What sort of low frequency response do you really need? Keeping in mind that extending the low frequencies is costly both in amp power and cabinet size, what is a realistic expectation?
I think a lot of the answer depends on what sort (genre) of music you will be mixing, and what the instrumentation is. For example, a traditional bluegrass band is going to have vastly different low end requirements than prog rock, metal, or pop. Why, you might ask?
Well, obviously, the lowest frequency you will have to be concerned with will come from the instrument that generates that frequency. It doesn’t take long to realize that the instruments you’ll have to look at are bass, kick drum, and synthesizer.
Let’s ignore the synth for a minute, and look at the other two, since those are present in nearly every situation. The lowest note on a four string bass (in standard tuning) is the low ‘E’, which is roughly 40Hz. Coincidentally, one of the most commonly used mics for a kick drum, the Shure SM-57, has a frequency response of 40-17,000Hz. So, in both cases, the lowest frequency is 40Hz. In this case, it wouldn’t make much sense to carry around a subwoofer system capable of generating 20Hz (or even 30Hz), since, as I stated before, extending the low frequency capabilities of your system means the cabinets are going to be MUCH larger and heavier. So, in a lot of cases, I would build my system to extend to 38 Hz (or so).
What if the bass player uses a 5 string? Well, the low ‘B’ (a fourth below ‘E’) is around 32Hz. You’re going to have to build the system so that 32Hz is still within the efficient frequency range. A lot of cabinets which are rated down to 40Hz will actually generate sound at 32Hz, but it’s not going to be NEARLY as efficient (i.e.-loud), and if it’s a vented enclosure, you run the risk of exceeding the excursion limits of the speakers, since the cabinet doesn’t provide any loading for the speaker at that frequency.
Many people say that you don’t absolutely have to have a system that is capable of generating these frequencies. Well, true, you don’t “absolutely” have to be able to do that. To be able to fully understand why you “should” have a system capable of reproducing everything, let’s look at some aspects of tone, waveforms, harmonics and overtones, etc.
A sine wave is a very smooth waveform that generates NO harmonics (overtones). It is what is called a ‘pure tone.’ As you sweep a sine wave down through the lowest frequency of the subwoofer, while it is above the cutoff frequency, the cabinet is operating at maximum efficiency, and you will have a loud sine wave. As you reduce the frequency of the sine wave to below the range where the cabinet operates, the resulting “loudness” will get softer and softer. If you are in an otherwise quiet environment, you may still hear the speaker cone “flapping” back and forth, since the amp is still dumping power into the speaker, and the cone still moves in response. But, since the cabinet is not efficient at this frequency, there is little sound (at least not a low frequency tone).
However, a bass guitar doesn’t generate a sine wave. It generates a very complex waveform full of harmonics (overtones). One of the strongest harmonics is the octave (an octave increase is exactly double the frequency). There are many many more overtones generated by the vibration of the string on the bass, but we’ll just look at this one for now. So, let’s say that the bass player plays a low ‘E’, which is around 40Hz (we’ll just round it to 40Hz for convenience right now). Let’s say that your bass cabinets only operate efficiently down to 50Hz. Can you guess what happens? You won’t hear the 40Hz low ‘E’ coming from the system. You will instead hear the next strongest overtone, which is the octave ‘E’ at 80Hz! As the bass player walks up the scale, he will reach a note where the fundamental is at or above 50Hz, and that note will sound lower than the low ‘E’, simply because your system could not reproduce 40Hz!
So, if the bass player uses a four string, and you use an SM-57 on the kick, I’d say 40Hz (or just slightly below) is a good target, no matter what genre of music it is.
If the band you work with consists of a couple of acoustic guitars, and singers, I’d say you can get by with a much higher frequency. The low ‘E’ on a guitar is 80Hz, and with an acoustic you are obviously not concerned with reproducing cabinet thump. I’d say that there is little reason to worry about much below 80Hz in that case.
The genre that can be of the most concern is “prog rock,” or just “prog.” In this often synthesizer-laden style, there are often one (or more) synths playing a “pad” sound (think background strings, only with other, meatier sounds), that typically extend down to the nether reaches of the frequency range. As an example, Emerson, Lake, & Palmer were the first group to tour with a system capable of generating 20Hz. (TWENTY HERTZ!!!) This was strictly because of the notes that Emerson played on his Moog (particularly on the song ‘Lucky Man‘). That system just also happened to be a quad system, and this was 1969 or 1970!
When you design the system for a band which has a synthesizer player, I’d study his use of it and see what frequency ranges he delves into. If he doesn’t play anything that goes below 40Hz, I’d just be concerned with the lowest note that bassist can hit (is he using a 4 or 5 string?).
It would be nice to be able to cover any contingency, but in the real world, it’s best to use “appropriate technology,” and tailor the system to your needs, rather than some outlandish ideal of perfection. If you want to build a system that extends down to 20Hz, go right ahead. You will be doubling the physical size of your system (since a cabinet that generates 20Hz needs a mouth area that is twice that of a cabinet that generates 40Hz), increase the weight, trucking requirements, and a host of other items. And, if nobody in the band (i.e.- the synth player) ever hits 20Hz, nobody in the audience will even know the difference.
However, if you try to “cheap out,” and build a system that only extends down to 80Hz, the kick drum and bass are going to sound really thin, since you’ve effectively eliminated the bottom octave of their ranges.