Limits: travel, heat and air

Travel: Xmax, Xmech and Xdamage#

At 200 W the vented example passes its 13.5 mm Xmax below 18.7 Hz, reaching 21.2 mm. At the 32 Hz tuning the port does the work and the cone moves only 2.5 mm. Below the tuning the box stops holding the cone, which is why a high-pass there helps: with a 25 Hz high-pass the peak falls to 6.3 mm.

Passive radiators have their own Xmax, on the PR Excursion tile, and it caps Max SPL too. A passive radiator can run out of travel while the driver is still inside its own.

Heat: Pe and the thermal rules#

Pe is the power the maker rates the driver for, 2,000 W for the Ciare 12.00SW. What damages a voice coil is heat, and a loudspeaker is not a resistor: its impedance swings with frequency, so the same watts mean a different voltage, current and heat at every frequency. The Max SPL tile reads the rating one of three ways, chosen under Thermal limit in its settings:

The coil heating budget is well under the rating because a power rating counts watts against the lowest impedance in its test band, while most of the band sits at higher impedance. Voice coil temperature (20–300 °C) raises Re for every rule: at 150 °C a copper coil’s resistance is 51% higher, which lowers the output where the impedance is close to Re. It applies to Max SPL only; the SPL tile stays cold.

Turn on Limits in the Max SPL tile’s header to draw both ceilings; the line is the lower of the two. In this box it follows Xmax below 30.9 Hz, heat from 30.9 to 34.8 Hz, Xmax from 34.8 to 86.6 Hz and heat above 86.6 Hz: heat takes over around the tuning, where the cone barely moves. Your input power and EQ do not move the line: they change what you send, not what the speaker can take. Port air speed is not part of Max SPL.

Air: port velocity#

Air through a port gets noisy when it moves too fast: it chuffs and whistles. The Port Velocity tile draws its peak speed at each frequency against a dashed warning line, 17 m/s by default. At 200 W the vented example’s port peaks at 28.4 m/s near 30.1 Hz, over the line. Air speed follows the square root of power: at a quarter of the power, 50 W, it peaks at 14.2 m/s.

• Change the warning line with Port velocity under Settings › Simulation…, from 10 to 30 m/s. The app’s hint puts the typical limit at 15 to 20 m/s, higher for flared ports.
• More port area lowers the speed; the port then gets longer to keep the tuning. Fix a noisy port walks through it.
• A sixth-order bandpass box has a second port, on the Port Velocity (Rear Chamber) tile.

All three at once: Max SPL and Headroom#

In this example the port is the tightest limit in the band the box plays: at 200 W it is over its line at the tuning, while the cone passes Xmax only below 18.7 Hz, beneath the box’s 30.3 Hz F3, and the power is 10% of Pe. Another design can be the other way round, which is why two tiles combine the limits:

• Max SPL (dB) shows the loudest each design can play at each frequency, a sine at a time, before Xmax or the thermal rule stops it. It ignores port air speed.
• Headroom starts from a listening target, an average level plus a crest factor for the peaks (85 dB and 12 dB by default), and colours each frequency Clean, Threshold or Over max against all the limits, port air speed included.

Ask your AI#

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