A Primer on Speaker Selection and Enclosure Design



 

While this discussion primarily concerns boats, the principles apply to any loudspeaker, whether it be in your home, auto, guitar, or boat.

 

Technically a speaker in its raw form; in other words, the speaker "element", is known as a "driver". The term "speaker" usually refers to the end product - a driver, cabinet, and associated electrical parts to make everything work.

There are a wide variety of drivers available today for the amateur speaker builder, ranging from the most inexpensive low quality drivers all the way to esoteric components that may rival some of the most expensive commercial systems. With this seemingly unlimited selection available, where do you start?

Fortunately, since we are dealing with a marine application, a lot of the selection criteria has already been chosen. Even then, there are a wide variety of automotive and marine grade speakers available, which can still make this task somewhat complicated. However, as we narrow our requirements, the list of available speakers quickly diminishes.

 

 

One thought that must be kept in mind that this is a boat - and has less than ideal acoustic conditions, speaker placement restrictions, and other factors. Therefore, don't expect music-hall quality improvements. For this reason, it is a bit pointless to spend hundreds of dollars on drivers when you are not going to reap their benefit in this environment. However, within this limitations, the goal is to still do the best job possible.

Before we come up with a set of requirements, a bit of speaker background knowledge is in order.


Infinite Baffle

If you pick up a speaker design book, one of the early concepts you will read about is "Infinite Baffle". Just like the transmission, tires, and pavement in an automobile present a load to the engine, a speaker must operate into a load. A load is the item of work that is performed by the speaker. For a speaker, the load is air pressure. As a driver moves in and out, it alternatively creates an area of high and low pressure. This occurs on both the front and rear of the driver.

There is a given amount of resistance in the air, and the ability of the driver to push the air is dependant on the size of the driver's cone, the distance the cone travels, and other mechanical dependancies.

One important factor here is that if the driver were not enclosed in a box, the air pressure at the front of the cone would become equalized with the air pressure at the rear of the cone. Since the air pressure at the front is always the opposite from the rear side, operating a driver outside of a box results in "decoupling" the driver from the surrounding air - and consequently, little air flow will occur. Using our automotive analogy, this is similiar to spinning tires on a wet pavement. And, if we spin our tires long enough, we can damage the engine. The same is true for the driver as well. If a large signal is applied during the uncoupled state, it can be damaged as well.

Therefore, all speaker drivers must have some means of preventing the air from the front of the cone from wrapping around to the rear. The most efficient way to do this is to mount the driver on a board - which obviously separates the air mass from each side of the cone.

In a theoretical sense, if this board was infinitely long and wide, you have the concept of "infinite baffle".


Optimum Enclosure

While it is not practical to have a true "infinite baffle", if we wrap the sides of the board around, we can make a box. This box has the characteristic of an infinite baffle, with one important difference. Depending on the size of the box, there is some pressurization that occurs when the driver's cone moves in. This has an effect on the frequency response of the driver. It is a general rule that the smaller the box, the more back pressure will occur, and will result in the driver having poorer low frequency response, and at the same time, more distorted "boominess".

At some point, the box size will be just right - neither too small or too large. This is the point where the driver will reproduce the sound as the designers intended, without the box influencing the result. This is referred to as the "Optimum Enclosure". In some speaker designs, this can be achieved, but in other designs, the optimum enclosure is simply too large to be practical, and a comprimise must be made.

A speaker having the characteristic of a sealed box is often refered to as a "sealed enclosure", or "acoustic suspension". Other speaker box types exist, including "ported" or "Bass Reflex", as well as many other designs. However, none of these are practical in the limited space environment of a boat.

Quite often, a comprimise must be made in the speaker box volume so that it will physically fit into the required location. This is done by using the Thiele/Small Model, which allows the calculation of various box sizes. This model is "normalized", which is to say, that regardless of what particular parameters and sizes are involved, a calculation can be made to give the characteristic response the box designer is looking for. This allows the model to be used for a wide range of differing drivers.

One important loudspeaker parameter is "Q", whith is the Quality of the entire system. The Q is the result of the matching of both the driver and the box. Q is a normalized factor, and can be thought of as a constant. An optimum enclosure as described above would have a Q of 1 (although some designers prefer 0.7). An "infinite baffle" would have a Q closer to 0.5, and an enclosure that is too small would have a Q of 2 or higher.

