F3, F6 and F10: how deep it goes

These three say how deep a design goes. Each is the frequency at which output has fallen by that many decibels relative to the passband: the flat region higher up, not the peak and not any particular note.

F3 is the usual headline figure. Quoting a speaker as "−3 dB at 42 Hz" means its F3 is 42 Hz.

F6 and F10 say how it gets there, and they are the more useful pair for judging a design. Two speakers with the same F3 can behave completely differently below it.

Why you should look at all three

The gap between them is the slope.

• F3 and F6 close together, F10 not far below: a steep rolloff. Typical of a vented box, which falls at 24 dB per octave below tuning.
• F3, F6 and F10 well spread out: a gentle rolloff. Typical of a sealed box at 12 dB per octave.

A sealed box with F3 of 55 Hz may still be producing useful output at 35 Hz. A vented box with the same F3 will be doing almost nothing there. In a real room the gentle one often sounds deeper, because room gain rises as the speaker falls and the two partly cancel.

So a speaker with a worse F3 can have better bass in practice. This is why comparing two designs on F3 alone is a mistake.

What these numbers are relative to

The passband, which is why they can move dramatically without the bass changing at all. Switch on the cabinet model and F3 for a 30 liter sealed box jumps from about 49 Hz to 264 Hz, because the baffle step drops the whole bass region 6 dB relative to the midrange, not because anything got worse. The shape below is unchanged.

Always read F3 together with what it is measured against.

What is a good F3?

It depends entirely on what you are building, which is why Baffle asks. A bookshelf speaker reaching 60 Hz is doing well for its size; a subwoofer reaching 60 Hz is not doing its job. And whatever you are building, what is available is set by your driver and the box you will tolerate; see Hofmann's Iron Law.