Why your loudspeaker is black, and other critical audio design concerns

Why your loudspeakers are black… and other audio design considerations

Omnex’s audio specialist Steve Case explores the details that make a professional grade cinema loudspeaker fit for purpose.

IN CINEMA, IMAGE is only half the picture. The audio plays an equal part in telling the story. A properly designed system will comply with industry standards to reproduce as closely as possible what the filmmaker heard when they mixed the master recordings. The screen channels are full range but, unlike many pro audio loudspeakers, they do not cross over to a subwoofer. The subwoofer in a cinema receives a separate recorded track, and it’s not normally used to reproduce very low frequency content from the Left, Centre and Right screen channels. However, because of the need to play low frequencies, screen channel speakers wil normally use 15” subwoofers in their LF sections. This will depend on the size of the auditorium. there may be multiple 15” drivers to achieve the right response.

A TYPICAL CINEMA speaker isn’t designed to be portable. It is only put into position once, so they are without handles and grilles and typically have screw-type barrier strip connectors rather that a Speakon connector you find on a portable speaker system. We normally paint a cinema specific speaker matt black to stop light reflecting off it. Since cinema speakers don’t need to be portable, and height and width are not usually an issue, the low frequency cabinets can be very large in cubic volume compared to pro audio speakers. This improves efficiency and extension at the very lowest frequencies, at the expense, however, of overall sensitivity.

In terms of construction, cinema cabinets are often of MDF instead of more expensive plywood. Also, the midrange and high-frequency horns and drivers of a screen channel are open at the rear, saving the cost of a full cabinet.

A KEY DIFFERENCE in configuration for cinema loudspeakers is the need to have excellent dialogue intelligibility. To achieve this, more screen speakers are now produced as a 3-way system which keeps most of the dialogue in the mid-range driver between the crossover points of the HF horn and the LF driver. A typical 3-way cinema speaker crosses over around 300Hz and 2kHz meaning most of the vocal

To take advantage of free acoustical

power, we try to place subwoofers on the floor behind the

screen

range is reproduced by the large midrange horn. This gives exceptional dialogue intelligibility, even in the back of the room. We optimise screen channels for typical auditoria, resulting in great sound, as well as ease of installation and aiming. Modern speakers may have custom voicing that achieve unmatched audio performance with little additional EQ on site.

To take advantage of free acoustical power, we try to place subwoofers on the floor behind the screen and not on a platform with the screen loudspeakers. Floor-positioned sub-woofers take advantage of “half-space” loading, which increases output by about 3dB, with no extra amplifier power. Even better if we place the subwoofers on the floor and push them all the way back against the solid wall behind the screen. That is called “quarter space” loading, and results in about 6dB greater SPL, without using a larger amp. Very low frequencies produced by the sub-woofers have no problem going right through any draping or masking below the screen.

LET’S NOT FORGET the surrounds. Modern surround speakers come in all shapes and sizes and colours. As these are the only speakers you may regularly see, some consideration needs to be placed on what they look like and whether they support the surrounding architecture. You may not want a white speaker on a black wall or vice versa. With the range available, there is one out there to support your sound system. In most modern cinemas there are four surround channels but many more working as arrays. This way you get the surround sound experience that keeps the audience glued to their seats.