of the cone, resulting in reduced lows, increased compression, worse transient response and greater harmonic distortion.

2. Voice coil temperature increasing

As the voice coil gets hotter and hotter the actual motor characteristics change resulting in less depth to the lows and more mid-bass boom, and meanwhile the increase in voice coil heat also increases the voice coil resistance and thus the total impedance the amp is driving, which means your bass rig gets less powerful as the night goes on.

3. Port air movement reaching too high a speed

The airspeed within the ports becomes so high that the port no longer acts like an ideal port, so the amount of extra lows it contributes to the tone becomes less and less, and in the absolute worse case scenario the port's phase response shifts so much that its contribution to the lows becomes subtractive rather than additive!

Is there a solution?

All the Barefaced models are designed to suffer less from these tonal changes at high SPL by having high excursion drivers which will reach a greater SPL before exceeding Xmax, large voice coils with extensive motor heatsinking to minimise voice coil temperature increases and large ports which remain linear up to Xmax. Furthermore the woofer alignments we have chosen may not sound hugely fat or thick in the lows at shop SPL but these low Q designs mean that even when they're being pushed hard the transient response remains good and the lows do not become overwhelmed by mid-bass boom.

That doesn't sound like a perfect solution though!

Exactly - every loudspeaker will run into these problems if you push it hard enough. The only real solution is to test the specific speaker cab at a demanding gig - if it performs for you in that situation then you can forget the science and be confident that your new cab does what it needs to do when it really matters!


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