I rather think I shortchanged everyone yesterday in my post about bass. I guess my bias towards adding subwoofers outweighed anything else and I realize not everyone agrees with me or can add a subwoofer. This is a long and hotly debated subject.
First there are those of you that believe your loudspeakers reach the depths of human hearing by themselves – in fact your owner’s manual and the marketing hype of the speakers told you so. It may be true but in my experience rarely so. Here’s a couple of things to contemplate: woofer size and position.
If your full range loudspeakers has a single woofer per side that is 10 inches in diameter or smaller, you’re not going to have a chance getting 20Hz bass in the room. Why? Because what a woofer/piston needs to do is couple the motion of the woofer to the air so that any change in motion of the woofer results in a 1:1 change in air pressure in the room at any frequency we’re interested in. What happens is that because air is not resistive, a 20Hz motion at the woofer will not move the air in the room as loudly as (say) a 60Hz motion will. This forces the loudspeaker designer to do one of two things: EQ the woofer so it is no longer flat and is significantly boosted at 20Hz, or measure the woofer’s response with a very closely placed microphone that ignores the coupling issue. Most loudspeaker manufacturers choose the latter. The first option is almost impossible to pull off effectively unless you separately amplify the woofer with another amp or accept a huge loss in overall loudspeaker efficiency.
The next problem one faces, even if the loudspeaker pair in play has a large woofer that can couple the air in the room at low frequencies, is placement. If you place the loudspeaker pair for best imaging chances are that is not the best place for bass. Bass is tricky and is one of the hardest problems in the room to solve. You set the loudspeaker pair in the best place for bass and quickly find that the imaging sucks or vice versa. Unless your loudspeaker pair has a built in subwoofer and the ability to turn the bass level up to match the room characteristics then you’re pretty much in trouble. This is why I recommend subwoofers in almost every case.
Ok, having said all that and realizing many of you will not go out and buy or use a subwoofer, what else can you do? My best advice is to get at least the upper low bass correct by speaker placement and, if need be, give up the quest for subterranean results. In other words, let’s focus on getting the sound of a good stand up bass or an electric bass to sound perfect and ignore the rare occasions where you want to have a pipe organ right in your room rattling the basement and family into submission. It’s indeed rare to have both without a good servo sub.
To get the best performance of a stand up bass you need the pluck of the bass coupled with the satisfying low bottom end to occur at exactly the same moment and sound as if they are coming from but one instrument. This isn’t that hard to achieve and you can get there with speaker placement.
Remembering to mark the position of your loudspeakers now – including the amount of toe in which can be as easy as writing the number of inches from the rear wall onto the tape on the floor for both sides of the speaker cabinet – first focus on the low part of the bass. Once the low end is right you can dial in the pluck which is a transient event and controlled by the upper frequencies.
The tricks for getting increased low end are moving the pair closer together and/or moving them closer to the rear wall. That usually gets what you want – but not always! As I mentioned, proper low end is tricky and you may have to move your listening position as well as the pair forward or backwards to to get it right. Remember that getting it “right” for low bass will only apply to your listening position in most rooms – because bass will be louder in some parts of the room, relative to the position of the pair and the listener, while not so loud in other parts of the room. The trick is to get the bass right in the part of the room you’re sitting in and ignore everything else.
Take notes, don’t bother moving the reference tape at all – leave it right where you placed it. Once you have the low end dialed in where you want it, go back to step one and test and readjust minutely for the single voice, then move up to the Tutti track and get it as best you can within the new area then check the low bass track until you have what you want – understanding that along the way we’re compromising each setup a little to get the best overall performance for all three – center image/proper placement behind the loudspeakers, outside spread of the orchestra/speaker disappearing, and now low bass/proper pluck. Pluck happens with toe in or reducing the distance between the two loudspeakers because it is a higher frequency event.
If after all these exercises you have everything working the way we’ve described, yet getting there has caused a bit of a brightness to the sound – which increased focus can sometimes do – here’s a neat trick you can use as long as it’s sparing. Tilt back the loudspeaker pair just a little. Start with a CD case placed under each loudspeaker – increasing to a couple if necessary. This will get the tweeter off axis and make it not quite so hot as well as increase the depth of the soundstage.
Paul McGowan – PS Audio, Intl.