Protecting Loudspeakers: Using Limiters To Enhance LF While Keeping Things Under Control

The input sensitivity required to fully drive this amplifier with a sine wave is 2.0 Vrms (2.83 Vpeak).

To keep the amplifier from clipping we need to set the peak threshold of our limiter to +9.0 dBV. This corresponds to the peak voltage of the amplifier input sensitivity.

When the amplifier output is at 89.4 Vpeak the loudspeaker displacement response would be close to that shown in Figure 3 if it had no inherent displacement limitation. We can see that the displacement would exceed 5.0 mm below 49 Hz and from 62-88 Hz.

This indicates we have to increase the signal level presented to the limiter side chain in these frequency regions.

The gain computer of the limiter will then see higher signal levels at these frequencies.

This will cause the limiter to reduce the signal level only when these frequencies are high enough to warrant it. The rest of the time the signal is unaltered.

Figure 3: Scaled peak displacement of a woofer in a vented enclosure with 63.2 Vrms swept sine input. Click to enlarge.

The displacement response is now displayed in dBV (corresponding to the RMS voltage output of the LP201 accelerometer preamp) in Figure 4.

We use the RMS value instead of the peak value here because we will be comparing this curve to measurements of some filters.

The EASERA measurements displayed measurements based on RMS values unless otherwise scaled. Since our measurement signal is a sine wave this all works out nicely.

Coincidentally, -10 dBV corresponds almost exactly to the 5.0 mm peak displacement shown in Figure 3. Anything above -10 dBV needs to be limited to -10 dBV in order for the displacement not to exceed 5.0 mm. Several filters are set up to yield the transfer function also shown in Figure 4.

The signal that will be presented to the side chain of the limiter will pass through these filters.

Figure 4: RMS displacement of a woofer in a vented enclosure (blue) and filter transfer function for limiter side chain (red). Click to enlarge.

The signal flow for this is shown in Figure 5 (below). The filters used are a second order shelving filter (low pass and mixer), a first order shelving filter and a 3-band parametric EQ.

These filters will increase the level of the signal in a manner corresponding directly to the desired frequency dependent limiting in order to control the woofer’s displacement.

The only thing remaining is to set the threshold of the limiter and measure the displacement of the woofer with the new limiter processing in place.

Since I want to show only the effects of the limiter and not any potential non-linearity in the suspension or motor of the woofer in decreasing the woofer’s displacement, I will not test at 89.4 Vpeak.