Bass Control, subwoofer optimization, and the future of home theater sound

Until recently, room correction solutions focused strictly on optimizing magnitude response, offering little help to correct phase response that is crucial for consistent performance throughout a room. However, with advancements in artificial intelligence and machine learning, combined with the development of all-pass filters, systems are now able to deliver spatially consistent bass response even from a multiple subwoofer setup. However, before we dive into the details of the future of bass control, let’s take a moment to review the basics of sound and speakers.

 Back to the basics

 The average human can hear sounds from about 20 Hz (very low bass) to 20,000 Hz (very high tones). Modern full-range speakers, such as left, center and right (LCR) speakers in a surround-sound system, combine a woofer and tweeter to reproduce the full range of audible frequencies. Subwoofers, on the other hand, are built with much larger woofers and no tweeters, specifically to reproduce bass tones from 20Hz to 200Hz. Professional-grade subwoofers may focus on the lower end, only reproducing tones in the 20Hz to 100Hz range, while some cinema certifications limit subwoofer signals to only 80Hz.

 In a system that combines full-range speakers and subwoofers, the speakers will produce every tone, while the subwoofers will only produce sounds lower than the frequency set by the system’s onboard low-pass filter – typically offering selectable cutoff points from 30Hz to 120Hz. A low-pass filter allows only low frequencies to pass to the speaker and blocks any signal higher than the cutoff.

 The sound these speakers produce, like all sound, travels in waves. Just like waves in water, sound waves reflect off edge surfaces and interact with other waves. These interactions change a wave’s magnitude (volume) and phase (timing) as perceived at a particular location. For instance, if two waves overlap perfectly in a given location in a room, the perceived sound is clearer and louder – they arrive at the same time. If the two waves overlap, where one wave’s valley is the same as the other wave’s peak, they cancel each other out and result in a poor listening experience.

 So why the need for bass control?

 Bass tones communicate feeling and physical power in music and film. To build an entertainment space in which each participant can experience fully a song or a film score, the magnitude and phase of each speaker must be controlled carefully to provide a consistently excellent listening environment regardless of listener location.

 To optimize a sound system’s bass performance and deliver room-wide improvement, each speaker’s output must be corrected for magnitude and phase response, particularly in the range of frequencies around the subwoofers’ upper limit. This “crossover point” is where the output of different speaker types overlap and it can result in wide variations in magnitude and phase at different locations in space. Any solution to correct these issues must take into account the shape of the room, the location and direction of each speaker, reflective or obstructive surfaces, and multiple observation points to ensure consistent performance throughout the space.

 While equalization (EQ) solutions can improve an audio system’s tone and dynamics, they don’t correct for phase. Because phase is effectively the timing and overlapping of sound waves, a system that is 100 percent “in-phase” delivers sound from every speaker to every listener at the same time. A system that is “out-of-phase” has crisscrossing soundwaves, meaning the same tone produced by different speakers will reach the listener at slightly different times, reducing clarity and accuracy and resulting in a sub-optimal experience.

 The more speakers a sound system has, the more reflections and crossovers are likely to occur. Because high-end home theaters and listening environments often demand multiple subwoofers – and the common best practice is to locate subwoofers separately – aligning all speakers optimally for each listener location can be complicated.

 Traditional challenges with bass optimization

 Historically, room correction tools have been developed to optimize full-range speakers. They have offered little support for subwoofers and bass control. This is partly due to the fact that most solutions have focused on improving the consistency of magnitude response, which can’t solve every problem. In the graph below, each blue line represents one location in a room, with the vertical axis denoting how much different the volume can sound from one listening location to the next. The ideal test result would show that all the blue lines overlap, meaning each location in the room receives the same magnitude for each frequency of sound.

Notice how the bass range from about 15 to 80 Hz shows a jumble of different values, indicating a wide variety of magnitudes throughout the room. This graph represents the best result attainable through EQ and basic room correction.

 To truly optimize bass performance, a system’s phase response must be corrected. But that can be very tricky. Determining the ideal phase response for each speaker is almost impossible with traditional measurement tools.

One existing solution, called “modal pattern prediction,” creates a heatmap of sound hotspots and dead spots based on the details of a room and sound system. The dead spots are not silent, but instead represent areas where certain tones may not be heard as well because of contradictory phasing. Although this heatmap can show what problems exist in a room, it offers no way to improve audio performance. It also requires significant setup time – a major downside to most room correction solutions – because no two rooms or two sounds systems are the same.

Overcoming challenges to bass optimization

One of the keys to solving the bass optimization problem is the introduction of an all-pass filter that allows for detailed control of a speaker’s phase response. Unlike high-pass and low-pass filters, an all-pass filter allows every signal through, but then also adjusts the phase response (timing). With this advancement, it’s possible to fine-tune the phase of each speaker so that the sound waves do not cancel each other out and create dead zones in a room.

Dirac Live Bass Control leverages all-pass filters, plus machine learning and artificial intelligence, to calculate gains and delays for every low-frequency contributor. In this way, the Dirac solution ensures that low frequencies add up so that the average is controlled, and most importantly, so seat-to-seat variation is minimized.

Looking at the lower frequencies in the graph below, we see how inconsistent the bass response is for different listener locations (where blue lines diverge heavily) when phase response is uncorrected.

As the following graph illustrates, using Dirac Live Bass Control with phase response correction and an all-pass filter improves the results greatly, with the system exhibiting a much more consistent bass response across all listening locations.

 A New Solution: Dirac Live Bass Control

Building on the strengths of the existing Dirac Live room correction solution, Dirac Live Bass Control introduces an automated method for determining the bass characteristics of a sound system at multiple listening locations. The solution comes in three versions.

●      The basic version of Dirac Live Bass Control is now bundled with Dirac Live, allowing users to manage a single subwoofer through custom crossovers and displaying measurement results to help users make speaker-by-speaker adjustments.

●      The second tier of Dirac Live Bass Control adds automatic calibration and filter adjustments for a single subwoofer.

●      The third tier of Dirac Live Bass Control is designed for multi-subwoofer management. It uses machine learning and AI to quickly determine the ideal settings for multiple subwoofers and can apply filters automatically, greatly simplifying a procedure that could otherwise take several days to complete.

What it all means

Innovations in home audio optimization provide unprecedented clarity and consistency for installers and end users. By adding phase-response correction to the process, our new bass control module for Dirac Live is the ideal tool to ensure that bass performance is the best it can be in every listening location throughout a room. Whether a system is using a single subwoofer in a standard-shaped room or multiple subwoofers in a complex room, the capabilities of Dirac Live Bass Control ensure that every listening experience is optimized like never before.