Blog entry by Adrienne Owens
It's not every fisherman who wants to listen to music while hauling gear. It can be distracting to some and downright unsafe for others. And no matter how good the music is, there will always be those who prefer the droning of the engine and the sounds of the sea to anything broadcast over the radio or recorded on a CD.
But let's say you're the type of person who eats, drinks, and sleeps with music. Maybe you still have a ticket stub from a Grateful Dead concert in your wallet, or own a 1,400-CD collection, or wake up every morning to Eddie Van Halen's version of "Pretty Woman." If this even remotely describes your level of interest in music, then by all means this article is for you.
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It's a lot like coming up with a power train for a boat. You wouldn't want to move a big boat with a small propeller or a small boat with a big propeller. You don't, for example, turn a 20-inch diameter propeller with an 8-92 Detroit Diesel, or try to muscle a 60-inch propeller with a 4-cylinder Westerbeke. Either way you're likely to burn up the engine faster than you can say, "Sergeant Peppers Lonely Hearts Club Band." Nor would you buy just any engine and gear combination.
The same thing's true for a stereo system. After all, a speaker, or driver, is a little like a propeller: It's an output device. Stereo components have to he balanced in terms of their power and impedance, the latter being a measure of overall electrical resistance.
"Compatibility is important," says John Keyser, a salesman for Harbor Audio in Camden, Maine. "If you run an improper speaker load, you can damage your amplifier."
For example, connect a pair of 2-ohm, 50-watt speakers to a 50-watt amplifier that is rated for a pair of hm speakers (An ohm is a measure of impedance.), and the amplifier will heat up and possibly even fry itself. It won't be very good for the speakers either, which can be permanently damaged by destructive "clipping," a form of distortion caused when an amplifier is overloaded or short on input voltage, a concern in many direct-current applications.
There's simply no arguing the fact that it's easier and often safer to buy a known system and have it installed by a knowledgeable technician.
"We just mounted a very nice system into a local lobster boat," says Karl Hupper, of Lew Grant and Associates, in Rockland, Maine. "It has a Jensen Maritime CD player, an Audiovox XM Satellite Receiver, and two pairs of Bose 151 speakers, one pair in the wheelhouse and one pair on deck."
A manufacturer might list a speaker with the following specification: 100-watts RMS at 4 ohms. In this instance, RMS stands for root mean square and, continuing with the marine propulsion analogy, it's a speaker's cruising speed. So, the speaker can be safely connected to an amplifier with 100-watt RMS per channel at 4 ohms.Another manufacturer may use a different rating for the speaker, as in 100 watts maximum at 4 ohms. This is a power rating stating that the speaker can handle a short-duration, high-intensity burst of music of 100 watts. As a guideline, to get a safe approximate RMS or continuous-duty rating for this speaker, you divide by two, which gives you 50 watts RMS. Hooking up a pair of these speakers to an amplifier with 100-watt RMS per channel at 4 ohms will give you substandard performance and probably cause premature death for both the speakers and the amplifier. What you need for the 100-watt max pair is a 50-watt RMS per channel at 4 ohms amplifier.
Incidentally, there are other ways manufacturers play with power ratings. For example, you might find a power amplifier that advertises 500 watts. When you look closer, it turns out to be a four-channel stereo, and the rating listed is a maximum power rating. Divide the 500 watts by 2 and you get 250-watts RMS, then divide by 4 and you come up with 62.5 watts RMS per channel.
Manufacturers also use peak power, full power, maximum output, instantaneous peak power, and peak music power output, for rating their equipment. These, as well as maximum power, are pretty much the same. The only rating that really matters is the RMS rating.
Many audiophiles will tell you it's all right and even desirable to have a power amplifier that is 10 to 20 percent over the RMS rated power of your speakers. This 10 -to 20-percent guideline means that a 100-watt RMS woofer can be safely plugged into a 120-watt-RMS per-channel amplifier, provided that the impedance ratings are matched. Another thing to bear in mind is that most stereos perform best at a volume setting of one-half or less, with the gain on the power amplifier, if one has been added, set at less than 60 percent.
