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The availability of sources of multi-channel audio DVD (“Digital Versatile Disk”), digital television broadcasting, high fidelity music, etc., to the average domestic consumer means that no home entertainment system, nowadays, is complete without the capability for digital surround sound. Dolby® Digital for example, is the de facto standard for surround sound technology, and allows 5 distinct sound channels, plus a sub-channel for LFE (“Low Frequency Effects”) devoted to frequencies between 3Hz and 120 Hz to be recorded, and reproduced on dedicated loudspeakers. Speakers are arranged (as front centre, front left and right, and rear left and right) around a listening space, such that sound emanates from a full 360° around the listener, provided an immersive lifelike, listening experience. DTS™ (“Digital Theatre Systems”) is similarly, a 5.1 channel digital surround sound technology, but 6.1 channel technologies, such as DTS-ES, and THX, and 7.1 channel technologies, such as Dolby TrueHD, and DTS Master Audio, also exist. These extended technologies add one or two extra speakers – typically at the rear of the listening space, to create a more complete 3D soundstage.
Wireless Speaker Features, Tips & Techniques
The rear speakers in a digital surround sound configuration typically need to be positioned anything up to 20′ or so from the other components of a home entertainment, and often mounted on a bookshelf, on a tall stand or directly on a wall, at a height of 6′ or so, above the floor. This means that it is nigh on impossible to position wired rear speakers in such a way as to avoid lengthy unsightly runs of speaker cable, without the inconvenience and expense of drilling holes and feeding cable through walls or trunking.
Wireless rear speakers have therefore become a popular solution for home entertainment configurations. Wireless speakers operate via an audio out transmitter connected to a digital A/V (“Audio Video”) receiver (the component that decodes the multiple audio tracks encoded onto the soundtrack of a DVD film, or digital television broadcast) at one end and a receiver and amplifier, at the other. Audio signal is transmitted using radio frequency (RF) signals; typically in the 2.4GHz frequency band, to the receiver and on to and out of the speakers. This means that no long front to back runs of line level speaker cable are required, so wireless speakers make sound sense, aesthetically, and economically, as well as providing a greater degree of freedom of speaker placement.
Of course until recently the term “wireless speaker” was a misnomer; there were not really any wireless speakers, per se (with the exception of early battery powered wireless speakers, which were typically low powered, poor quality models, unsuitable for digital surround sound applications) because, even in those speakers officially described as “wireless”, mains power was still required. Nowadays however, battery operated wireless speakers, powered by 6 standard “AA” alkaline batteries, or a 12-volt DC adaptor, as an alternative, are capable of delivering 3 Watts per channel RMS (“Root Mean Square”), with an SNR (“Signal to Noise Ratio”) of over 50dB, and distortion at less than 1.5%, at a range of over 300′ under ideal conditions. Incidentally, RMS represents the maximum power output of a speaker, throughout its lifetime, and is therefore a more realistic measurement of speaker output, and quality, than so-called “peak power” which is a theoretical, instantaneous maximum, under optimum conditions.
Wireless Speaker Placement
Wireless speakers in a home entertainment system, because of the inherent advantages of wireless technology, tend to be rear speakers. Rear speakers often set the stage, in fact, the “soundstage”, the 3D space that appears, to the listener, to be occupied by sonic images – for the remaining speakers, and other components, of a digital surround sound system, so the placement of wireless speakers is critical to achieving the desired effect.
Fundamental principles apply to the placement of wireless speakers, as they do to wired speakers, but given that wireless speakers are more flexible in this respect, their positioning can be tailored at least to some degree, to suit the physical layout, and acoustic characteristics, of your listening space.
Broadly speaking however, height is the key to the successful placement of wireless rear speakers; rear surround sound speakers should typically be positioned high on a wall or on a stand of sufficient height, if wall mounting is undesirable, or impractical, at a height of 3′, or 4′, higher than the front speakers, and tilted downwards towards the listening position.
Wireless speakers should also be positioned behind the listening position, so that there is a natural acoustic delay (exacerbated by the height of the speakers) before sound reaches the ears of the listener. In a particularly long room however, mounting wireless speakers on the back wall can result in a narrow, unrealistic soundstage, so mounting the left and right rear speakers on side walls, facing inwards towards the listening position, may produce better results. Additional caveats also apply to so-called “bipole” or “dipole” rear speakers (which emanate sound, simultaneously, in two directions, and can therefore be positioned behind, or alongside, the listening position, but should not be tilted), and to high fidelity music (from DVD-Audio or SACD for example), which may sound better if reproduced by rear speakers positioned at the same height as the front speakers.
As more and more electronic devices in the home (e.g. microwave ovens, cordless phones, etc.) incorporate wireless RF interfaces, it is understandable that some interference is to be expected, from place to place, in the operation of wireless speakers. In addition, certain construction, and other materials (e.g.metal, concrete, plastic, etc.) may absorb or interfere with RF signals. This interference can compromise the reliability and performance of wireless speakers, so some adjustment in the physical positioning of speaker units is normally required for optimal reception and audio quality. If wireless speakers are not to be permanent fixture is your listening space, marking the position(s) where reception is at its strongest, possibly with a shelf, or wall bracket, may save you some time, and frustration, in the long term. Do bear in mind, too, that the “operating distance” figure quoted in wireless speaker specifications is usually the maximum distance, given a direct line of sight between the transmitter and receiver. If you want to transmit audio around corners, or through intervening walls, ceilings, etc., you may find that there is a significant reduction in the quoted operating range.
Technological advances in wireless speaker technology, and in digital technology, generally however, mean that interference is less of an issue than it once was. Some wireless speakers share the same 2.4GHz frequency band as “WiFi” (short for “Wireless Fidelity”) wireless networking devices, but by the same token, also share the same “channel hopping” technology. Essentially, error correction data is transmitted alongside the basic RF signal, such that if signals transmitted on a particular frequency suffer interference, transmission switches, automatically, to another, clearer, frequency and signals “fragments” are reconstructed, with a high degree of accuracy, into the original signal, at the receiver. Digital amplifiers, nowadays, are also highly compact and efficient, and generate minimal heat in relation to their output power.
You should however consider the other wireless technologies in your home, in relation to your implementation of wireless speakers. If you have a 2.4GHz cordless phone, for example, you may like to consider a different frequency band for your wireless speakers; the 900MHz frequency band is somewhat crowded (one of the reasons alternative frequencies, including 2.4GHz, were adopted in the first place) but is, nevertheless, still a possibility. Many wireless speaker systems quote the number of individual channels that can be transmitted, in their specifications, and this can be useful to know if you have a number of different wireless devices potentially broadcasting on the same channel.