In a previous article I talked about the many ways to skin a cat when it comes to loudspeaker design. The fun thing about our profession is that there are many different routes to get to the goal and everything has a trade-off, get this but lose that. That article was about the fundamentally different approaches between bass reflex and quarter wave, as well as sealed box versus open baffles. It was not about the design considerations of the cabinet itself, materials chosen, shapes given , etc. The previous design considerations were more about how to achieve a clean bass extension. This time it's going to be about stereo imaging.
Very early reflections (VER)
Very early reflections are reflections of sound that occur within a time window of a few milliseconds after the original time window. If a reflection occurs after that time window, the brain incorporates it into the acoustic image as a reflection and stereo image is affected less. If it occurs within this time, the brain processes the reflected sound as part of the original sound and the image is blurred.
The problem with very early reflections is that they occur out of phase with the original signal, with the original wave being on the rise while the reflection is falling, causing cancellations and super-impositions that depend on their distribution and location on the frequency of the wave.
Loudspeakers that are not designed to minimize very early reflections, and even many very high-end loudspeakers are not, tend to present the need for a lot of tinkering around to find the sweet spot for the listener position. Loudspeakers that struggle less with very early reflections. Are more prone to a stable stereo image that presents well from a large variety of listening positions.
Very early reflections occur before 0.05 ms after the primary wave front and at the speed of sound of 340 m/s. This equates to a distance of 1.7 m, since sound is reflected first towards the wall and then back we should account for about half of this distance or 0.8 m which is the minimum distance for a speaker to be placed from any wall for the brain not to get confused. Please note, that reflections even after that contribute to the stereo image in the form of reverb, but the auditory cortex computes these echoes as a sense of depth and size of the room not as an undefined location of sound.
Besides removing a loudspeaker from the wall there are several design measures of the speakers that can be taken to prevent very early reflections from blurring the image. For that the designer needs to know what is causing the very early reflections from the speakers themselves.Very early reflections also affect the sound coloration because, contingent on wavelength, cancellations and super impositions occur.
Reflection from the front baffle:
The sound emanating from a loudspeaker is always to a differing degree omni-directional. The sound is reflected back onto the speaker cabinet and back to the listener’s ear at a slightly delayed time causing a blur in image. To prevent that, many great designs find a way to have a very narrow front face. Designs having a large woofer that would otherwise prevent a slender design, place the woofer sideways as shown in this picture. This works only if the cut off frequency for that driver is below 160 Hz where human hearing starts to be able to detect the origin of the sound. Designs using that configuration are often three-way as shown, leading to a mid range driver still claiming considerable baffle real estate.
Other designs create a pyramid shape narrowing the cabinet to provide only the diffractive surface that is needed for each driver. This shape makes the baffle no wider than need be and helps to eliminate very early reflections. The esthetics are of such a masonic design that their beauty is certainly in the eye of the beholder, but technically they are a good solution.
The third option is to place the driver in a cabinet that does not have a front wall or one that is curved. B&W epic Nautilus speaker is a great example of clever design for good stereo imaging.
The fourth option is to use a rather small driver and get good bass extension with a quarter wave or transmission line design without the need of a larger woofer. This point source design also has the advantage of not requiring an electronic crossover, which we will talk about later. A design like our SoundSommeliers speakers uses this approach and gets good but not excellent bass extension. Such designs are very good for acoustic music and music that is produced from “real instruments” vs. electronic and synthetic wave forms that intentionally drive bass below what is known to occur in nature.
Reflections from the back of the cabinet through the membrane.
A loudspeaker driver will always emit as much sound in the backward direction as facing forward and one of the three reasons to even have a cabinet is to keep the sound radiating backward from hitting the ear at a different time or creating acoustic short circuits by having air flow from the back to the front and vice versa, deteriorating loudness. The other two reasons to have a cabinet is to hold the loudspeaker at a good listening height and to help extend bass through a quarter wave or resonance beyond the capabilities of the driver.
