spat3di — Positions the input sound in a 3D space with the sound source position set at i-time.


This opcode positions the input sound in a 3D space, with optional simulation of room acoustics, in various output formats. With spat3di, sound source position is set at i-time.


aW, aX, aY, aZ spat3di ain, iX, iY, iZ, idist, ift, imode [, istor]


iX -- Sound source X coordinate in meters (positive: right, negative: left)

iY -- Sound source Y coordinate in meters (positive: front, negative: back)

iZ -- Sound source Z coordinate in meters (positive: up, negative: down)

idist -- For modes 0 to 3, idist is the unit circle distance in meters. For mode 4, idist is the distance between microphones.

The following formulas describe amplitude and delay as a function of sound source distance from microphone(s):

amplitude = 1 / (0.1 + distance)

delay = distance / 340 (in seconds)

Distance can be calculated as:

distance = sqrt(iX^2 + iY^2 + iZ^2)

In Mode 4, distance can be calculated as:

distance from left mic = sqrt((iX + idist/2)^2 + iY^2 + iZ^2)
distance from right mic = sqrt((iX - idist/2)^2 + iY^2 + iZ^2)

With spat3d the distance between the sound source and any microphone should be at least (340 * 18) / sr meters. Shorter distances will work, but may produce artifacts in some cases. There is no such limitation for spat3di and spat3dt.

Sudden changes or discontinuities in sound source location can result in pops or clicks. Very fast movement may also degrade quality.

ift -- Function table storing room parameters (for free field spatialization, set it to zero or negative). Table size is 54. The values in the table are:

Room Parameter Purpose
0 Early reflection recursion depth (0 is the sound source, 1 is the first reflection etc.) for spat3d and spat3di. The number of echoes for four walls (front, back, right, left) is: N = (2*R + 2) * R. If all six walls are enabled: N = (((4*R + 6)*R + 8)*R) / 3
1 Late reflection recursion depth (used by spat3dt only). spat3dt skips early reflections and renders echoes up to this level. If early reflection depth is negative, spat3d and spat3di will output zero, while spat3dt will start rendering from the sound source.
2 imdel for spat3d. Overrides opcode parameter if non-negative.
3 irlen for spat3dt. Overrides opcode parameter if non-negative.
4 idist value. Overrides opcode parameter if >= 0.
5 Random seed (0 - 65535) -1 seeds from current time.
6 - 53 wall parameters (w = 6: ceil, w = 14: floor, w = 22: front, w = 30: back, w = 38: right, w = 46: left)
w + 0 Enable reflections from this wall (0: no, 1: yes)
w + 1 Wall distance from listener (in meters)
w + 2 Randomization of wall distance (0 - 1) (in units of 1 / (wall distance))
w + 3 Reflection level (-1 - 1)
w + 4 Parametric equalizer frequency in Hz.
w + 5 Parametric equalizer level (1.0: no filtering)
w + 6 Parametric equalizer Q (0.7071: no resonance)
w + 7 Parametric equalizer mode (0: peak EQ, 1: low shelf, 2: high shelf)

imode -- Output mode

  • 0: B format with W output only (mono)

    aout    =  aW

  • 1: B format with W and Y output (stereo)

    aleft   =  aW + 0.7071*aY
    aright  =  aW - 0.7071*aY

  • 2: B format with W, X, and Y output (2D). This can be converted to UHJ:

    aWre, aWim      hilbert aW
    aXre, aXim      hilbert aX
    aYre, aYim      hilbert aY
    aWXr    =  0.0928*aXre + 0.4699*aWre
    aWXiYr  =  0.2550*aXim - 0.1710*aWim + 0.3277*aYre
    aleft   =  aWXr + aWXiYr
    aright  =  aWXr - aWXiYr

  • 3: B format with all outputs (3D)

  • 4: Simulates a pair of microphones (stereo output)

    aW      butterlp aW, ifreq      ; recommended values for ifreq
    aY      butterlp aY, ifreq      ; are around 1000 Hz
    aleft   =  aW + aX
    aright  =  aY + aZ

Mode 0 is the cheapest to calculate, while mode 4 is the most expensive.

In Mode 4, The optional lowpass filters can change the frequency response depending on direction. For example, if the sound source is located left to the listener then the high frequencies are attenuated in the right channel and slightly increased in the left. This effect can be disabled by not using filters. You can also experiment with other filters (tone etc.) for better effect.

Note that mode 4 is most useful for listening with headphones, and is also more expensive to calculate than the B-format (0 to 3) modes. The idist parameter in this case sets the distance between left and right microphone; for headphones, values between 0.2 - 0.25 are recommended, although higher settings up to 0.4 may be used for wide stereo effects.

More information about B format can be found here:

istor (optional, default=0) -- Skip initialization if non-zero (default: 0).


ain -- Input signal

aW, aX, aY, aZ -- Output signals

  mode 0 mode 1 mode 2 mode 3 mode 4
aW W out W out W out W out left chn / low freq.
aX 0 0 X out X out left chn / high frq.
aY 0 Y out Y out Y out right chn / low frq.
aZ 0 0 0 Z out right chn / high fr.

If you encounter very slow performance (up to 100 times slower), it may be caused by denormals (this is also true of many other IIR opcodes, including butterlp, pareq, hilbert, and many others). Underflows can be avoided by:

  • Using the denorm opcode on ain before spat3di.

  • mixing low level DC or noise to the input signal, e.g.

    atmp rnd31 1/1e24, 0, 0

    aW, aX, aY, aZ spat3di ain + atmp, ...


    aW, aX, aY, aZ spa3di ain + 1/1e24, ...

  • reducing irlen in the case of spat3dt (which does not have an input signal). A value of about 0.005 is suitable for most uses, although it also depends on EQ settings. If the equalizer is not used, irlen can be set to 0.


See the examples for spat3d.

See also

Panning and Spatialization: Simulation of room acoustics


Author: Istvan Varga

New in version 4.12

Updated April 2002 by Istvan Varga