# Max SPL (dB) > How to read the Max SPL tile in 00 Simulator: the loudest a design can play at each frequency before Xmax or its thermal limit, and the three thermal rules. Source: https://simulator.00aud.io/docs/reference/tiles/max-spl The loudest each design can play at each frequency before a limit stops it: the cone reaching Xmax, or the driver reaching its thermal limit. It describes what the driver and box can take, so your power setting and EQ do not move it. - **Shows:** The loudest each design can play at each frequency, one sine at a time, before the cone reaches Xmax or the driver reaches its thermal limit. - **Healthy when:** The line sits above the level you want, with room to spare, across the band you will play. - **If it is not:** More cone area where Xmax sets the line, more drivers to share the power where heat sets it, or a lower level. ## The tile _Figure: The Max SPL tile for the vented example with Limits on: the Max SPL line and the two ceilings it follows, the power limit dashed and Xmax dotted._ Ciare 12.00SW in the 55 L vented example tuned to 32 Hz, half space, 1 m, with the default thermal rule. Max SPL does not depend on the input power you set. Simulated when this page was built. | Frequency | Max SPL | Power limit ceiling | Xmax ceiling | Set by | | --- | --- | --- | --- | --- | | 20 Hz | 93.7 dB | 111.3 dB | 93.7 dB | Xmax | | 32 Hz | 120.7 dB | 120.7 dB | 123.6 dB | Heat | | 50 Hz | 117.9 dB | 127.4 dB | 117.9 dB | Xmax | | 100 Hz | 122.4 dB | 122.4 dB | 125.4 dB | Heat | | 200 Hz | 121.3 dB | 121.3 dB | 137.1 dB | Heat | 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](https://simulator.00aud.io/app?template=vented) 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. Which ceiling sets the vented example’s line, from 10 to 400 Hz. | Frequencies | Set by | | --- | --- | | Below 30.9 Hz | Xmax: the cone runs out of travel | | 30.9 Hz to 34.8 Hz | Heat: the power limit | | 34.8 Hz to 86.6 Hz | Xmax: the cone runs out of travel | | Above 86.6 Hz | Heat: the power limit | ## 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](https://simulator.00aud.io/docs/reference/tiles/port-velocity) tile too, or use [Headroom](https://simulator.00aud.io/docs/reference/tiles/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. > **Xmax is a distortion reference** Makers define Xmax in different ways, and passing it is not a damage point. [Klippel’s assessment of Xmax](https://www.klippel.de/fileadmin/klippel/Files/Know_How/Literature/Papers/Assessment_of_Voice_coil_peak_displacement_XMAX_02.pdf) explains why the definition matters. ## 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**. | Rule | Matches | Holds at the limit | Vented example | Lonely.TH21-30 horn | | --- | --- | --- | --- | --- | | Real input power (the default) | Hornresp | The real power going into the driver, at its rating. The voltage climbs at every impedance peak. | 2,000 W | 1,700 W | | Constant voltage | WinISD | One amplifier voltage at every frequency, by default √(rating × Re) per driver. | 81.2 V | 93.1 V | | Coil heating | An estimate | The heat in the voice coil, at a budget re-created from the driver’s own rating test. | 1,029 W of heat | 739 W of heat | 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, the default This rule holds the electrical power going into the driver at its rating, the rule Hornresp’s maximum SPL tool uses. Where the impedance is high the driver draws little current, so reaching the rated power takes a high voltage: the thermal ceiling rises at every impedance peak, and Xmax usually caps it there. **Power limit** sets the limit as a percentage of each driver’s rating, from 5 to 1,000%, so one setting suits every enclosure in a workspace. To match a chart drawn at another power, set it to that power over the rating: the Lonely.TH21-30’s designer used 2,160 W, 127% of the 1,700 W rating. In a multiple-entry horn, a driver moved by the others is charged at least its own coil heat. ### Constant voltage This rule holds one amplifier voltage at every frequency, WinISD’s convention. Left blank, **Voltage ceiling** is √(rating × Re) for each enclosure, the rated power into the coil’s resistance. Type your amplifier’s voltage, from 0.1 to 2,000 V, to see what that amplifier can do with every enclosure. The result is an amplifier envelope rather than a driver limit: at the impedance peaks the driver could take much more. ### Coil heating, an estimate What damages a voice coil is heat, the current squared times the