Howto:Add sound effects to aircraft

You can add sound effects to aircraft by adding the sound samples and configuring the sound effects in XML files. The sound effects can be conditional and the sound's pitch and volume can depend on properties in the property tree.

Note  This text is partially incorrect/outdated. Especially how the final volume/pitch is calculated. Make sure to also read the official documentation.

Some sound effects are added other ways. For example, sounds from avionics like navigation radio receivers are automatically added when you add those radios.

The volume calculation due to distance and orientation of the sounds source ONLY work on mono samples! — syd adams (Dec 7th, 2011). Re: [Flightgear-devel] Stereo sounds no longer supported?. (powered by Instant-Cquotes)

The old way was that the stereo sound would always play the left channel on the left side. Now you split the sound and place it in 3D space. When you turn your head, the speaker the left side sound comes out of changes with you — bugman (Sep 14th, 2015). Re: 787-8 Boarding music. (powered by Instant-Cquotes)

All sounds must now be mono point sources, and stereo simply requires the two sound sources to be positioned in the 3D environment. — Edward d'Auvergne (May 10th, 2015). [Flightgear-devel] Audit and proposal for handling non-supported stereo sound files in FGAddon via the 3D sound engine.. (powered by Instant-Cquotes)

We use OpenAL for 3D positional audio and most OpenAL implementations can only handle one stereo file.Also Stereo files will not work in 3d space, so they will not be positional.But most aircraft developers did not know that and used several stereo files expecting everything to work properly but the truth was that in most implementations just one of those stereo files was played.And if you think of it, stereo separation does not work for boarding music either since it plays left channel to your left ear and right channel to your right ear. But in reality boarding music is played left channel to the left audio channel in the aircraft and right channel to the right audio channel in the aircraft.This works the same as normal stereo if you enter the aircraft, but will, reverse once you turn around.So the solution is to split up the stereo file into two mono files and play the proper channel at the proper side of the passenger compartment. — erik (Sep 14th, 2015). Re: 787-8 Boarding music. (powered by Instant-Cquotes)

Sound configuration files

The sound configuration files are PropertyList XML files.

The top level sound configuration file is composed of a <fx> and a <name> tag, a <path> to a sound sample file and zero or more <volume> and/or <pitch>definitions.

Paths are relative to $FG_ROOT, but absolute paths may also be used.

Comments are bracketed with <!-- -->.

Example

This example would define an engine sound effect for a piston engine driven aeroplane.

The sound effect representing the engine is located in $FG_ROOT/Sounds and is named wasp.wav. The effect is started when the property /engines/engine/running becomes non-zero.

When that happens, the sound will be played looped (see <mode>) until the property returns zero again. As you can see the volume is dependent on the property /engines/engine/mp-osi, and the pitch of the sound depends on the property /engines/engine/rpm.

<?xml version="1.0" encoding="UTF-8"?>
<PropertyList>
 <fx>
  <engine>
   <name>engine</name>
   <path>Sounds/wasp.wav</path>
   <mode>looped</mode>
   <condition>
    <property>/engines/engine/running</property>
   </condition>
   <volume>
    <property>/engines/engine/mp-osi</property>
    <factor>0.005</factor>
    <min>0.15</min>
    <max>0.5</max>
    <offset>0.15</offset>
   </volume>
   <pitch>
    <property>/engines/engine/rpm</property>
    <factor>0.0012</factor>
    <min>0.3</min>
    <max>5.0</max>
    <offset>0.3</offset>
   </pitch>
  </engine>
 </fx>
</PropertyList>

Inclusion from the aircraft-set.xml file

In order for the sound configuration to be used, it has to be included from the aircraft-set.xml file.

This can be done by adding the path to the file relative to the aircraft's base path within the <sim> tags in the aircraft-set.xml file..

<?xml version="1.0" encoding="UTF-8"?>
<PropertyList>
 <!-- ... --->
 <sim>
  <!-- ... --->
   <sound>
    <path type="string">My-sound-config.xml</path>
   </sound>
  <!-- ... --->
 </sim>
 <!-- ... --->
</PropertyList>

Configuration tags

<fx>

Named FX subtree living under /sim/sound

< ... >

This is the event separator. The text inside the brackets can be anything, but it is advised to give it a meaningful name like: crank, engine, rumble, gear, squeal, flap, wind or stall.

The value can be defined multiple times, thus anything which is related may have the same name (grouping them together).

<name>

This defines the name of the event. This name is used internally and, although it can me defined multiple times in the same file, should normally have an unique value.

Multiple definitions of the same name will allow multiple sections to interfere in the starting and stopping of the sample.

This method can't be used to control the pitch or volume of the sample, but instead multiple volume or pitch section should be included inside the same event.

