Sub-bass: what lives below 60 Hz, and what can play it

The lowest notes in music

Sub-bass runs from the bottom of human hearing, around 20 Hz, up to about 60 Hz, and the upper boundary is a loose convention. Few instruments reach down there, and those that do only on their lowest notes.

The organ's lowest pipe sits below 20 Hz, where a tone comes across more as pressure or vibration than as pitch. Electronic music, hip-hop and film scores put a lot of energy in this range, with synth basses and drops tuned wherever the producer likes. Much acoustic music has very little: in standard tuning, the lowest note a string quartet can play is the cello's open C, at 65.4 Hz.

The ear helps small speakers out. Play a bass note with its fundamental missing and you still hear its pitch from the harmonics above it, which is why a bass line survives on a phone speaker. What goes missing is weight: the pressure of an organ pedal note, the floor under a mix.

Why so many products roll it off

A speaker makes bass by pushing air, and holding the same level an octave lower takes four times as much cone travel. Small woofers run out of travel, or distort trying, so small speakers roll off above the sub-bass or reach into it with help. That help is a port, which the bass reflex explainer covers, or electronic bass boost in an active design, which asks even more travel of the woofer.

Headphones have it easier. A sealed ear cup or ear canal is a small closed space, and in a closed space the pressure follows the diaphragm's movement directly, so even a small driver can hold its level down to 20 Hz. The seal is the condition. Glasses arms, hair, tired pads or an ear tip one size too small open a leak, and air escapes through a leak more easily the lower the frequency, so the deepest bass goes first.

Bass extension and bass quantity

Bass extension is how low a product plays before its output falls away. Output rarely stops at a cliff. It slopes, so an extension figure only means something beside the drop it was taken at, most often 3 dB and sometimes a more generous 6 or 10, and the frequency response explainer shows why a range printed without one tells you little. The figure is also measured at a modest level, so it says nothing about how loud the lowest notes can play before the woofer or diaphragm runs out of travel.

Reviews describe the slope in words. The HIFIMAN Sundara review hears "good sub-bass extension" that slopes down from about 50 Hz with 20 Hz still audible, and bass that is present without being boosted, which is the second question.

Quantity is how loud the bass sits against the midrange, and a product can have plenty of one and little of the other:

• The HIFIMAN Arya Stealth reaches 20 Hz, fast and transparent, and its review says it "doesn't offer a sense of weight". It's one of several planars in the best planar magnetic headphones guide that are heard reaching 20 Hz.
• The Sivga Robin SV021 goes the other way, with a big mid-bass boost "that doesn't extend to the sub-bass". Its extra bass all sits above the deepest octave.
• The HarmonicDyne Eris has both, with a deliberate, overwhelming emphasis on the lowest octaves. Its review hears the midrange pushed back and small details swallowed by the bass, and suggests a light EQ cut in the sub-bass.

Sub-bass in a room

For speakers, the room shapes the sub-bass at your seat as much as the speaker does. A room's lowest resonances fall in this range: in a room 3 to 7 m long, the first mode along its length sits between about 25 and 57 Hz. The room modes explainer shows why that makes the deepest notes swell at some seats and all but vanish at others, and what helps.

A subwoofer helps in two ways. It adds output a bookshelf speaker can't make, and because the ear can barely tell where the deepest bass comes from, you can place it wherever gives the smoothest bass at your seat. Headphones skip the room altogether, so what you hear from them down there comes from the product and its seal.

Testing how low your system plays

The signal generator plays sine sweeps in your browser. Set a moderate volume on a midrange tone first, then sweep slowly from 100 Hz down to 20 Hz on headphones or speakers and listen for where the tone fades. A phone or laptop speaker can't play that range, so don't test on one.

Three things can mislead you:

• Your hearing can fade before the product does. Thresholds rise steeply at low frequencies, so at 20 Hz a tone needs tens of decibels more level than at 1 kHz before you hear it at all, and turning the volume down makes the lowest octave fade from perception first.
• A deep tone that turns buzzy, or seems to jump up in pitch, usually means distortion. You're hearing the driver's harmonics as it runs out of travel, so turn it down. On speakers, rule out a rattling shelf or window first.
• On speakers, a sweep that swells and dips at fixed frequencies is showing you the room's modes, and the pattern changes when you change seats.

Don't turn the volume up to chase the lowest tone. The next track will play at that setting, and a woofer driven past its travel can be damaged.