Loudspeaker Linearity in Cinema: Why Does It Matter?
When deploying linear components, you have a mathematically correct and predictable system that can be measured individually or as a whole. You can experiment with science-based design concepts with predictable results that are repeatable. Once any of the components behaves in a non-linear fashion, this predictability falls apart.
So, how does the linearity principle translate for the cinema?
In this interview, John Meyer, CEO and cofounder of Meyer Sound, looks at the importance of linearity, why linear systems are the only way to go in cinema sound, and how digital processing simply cannot patch up problems caused by a distorted loudspeaker.
Q: Is linearity a recent concept?
Meyer: No. In the 1950s power amplifiers made a big advancement in technology—they became linear, which means that the amplifiers didn't modify the signals and made it possible to have several signals processed by the amplifier simultaneously while remaining independent. With this advancement, a violin and a trumpet could now be reproduced from the same loudspeaker at the same time and the listener could still distinguish between the two.
Q: Who were the early adopters of linear technology?
Meyer: The Hi-Fi, stereo, FM radio, and cinema industries all benefited when they became the first adopters to take linearity seriously. Series of tests were developed to evaluate the linearity of amplifiers and loudspeakers and these findings would be published in audio magazines. By the 1980s, this testing of products disappeared. Today too much of the speaker evaluation is done subjectively, without any linearity data as reference.
Q: What is the problem when loudspeakers are not tested for linearity?
Meyer: When the user has no information about whether a loudspeaker is linear, system selection becomes subjective. So when someone chooses one product over another, oftentimes it's because they like a certain sound or tone in a speaker, which means the signal is enhanced in some way. With this tone, a speaker system might sound good on one type of music and not so good on other music. This is a big problem for the cinema.
Linear speaker systems are the next jump in technology. Everything we have achieved in digital processing can now be implemented. There are some who would have you believe that with digital signal processing you can fix any problems that loudspeakers have. This is simply not possible."
— John Meyer
Q: What happens when cinema loudspeakers are not linear?
Meyer: Cinema power amplifiers, filters, and playback from uncompressed digital systems today are quite low in distortion, all except the loudspeaker. To make the speech stand out, most cinema sound systems typically generate large amounts of second harmonics, which cinema sound mixers are accustomed to. The problem with forcing the harmonics from the loudspeaker is if you play complex music through such a system the sound becomes smeared and it is difficult to hear the separate musical instruments. Today there are outboard harmonic-enhancing hardware and plugins to add second harmonics upstream if desired by the creators.
Q: Do most cinema sound systems distort only when played at high levels?
Meyer: Most cinema sound systems are comparatively low in distortion at low levels, so most distortion is not revealed in tunings. Cinema sound systems are set up at 85 dB SPL in the room when fed a signal which is supposed to be 10% of maximum level or -20dB. However this varies from the true level by several dB because of various references that are used by the cinema industry. [See Pete Soper's paper on calibration of cinema loudspeakers here] When a system is not checked at the appropriate high levels, it could distort when playing back loud movie scenes for patrons.
Q: How does linearity impact low-frequency reproduction?
Meyer: Linearity impacts playback of all types of sources. Linear subwoofers can reproduce several low notes at the same time, meaning they can also reproduce the detail in explosions. Non-linear loudspeakers change the sonic information into something different, and cannot reproduce the original notes separately.
Another big advantage of linear systems is in the control of low frequencies in cinemas. Directional subwoofer configurations can be used to keep the sound from leaking into the adjoining room. If a single subwoofer component is not linear at high sound levels, the directional control of the system is lost and the sound will punch through the wall to the other rooms.
Q: What other advantages do linear systems bring to the cinema?
Meyer: All the filters, analog or digital, are based on linear theory and only will work on systems that are linear. These filters are a big part in cinema tuning. Let's say we want to tune a speaker system in a cinema. We set up a microphone in the room a little further back from the center of the room. We play the -20dB pink noise source into each screen channel speaker and set the frequency response using filters and set the frequency spectrum to have the same magnitude or SPL for each frequency band using filters. The high frequencies have a roll-off caused by the screen, which can now be corrected by adding a simple filter when showing movies. The only way for this filter to work is when the loudspeaker responds in a linear manner.
Another advantage of linearity is a coherent phase response which results in better imaging and smoother movement of panning on screen and out to the surrounds.
Q: Can digital processing fix the problems of a non-linear loudspeaker?
Meyer: This is not possible. If a voice coil is rubbing and causes a buzzing sound, this simply cannot be solved by some clever DSP algorithm. There are many examples of speaker distortions that don't have a DSP solution, and DSP certainly cannot correct the directionality of a horn loudspeaker, although such claims have been made. Loudspeakers need to be built to be linear and be able to stay linear up to the maximum operation level. This is what we are proposing with our EXP cinema systems.
VIDEOSJohn Meyer: Linear, Self-Powered Systems
TECHNICAL REPORTSCalibrating Cinema Sound Systems (PDF)