The types "raise" and "fall" will stop the playback of the sample regardless of any other event. This means that when the type "raise" is supplied, sample playback will stop when the event turns false. Using the type "fall" will stop playback when the event turns true.

IMPORTANT

If the trigger is used for anything else but stopping the sound at a certain event, all sections with the same name should have exactly the same sections for everything but property and type.

In the case of just stopping the sample at a certain event, the sections for path, volume and pitch may be omitted.

<path>

This defined the path to the sound file. The path is relative to the FlightGear root directory but could be specified absolute.

The sound format must be mono.

<condition>

Define a condition that triggers the event.

For a complete description of the FlightGear conditions, please read docs-mini/README.conditions or read the Conditions article on the wikie.

An event should define either a condition or a property.

<property>

Define which property triggers the event, and refers to a node in the FlightGear property tree.

Action is taken when the property is non zero.

A more sophisticated mechanism to trigger the event is described in <condition>

<mode>

This defines how the sample should be played:

once

The sample is played once. This is the default.

looped

The sample plays continuously, until the event turns false.

in-transit

the sample plays continuously, while the property is changing its value.

<type>

This defines the type of this sample, each type also have seperate volume slider in sound settings menu:

fx

This is the default type and doesn't need to be defined.

avionics

Sounds set to this type don't have a position and orientation but are treated as if it's mounted to the aircraft panel.

It is up to the user to define if it can always be heard or only when in cockpit view.

atc

Is used by the ATC chatter add-on.

<delay-sec>

Delay after which the sound starts playing.

This is useful to let a property start two sounds at the same time, where the second is delayed until the first stopped playing.

<volume> / <pitch>

Volume or Pitch definition.

Currently there may be up to 5 volume and up to 5 pitch definitions defined within one sound event.

There are two important results from each <volume> and <pitch> section: the offset and the value. Normally all offset values from the different volume/pitch are added together to create the total offset. The values from each <volume> or <pitch> section are multiplied together to create an final value. Then the volume (or pitch, respectively) is set to total offset + final value.

A special condition occurs when the value of factor is negative, in which case the offset doesn't get added to the other offset values but instead will be used in the multiplication section.

Pitch final output can range from 0 to 2. 1 represents the original pitch of the sound file. Offset defaults to 1, meaning that if no specific <pitch> parameters are given the file will be played at its original pitch.

Volume final output can range from 0 to 1. 1 represents full volume and 0 is silence.

<expression>

Defines an expression, and will replace all other tags in this volume/pitch tag.

<property>

Defines which property supplies the value for the calculation.

Either a <property> or an <internal> should be defined. If none is defined, the property reverts to its default. For volume this is 0, meaning the sound will not be audible at all.

The value is treated as a floating point number.

<internal>

Defines which internal variable should be used for the calculation.

The value is treated as a floating point number.

The following internals are available at this time:

dt_play

The number of seconds since the sound started playing.

dt_stop

The number of seconds after the sound has stopped.

<type>

Defines the function that should be used upon the property before it is used for calculating the net result:

lin

Linear handling of the property value. This is the default.

ln

Convert the property value to a natural logarithmic value before scaling it.

Anything below 1 will return zero.

log

convert the property value to a true logarithmic value before scaling it. (log base 10)

Anything below 1 will return zero.

inv

Inverse linear handling (1/x).

abs

Absolute handling of the value (always positive).

sqrt

Calculate the square root of the absolute value before scaling it.

<factor>

Defines the multiplication factor for the property value.

A special condition is when scale is defined as a negative value. In this case the result of |<scale>| * <property) will be subtracted from <default>

<offset>

The initial value for this sound.

This value is also used as an offset value for calculating the end result.

<random>

Add a bit of randomness to the offset. Only used for pitch.

<min>

Minimum allowed value.

This is useful if sounds start to sound funny. Anything lower will be truncated to this value.

<max>

Maximum allowed value.

This is useful if sounds gets to loud. Anything higher will be truncated to this value.

Keep in mind that min & max are applied first, then offset. So for example if min = 0, max = 0.5 and offset = 1.0, then the resulting values will range 1 to 1.5.

<position>

Specify the position of the sounds source relative to the 3D models center.

The coordinate system used is a left hand coordinate system where +Y = left, -Y = right, -Z = down, +Z = up, -X = forward, +X = aft. Distances are in meters.

It's important to note that for sounds the Y axis is inverted compared to e.g. object animation coordinates (see also Howto:3D_Aircraft_Models).

The volume calculation due to distance and orientation of the sounds source ONLY work on mono samples!

Also take notice that the values should not be 0.0, since that can produce sound artifacts on some sound cards.

