June 18, 2012

Does home theatre require different speakers?

How well has the audio industry responded to the demand for home theatre? Not very well at all! The solutions offered would seem like a knee jerk reaction to the need for multi-channel audio. We've added centre speakers and small surrounds that can be mounted on walls, but there is little consideration given to what is required to create the genuine experience. Virtually none of the speakers marketed for the purpose have the capability to meet THX standards for home cinema adequately. New designs are needed that reflect the opportunities afforded by having a dedicated room, along with re-thinking the science of reproduction as it applies here. Current products here reflect two channel thinking.

Stereo listening

In a serious stereo system the focus is on music reproduction and that involves both good design and personal preference. The speakers are more likely to be placed into the room and usually there won't be a dedicated room. Room treatment most often won't be included. Only one seat needs to be optimised, so a small single-seat sweet spot is fine. In that seat the imaging needs to work and the sound balanced. The bass requires optimisation only in that spot. Personal preference plays a greater role in the choice of speakers. One might prefer the delicate top end of ribbons or exotic tweeters. The deep sound stage of dipoles. The slam of horns. Or perhaps a slightly warm sound coming from the right mix of speakers and electronics. The imaging of single fullrangers. The range of valid choices for stereo listening is vast, but home cinema suggests different challenges, specific requirements and a much smaller range of options that are well designed for the purpose.

What we need for home cinema

When setting up a dedicated home cinema room, the requirements are quite different. We want to achieve:

  • Much greater clean dynamic output - 105 dB in each seat, with 115 dB in the bass
  • Balanced sound in every seat with minimal seat to seat variation in response
  • Good imaging in every seat, rather than just in one central sweet spot
  • A room that has less reverberation
  • A good view of the screen
  • An enveloping sound stage that extends beyond the walls, coming from any place in the room as well as outside it
In order to achieve this we need:
  • Speakers with high efficiency, high power handling and controlled directivity
  • Multiple subwoofers optimised in placement and response with EQ
  • Acoustic treatment for the bass so that the much greater amount of bass will remain under control, rather than sound boomy and bloated
  • Surrounds installed for maximum envelopment (not floorstanders on the floor!)
  • Centre speaker oriented correctly and installed appropriately
  • Calibration with measurement tools and all EQ and high and low pass filtering correctly setup and tested
  • Acoustic treatment to an appropriate level of reverberation
In other words, the requirements of home cinema converge on a very limited range of solutions in terms of speakers. The ideal speakers for home cinema are those with constant directivity, with compression drivers and waveguides. A point source horn is the ideal choice for front main speakers, or a speaker based on a waveguide loaded compression driver matched to a high sensitivity mid. Coaxial speakers have a desirable point source characteristic, but care is required in their selection as some of them have a nasty harsh sound and poor sounding coax drivers are abundant.

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