So the much bigger question here, to my mind, is why you would want speakers that can play linearly all the way down to 20Hz or lower in the first place.
I sense that Irishman might have fallen into the same trap of thought and intuition that afflicts so many fans of audio - namely that the "ultimate" goal would be to have each and every speaker playing the entire audible frequency range from one location. Believe me, at one point, I fell into that line of thinking, too, going so far as to make plans for using 5 or 7 subwoofers in addition to 4 dedicated LFE subwoofers so that each individual speaker could have a subwoofer placed beneath it that would act as the bass driver so that every single speaker could be run full range.
But here's the thing: what is it that we're actually trying to accomplish?
To my mind, what I want to be able to do is to sit in any seat in my theater. And from any seat in my theater, I want to be able to play a frequency sweep from below 20Hz all the way up to above 20 kHz. I would, of course, play this sweep at 85 dB SPL so that my hearing would be at its most linear. And what I would want to hear would be as even and uniform a sweep as possible.
In other words, if the signal in the recording is telling a speaker to play any given note at any given loudness, that is what I want to reach my ears. Naturally, as I move from seat to seat, certain speakers in a surround sound setup would be physically closer to me, and thus louder. But from the primary seat, every single speaker ought to sound equally loud, and perfectly even throughout the entire sweep. And then if I move to a different seat, I can expect that physically closer speakers will sound louder, but the sweeps from each indivdual speaker should still play evenly in loudness from the lowest note to the highest. To me, that is simply accuracy. The signal said play whatever given note at whatever loudness. That's what I want to hear. No rollercoaster of louder and quieter during the sweep. And no lack of uniformity from seat to seat.
So what actually happens if each and every speaker plays to below 20 Hz all on its own? You can try this for yourself. Create a bass sweep and play it through only one speaker at a time. You will discover that the bass sweep sounds like a rollercoaster and is not uniform from seat to seat.
The bottom line is that genuinely full range speakers are not the goal! It doesn't matter how they play in a wide open field or in an anechoic chamber. It matters how they sound in a real room. But it is not the speaker's fault! It is playing linearly and accurately. So what went wrong?
It comes down to the fact that bass sound waves are physically very long. 20 Hz, for example, has a wavelength of about 55 feet (that's sound travelling through air, room temperature, sea level air pressure, all that sort of thing. There are small changes that can happen due to temperature, air pressure, etc. But the wavelength of 20 Hz in air is roughly 55 feet).
So let's think about what happens in a room:
The speaker's driver starts to move outward from the speaker causing a change in pressure that propegates outward. Before the speaker driver even reaches its maximum travel outward, that sound wave has already reached a wall and reflected. As the speaker driver starts to move back inward, some of that original outward pressure is already reflecting back in the same direction, having travelled outward, reflected off of a wall, and is now moving back in the opposite direction. With sound travelling at about 1100 feet per second, the sound wave can move past your ear, reflect off the wall behind you, and return as a reflection before the speaker driver itself is even moving back into the cabinet! And this reflection just compounds and compounds as the speaker driver continues to move in and out while the sound waves continue to reflect back and forth and all around the boundaries of your room.
And THIS is why it is said that bass is "omnidirectional". Bass isn't really omnidirectional. If you were to suspend yourself and your speakers more than 55 feet above the ground, with no ceiling above you, and no walls around you at all, and all of the speakers at least 55 feet away from you, then even 20 Hz bass would be directional, and you really would want genuinely full range speakers at every position so that the bass would come from the correct direction! But NO ONE has that setup
So in any room, we are never hearing direct sound when it comes to bass. The wavelengths are too long, and we are only ever hearing reflected sound. That is why it seems to be omnidirectional - or directionless. But more importantly, that is why we do not actually want full range speakers! It is totally intuitive to want them: the signal is saying to play a given bass note from a given speaker - doesn't it make perfect sense to then have that speaker play that bass note? Make perfect intuitive sense. But the end goal is not about having the bass COME from the location called for in the recording, the end goal is about HEARING the bass that is called for in the recording. And your room won't let you do that.
So it doesn't matter how low your speakers can actually play themselves - at some point, due to the positioning of your speakers, your seats, and the dimensions of your room, the wavelength of a given bass frequency - and all frequencies below it - will exceed at least one of the dimensions of your room, or the distance between you and the speaker. And at that point, you will cease to hear direct sound and only hear reflected sound. And once you start hearing only reflected sound, it will sound like a rollercoaster, and it will sound different from seat to seat.
The solution is subwoofers. Multiple subwoofers. The goal is not to eliminate subwoofers from your system. Subwoofers just take a 2-way speaker and turn it into a 4-way speaker, or a 3-way speaker into a 4-way speaker, and so on. There is absolutely nothing less "pure" about having subwoofers play the lower frequencies. By that logic, the only type of speaker anyone should ever use would be a single driver. Otherwise, the sound is less "pure" because the tweeter is only playing a certain range of the audible frequencies while the mid-range and mid-bass drivers play other select ranges. We don't go around disconnecting our mid-bass drivers in our speakers just to make the mid-range and treble more "pure". So why do some of us have it in our heads that disconnecting our subwoofers should be some sort of lofty goal?
No, the solution is to use multiple subwoofers positioned carefully around the room so that those long bass wavelengths have so many interactions amongst themselves that they just even out and become uniform throughout the room. Bass sound waves emminating from any one location will always produce bass modes and peaks and dips that are non-uniform from seat to seat. But multiple subwoofers can create so much sound wave interference that it all just evens out and becomes uniform.
For all frequencies above the point where they become uneven at even one seating location, we want the individual speakers playing those. So some level of bass extension in the speakers is certainly necessary. But not full range.
I get it. I get the intuition. I get the appeal of the idea of having each speaker play the entire audible frequency range from its one location. If you super duper want to do that, why worry about having it all built into one cabinet? Put a subwoofer beneath each and every speaker and use whatever crossover you like best. It's no different than the crossover between your tweeter and the mid-range driver or the mid-range driver and the mid-bass driver(s). You can take any 2-way speaker and turn it into a 3-way by putting a subwoofer with a crossover underneath it. Simple. cost-effective. But it still should not be your goal. It doesn't work. Go ahead and play a bass sweep to hear it for yourself
- Rob H.