The sound reflected backwards will then hit the back wall of the cabinet and some will reflect back through the membrane. There are again several design solutions that can help mediate this problem. One is to provide acoustic damping on the inside of the cabinet to absorb the sound. Many high-end loudspeakers like our SoundSommeliers line of speakers provide a curved wall approach. By creating a parabolic shape on the side walls of the speaker, the sound waves are always reflected back into the center of the driver and do not reach the membrane.
Phase shifts
The phase, or the part of the cycle that the wave is in, is a very important cue for human hearing to determine the origin of a sound. Many loudspeakers that use multi drivers employ higher order crossovers that separate the signal between the drivers. This design will show a 180 degree phase shift around the crossover point that causes one loudspeaker driver moving in while the other moves out leading to cancellations and a confused sense of space. 1st order crossovers, however, do not have this problem but they are rather difficult to design and get right for different reasons. Single driver loudspeakers like the SoundSommeliers do not suffer from this problem either, as they simply do not employ a crossover.
Another source of phase shifts in multi range drivers comes from the concept of acoustic center. A large driver like a woofer will normally have a significantly larger excursion than a neighboring mid range or tweeter. This leads to the design challenge where the same frequency from a crossover point originates slightly further back at the low range driver or woofer than it does from the midrange or higher frequency driver.
This different sound origin causes cancellation and superimposing an image and can significantly through of human hearing. The way that hi-end speakers solve this is by moving the woofer a little further to the front in what is known as a slanted design. By this the acoustic centers are aligned and the phase shift is evaded. Lower end speakers may solve this problem by introducing a deliberate electronic delay of the lower frequencies to make sure that they reach the ear at the same time. It is my opinion that the best electronics in a loudspeaker are the ones that were never installed, but I am looking forward to a heated debate on this issue.
Lastly a source of phase shift that applies to all two and three-way speakers that do not use a coaxial design stems from the simple fact that the sound from different drivers originates from different physical locations. Especially in near field listening where the triangle formed between the listener, the woofer and a higher frequency driver is not unilateral, once again leading to phase shifts. This could in theory be solved with timing delays, but the designer would need to make binding assumptions about the listening position and height and the stereo image would be unaffected except in a very narrow sweet spot.
Cabinet Materials
When choosing materials for a cabinet, it is important that these materials have three properties. First, they should be sufficiently rigid and stiff enough to not move with the driver. Second, the design should not have a frequency of its own and there should not be a distinguishable ping when the cabinet is excited. Lastly, the cabinet should not leak sound to the outside. The last factor is more important with bass reflex boxes or sealed box designs compared to quarter waves. The reason for that is that the formation of a standing wave compared to Helmholtz oscillations excites the cabinet much less.
There are many good choices the most popular being medium density particle-board. This material dampens the sound well and provides a good barrier. Curved walls and other design features are difficult to accomplish with this material, however, and a plain box is the usual outcome.
Some companies use different materials like glass or concrete. While these materials emphasize rigidity, they also are also prone to having their own resonances, and design measures must be taken to prevent coloration.
We are getting good results with an aluminum frame that is laminated with real wood veneer or printed vinyl for our custom designs. The combination of these materials provides great rigidity while at the same time damping against resonant modes.
Fiberglass, real wood or different types of plastics are also good choices.
Conclusion
In conclusion, a precise congruent stereo image is one of the many sublime about enjoying music. Image might be the attribute that most people miss when selecting a loudspeaker primarily focused on bass extension, a flat response, and sufficient loudness. Those loudspeakers are rather easy to make and many make them. Where it gets more complicated is when it comes to giving our musical souls a sense of space. We at KVART & BOLGE are convinced that a precise special image creates an emotional response and a deeper connection to the music. Our hearing has also evolved to keep us safe in an environment were little noises alert us to predators and the feeling of understanding the acoustic world around us, and lets us put our minds at ease at a deeper meditative level and relax into the music.
Arved Deecke is founder of the Danish / Mexican Loudspeaker company KVART & BØLGE that makes audiophile quarter wave loudspeakers and sound systems at a price anyone can afford. In his free time he blogs about all things related to sound, music and audio.
