Documentation sections

Subwoofer Enclosure Volume

POSThttps://rftools.io/api/py/v1/calculate

Slug subwoofer-box. Calculate subwoofer box volume for sealed and ported enclosures from Thiele-Small parameters (Vas, Qts, Fs). Get optimal internal volume, port tuning frequency, and −3 dB cutoff instantly.

Inputs

Send every one of these in inputs, in the units shown. Most are required: a call without one is refused (400) and costs nothing; one the calculator can do without takes the default shown. An input outside its range is still computed and flagged in the provenance.

Inputs of subwoofer-box
Name	Unit	Range	Default	Description
vasnumber	L	0.1 … 2000	50	From driver datasheet
qtsnumber	none	0.01 … 2	0.35	Qts < 0.35 → vented, 0.35–0.7 → sealed
fsnumber	Hz	5 … 200	35	Fs (Resonant Frequency)

Outputs

Returned in values, by key. A value that is unbounded or undefined is null.

Outputs of subwoofer-box
Field	Unit	Description
vbSealedheadline	L	Sealed Box Volume
vbVented	L	Ported Box Volume
portTuning	Hz	Port Tuning Frequency
f3Sealed	Hz	-3 dB (Sealed)
f3Vented	Hz	-3 dB (Ported)

Calling it

Calculate: one call, the request and response in full, and its errors.
Batch: this calculator with others, or a sweep, in one request.
Solve for a target: the value of one input that brings an output to a target.

curl -X POST https://rftools.io/api/py/v1/calculate \
  -H "X-API-Key: $RFTOOLS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "slug": "subwoofer-box",
    "inputs": {
      "vas": 50,
      "qts": 0.35,
      "fs": 35
    }
  }'

import rftools

client = rftools.Client()  # reads RFTOOLS_API_KEY, or the key `rftools login` saved

result = client.calculate("subwoofer-box", {"vas": 50, "qts": 0.35, "fs": 35})
print(result.values)
print(result.warnings, result.provenance.formula_ref)

const response = await fetch("https://rftools.io/api/py/v1/calculate", {
  method: "POST",
  headers: {
    "X-API-Key": process.env.RFTOOLS_API_KEY,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "slug": "subwoofer-box",
    "inputs": {
      "vas": 50,
      "qts": 0.35,
      "fs": 35
    }
  }),
});
console.log(response.status, await response.json());

package main

import (
	"bytes"
	"fmt"
	"io"
	"net/http"
	"os"
)

func main() {
	body := []byte(`{
	  "slug": "subwoofer-box",
	  "inputs": {
	    "vas": 50,
	    "qts": 0.35,
	    "fs": 35
	  }
	}`)
	req, err := http.NewRequest("POST", "https://rftools.io/api/py/v1/calculate", bytes.NewReader(body))
	if err != nil {
		panic(err)
	}
	req.Header.Set("X-API-Key", os.Getenv("RFTOOLS_API_KEY"))
	req.Header.Set("Content-Type", "application/json")
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	fmt.Println(res.Status, string(out))
}

application/json

{
  "slug": "subwoofer-box",
  "values": {
    "vbSealed": 16.231658226204388,
    "vbVented": 31.291367730051,
    "portTuning": 40.27281732492783,
    "f3Sealed": 70.71067973141594,
    "f3Vented": 42.611891604828415
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:subwoofer-box",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "R. H. Small, \"Closed-Box Loudspeaker Systems\", JAES 20(10), 1972; G. Margolis & R. H. Small, \"Personal Calculator Programs for Approximate Vented-Box and Closed-Box Loudspeaker System Design\", JAES 29(6):421–441, 1981",
    "assumptions": [
      {
        "code": "sealed-butterworth",
        "text": "The sealed box targets a Butterworth Qtc of 0.707 and is not reported for a driver whose Qts is 0.707 or more."
      },
      {
        "code": "margolis-small-1981",
        "text": "The ported volume, tuning and −3 dB frequency are Margolis & Small's 1981 approximations, not an exact Thiele alignment; port losses and room gain are not modelled."
      }
    ],
    "validRange": {
      "status": "inside",
      "bounds": {
        "vas": {
          "min": 0.1,
          "max": 2000,
          "unit": "L"
        },
        "qts": {
          "min": 0.01,
          "max": 2,
          "unit": ""
        },
        "fs": {
          "min": 5,
          "max": 200,
          "unit": "Hz"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "vas": 50,
      "qts": 0.35,
      "fs": 35
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "vas"
      ],
      "msg": "Input should be a valid number, unable to parse string as a number",
      "input": "abc"
    }
  ]
}

An input sent as text. Nothing was counted.

{
  "detail": "More than 120 requests in 60 seconds from this address.",
  "errorKind": "rate_limited",
  "limit": 120,
  "windowSeconds": 60,
  "upgradeUrl": "https://rftools.io/pricing/"
}

Too many requests in a minute, with Retry-After.

Same inputs on the calculator pageSubwoofer Enclosure Volume, at these values