Low freq. from small drivers? Is it possible

I am a believer in large area LF drivers. My subwoofer system sports three 15" drivers and three 12" drivers. I have some small monitor speakers that will play almost as low in frequency, but there is obviously no comparison in the impact of the sound. Large diameter (area) LF drivers have an effortless quality to their sound.

It's possible to get excellent bass by using multiple smaller woofers as opposed to fewer larger ones. The surface area works out to be about the same but the smaller drivers are generally quicker.

For example, the bass specification of the Dali Megaline is 35 HZ.

That specification did not particularly astound me until I was face to face with the designer at the London audio show. When I found the measurement was taken at center line of an anechoic chamber I was impressed.

In a typical listening room the boundary reinforcement at the floor, sides and rear wall would improve those numbers substantially.

The Dali is a line source two way (ribbon plus woofers) with twenty four 6.5 inch bass drivers to achieve the desired surface area. They are rated in excess of 131 DB before breakup.

I will try to verify when mine get past the required 500 hours and I get some serious power on them.

Smaller drivers can produce low bass IF the cone is of a high mass design and capable of long excursion. There are several problems here though. That is, why would you want to use a smaller woofer of higher mass? This reduces both transient response and high frequency bandwidth, negating much of the benefit of using a smaller driver. On top of that, a driver with more surface area reproducing the same note at the same amplitude of a smaller driver with longer excursion will produce lower distortion. The more "throw" that the driver makes, the more distortion that it produces. On top of that, longer throw woofers produce more reflected EMF, making them harder to control. On top of that, the smaller driver will have to move more air to produce the same spl, meaning that it will be producing more distortion at all times while running into Xmax ( linear excursion capabilities ) sooner than a larger driver. Obviously, there are a lot of "on top of's" in this equation : )

In plain English, this means that if you want deep bass out of a small driver, you'll have to make several compromises in other areas to get it. The only way to get low distortion and deep bass while retaining good linearity at high volumes is to use large woofers in a large cabinet or a multitude of smaller woofers in a large cabinet.

To quote speaker designer Bill Fitzmaurice: "The bottom line for speaker cabinets is that for the goals of a small box, a reasonably efficient system, and good bass extension, you may achieve any two of those goals at the same time, but not all three". Sean
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hmmm, very interesting!

For example: So the base out of a twin 6.5 like the b&W Naut 804, could be good, but not great.

>Here is the frequency response of the Ultimate Monitor from >Karl Schuemann

The specs don't tell you enough to be useful.

1. They don't tell you about distortion. The mid-ranges used in my speakers are good for .3% THD @ 96dB/1 meter in their operating range with an increase to 1% at the tweeter cross over frequency where they're 6dB down and distortion should be less. Run at lower frequencies they'll have horrible distortion at much lower output levels - maybe 10% at 70dB one you reach 30Hz. This is especially bad at low frequencies where tight spacing of the equal loudness curves makes the harmonics sound louder than the fundamentals. IM distortion is even more of a problem.

2. They don't tell you what the maximum output level is at those frequencies. Using a pair of 6.5" scan speak mid-bass units (Sd = 145 cm^2, xmax = 5mm) in a sealed box the linear limits are about

88dB @ 35Hz
94dB @ 50Hz
100dB @ 71Hz

3. They don't tell you what the off-axis response looks like. At reasonable listening distances you're picking up more sound from the reverberant field than direct sound. The shape of the off-axis curves has a _huge_ effect on what you hear.

4. The don't quantify thermal compression. This is especially important where you start equalizing. The BOMB is a Linkwitz Transform which allows you to change the F3 point and Q of a speaker thus getting you lower bass extension and less group delay at higher frequencies. Low group delay means "fast bass." The problem is that your power requirements go up. The extra power increases voice coil heating and therefore resistance. That means more thermal compression and changes in the cross-over response with output level than you'd have in a speaker without equalization.

I'm sure the Ultimate Monitors are exceptional speakers although they can't break the laws of physics. If you want natural sounding bass at realistic output levels you need a 3-way or sub-woofers, the later being better because high and low frequency transducers interact with the room differently and therefore work best with different placement.