coil’s resistance. Power that moves the cone and the air is not coil heat, so this rule credits a design, a horn above all, that turns more of its input into sound. A power rating is not the heat a coil can shed. A rating counts watts as the test voltage squared over the lowest impedance in the test band, while the pink noise spreads across frequencies where the impedance is well above that minimum, so the coil takes much less than the rating says. The tile re-creates the test in the model, driver in free air and coil cold, to estimate the heat the driver survived. That is the **Coil heat budget**, 100% by default. B&C 21DS115-8, rated 1,700 W: coil heat in its re-created rating test, by **Rating test band**. | Assumed test band | Coil heat | Share of the rating | | --- | --- | --- | | 20–200 Hz | 592 W | 35% | | 30–300 Hz (default) | 739 W | 43% | | 50–500 Hz | 812 W | 48% | Drivers do not store their test band, so the band is an assumption. If you have better data, scale the budget as a percentage. The rule stays an estimate because the real limit is temperature, not heat, and cooling depends on how the cone moves and air flows past the coil. Use it to compare designs, not as a rating. ### The rules as formulas ``` 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)) ``` I is the coil current, Z the driver’s impedance and Re{Z} its real part. The model is linear, so one solve per frequency scales to every limit: a drive ratio s multiplies power and heat by s, and voltage and excursion by √s. ## 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](https://www.diyaudio.com/community/threads/lonely-th21-30-21-30hz_tapped-horn-b-c-21ds115-8.439970/) 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. _Figure: An interactive Max SPL chart of the Lonely.TH21-30 tapped horn: choose a thermal rule, a power limit and a voice coil temperature, and compare with the designer’s Hornresp chart._ | Frequency | Real input power | Constant voltage | Coil heating | | --- | --- | --- | --- | | 30 Hz | 128.8 dB | 128.7 dB | 125.3 dB | | 35 Hz | 133.3 dB | 129.9 dB | 130.2 dB | | 43 Hz | 132.3 dB (Xmax) | 129.2 dB | 132.3 dB (Xmax) | | 55 Hz | 135.2 dB | 130.8 dB | 132.5 dB | | 91 Hz | 140.1 dB | 130.3 dB | 139.9 dB | | 121 Hz | 138.5 dB | 136.0 dB | 136.4 dB | | 178 Hz | 142.3 dB | 137.7 dB | 143.1 dB | Lonely.TH21-30 tapped horn with a B&C 21DS115-8 (rated 1,700 W, Xmax 16.5 mm), half space, 1 m, at the default settings. “(Xmax)” marks where Xmax, not the thermal limit, sets the level. 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 | To match | Thermal limit | Then | | --- | --- | --- | | A Hornresp maximum SPL chart | Real input power | Set **Power limit** to the chart’s power as a percentage of the rating. | | A WinISD maximum SPL chart | Constant voltage | Leave **Voltage ceiling** at √(rating × Re). | 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](https://simulator.00aud.io/validation/winisd) compares the solver with WinISD on a public set of designs. ## Limits of the model > **Read it as a ceiling to stay under** The model is linear. Motor force, suspension and ports change at high levels, Xmax is a distortion reference, and a hot coil compresses the output. Amplifier voltage and current limits belong to the [Headroom](https://simulator.00aud.io/docs/reference/tiles/headroom) tile. ## What to try | Try | What it changes | | --- | --- | | [Turn on Limits](https://simulator.00aud.io/docs/reference/tiles/max-spl#how-to-read-it) | Click **Limits** in the tile header to draw the thermal ceiling dashed and the Xmax ceiling dotted. The line follows whichever is lower. | | [Check the thermal rule](https://simulator.00aud.io/docs/reference/tiles/max-spl#thermal-rules) | The tile’s **Settings** choose how the power rating becomes a ceiling: real input power as in Hornresp (the default), constant voltage as in WinISD, or coil heating. | | [Compare with what you ask for](https://simulator.00aud.io/docs/guides/power-and-limits) | Max SPL does not move with **Input power** or EQ. Put the **SPL (dB)** tile beside it: where your SPL crosses the line, the design is asked for more than it can give. | ## Ask your AI **Ask your AI** Your assistant reads the same Max SPL, with your workspace’s thermal rule, through the 00 Simulator connector. > Using 00 Simulator, simulate every enclosure in my open workspace and tell me its Max SPL at 30, 50 and 100 Hz, whether Xmax or the thermal limit sets it at each, and how far its SPL at the set power sits below it.