Sub-woofers: how to extend the bass response of your micro-monitors.
Ben Duncan provides an insight into a useful method for extending the bass response of your studio's less-than-efficient micro monitors.
Inevitably, small loudspeakers lose out - in one way or another - at the bottom end. If this wasn't the case, nobody would bother to employ the more monstrous variety. The small speaker's setback can be overcome though, by adding a single, specialised bass speaker - a subwoofer. This scheme is displayed in Figure 1.
But perhaps you're doubtful about the value of this. If you're using mini or micromonitors, it's probable that (i) you're on a tight budget, or (ii) your working space is cramped, or at least insufficiently big to accommodate a pair of wardrobe-sized enclosures. Or (iii), you're a musician, and you don't see the point in big speakers, because small ones sound 'just fine', or (iv), an extrapolation of (iii), because small speakers help you see the stereo sound-stage most clearly. If so, don't worry: the concept of adding a second, auxiliary speaker to soup up the bass-end needn't contradict these principles.
First, if your budget is very tight, forget it. But if you have £30 or more to put towards a DIY crossover, and can lay your hands on a redundant PA or back-line speaker, the project is feasible. On a more sophisticated level, meanwhile, you can put together a bi-amped sub-woofer system for less money than a pair of full-range monitors, and save valuable floor space while you're at it. Reasons why a sub-woofer needn't contribute to acute control-room claustrophobia include the fact that we only need one (for a stereo set up), and moreover, sub-woofers can be tucked away in outright strange places, without detriment to their performance. If, on the other hand your situation is in category three, (above) you may feel 'to hell with sub-woofers', but just in case you're curious, rest assured a single sub-woofer won't screw up the wondrous imaging qualities of your existing monitors, save any hard panning on bass instruments. Then, there's always the option of stereo sub-woofers...
Figure 1 contrasts the low frequency performance of 3 broad classes of speaker. The first has an 8", Thiele-loaded driver, the second and third, 6" and 4" drivers respectively, in infinite baffle (sealed) enclosures. Speaker No. 3 is better known by another name, of course - it's an archetypal micro-monitor. And as you can deduce from the graph in Figure 2, its salient bass-end characteristic is that its output dies off rapidly below 250Hz. In speaker jargon, we point to the frequency where the level has fallen by 3dB, and call this the 'f3' frequency: f3 = 250Hz.
The practical result is that even if the sound is loud enough, and you can hear the bass instruments clearly, something is nevertheless missing. Indeed it is - around 20 to 25% of the audible spectrum is absent, or at best, considerably muted. So how come we're able to put up with this sorry state of affairs?
The answer lies in our hearing mechanisms' adeptness at error-correction: when we hear what sounds like a kick-drum, or bass, or a bass synth - with the fundamental and the first few harmonics either absent, or muted - our minds are able to synthesise the missing bits.
Figure 3 shows this pictorially. The solid vertical lines represent the fundamental and the harmonic series of a bass instrument. A curve has been drawn across their tops to symbolise the instrument's characteristic timbre. The second set of (hatched) verticals shows the output of a micro-monitor, reproducing the same sound. Note how the roll-off below 250Hz has muted the fundamental, together with the 2nd and 3rd harmonics. The curve along the top, contrasted against the original, suggests how radically the attenuated bass response has changed the timbre of the instrument, though the pattern of the higher harmonics remains unchanged.
The quality and accuracy of the 'error correction' is naturally down to our individual aural experience. If you'd never heard a kickdrum before, hearing one for the first time on a 4" micro-monitor would lead to a perception very different to that of a musician, who's exposed to said instrument all the time, and has become very adept at mental reconstruction, albeit largely subconscious.
Other than having an extensive 'library' of instrumental sounds to draw upon, this 'bass in the head' trick depends, of course, on how well the higher harmonics are put across. Looking back at Figure 3, you can see that harmonics above the 4th are reproduced in the correct perspective, and providing this ties in with a flat response across the midrange, and low distortion, the overall reconstruction of the missing bass will be sharp and realistic. This leads us on to the fact that the midrange performance of any speaker says a lot about the bass sound. From another angle, our perception of sound is akin to holography, ('The part contains the whole'), except that all the equipment resides in the human brain.
