{
  "id": "subwoofer-box",
  "version": "04d0000436b3",
  "status": "published",
  "name": "Subwoofer Box Calculator",
  "question": "What is my subwoofer box volume?",
  "summary": "Finds the net inside volume of a sealed or ported subwoofer box from its outside size, wall thickness and driver displacement, and the port length that tunes a ported box to a frequency.",
  "category": "physics",
  "subcategory": "mechanics",
  "url": "https://www.acalculator.org/physics/subwoofer-box-calculator",
  "markdown": "https://www.acalculator.org/physics/subwoofer-box-calculator.md",
  "kind": "function",
  "method": "Inside size = outside − 2 × wall; net volume V = inside width × height × depth − driver displacement. Port: L_eff = c² × A ÷ (4π² × f² × V), with A the total port area and c = 343 m/s; port length = L_eff − k × D.",
  "assumptions": [
    "Rectangular box with all six walls the same thickness; bracing and the port tubes are not subtracted (the port volume is shown so you can add it).",
    "Speed of sound 343 m/s (air at 20 °C). Round ports; several ports act as one port of their total area.",
    "The end correction k × D (default 0.732, one flanged and one free end) is the usual design value; real boxes may tune a few hertz off."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "type": {
        "title": "Box type",
        "description": "A sealed box, or a ported (vented) box tuned by a round port.",
        "type": "string",
        "enum": [
          "sealed",
          "ported"
        ]
      },
      "w": {
        "title": "Outside width",
        "description": "The box’s outside width.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "length",
        "minimum": 0.001,
        "maximum": 10
      },
      "h": {
        "title": "Outside height",
        "description": "The box’s outside height.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "length",
        "minimum": 0.001,
        "maximum": 10
      },
      "d": {
        "title": "Outside depth",
        "description": "The box’s outside depth, front to back.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "length",
        "minimum": 0.001,
        "maximum": 10
      },
      "t": {
        "title": "Wall thickness",
        "description": "The thickness of the panels; 3/4 in (19 mm) MDF is common.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "length",
        "minimum": 0,
        "maximum": 10
      },
      "disp": {
        "title": "Driver displacement",
        "description": "The volume the subwoofer itself takes up inside the box, from its spec sheet; 0 if unknown.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "volume",
        "minimum": 0,
        "maximum": 10
      },
      "fb": {
        "title": "Tuning frequency (Hz)",
        "description": "The frequency the port tunes the box to, in hertz; 30 to 40 Hz is common for subwoofers.",
        "type": "number",
        "minimum": 5,
        "maximum": 200
      },
      "pd": {
        "title": "Port diameter",
        "description": "The inside diameter of a round port tube.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "length",
        "minimum": 0.001,
        "maximum": 10
      },
      "np": {
        "title": "Number of ports",
        "description": "How many identical round ports.",
        "type": "integer",
        "minimum": 1,
        "maximum": 4
      },
      "k": {
        "title": "End correction factor",
        "description": "Added acoustic length per port diameter: 0.732 for one end flush with the baffle and one free end.",
        "type": "number",
        "minimum": 0,
        "maximum": 1
      }
    }
  },
  "outputs": {
    "net": {
      "label": "Net volume (ft³)",
      "description": "Inside volume minus the driver displacement, in cubic feet.",
      "format": "number"
    },
    "netL": {
      "label": "Net volume (liters)",
      "description": "The net volume in liters.",
      "format": "number"
    },
    "gross": {
      "label": "Inside volume (ft³)",
      "description": "Inside width × height × depth, before the driver.",
      "format": "number"
    },
    "inside": {
      "label": "Inside size",
      "description": "Each outside size minus two wall thicknesses.",
      "format": "text"
    },
    "portLength": {
      "label": "Port length (in)",
      "description": "The length of each port tube for the tuning frequency.",
      "format": "number"
    },
    "portCm": {
      "label": "Port length (cm)",
      "description": "The port length in centimeters.",
      "format": "number"
    },
    "portVolume": {
      "label": "Port volume (ft³)",
      "description": "The volume the port tubes take up; add it to the box if you want this net volume after the ports.",
      "format": "number"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "type": "ported",
      "w": "24 in",
      "h": "15 in",
      "d": "14 in",
      "t": "0.75 in",
      "disp": "0.1 ft3",
      "fb": 35,
      "pd": "4 in",
      "np": 1,
      "k": 0.732
    },
    "outputs": {
      "net": 2.097265624999999,
      "netL": 59.387948965799986,
      "gross": 2.197265624999999,
      "inside": "22.5 in × 13.5 in × 12.5 in",
      "portLength": 10.146878705814185,
      "portCm": 25.77307191276803,
      "portVolume": 0.07379018425706607
    },
    "text": "The net volume is 2.097 ft³ (59.39 L)."
