Where to get driver data: datasheets, measurement databases and measuring it yourself

Every loudspeaker calculation is only as good as the data it starts from. The driver parameters on a datasheet are a good start, but not the last word. Let's look at where the data comes from, where to find independent measurements and when it pays to measure yourself.

A datasheet is not the law

Manufacturers measure the parameters on one or a few units, often new and not broken in, and the response on an infinite baffle. The production tolerance of the resonance is typically ±15 %, and once the suspension has broken in, fs usually drops.

A good example is the Dayton RS100-4 from the Needle. The catalogue gives fs as 84.8 Hz, while Acoustic Design Magazin writes of a resonance around 75 Hz for it. A 10 Hz difference shifts both the tuning and the ideal box volume. With a line like the Needle it matters less, with a precisely tuned bass reflex more.

Where to find driver data and measurements

Datasheets and manufacturer files. Several manufacturers, such as SB Acoustics, Scan-Speak and Dayton Audio, also offer FRD and ZMA files for download. They are measured on a baffle, not in your cabinet, but they are good enough for a first crossover design.
Zaph Audio. John Krutke's archive of comparative driver measurements: frequency response, distortion, cone decay (CSD) and parameters. For example the 8" woofer comparison. The archive is older, but many of the drivers are still on sale.
HiFiCompass. A large and continuously growing database of driver measurements, with many Tang Band, Dayton and SEAS models. Forum users call it one of the most thorough on the internet.
Loudspeaker Database. Clear Thiele-Small parameters for thousands of drivers in one place.
The Loudsmith catalogue. 1,604 drivers with complete parameters, dimensions and prices, right in the app.

Measurements of finished speakers: the spinorama

It is also worth looking at measurements of finished loudspeakers. They show what a well-designed speaker looks like. Reviewers such as Erin's Audio Corner and Audio Science Review measure with a Klippel Near-Field Scanner, which calculates the response as if in an anechoic chamber from many measurements around the speaker.

The result is shown according to the CTA-2034 standard, the so-called spinorama, with four curves:

On-axis: the response on the axis.
Listening window: the average over ±30° horizontally and ±10° vertically, what you hear from a normal seat.
Early reflections: the sound that arrives after the first bounce off the walls, floor and ceiling.
Sound power: all the sound radiated in every direction.

A good speaker has flat on-axis and listening-window curves and smoothly falling reflection and power curves, without humps or dips. The reviews include the DIY kit Audio First Fidelia, which shows that a home build can match commercial speakers.

Measure it yourself

Frequency response (FRD)

The biggest jump in crossover quality comes from measuring the drivers in the finished cabinet. It captures edge diffraction and the effect of the cabinet, which a datasheet cannot know. All you need is a measurement microphone such as the UMIK-1 and Loudsmith's speaker measurement or REW. Each driver is measured separately, far field for the mids and treble and near field for the bass, and the result is saved as an FRD.

Impedance (ZMA)

For impedance you need a simple jig: a resistor of about 100 Ω in series with the driver and a sound card that measures the voltage on both sides. A program such as REW calculates the impedance from the ratio. The impedance shows the actual resonance in the box and the port tuning, which is the best check that the cabinet came out as calculated.

Thiele-Small parameters by the added-mass method

Two impedance measurements are enough to calculate the driver's parameters. First measure the free-air resonance fs, then stick a known mass M on the cone (modelling clay or coins, for example) and measure the new resonance fs'. Then:

moving mass Mms = M / ((fs / fs')² − 1),
compliance Cms = 1 / ((2π · fs)² · Mms),
equivalent volume Vas = ρ · c² · Sd² · Cms, where ρ · c² is about 1.42 · 10⁵ Pa.

The program reads Qms and Qes from the shape of the resonance peak. REW does this automatically.

How Loudsmith uses the data

For every driver it compares the datasheet sensitivity with the one calculated from fs, Vas and Qes, and checks Re against the impedance. If the data does not agree, it warns you and ranks the driver lower in its recommendations. In the catalogue it catches about 50 drivers with an error in the datasheet this way.
Without measurements it works from the catalogue parameters and estimates edge diffraction from the cabinet dimensions.
When you load FRD and ZMA files for the individual drivers, they replace the model and the crossover is recalculated on real data.
From the speaker measurement page you send a measured driver to the app with one button, without copying files around.

This is exactly how I want to verify both my builds, the Needle and the Tang Band with the Peerless. I will add the measurements to the articles as soon as I have them.