Every home cinema enthusiast knows that surround speakers are essential to reproduce directional and ambient effects in the movie's original score. There are four different types of surround speakers available: direct-rating, dipole, bipole and quadpole speakers. All of them are used for surround channel sound reproduction, but what are the differences between them? What are the advantages and disadvantages of dipoles and direct-rating speakers? Is there a best solution for everybody, or does it - as always - depend on the circumstances in the individual listening room?
Every multichannel cinema has many surround speakers along each side wall and along the rear wall. To avoid the sound being exactly localised, a high number of surround speakers produces various reflections and creates an impressive surround athmosphere.
Since this high number of speakers doesn't make sense in home cinemas, dipole speakers simulate the real cinema speaker setting by firing to the front and to the back. When placed correctly, this also creates reflections and prevents movie effects from being localised.
By definition, dipole speakers radiate sound in two, usually opposite, directions with the sound travelling in one direction being out of phase with the other. When one cone moves outward, it compresses the air molecules, while the other cone moves inward, resulting in a rarefaction or expansion of air molecules. These opposing bass waves cancel at the sides of the speaker, producing a "null" in the listening area to the sides, whereas the midrange and treble sounds fire forward and to the rear.
In contrast to dipoles, direct-rating speakers can radiate sound in one direction only. When aimed directly to the listener, localizing the sound is very easy. For that reason they are not able to create a reflected and ambient wash of sound like that heard in a big movie auditorium. Traditional home cinema speakers systems use two or more direct-rating speaker on each side of the listening area to help reduce this effect.
Some surround sound experts claim that dipoles simply sound out of phase, and to eliminate this effect, they prefer multidirectional bipole surround speakers. In bipole speakers, the front and rear bass waves are in phase, and fire both forward and to the rear. They create a similar (but not identical) wash of atmospheric sounds without bass cancellation or null area like that produced by a dipole. Because this difference can be very fine, some manufacturers offer speakers with a switch to allow the user to choose between dipole or bipole effects.
In quadpole surrounds, woofers firing upwards and downwards are placed on the top and bottom panels. In addition, a pair of tweeters firing at 45-degree angles is used, on angled portions of the speaker baffle. The result is a multidirectional-radiating surround speaker rating in four different directions. When a quadpole is mounted on the sidewall, the net effect is both an ambient wash of atmospheric sound coupled with higher-frequency directional acuity provided by each speaker's dual tweeters firing at diagonal angles across the listening area.
Advice for rear speaker placement
The type of surround speaker that can deliver best results depends on both room size and actual listening conditions in your home cinema. For optimum performance, dipole speakers have to be mounted on the side wall - preferably close to a corner of the room. When placed like this the sound will be reflected by the back wall and create a diffuse sound more in keeping with the true cinema atmosphere. Ideal positioning should prevent single sounds to be localized directly. This is not a set-up that is recommended for hi-fi listening, or indeed for film fans who like to hear precision in their soundtracks - but it does help you to achieve that cinema atmosphere without making use of your front door.
If fixing rear speakers directly on the side walls in your listening room proves to be impossible, you would be direct rating surround speakers and place them on a stand at the sides of the listening position. Placing your satellite speakers beside, rather than behind, the listening position tends to deliver the best results, as speakers mounted too far behind the listening position can result in a loss of envelopment and integration of the surround information within the whole sound field. If there's no alternative to the back you might want to try the following options:
Mount your rear speakers well above ear level, and try aiming them at each other, towards the front, or to reflect off the sidewalls. Experiment with placing and aiming the speakers until the surround sound field seems to envelop you, rather than coming from behind.
The experts at Dolby labs specify that surround speakers should be mounted above the level of seated listeners at each side of the listening area. In domestic installations of THX speaker systems, licensor Lucasfilm strongly recommends the use of dipole speakers for the best listening experience. Personal taste rather than mathematic measurements will however dicate what suits you best - try before you buy.
There are many advanced placement guides are available on the web, but this one from Jamo is a good place to start: www.jamo.de.
Have you already tried to optimise your speaker setting? What methods did you use? Which proved to be the best approach and did you succeed? Share your experience with others in our forum to help them find their way through the placement jungle.