<x>

X dimension offset (-X = forward, +X = aft, in meters)

<y>

Y dimension offset (+Y = left, -Y = right)

<z>

Z dimension offset (-Z = down, +Z = up)

<orientation>

Specify the orientation of the sounds source.

The zero vector is default, indicating that a Source is not directional.

Specifying a non-zero vector will make the Source directional in the X,Y,Z direction

<x>

X dimension

<y>

Y dimension

<z>

Z dimension

<inner-angle>

The inner edge of the audio cone in degrees (0.0 - 360.0).

Any sound withing that angle will be played at the current gain.

<outer-angle>

The outer edge of the audio cone in degrees (0.0 - 360.0).

Any sound beyond the outer cone will be played at "outer-gain" volume.

<outer-gain>

The gain at the outer edge of the cone.

<reference-dist>

Set a reference distance of sound in meters. This is the distance where the volume is at its maximum. Default is 60.

Volume is clamped to this maximum for any distance below.

Volume is attenuated for any distance above.

Attenuation depends on reference and maximum distance. See OpenAL specification on "AL_INVERSE_DISTANCE_CLAMPED" mode for details on exact computation.

<max-dist>

Set the maximum audible distance for the sound in meters. Default is 6000.

Sound is cut-off above this distance.

Creating a configuration file

To make things easy, there is a default value for most entries to allow a sane configuration when a certain entry is omitted.

Tag	Default value
<type>	lin
<factor>	1.0
<offset>	0.0 for volume, 1.0 for pitch
<min>	0.0
<max>	0.0 (don't check)

Calculations are made the following way (for both pitch and volume):

value = 0;
offs = 0;

for (n = 0; n < max; n++) {
    if (factor < 0) {
        value += offset[n] - abs(factor[n]) * function(property[n]);
    } else {
        value *= factor[n] * function(property[n]);
        offs += offset[n];
    }
}

volume = offs + value;
if (volume > 1.0) vol = 1.0;

where function can be one of: lin, ln, log, inv, abs or sqrt

Multiplayer sound

The multiplayer sound file works just as the normal sound file, except it should reside in /sim/model/sound instead of /sim/sound. Make sure it depends on properties that are transmitted over MP.

An example could be found in FGAddon 777-200: https://sourceforge.net/p/flightgear/fgaddon/HEAD/tree/trunk/Aircraft/777

• 777-200-set.xml extends 777-200-common.xml (this is not mandatory, just here to explain the inheritance. The -set.xml does/can include the model directly)
• 777-200-common.xml references Aircraft/777/Models/777-200.xml
• Models/777-200.xml has an <sound><path>Aircraft/777/Sounds/mp-sound.xml</path></sound> reference
• Sounds/mp-sound.xml contains an ordinary sound config file with relative property paths resolved to the relative /ai/models/multiplayer/ nodes of the multiplayer instance.
• The result is a subnode /ai/models/multiplayer/sim/sound, with a "path" node pointing to Aircraft/777/Sounds/mps-sound.xml.

Recommended audio file formats

Note  The sound sample must be in mono format. This is because it can not be put in a 3D context otherwise. Also, as of at least June 2015 stereo samples will not be played.

There some considerations on the quality to the audio files. As a rule of thumb, any computer is able to playback sounds in DVD-quality (48 kHz/24 bits). Macs still has trouble reading 32 bit files. Most standard sound cards also can't playback 32 bit files.

From a listeners point of view, anything above 44.1 kHz and 24 bits is only useful for high end music. Personally, I would think that 44.1 kHz with 16 bits should be more than enough to play engine sounds and alarms from the cockpit. Consider the disk-space against the gain in audio quality!

Preventing sounds from playing when sim starts

Some times it can be hard to prevent sounds from playing right away, if the condition starts to evaluate to true. A hack to get this to work is to multiply the volume with a property that you in nasal set to true some seconds after the sim has been initialized.

Bugs in the sound system

• The Doppler effect if you pass an aircraft in high speed sometimes is way too high pitch.
• None of the position coordinates must not be 0.0 as described above.
• Sounds using <mode> 'once' together with <max-dist> behave incorrectly when going out of range of the sound, then back in range, all while the trigger condition remains true:
  • Until January 2023 (Flightgear 2020.3): the above causes the sound to incorrectly restart playing.
  • Since January 2023 (for development versions): the sound stops entirely when going out of range, it is not possible to come back in range to hear the end of the sound.

Related content

Wiki articles

• Conditions
• Expressions
• Howto:Reload sound configuration without restarting

Readme file

• $FG_ROOT/Docs/README.xmlsound

Source code

• simgear/simgear/sound/xmlsound.hxx
• simgear/simgear/sound/xmlsound.cxx
• flightgear/src/Sound/fg_fx.hxx
• flightgear/src/Sound/fg_fx.cxx