Bass 'in the head' can be a subtle and elusive thing. If you try focusing on it - instead of the music - it'll most likely vanish; in this sense, it's like peripheral night vision, and objects that disappear when you turn to face them. Another reason why 'bass in the head' is second best is that it's precisely that; it has no physical side-kick. And self-evidently, the physical sensation of low bass, however subtle it may be, is more often than not all part of the music's feel. Literally so!
Knowledge of this psychoacoustic dimension is helpful if we're to come to terms with a contradiction: on the one hand, large speakers are emphatically a must for wholesome bass, yet on the other, it's possible to live and work quite happily with tiny, bass-less speakers. The main reason for favouring an extended bass response is not for its own sake, but because it leaves less work for the imagination. In other words, it releases mental energies, which can be more usefully channelled into the central task of producing or assessing music.
Going back to Figure 2, speaker 1 has a 50Hz f3, so it shouldn't have any significant low frequency problems. Or perhaps not, as the family of curves in Figure 4 reveal.
Here, the speaker's frequency response is shown at a range of drive levels.
As you can see, the response to 50Hz, quoted in the specification is only true for levels of 1 watt (= 90dBSPL), or below. At higher levels, though, we're no longer looking at frequency response alone: the zone bounded by the tinted line represents the speaker's physical limits, the maximum PHC. (See HSR May 84, p. 59 for more details on this).
Once we hit this limit, the bass has to stand aside, if the speaker isn't to be damaged. In effect, this means we have to roll off the low bass with EQ. For example, with the speaker shown, we'd have to inflict a 3dB attenuation at 190Hz, rolling off at about 6dB/octave below this frequency, in order to safely drive the speaker at (short term) 115dBSPL levels. This, incidentally, corresponds to a 98 to 102dB mean sound level - nothing special, that is. And ironically, the instruments which most need plenty of dynamic headroom, that's slapped bass and kick drum, are precisely the ones that most stress the low bass power handling capabilities of small speakers.
From this examination, we can see why a sub-woofer may be desirable, even if the frequency response of the existing monitors is quoted to a 50Hz f3, or lower. By handing over the heavy bit (the low bass) to our sub-woofer, we can make full use of the existing monitors without sacrificing a full-range sound.
At frequencies below 800Hz, our hearing gradually loses its sense of direction, and equally, the output of all but the biggest speakers becomes progressively omnidirectional. Down at 100Hz, it's no longer possible to tell the exact whereabouts of a source reproducing this frequency by static, audible clues alone. Sure, we may be able to say with certainty that it's above or below, or in front, or behind, but without roving around the room (no peeking under that blindfold please!) pinpointing the speaker isn't possible.
By contrast, incidentally, a skilled listener + blindfold can typically walk straight towards a midrange speaker from 15 feet, after being disorientated, and point directly at the middle of the cone! Our sonic direction-finding apparatus is aided by the fact that at 3kHz, many speakers have a fairly narrow dispersion in the high midrange, the frequencies where our sense of direction is most acute.
Given that monitoring or listening excludes wandering around the control room, this psychoacoustic trick can be put to good use. First, provided the path length from sub-woofer to your ears is roughly the same as the existing monitors, you're free to place the sub-woofer anywhere in the intervening space.
"Second", announced the professor, with a dramatic flourish, "Sub-woofers can be made to disappear" Alice was still puzzling over this assertion, when, with an eldritch scream, the control room floor gave away, the professor collapsed, and his left foot shot through into the bowels of the building, via the cone of a 24" drive unit, which had been lurking under the floor.
Yes: The good news is that sub-woofers can be buried under the floor, and even covered with a light carpet, but be sure to lay down some strong weld mesh! Alternatively, the enclosure can be buried in the wall or the ceiling, with the proviso that the structure is solid enough. Otherwise, the sub-woofer may amplify any underlying acoustic nasties. But we'll look into this later.
This leads us on to the "Look, 2 speakers but only one sub-woofer, isn't the stereo great!" trick. With our hearing losing its sense of direction below about 250Hz, there's little or no natural stereo perception below this frequency. So with the exception of (unnatural) hard left/right bass panning, one sub-woofer is as good as two. Cynics amongst you will have observed that this is one of the few anomalies of the human ear that can save us money and space...
The next step is a practical one - choosing a speaker. Surprise, surprise, this need not cost the earth, and needn't be a specialist unit. But this, and more about bi-amping must wait until next month.