Saturday, June 7, 2014

Speaker Enclosures - Part 4

As all things must come to an end, this is the last of a 4-part series discussing issues with speaker cabinet designs. By now you should understand a little bit about how cabinets influence the sound of a speaker by what is called internal resonances. These resonances are challenging to control but easy to predict in solid rectangular cabinets. Changing the shape of a speaker enclosure from a solid rectangle to one with fewer parallel sides eliminates some of the internal resonance issues and eliminating all of the parallel surfaces eliminates one of the modes that an enclosure resonates (axial mode).

Truncated pyramids with their totally non-parallel wall structure are good choices for such designs as are variations on the basic wedge shape. But again there is more about cabinets that can be visually discerned. Scrutinize the way the baffle board is designed (the board on which the drivers are mounted) in the two cabinet designs below and note the differences. Take your time and see how many you can find just by looking at them.

Baffle Boards 1 and 2

The first thing you should notice is that the RH baffle board flush mounts the drivers and the LH does not. Why do you suppose this is? Could there be a reason? You bet! It's called the First Reflection. As the sound moves away from the driver, it strikes something - anything - from the edges of nearby drivers to screws on the baffle board to - well you get the idea. Anything that sound can be reflected from will bounce back to the driver and disrupt the sound coming from it. The simplest thing to do is to make the front of the baffle board appear as flat and smooth as possible thereby eliminating as much of the driver-induced first reflections as possible. You can do this by recessing the drivers into the baffle board (countersink them) and to use flat-head screws instead of round-head, etc.

Good job! Now, look at how the drivers themselves. Why did the RH designer use three drivers and the LH designer two? Good question. Air is moved by the driver thereby producing sound but just like bicycle racers ride lightweight bikes to go fast, two smaller drivers are typically lighter than one bigger driver and they will move faster (have better transient response) than using a single driver. However, you never get anything for nothing. You double the cost by adding a second driver and the free-air resonance of the cone will be higher (less deep bass, all things being equal).

Good. So far you have noticed how drivers are attached, recessed, and arranged on the baffle board. BTW, why did the RH designer put the tweeter in between the two woofers? Doing so causes the relative sound wave created by the woofers to appear to emanate from the same point as that of the tweeter (called a point-source). Unfortunately, there is a compromise with such an arrangement that disperses the sound by the two drivers well horizontally but not so much vertically. So to get good listening from a wider range of listening positions, you should orient the speakers vertically as shown rather than laying them horizontally on their side.

OK, let's look at some more examples and see what differences can be observed. Again take your time and see what you notice just from the appearance of the drivers on the baffle board.

Baffle Boards 3 and 4

The most obvious difference is the way the woofers are mounted. These are both KEF speakers and it shows how the evolution of design takes place. The model on the left shows the typical convention for mounting a driver: put a screw in every hole in the driver basket. The model on the right shows a marked departure from this philosophy as a result of some serious research: 3 screws only in a 120-degree pattern. KEF found back in the early 1980s when trying to minimize basket resonances that by using three screws to mount a driver created the minimum amount of mechanical resonance. This is something to note that seems to elude speaker manufacturers today. Most still plug all of the holes with screws.

Here is a free hint: if your speakers have 8 screws in them, back off the 5 screws that are not in a triangular pattern and see if you hear a difference in the way they sound. If so, remove the driver, plug the holes with RTV, and replace the driver using only 3 screws (one on top and two at the bottom).

One last thing that you need to understand about the baffle board and that is shown in the next picture.

Baffle Boards 5 and 6

These are two versions of the B&W model 801 as they have evolved over time. The LH older model shows an early attempt to round the edges of the enclosure at the baffle board. The RH newer version shows the implementation of a no parallel sides enclosure with much more radically rounded midrange and tweeter surfaces. What B&W and many others are doing here is to remove the effects of what is called edge diffraction where the mere presence of a square edge influences how the sound radiates into free space.

Rounding the corners of the baffle board is pretty common practice today and something you can easily see when making your next loudspeaker purchase. Those with well designed boxes using flush-mounted drivers, non-parallel surfaces, and rounded baffle boards at least give the loudspeaker a chance at sounding better by minimizing the physical interactions of first reflections and edge diffractions. It does amaze me that manufacturers still insist on using more than three screws to install drivers but hey, some folks never do learn from history.

Anyway, I hope you have enjoyed this brief explanation of speaker enclosures. At least now some of the mystery behind the curtain has been exposed and you can better understand why designers make the choices they do. Most is cost driven but some are aesthetic compromises. Regardless, all are just that: compromises. And BTW, compromise is not a bad word as some may lead you to believe.

Related Articles
See all entries about speaker enclosures in Part 1, Part 2, Part 3, and Part 4. Also, a related article on the effects of crossover network components on driver performance.

Yours for higher fidelity,
Philip Rastocny

I do not use ads in this blog to help support my efforts. If you like what you are reading, please remember to reciprocate, My newest title is called Where, oh Where did the Star of Bethlehem Go? It’s an astronomer’s look at what this celestial object may have been, who the "Wise Men" were, and where they came from. Written in an investigative journalism style, it targets one star that has never been considered before and builds a solid case for its candidacy.

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