Speakers with fullness and weight?

Thanks for your kind words, Ngjockey and Mapman and Seikosha and Hesson11.

Bob mentioned the Allison effect, which has to do with the impact of nearby room boundaries on the frequency response curve at lower frequencies, focusing on eliminating the dips that normally occur.

If we consider the wall behind the speaker, there will be a frequency at which the energy that bounces off that wall arrives 1/2 wavelength behind the direct on-axis sound, and we'll have a dip at that frequency. Along the same lines, we can expect a floor-bounce dip at the frequency the bounce off the floor (between speaker and listener) arrives 1/2 wavelength behind the direct sound. The longer the path length difference between the direct sound and the reflection, the lower in frequency the dip.

Roy Allison's ingenious approach was to:

1) Use fairly shallow speakers and place them flush up against the wall. This moves the wall-bounce dip up high enough in frequency that the inherent directionality of the speaker minimizes it.

2). Place the woofer down near the floor, and the midrange/tweeter up much higher, and choose the crossover point such that the floor-bounce dip for the woofer occurs above the range where the woofer is active, but below the range where the midrange is active, so that the output of neither gets floor-bounce-notched. This is so elegant it almost hurts.

3) Because the woofer is very close to the intersection of floor and wall, it gets the benefit of a lot of boundary reinforcement, so we can get either significantly deeper bass or significantly higher efficiency that we'd otherwise get from an enclosure that size.

One of my absolute favorite speakers of yesteryear is the Snell Acoustics Type A, Peter Snell's masterful adaptation of Roy Allison's concepts.

Another favorite speaker of yesteryear was the little Meridian M2, a small active "D'Appolito configuration" MTM that predated Joseph D'Appolito's landmark article. Placed on a short stand, the floor bounce distance for the top woofer was significantly different from the floor bounce distance for the bottom woofer, such that each one filled in the other's dip. I haven't seen anyone do a low-altitude MTM since then, but imo it makes a lot of sense.

All of that being said, the floor-bounce dip in particular is one of those things that looks worse on paper than it sounds to the ears. Our ear/brain systems are quite accustomed to it, as it's present every time you talk to someone in person outside of an anechoic chamber (part of the energy of your voice bounces off the floor or table or whatever is in between you and the other person, arriving at his ears later in time, and causing a dip in the frequency response of your voice).

The hard part for the speaker designer is figuring out which of all the many problems speakers have are the ones that most need addressing, and there are probably as many well thought-out opinions about that as there are speaker designers + audiophiles combined!

Duke

Amps + speakers + room form a "system within a system", of sorts. Boundary reinforcement can warm up otherwise thin-sounding speakers. Likewise a (low damping factor) tube amp can warm up otherwise thin-sounding speakers.

Most speakers exhibit a phenomenon called the "baffle step" that can contribute to thinness of sound. Briefly, as the wavelengths get long in relation to the front baffle width, the energy is not so much concentrated out in front of the speakers, but starts to wrap around more and more, until in the bass region the speaker is essentialy omnidirectional.

Let's walk through an example. Suppose we have a mini-monitor on a stand with a baffle width of 8.5 inches. This corresponds to 1/2 wavelength at 800 Hz. So beginning at 800 Hz, the on-axis SPL gradually shelves down, to approximately -5 dB at 200 Hz or so (in theory we'd be -6 dB at 100 Hz, but in practice we start getting some boundary reinforcement from the floor by then).

Now the baffle step is not as bad as this appears at first glance, because the energy that wraps around is still present in the reverberant sound field. So we end up with a thin spectral balance in the on-axis sound, which is compensated to some degree by the overly warm spectral balance of the off-axis sound (which in turn is what dominates the reverberant sound within the room). How much effect on the percieved tonal balance the direct vs reverberant sound has at the listening position depends on a number of factors, not least of which is listening distance: The on-axis sound falls off with distance much more rapidly than the reverberant sound does (assuming a semi-normal listening room).

What can be done to deal with the baffle step? We can make the baffle wider, pushing the wrap-around frequency lower, so that its effect is reduced. We can move the speakers closer to the room boundaries (close to the wall and/or put them on shorter stands) so that boundary reinforcement kicks in higher up than it otherwise would have. We can compensate by equalizing the output of the speaker to be the approximate inverse of the baffle step, at the risk of ending up with too much in-room low-end energy (so we don't want to overdo it). We can use an enclosure type that is inherently less susceptible to the baffle step (like a bipolar, wherein the output of the rear woofer wraps around and helps out the front woofer).

So if you like your present speakers aside from a bit of thinness, but you don't want to lose soundstage depth by moving them back against the wall, consider using a significantly lower stand so that they get more boundary reinforcement from the floor. Some people don't like the perception that the sound is coming from lower than seated ear level, but try closing your eyes and pretending you're in a balcony seat.

Duke
dealer/manufacturer