Spatial audio, HRTF and binaural tones.

HRTF: how a direction is heard

A sound from your left reaches your left ear slightly earlier and slightly louder than your right. Your outer ears, head and shoulders also colour it, differently for each direction: a sound from behind loses some brightness, a sound from above has other notches in its spectrum. The brain reads these differences as a place.

A head-related transfer function, or HRTF, is a measured description of that filtering for every direction. A spatial renderer passes each sound through the left and right HRTF for the direction it should come from. Heard on headphones, the sound then appears outside the head, at that direction.

Because everyone’s ears differ, a general HRTF fits some listeners better than others; front and back are the directions most often confused.

How Raumo places sounds

Every sound in a Raumo soundscape has a position in a room around you: up to six sounds and a Ground beneath them. On headphones, Raumo renders them with Apple’s high-quality HRTF renderer. On the iPhone’s speakers it uses Apple’s speaker rendering for the same positions and distances.

This is different from music mixed in advance for a spatial format, such as a Dolby Atmos album. Raumo’s sounds are positioned live, as objects, and you can move them while they play. A Place then shapes the space around all of them at once: how long it rings and where its echoes go.

Distance is volume

In Raumo the distance of a sound is its volume control. Bring it closer and it becomes louder; move it out and it becomes quieter, until it is silent at the edge of the room, three metres away.

Head tracking

With AirPods and other headphones that report their movement, Raumo can keep the room in place while you turn your head: the sound in front of you moves to your side as you look away. It follows the turn of the head only. If you turn your whole body, or lie down, the room slowly comes round with you over about half a minute, so the soundscape is not left behind.

Head tracking is optional. You turn it on in the player’s menu, and iOS may ask for motion access. Raumo uses the movement only to turn the room.

Binaural tones, physically

A binaural tone is a pair of steady tones, one in each ear, a few hertz apart. Played on headphones, each ear hears one steady tone. Listeners usually perceive a slow beating at the difference between them, although no sound in the air beats: the beat arises because the two ears’ signals are combined in hearing.

For example, with a 240 Hz tone and a 10 Hz difference, the left ear hears 235 Hz and the right ear 245 Hz. The difference is usually called the beat frequency.

Over loudspeakers the two tones meet in the air and interfere, so you hear an ordinary amplitude pulse instead. That is why binaural tones need headphones.

In Raumo

• 34 tones in the Tones family: seven bands with beat frequencies from 1 to 45 Hz, each in three timbres (a soft pad, a vowel choir and a narrow noise band), three slow glides between bands, and ten single tones from the Solfeggio set and 432 Hz, which are the same in both ears and have no beat.
• In the key of the music. A band’s centre tone is a note of the soundscape’s chord, between about 120 and 580 Hz. The left ear sits half the beat below it and the right ear half the beat above, so the pitch you hear is the chord tone itself.
• Kept out of the room’s acoustics. Tones bypass the HRTF, which would mix both into each ear, and get no echo or reverb. Their distance sets only their level, and head tracking does not move them.
• Best with headphones. The app says so where you choose a tone. The Binaural playlist plays calm music over one tone.

Raumo makes no claim that any binaural tone, band or frequency affects sleep, focus, mood or health. They are sounds to choose, described here by what they are.