Why measured T/S beats the datasheet

A datasheet is a production average, often measured on a new driver. Your driver might be broken in, from a different batch, or just different. A box modeled on the wrong Vas or Qts is a box you cut twice. Read the datasheet first (how to read a driver datasheet), then confirm on your bench.

Gear you need

  • An impedance measurement setup: REW with an audio interface and a simple jig (a sense resistor plus calibration), or a dedicated DATS-style tester that does the sweep for you.
  • A scale accurate enough to weigh small added masses (blu-tack, or nuts in blu-tack).
  • A way to hold the driver rigidly, in free air, with the cone firing horizontally.
  • A quiet room. Drivers act as microphones and pick up noise and vibration.

New to REW? Start with REW basics.

Fs and Qts from an impedance sweep

  1. Run REW’s impedance calibration before you measure the driver.
  2. Exercise the driver for a while at medium level to settle the suspension.
  3. Hold it rigidly in free air, with no baffle and the cone firing horizontally.
  4. Sweep up to 20 kHz so REW can model the voice-coil inductance.
  5. In REW’s Thiele-Small window, enter the voice coil’s DC resistance, the effective cone area (or diameter), and the air temperature and pressure.

The free-air fit gives Fs and the Q values (Qms, Qes, Qts). It can’t give Vas: for that, you need a second measurement.

Vas: added-mass vs. sealed-box method

  • Added mass: add roughly half the expected Mms, split into four equal pieces spaced evenly near the voice coil (not at the outer edge), and sweep again. Weigh the pieces precisely, since the result is only as good as the scale.
  • Sealed box: mount the driver airtight on a test box a bit smaller than the expected Vas and sweep again.
  • REW also offers a dual added-mass method, which its documentation calls the most accurate. All methods depend on measurement quality.

Sanity-check against the datasheet

Put your Fs, Qts and Vas next to the published ones. Small differences are normal. Large ones mean re-measure first: check calibration, how rigidly the driver is held, the masses, and noise. Only then trust your numbers over the sheet.

Feeding the box model

Use your measured set in the box model and compare alignments (sealed vs ported). Then sanity-check the plan against enclosure mistakes that kill DIY speakers before you cut.