Paul's Posts — 20 August 2012
By Paul McGowan
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.
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Soundminded
Because the wavelengths of low frequencies is very large compared to home listening rooms,there’s a characteristic of cancellations away from room boundaries and a lack of acoustic reinforcement below the room cutoff frequency. I’ve read response can varay up to 40 db in just a few feet. It wouldn’t surrprise me. Toole seems to have solved this problem. By placing 4 subwoofers in the room corners or on the middle of all 4 walls uniformity throghout the room is greaty improved. I haven’t tried his solution yet but maybe I will.
Interesting how so many people who have so much disdain for negative feedback in amplifier circuits where successful execution is a matter of expertise (which admittedly only a small percentage of amplifier designers have owing to its diffuculty) think nothing of tackling the much more difficult to execute electromagnetic servo feedback system. But even when optimized, the servo aspect can only solve so much. What it can do is extend the motion of the voice coil to be a closer analog of the electrical input signal. It can’t solve room resonance and cancellation problems, it can’t overcome harmonic distortion due to a host of sources such as cone flexing. It is in effect an automatic equalizer where the signal is boosted where and to the degree that the mechanical response is weak. But even there it has limitations.
To optimize the potential of servo feedback start with the best performing conventional non servo system. IMO for many reasons that would be the acoustic suspension design. It has multiple advantages over other designs in general and over ported systems in particular. I know that’s controversial these days but I’ll stick to my guns.
All woofer/enclosure systems are explained by two equations. These equations are often combined into one monster equation and their application is simplified by the Thiel Small parameters which turns them into a cookbook recipe, that is you don’t have to understand the theory to apply them successfully, you just plug in the numbers into the software and the right answer pops out. The first equation is Newton’s second law of motion applied to forced oscillation. This equation explains the mechanical behavior of the system, especially frequency response. The second equaton is the LCR equation which explains the speaker’s electrical behvior…sorta. Ironically the two equations are exactly the same and they’re called ordinary secondary order differential equations. Their solutions are well known. The one kicker in the LCR equation is often the omission of the reverse emf component, a signal generator that operates out of phase to the input. It tends to generate a signal at the system resonance frequency from stored mechanical energy when the cone is displaced being converted back into electrical energy no matter what the driving signal frequency. It’s always bucking the input signal. This can be minimized by both mechanical damping in the speaker/enclosure design and electrical damping by the amplifier. In this regard the low output impedance of solid state amplifiers used with heavy gage wire has a huge advantage over tube amps.
One advantage is that the mechanical springiness represented by the letter “k” in the mechanical system equation is not frequency dependent in an acoustic suspension design (it follows the gas laws P1*V1=P2*V2) but is strongly frequency dependent in the resonant air column of a ported design. This makes AS systems relatively easy to design to adjust their frequency reponse to be flat down to any arbitrary frequency and system Q to be controllable. It’s much harder in a ported system. The AS system has another huge advantage that’s often overlooked. Because the restoring force on the cone is distributed uniformly over the area of the cone by air pressure there is no force gradient radially or circumferentially tending to shear or twist the cone into harmonic breakup modes. This is one reason why in 1955 AR1W and its descendants had only 5% THD at 30 hz and can be equalized flat to extremely low frequencies. AS systems have only a 12db per octave falloff below system resonance. It’s also easy to adjust system mechanical Q optimally to the ideal of 0.707. Heavier cone material required for an AS systems also results in a stronger structure for a given material. Ported systems by contrast have a series of resonance nodes at the resonance frequncy and at every harmonic while they have an anti-resonance nodes at every halfway point between. This must be overcome to obtain flat response and the system falls off at 24db per octave below system resonance manking extening FR using equalizatoin next to impossible.
The supposed “speed” of a woofer is IMO a bogus argument against AS systems. What may increase is propagation delay, that is the time between application of the electrical input and mechanical response. This only matters theoretically at the crossover frequency where two drivers are operating at the same frequency, one lagging behind the other. This would have an audible effect of phase cancellation and hence an FR dip. This can be overcome by using a compensating time delay in a multiamplifer arranged system for the “faster” drivers.
hahax
I agree with you about fast woofers. Bass is simply not fast. But what I think people refer to is well damped woofers that don’t ring, woofers with low Q. The problem with low Q can be sales. They sound lean to people who are used to higher Q speakers, which is most of them.
And thenI have a friend(a designer) who likes to say if you want a faster woofer, get a better tweeter. Of course he’s saying that woofer speed is also a function of a bass note’s overtones.
hahax
In respect to the need for large bass drivers there’s the need for a bass driver to play loud, loud enough to be heard and that means large. A small woofer can be designed to go very deep but it will sound way down at the levels it can play. Look at the fletcher Munson curves. Our ears are most sensitive at mid frequencies but that difference begins to reduce at high levels. At 80 db, pretty loud as an average, our deep bass is down about 10 db to our ears. It takes about 100 db, very loud and unsafe to equalize bass and mids.
johnalgodones
I agree with Paul. I’ve set up systems in many rooms and have never found that the best imaging and deep bass coincide in one speaker position. Even when using a sub, finding the best location can be very tricky. Paul may write about locating subs tomorrow, but in the meantime, you might want to try computer aided setup. I use RPG Diffusor Systems “Room Optimizer Software” which will locate the best position for as many as three subwoofers. My two ended up (roughly) in the rear corners, where, with some care, they blend perfectly with the front speakers. This did require moving the listening position forward about 6″. But, because you enter the location of the front speakers in their best imaging position into the computer program, you end up with the subs in their best position without having to move the main speakers.
hahax
The problem with the woofers seperated from the main modules can be properly integrating them together. The long wave lengths at cross over involved help but don’t always lead to ideal integration. I do like the solution the Pipedreams and Scaena speakers(some of the same designers) used, a crossover with variable phase adjustment. You find the positions for both sets of boxes and null a sgnal at crossover varying phase and then flip the phase 180 degrees.
oliver T. Finch
A 15′ sub woofer and a bass tower with four 18” woofers do not sound the same even when matched for volume. Why? Even the sound stage is much larger and the whole performance is more life size. I think it has to do with the amount of air moved. Frequencies around 100 c\s and below effect the mid range and above. Also,very low frequencies are felt more than heard. Paul has hit the nail on the head when he talks about the amount of air movement. Regards.