In audio and room acoustics two desireable types of sound fields exist. Direct sound travels from the loudspeaker directly to the listeners ear. Multichannel sound systems such as stereo use time delays and level differences of the various parts of the signal to create the illusion of a sonic image with the sound playing through two or only a few loudspeakers. The auditory clues for this sonic image are contained within the direct sound.

Ambient or reverberant sound is sound that has reflected from multiple room surfaces before reaching the listener and typically arrives 10 ms - 30 ms or more after the direct sound. The reflected sound has changed because it is the sum of multiple reflections. If the sound has changed enough so that it no longer visually appears to look like the original sound in a wave editor graph it is said to be decorrelated. Decorrelated sound can be generated from a stereo recording by subtracting the left signal from the right signal. This is how ambience is generated from a two channel recording.

Correlation is a basic statistical term used to describe the similarity between two sets of data. In this case correlation describes the similarity in time between the direct sound and the ambient sound. In this case there is no correlation.

High quality sound reproduction requires the presence of both types of sound. A problem in modern hifi sound is to eliminate the early reflections that do not fall into the categories of direct (correlated) and ambient (decorrelated) sound. It is well known that early reflections, those that arrive a few milliseconds after the direct sound (up to 20 ms +) distort the sonic image.

A great source of confusion arises between the various standards in record playback of multichannel systems. Standards such as THX, Dolby AC3, are often confused as being competing brands. This results in further confusion about sound field types required. Some multichannel systems require correlated sound from the rear, others require decorrelated sound. The difference between these type types of sound fields required often dictates design principles in the loudspeaker and the acoustical layout of the room.

Dolby Surround and Pro Logic systems require decorrelated sound from which require rear speakers to be in a reflective environment at the very least. The best option in this case is to have dipole radiators in the rear with the listener placed in the null of the dipole. This prevents the listener from hearing where this decorrelated sound originates from. This is the format that is most often used as soundstages and movie sounds normally occur from the front.

High end systems tend to be built to THX standards. Movies most often use Dolbly AC-3 encoding for Dolbly Pro Logic surround sound. The rear channels both contain identical (L-R) decorrelated ambience signals. The center channel contains (L+R). Dolby Pro Logic is essentially the same logic as Dolby Surround but with additional processing to increase channel separation. "5.1" is 5 channels + a subwoofer output.

In the case of correlated sound comming from the rear, the rear loudspeakers should be treated the same way as the front loudspeakers; that is to avoid early reflections.

In both cases the signal from the rear should be aligned in time so that the sound from the rear arrives at the same time as the front loudspeakers and the proper delays as present in the recordings are heard.

Connect the two positive terminals of your amplifier to a single loudspeaker and disconnect all other channels to generate a Left - Right ambience signal at the speaker. Play some music and give it a listen, this signal is used to deliver ambience to rear loudspeakers in Dolby Surround.

*Before doing this, connect one loudspeaker and check that it still works when the ground connection is moved to another channel. If this works, the amplifier has a common ground and the above experiment can be performed.

*This reduces speaker impedance as measured at the amplifier terminals. Be careful of levels if the ambience speaker is to be permanently connected.

*The responsibilty of this experiment lies soley on the experimenter. The amplifier manufacterer of any particular unit should always be contacted before this is attempted.