Bow wow

October 2, 2014
 by Paul McGowan

Woofers. Bass drivers. Big cones. All used to produce about 70% of our music’s content in terms of quantity.

If you look at an amplitude spectrum of music you’ll find the vast majority is well below 1,000Hz and most of the energy our power amplifiers deliver to our loudspeakers is used by the woofer to reproduce these lower frequencies. And as you go down in frequency you need bigger sizes and more power to push them.

In our continuing series of subwoofers, we’ll start to take a look at the actual woofer driver itself. While my field of expertise is in designing analog electronics and I am no expert on the design of woofer drivers, I do know enough to get us a bit of understanding. So let’s move forward.

We understand that all speaker drivers are pistons; perhaps better put as ‘air pumps’. They push and pull back and forth, pressurizing and depressurizing the air in the room to make sound. The quality of that piston has a lot to do with how well the air is pressurized in the room.

The amplifier sends out power to move the piston a specific distance, then brings it back again. The expectation is that the piston (speaker driver) will move the air in an exact duplicate of the amp’s output. No deviations allowed. Any deviation from what the amplifier intended is called distortion. We hope for a perfect piston but, alas, nothing is perfect. Imagine, for a moment, a poor piston. A really poor piston. One that perhaps cannot move as far as it needs to, or not as quickly as the amplifier demands, or changes its shape from that of a perfect piston to something else. In this case we wind up with a lot of wrong air pressure in the room and get a noticeably distorted output.

Most modern woofers do not have loads of distortion. Even the cheapest crap out there is pretty good. But we’re not interested in pretty good. At least I am not. We’re searching for varying degrees of really good and, as such, we need to look at several parameters of woofers. How they are constructed, what materials they are made from, and how they are driven.

If we want to make the ‘perfect piston’ or ‘air pump’ we want to make sure, at a minimum, our piston follows exactly the instructions from the amplifier and does nothing on its own. The first requirement, following exactly the amplifier’s instructions, is perhaps the most difficult task for a woofer of all. Let’s put that aside and focus on the second; what materials they are made from.

A perfect woofer might be made from a thick steel cone. Why? Because it is so stiff and unbending that the air it is attempting to pressurize cannot push back hard enough to change its shape. If we make our cone instead out of something more flexible, such as paper, our chances of the air pushing back and distorting that paper go up dramatically.

Yet, there are no thick steel cones and there are many paper cones, and everything in between. Why is that?

Tomorrow we’ll take a look.

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10 comments on “Bow wow”

  1. One criteria for cone material is that it must be strong. Steel (one of my favorite materials for many things but not speaker cones) meets that test. But another is that it must dissipate energy within itself efficiently, that is it must not resonate. Flick an aluminum pie tin with your finger and hear it sing. Energy takes a long time to dissipate as it resonates. Paper by contrast doesn’t do that to nearly the same degree. Energy is lost quickly between the cellulose fibers. Paper can be made remarkably strong for its weight. Plastic is better than metals such as aluminum but not as good as the most suitable paper. The resonances of metal are most noticeable in tweeters where their ringing gives music a bright harsh sound. Steel has another disadvantage for a cone, it is magnetic and can affect the magnetic field at the voice coil distorting it. Where mass is a consideration, steel also strikes out as it is relatively heavy.

    YG Acoustics machines aluminum cones for its speakers from a solid billet. Here are videos that constitute a factory tour starting with the third video from the top.

    “Once you’ve lived with a (1/2 ton) crane in your listening room it’s hard to go back.”
    Dick Diamond

    http://www.avshowrooms.com/YG_Acoustics_Co.html

  2. I have often wondered what effect two loads on a preamp has If I added a powered subwoofer to my full range system, both out of the same preamp. What about impedance etc?

  3. Paul, you stated, “Woofers . . . produce about 70% of our music’s content in terms of quantity”.

    I’ve read a couple of sources that state that the mid-point of acoustical energy produced by a symphony orchestra falls somewhere around 260 Hz. If this is true and half of the content quantity is below that frequency then I don’t understand your statement. And even if we expand our consideration to woofers in general to come closer to that frequency, how does that relate to your subject of sub-woofers?

    There is another point here. You have been very clear about your preference for operation the main speakers and amp full range (to whatever limit they have) and overlaying a sub-woofer to that. Yet you acknowledge here, “most of the energy our power amplifiers deliver to our loudspeakers is used by the woofer to reproduce these lower frequencies.” So why would we not want to relieve our main woofers and amplifier of the task of producing or attempting to produce those lower frequencies if they can be handled by a powered sub?

    It seems to me there may be two benefits from adding a subwoofer system. The first is the obvious ability to extend the low frequency capability of the system. But might a second be the removal of some of the mid-bass (40-80 Hz, but possibly lower depending on the system) from the main system woofers and amplifier, thus lowering distortion and improving clarity for the upper bass and lower midrange?

    Thanks if you can shed more light on these points.

    1. I believe the optimum crossover point for a woofer is 400Hz. This is where your ears start to roll off bass, needing sensing via other mechanisms (body cavities) and compensation in your brain to maintain sensitivity. It is also where the wavelength is long enough that inter-aural level differences more or less stop – it is like your head is not there! This accounts for the 70% number. I further like to cross subs at 100Hz or above, letting the woofer serve as a mid-bass coupler. Most speakers have woofers of 10″ or less, which is not enough surface area for symphonic bass or bass guitar. Bass guitar amplifiers typically have four 10″ speakers, two 12″ speakers or one 15″.

      1. Quite interesting, this 400 Hz crossover point. Peter Lyngdorf from Lyngdorf Audio (former TacT Audio) and specialized in room correction processors advocates a ‘sub’woofer concept having just this high crossover point! 🙂

    2. Indeed, and in theory, removing some of the tasks of the main speaker might be helpful, but it’s just been my experience that when you do this you cause more hard than damage. Plus, moving it from one woofer to another doesn’t really solve the problem, it just moves it to another device that may not be as well suited to handle the task at hand.

      I believe a well designed full range loudspeaker should be left alone and augmented in its lower frequencies by a sub, as you know. We cannot escape the problems of multiple frequencies on the same driver, unless we want to keep splitting the duties up more and more and, I think, there’s a point of no return in all this.

      1. Subwoofers in one place, the rest of the speaker somewhere else, line arrays, flat panels, directional tweeters that increasingly beam as frequency goes up all seem to blow Olsen’s idea that the perfect speaker is a pulsating sphere out of the water.

        As usual, I look through the other end of the telescope. Instead of looking at the speaker end, I look at the listener end, that is at the sound field reaching him and ask; “what is the designer trying to achieve?”

  4. brown cow theDISH ran away with the spon the little dog clarked Anst cow catpulted over ver horizon ic ic ic und day no no buny mo ma no ra,in making jose u c uc uc i varned u uc uc, some times i feel like you no u no yada yada yada ding, un symbell holaqui oui oui mon sweet litke etc. bon o nobonbon rocy o nyet rocy mit mein pict shure really big show!!!!!!!!!!!!!!!!!!!!

  5. Woofers today have a lot of distortion – TIME distortion. It is usually measured as “group delay”, but it can also be seen as waveform distortion of rectangular gated sine waves, which are closer to music than any other signals used in audio. Harry F. Olson was a particular advocate of the rectangular gated sine, and manufacturers like General Radio and Rohde & Schwartz made rectangular gated sine generators in the late ’60s and early ’70s. My speaker simulation software LspCAD has a transient response option to view rectangular gated sine response.

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