Dr Adam Hill, sound engineer and lecturer in Audio Engineering at the University of Derby advises on effective bass delivery...
Whether it’s a body-shaking live sound system, a dance-inducing club PA or an all-immersive home theatre, we crave bass.
There’s no such thing as a one-size-fits-all method for tuning subwoofer systems. Unlike with high frequencies, low-frequency sound reproduction is equally – if not more – affected by room acoustics as it is by the loudspeakers.
Without a good understanding of the loudspeaker-room interaction, it is difficult to get the most out of your subs. We’ll take a quick look here of the basics to deliver intestine-churning, but tasteful low-end to an audience, or whatever experience you’re targeting.
In small spaces like home theatres, room modes are king. These are standing wave patterns at various frequencies, dependent on the room dimensions, that cause severe seat-to-seat variations in the frequency response, as well as unwelcome transient smearing.
While room modes exist throughout most of the audible frequency range, they are only perceptible below the Schroeder frequency, which is calculated based on reverb time and volume. In small rooms, this comes to around 200Hz.
So how can we reduce the negative effects of room modes to provide a consistent listening experience across a room?
Most home theatre systems come with a measurement mic and an automatic calibration procedure. These methods typically employ an inverse-filter, based on a single measurement to give a flat frequency response. These achieve precisely that, but only at the measurement location. In terms of bass coverage, they provide no benefit whatsoever. The same goes for spatially averaged measurements. You cannot EQ away room modes. Something different is required.
A good first step is subwoofer positioning. Sticking a subwoofer in a corner excites all room modes, resulting in a boomy bass response as well as severe seat-to-seat variations. If you have one subwoofer, try moving it to a wall midpoint. This prevents certain modes from being excited and gives a less boomy sound with more even coverage. If you can get away with it, get a second subwoofer and place the two at opposite wall midpoints. This further improves the situation. In essence, the more subwoofers you have, the less room mode issues you’ll encounter.
Low-frequency reproduction in live sound is a different beast than in small rooms. In venues we typically work in, the Schroeder frequency is usually below 20Hz, therefore room modes are not an issue. So what are the challenges in live sound?
Instead of room modes causing the issue, it’s comb-filtering between subwoofers and early reflections (for indoor venues) that is the culprit.
When you send the same signal to spaced loudspeakers, the signals recombine constructively or destructively, depending on the phase relationship between the arriving sounds at a given location. When subs are deployed as left and right stacks, this causes dead spots in off-centre areas of the audience and a power-alley down the centre. This means the FOH engineer receives more bass, which can result in a bass-light mix for off-centre areas of the audience.
To avoid this, it is fairly well known that spacing your subs across the front of the stage gives more even coverage due to the small phase differences from the arriving waves. Typically, you want to aim for no more than ½ wavelength spacing of your subs (two metres @ 85Hz).
Without any other DSP, this gives even coverage across an audience that is between the horizontal array of subs, with a gradual roll-off of energy outside of this area. To widen this coverage, you apply delay to the outside stacks. To narrow the coverage, delay the inside stacks.
If using cardioid subs, it is essential to give adequate free space behind the units – at least two metres or so. We’ve found at Gand that if you don’t have this, you lose system efficiency as well as the cardioid pattern.
If you want to further reduce comb filtering, you must find a way to decorrelate the signals from each subwoofer. Based on my experience, I see more and more system techs configuring PAs so the subs are fed by a mono aux. This means that the signals coming from all subs are highly correlated and will therefore cause severe comb filtering.
In cinema, there are similar issues to live sound. What I’ve found, though, is that cinema is lagging behind live sound in its subwoofer calibration methods. They rely largely on single or spatially-averaged measurements to tune their systems, which, as I’ve mentioned, does nothing to improve consistency of bass coverage.
Cinemas have the benefit of speakers surrounding an audience. The system naturally exhibits a degree of decorrelation, but comb filtering is still a serious issue, much more so than room modes – they’re only a problem below around 35Hz.
While it’s possible to use an optimisation routine, requiring precise measurements and signal processing, I’m developing a method of automatically decorrelating the low-frequency signals feeding the loudspeakers – a method known as Diffuse Signal Processing.
This technique applies an imperceptible, low-level noise-like decay onto source signals, which when applied correctly avoids significant comb filtering. This requires no on-site calibration.
Although work is ongoing to perfect this method, I see applications not only in cinema sound, but also in live sound and home theatres.
The ultimate goal is to deliver an equal listening experience to all. Clearly, the challenges at low frequencies are very different from those at high frequencies.
While this article only scratches the surface of the tools available, I hope it gets you thinking about simple fixes you can apply to whatever system you work with to give the best possible listening experience to as many people as possible.
Dr Adam Hill is a lecturer in Audio Engineering at the University of Derby in the UK, and a live sound engineer for Gand Concert Sound.