Andy W

What is a dipole speaker, and how and why are they used in a home theater?

Allow me to confine my answer to the more ubiquitous conventional box type speaker basic designs and 5.1 channel movie (not music) surround sound.

As most of you already know, a conventional speaker has drivers (woofers, tweeters, etc.) installed on a single face of the speaker’s cabinet, and the drivers are usually pointed toward the listening area. This type of speaker is called a monopole as it emits sound in only one direction, forward or toward the listening area. Monopoles are commonly used to deliver front left, center and right channel sounds of a multi-channel soundtrack, although other types of speaker designs can also be used effectively in the front channels.

A dipole speaker is one that has drivers facing in opposite directions from each other. One set of drivers is mounted on one side of the speaker cabinet (let’s call this side Face A). Another set of drivers (usually identical to those mounted on Face A) is mounted on the opposite side of the speaker cabinet relative to Face A (let’s call this side Face B). Therefore, a dipole speaker emits sound in two opposite directions - one sound field coming from Face A, and the other sound field pointed in a 180 degree opposite direction from Face A (coming from Face B).

In addition, dipole speakers have one other defining characteristic not visible by simply looking at its construction. The opposing drivers are internally wired to operate “out-of-phase” relative to each other. This means that as the Face A drivers are moving out or away from the cabinet at a specific moment to produce part of a sound wave, the Face B drivers are moving in or into the cabinet at the exact same time. This produces a diffused, or hard to localize sound effect and can be very beneficial for use as surround sound channels of multi-channel movie soundtracks.

When dipoles are used for surround sound channel duties, it is recommended to locate them directly to the sides of the listening area. Face A would point toward the front of the room, and Face B would point to the rear of the room. Therefore, one side of the speaker cabinet with no drivers would point toward the chair or couch where you sit to watch and listen to surround sound movies. This location of the dipole speakers relative to your seating location would put you in the “null” area of sound distribution coming from the surround sound channels. The “null” area is where it is difficult to locate where the sound is coming from.

When dipole speakers are used for surround sound and located in your room as described above, they can be most effective in recreating surround sound effects that are meant to sound as if the sound is all around you, as in a rain storm, wind, thunder, in the middle of a crowd, etc.

It appears then that dipoles should be used for the surround channels in lieu of conventional monopoles for home theater, yes?

In the days of Dolby Pro-Logic matrix surround (before Dolby Digital and DTS discrete multi-channel surround sound existed for home theater), the answer was a resounding yes. Old Dolby Pro-Logic soundtracks were monophonic (sound coming from the left and right surround channels were identical), were limited in channel separation relative to the front speakers, had limited lower and upper frequency response and limited dynamic range (could not get loud relative to the front channels). Back then, because of these limitations, surround sound was used primarily to put you into a diffused sounding environment, such as a cavernous room, a stadium, a rain storm, etc. Dipoles were highly recommended for use as surround channels by Dolby Labs because they reinforced the diffused surround effects that Dolby Pro-Logic was capable of reproducing.

Since the technology for Dolby Digital and DTS discrete multi-channel surround sound became a reality for home theater, the answer of whether dipoles are better than monopoles for surround sound is not so clear anymore. Dolby Digital and DTS are now capable of reproducing discrete stereo sound effects in the surround channels with the same full frequency bandwidth and dynamic range of the front channels. This opened new opportunities for movie soundtrack designers to put discrete sound effects (meant to be localized) into the surround channels, such as bullets whizzing by from different locations, a door knocking behind and to the right or left of you, distinct conversations coming from all around you, much more powerful explosion sounds, jets flying around your room, etc.

Yes, dipoles are still better at reproducing intended diffused surround effects, and Dolby Labs and other respected organizations still recommend dipoles be used for surround sound channel duties of Dolby Digital movie soundtracks. But what about those discrete sounds being put into Dolby Digital and DTS movie soundtracks – the ones that are meant to sound like they are coming from a specific direction? Since monopoles are generally accepted as being better able to help you pinpoint where a sound is coming from (provided the monopole speakers are designed to produce pinpoint imaging and are properly installed and located in your room, which is another discussion entirely), why not use monopoles for these newer soundtracks? The answer now, because of the newer surround sound technology is no longer so easy as it was just a few years ago.

Today, the debate continues whether dipoles should be preferred over monopoles for surround sound duties. Dipoles are better at reproducing diffused surround effects. Monopoles are better for pinpoint surround effects. However, both diffused and pinpoint surround effects are now utilized in today’s movie soundtracks. So which type of speaker should you go with for your surround channels? The answer has to be whatever your personal preference is. Listen to both types of speakers and decide for yourself which type of surround sound you like best before you make a purchase.

Some speakers are designed to be changed with the flip of a switch to behave as a dipole or as a monopole. Also, a few receivers (the more expensive ones) allow you to connect two sets of surround speakers, one set dipole and one set monopole, for you to decide which type of speaker you want to use at any given time.

One other thing – monopole speakers used for surround channels should be located to the sides, and unlike dipoles, somewhat behind the listening position. You should also experiment with what direction you should face the monopole speaker - toward the listener, toward the front of the room, toward the rear of the room or ceiling (for a reflective bounce effect), toward the opposite diagonal front speaker, etc. You’ll be surprised at the vast differences you will hear in the performance of the speakers. And its fun to experiment.

P.S. – Regarding discrete multi-channel surround sound music formats, such as DVD Audio and SACD, the conventional wisdom is to use monopoles, preferably identical model speakers all around (except subwoofer). But this is another discussion.

Happy listening.