Max SPL (dB)

The tile#

In the vented example the line reads 120.7 dB at the 32 Hz tuning, where heat sets it: the port does the work there and the cone barely moves, so the Xmax ceiling shoots up to 123.6 dB. Either side of the tuning the cone moves more and Xmax takes over.

• Open this example in the simulator and add Max SPL (dB) with Add tile. Hover the line for the level at a frequency.
• Click Limits in the tile header to draw both ceilings: the thermal ceiling dashed and the Xmax ceiling dotted. Hover with Limits on and the legend lists both ceilings, the excursion against Xmax, and the rule’s own quantity (power, voltage or coil heat) against its limit.
• The tile’s Settings choose the thermal rule and the voice coil temperature, for every enclosure in the workspace. Pin settings to side keeps them open beside the tile.
• When a setting differs from the default, a small chip beside the tile title names it, so a shared chart says how it was drawn. The settings are saved with the workspace and used by share links and your assistant.

How to read it#

• The line is the lower of two ceilings at each frequency. Where they cross it turns a corner, because the limit that sets it has changed.
• Each frequency is a steady sine at its own limit. Music spreads its energy across frequencies and shares one thermal budget, so no programme reaches every point at once, and the highest point on the line is not a broadband maximum.
• Compare it with what you ask for. Put the SPL (dB) tile beside it at the power you plan to use: where the SPL rises above this line, the design is asked for more than it can give.
• Your Input power and EQ do not move the line: they change what you send, not what the speaker can take. Listening distance, radiation space and the baffle step do move it, because they change the level at the listener.
• Port air speed is not part of Max SPL. Check the Port Velocity tile too, or use Headroom, which counts it.

Xmax#

Excursion grows with the drive, so the Xmax ceiling is the level at which the cone reaches its rated one-way excursion. Passive radiators have their own Xmax and cap the line too, and a multiple-entry horn checks every driver. Xmax applies under every thermal rule.

In a vented box or a tapped horn the cone barely moves at the tuning, because the port or horn does the work. The Xmax ceiling shoots up there and heat takes over. Either side, excursion rises quickly and Xmax usually sets the line, which is why horn curves are angular.

Three ways to read the thermal limit#

A power rating is one number of watts, but a loudspeaker is not a resistor. Its impedance rises and falls with frequency, so the same watts mean a different voltage, current and coil heat at every frequency. Each rule holds a different quantity at the limit; choose one under Thermal limit in the tile’s Settings.

The rules part most where the impedance swings most. On the vented example real input power and constant voltage differ by up to 3.6 dB; on the tapped horn below, by up to 10.4 dB.

Real input power:  I² × Re{Z} = rating × power limit
Constant voltage:  V = voltage ceiling
Coil heating:  I² × Re(T) = heat budget
Xmax:  |x| = Xmax, for every diaphragm
Max SPL = SPL at the solve + 10 log10(min(thermal ratio, Xmax ratio))

Try the rules on a tapped horn#

Horns show the rules at their most different, because their impedance swings widely. This is the Lonely.TH21-30, a tapped horn for the B&C 21DS115-8 (the designer’s thread and charts on diyAudio), drawn under each rule in your browser. Tick the designer’s Hornresp chart to compare, or match the designer’s settings: real input power at 2,160 W with a cold coil.

Under real input power at 2,160 W, the model lands within 0.4 dB of every point read off the designer’s published chart from 20 Hz to 178 Hz.

Voice coil temperature#

A hot coil has a higher resistance: for copper, Re(T) = Re × (1 + 0.00393 × (T − 20 °C)), so at 150 °C it is 51% higher. The same voltage then drives less current, and output falls most where the impedance is close to Re. This is power compression.

Set Voice coil temperature from 20 to 300 °C in the tile’s Settings. It applies under every rule and only to Max SPL; the SPL (dB) tile stays cold. At 150 °C the vented example’s line drops 1.6 dB at 100 Hz and 1.4 dB at its 32 Hz tuning. Like Hornresp’s power compression tool, it sets a temperature; it does not predict one.

With a series capacitor#

A capacitor between the amplifier and the driver uses no power, so under real input power and coil heating the line does not move: the capacitor changes the voltage the amplifier needs, not what the driver and box can take. Under constant voltage the line follows the capacitor. It falls below the capacitor’s resonance with the driver, and rises slightly around it, where the driver sees more voltage than the amplifier gives.

A multiple-entry horn scales all its amplifiers together, so a capacitor on one driver changes the balance between them, and Max SPL moves with it under every rule.

Matching Hornresp and WinISD#

If the curves still differ, compare the SPL and impedance at one fixed power first: a difference there comes from the model, not the rule. The WinISD validation compares the solver with WinISD on a public set of designs.

Limits of the model#

What to try#

Ask your AI#

Related#