Why is the rear delay seventeen milliseconds?

Because that is below the point at which a delayed copy is heard as a separate echo. Under roughly twenty to thirty milliseconds the copy fuses with the original, so the delay changes where the sound seems to be rather than adding an event to it, which is exactly the cue the rear channels need.

Why are the rear channels cross-fed?

Because a rear channel carrying the same side it sits behind would only sound like a louder version of the front. Building each rear mainly from the opposite side puts the difference between the channels into the rear pair, which is what creates the sense of a room rather than a duplicated image.

What happens on a normal stereo system?

The front pair is untouched, so the recording plays back exactly as it was and the rear channels are simply not used. That is the reason for leaving the fronts bit-identical rather than folding the rears into them, and it means an upmix carries no risk for listeners without four speakers.

Is this real surround sound?

It is an upmix rather than a surround mix. Nothing in a stereo recording contains genuine information about what was behind the performers, so the rear channels are synthesized from what is present. What they add is a plausible sense of space, not recovered detail.

Why is the rear level so conservative?

Because the rears are meant to be felt rather than heard. Pushing them up makes the effect obvious and pulls the listener's attention backward, while six to twelve decibels down keeps a voice or a lead instrument in front where it belongs while still suggesting the room around it.

Surround Sound Simulator

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Surround Sound Simulator turns a stereo recording into four channels, keeping the original pair in front and placing a quieter derivative behind. The rear channels carry ambience rather than content, which is what makes the brain hear them as around you.

What is Surround Sound Simulator?

Drop a stereo file, set the rear level, and upmix; the output is a four-channel WAV in the order front left, front right, rear left, rear right.

What the tool does:

It keeps the original left and right channels as the front pair, bit for bit, with nothing added to them.
It derives the two rear channels from the same material: delayed by seventeen milliseconds (17 ms), cross-fed so each rear is built mainly from the opposite side, and attenuated.
A rear level control from eighteen decibels down to nothing, defaulting to six decibels down.
The report gives the rear trim and the peak level the rear channels actually reached.

The design rests on one piece of perception. A delay under roughly twenty to thirty milliseconds is heard not as a separate echo but as part of the same sound, so a delayed copy adds no new event to the signal; what changes is the sense of where the sound is. That is the fusion threshold the rear channels sit below, and it is why seventeen milliseconds was chosen: far enough behind to be placed elsewhere in space, close enough that it never reads as an echo. Cross-feeding each rear channel from the opposite side is the second part. A rear that carried the same channel it sat behind would simply sound like a louder version of the front; building each rear mainly from the other side puts the difference between the channels into the rear pair, which is what gives the impression of a room rather than a duplicated stereo image. Keeping the level low is the third part and the one the panel is explicit about: the rear channels are meant to be felt rather than heard, and the guidance of six to twelve decibels down is what keeps a voice in front of the listener instead of behind them. Because the front pair is untouched, a stereo system ignores the rear channels entirely and plays back exactly the original recording, so the upmix carries no risk for listeners without the extra speakers.

Where it is limited:

It is an upmix rather than a true surround mix, so no genuine spatial information is recovered.
The output is a four-channel WAV, which not every player or editor accepts.
The rear delay is fixed rather than adjustable.
No centre channel is produced.
On a stereo system the result sounds like the original, since the rears are simply not played.
Output is sixteen bit WAV.

Use it to give a stereo mix a sense of space on a quad or four-channel system, to prepare ambience for an installation, or to check how a mix sounds when its rears are separated out. When the goal is to widen an existing stereo image rather than to add channels, the stereo widener does that within two channels, and when the rears should carry a genuine tail rather than a delayed copy, the surround effect is better served by a reverb and the convolution reverb before the upmix.

How to use Surround Sound Simulator

Drop a stereo file onto the panel; a mono source is duplicated to a front pair.
Choose the rear level, keeping it between six and twelve decibels down.
Upmix to quad and check the reported rear peak.
Download the four-channel WAV, which plays as ordinary stereo on a two-speaker system.

When to use Surround Sound Simulator vs related tools

Surround Sound Simulator is the tool for making four channels from two without altering the front pair. Stereo Widener changes the width within a stereo pair instead, and 8D Audio Converter creates a rotating effect for headphones rather than for speakers. When the space should come from a reverb tail rather than from discrete rear channels, Reverb Simulator and Convolution Reverb are the tools for that.

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