Comparing various "Q's" from the frequency response graph above, you should be able to see that a Q of 1 generally results in a combination of the flattest response and extended low frequency response. A Q lower than 1 will have an extended but reduced low frequency response, and a Q of higher than 1 will have a boomy sound, with a reduced low frequency response.

It's this graph that indicates what the expected change of response will be if you have to use other than an optimum enclosure - which is often the case. For example, you might find that although a Q of 1 is the optimum, the enclosure is just too large for your application. But, if a slightly smaller enclosure could be used - one which results in a Q of 1.5, this might be acceptable. There is a slight reduction in low frequency response and boominess, but it might still be acceptable. So in a sense, the box designer chooses the Q they can live with.

An option to reduce the Q of the box is to fill it with damping material. This must be done on an experimental basis, with a judgement given for the overall change in quality. Still, it can help when sufficient space is not available for the box size.


Automotive vs. Home Systems

There are a few differences between automotive and home systems that might not be readily apparent. One difference is that drivers designed for home systems are typically intended to be mounted in closed or vented box. In contrast, drivers for automotive use are often designed to also work acceptably in an infinite baffle situation - simply because areas such as a kick panel on a door has no space to construct a suitable box.

Another difference is that while most home systems are rated for a characteristic impedance of 8 ohms, automotive systems are typically rated for 4 ohms. This is primarly due to the fact that auto systems use a reduced voltage for their amplifiers (12VDC), while home systems can use a higher voltage for their power supplies, since they are powered by 120VAC household current. Using an 8 ohm driver in a 4 ohm envronment will work, but will require additional amplifier voltage for the same loudness. The result is that an 8 ohm speaker won't play as "loud".

A third difference, especially when comparing marine rated drivers to home versions is the construction. Automotive and marine systems typically are designed to withstand the harsh environment of the outside world, while home systems might be able to use more delicate materials in their design. There is so much crossover in this area that it may not really be an issue.

The last major difference is that many automotive drivers have built-in tweeters (high-frequency drivers) that are coaxially mounted into the driver, while home systems are almost always separate components. Some auto systems use separately mounted tweeters as well - and we'll cover this in more detail.

Due to these differences, an automotive or marine-rated driver will likely perform better in a marine boat than a driver designed for a home system.


Coaxially Mounted Tweeters

The photo shown below is a co-axial speaker. The larger cone - the woofer - is constructed in a normal matter, except that the center of the speaker is open. At the center, a fixed post containing a smaller tweeter is constructed. The term co-axial is used to describe that both speaker's are along the same center line. This almost always results in better stereo imaging as you move about the boat. If this were not done, some loss of stereo information may be apparent depending on where your ears are in relation to the speaker. This is simply because when the tweeter and woofer are not the same distance from your ear, phase differences due to the distances travelled from the speaker to your ear will result in loss of stereo imaging.

Some expensive automotive drivers do have separately mounted tweeters, but could end up having less stereo imaging simply due to the different distances from the voice coils to your ears. Before you spend the extra money for these speakers, consider where you will be placing them and where you intend to sit most of the time.

In contrast, a home stereo system typically has the tweeter separately mounted above the woofer along the same vertical centerline. In a home environment, there is typically a "favorite" listening spot, usually seated between the speakers with both tweeter and woofer at eye-level. This layout, along with careful placement of the speakers is used to minimize phase differences. The notion here is that listener will always be seated in a single location.

Some really inexpensive drivers have what is referred to as a "whizzer" cone. This is simply a smaller cone glued to the center of the main cone in an attempt to reproduce high frequencys. Its advantage is somewhat suspect.

But, it again must be remembered that a boat is less than an ideal acoustic environment. Regardless of this fact, isn't it better to buy a co-axially mounted speaker rather than a separate tweeter component? An audiophile might disagree, and point out that a coaxially mounted speaker cannot provide the ability to tune a crossover network like a separate tweeter can. But we are still talking about a boat, right?


Speaker Selection

Now that some background has been covered, the requirements I thought were important for the replacement speakers are:

I chose Polk db650s, which are 6 1/2in speakers with co-axial tweeters. Polk is a well-known home and automotive speaker company, so this would be considered "name-brand". The characteristics of this speaker are:

The street price for these speakers is $90-$100 per pair, which puts them in the moderately inexpensive category. They are really nice speakers for the price.


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