  },
  "examples": [
    {
      "given": {
        "type": "sealed",
        "w": 0.6095999999999999,
        "h": 0.381,
        "d": 0.35559999999999997,
        "t": 0.019049999999999997,
        "disp": 0.0028316846592000007
      },
      "expect": {
        "gross": 2.197265625,
        "net": 2.097265625,
        "inside": "22.5 in × 13.5 in × 12.5 in"
      },
      "source": "hand calculation in content.mdx: 22.5 × 13.5 × 12.5 = 3,796.875 in³ ÷ 1,728 = 2.1973 ft³; NIST Special Publication 811 (1 in = 2.54 cm exactly, so 1 ft³ = 1,728 in³), https://www.nist.gov/pml/special-publication-811; Python 3: 22.5*13.5*12.5/1728"
    },
    {
      "given": {
        "type": "ported",
        "w": 0.5,
        "h": 0.5,
        "d": 0.5,
        "t": 0,
        "disp": 0,
        "fb": 35,
        "pd": 0.1016,
        "np": 1,
        "k": 0.732
      },
      "expect": {
        "netL": 125,
        "portLength": 3.283921834519343
      },
      "source": "Wikipedia, Helmholtz resonance (f = (v ÷ 2π) × √(A ÷ (V₀ × L_eq)), with L_eq the neck length plus an end correction), https://en.wikipedia.org/wiki/Helmholtz_resonance (retrieved 2026-10-02); OpenStax, University Physics Volume 1, §17.2 Speed of Sound (343 m/s in air at 20 °C), https://openstax.org/books/university-physics-volume-1/pages/17-2-speed-of-sound (retrieved 2026-10-02); Python 3: 343**2*math.pi*0.0508**2/(4*math.pi**2*35**2*0.125) - 0.732*0.1016, in inches"
    },
    {
      "given": {
        "type": "ported",
        "w": 0.6096,
        "h": 0.3048,
        "d": 0.3048,
        "t": 0,
        "disp": 0,
        "fb": 35,
        "pd": 0.1016,
        "np": 1,
        "k": 0.732
      },
      "expect": {
        "net": 2,
        "portLength": 10.782746830374283
      },
      "source": "Wikipedia, Helmholtz resonance (f = (v ÷ 2π) × √(A ÷ (V₀ × L_eq)), with L_eq the neck length plus an end correction), https://en.wikipedia.org/wiki/Helmholtz_resonance (retrieved 2026-10-02); OpenStax, University Physics Volume 1, §17.2 Speed of Sound (343 m/s in air at 20 °C), https://openstax.org/books/university-physics-volume-1/pages/17-2-speed-of-sound (retrieved 2026-10-02); hand calculation in content.mdx: L_eff = 343² × A ÷ (4π² × 35² × 2 ft³) = 13.71 in, minus 0.732 × 4 in; Python 3"
    }
  ],
  "sources": [
    "Wikipedia, Helmholtz resonance (resonant frequency f = (v ÷ 2π) × √(A ÷ (V₀ × L_eq)), with L_eq the neck length plus an end correction). https://en.wikipedia.org/wiki/Helmholtz_resonance (retrieved 2026-10-02)",
    "OpenStax, University Physics Volume 1, §17.2 Speed of Sound (the speed of sound in air is 343 m/s at 20 °C). https://openstax.org/books/university-physics-volume-1/pages/17-2-speed-of-sound (retrieved 2026-10-02)",
    "Wikipedia, End correction (the added length at an open pipe end; about 0.6133 × the radius for an unflanged end, Levine and Schwinger 1948). https://en.wikipedia.org/wiki/End_correction (retrieved 2026-10-02)",
    "NIST Special Publication 811, Guide for the Use of the International System of Units, Appendix B (1 in = 2.54 cm exactly). https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-02)"
  ],
  "related": [
    "density",
    "wavelength",
    "ohms-law"
  ],
  "changelog": []
}