Now that it's agreed everything has to be matched and balanced with respect to power and impedance, it's time to look at some speakers.
"We always start with the speakers," says Scott Morris, Coordinator for Ocean Alexander Marine Service, in Seattle. "Will they be flush mount or box type? How well will they fit the area? Where will the wiring go? Does the customer want to drown out the sound of the engine, or is this just music for dockside?"
Another question you might-ask yourself is, how discerning or sensitive are my ears?
"I installed a system for a concert violinist that cost thousands of dollars," says Morris. "I've also installed off-the-rack car stereo systems for a few hundred. If you don't have the range of hearing, you can't tell the difference between the two."
You might also want to keep in mind:
* Deep bass sounds are of the long wavelength variety. They require the most power to reproduce because they need a comparatively big driver (woofer) with a long stroke.
* Not all music is heavy on bass. If your interest is more in the big band or folk area, you may not need power-hungry woofers to get the sound you want. Small two-way or three-way speakers or a pair of midrange drivers may suffice.
* A general rule of thumb is that to double the sound of what you're currently listening to, you have to multiply the number of watts by 10. In other words, if you're currently listening to a 10-watt-RMS per-channel marine stereo matched to a pair of 10-watt-RMS marine speakers, and you want to double the loudness, you'll need a 100-watt-RMS per-channel system.
For a small boat, however, a two-channel 100-watt RMS per-channel stereo is getting up there in power. Here's why: To find out the power drain on the boat's electrical charging system, double the total RMS value to take into account peak power bursts, then divide by the voltage put out by the alternator to get the current draw. In this case, 400 watts total peak power divided by 13.8 volts--a good average charge a factory installed marine alternator--results in a current draw of 29 amps. With a standard 45-amp alternator, and the electrical needs of sounder, radar, GPS, plotter, bilge pumps, etc. to factor in, doubling your loudness may mean having to buy a new higher capacity alternator, although there are other alternatives. For instance, you could install an audio capacitor (an energy reservoir) between the battery and the power amplifier.
Having decided on the amount of power you need, it's time to figure out what you want for speaker. You can go with boxed or enclosed speakers, or with component speakers. Then buy woofers, midrange drivers, and tweeters individually and mount them yourself. The latter method provides the best possible sound reproduction for a given application, but it is also considerably more complicated, particularly when it comes to the woofers.
Unlike tweeters and small midrange drivers, which can be mounted in a cabinet or against a bulkhead without concern for altering their performance (We're not talking placement with respect to each other or the listener here, just the actual mounting), woofers have special mounting needs. As the woofer generates sound, it pushes air out its front and back--and herein lays one of its problems for small workboat installations.
The sound coming out the back of a woofer is 180 degrees out of phase with the sound coming out the front of the woofer. Consequently, if the woofer is mounted incorrectly, the sounds from the back and the front will interfere with each other and possibly cancel each other out. At the very least, improper installation will cause you to lose much of your low frequency performance. To solve this problem on a small workboat, where cabinet space and bulkhead depth is either nonexistent or at a premium, you'll need to build a custom speaker box for the woofer, and that brings up an interesting conundrum. Why not just buy a good marine boxed speaker of the two-way or three-way variety?
"In most of the boats we're doing now," says Morris, "we're using two-way and sometimes three-way speakers."
A two-way speaker is one that has a separate tweeter and woofer mounted in the same box, sometimes one over the other to save space. Your typical two-way car speaker has a tweeter mounted either on a post (center mount) or bridge (perimeter mount) directly over the woofer. There's also the dual-cone design, a speaker that consists of two cones, a main cone for lower frequency sound, and a whizzer cone for the high frequency sound.
Three-way and four-way speakers round out the category. The former is a two-way speaker with the addition of a midrange driver, the latter a three-way speaker with the addition of an even smaller tweeter. On a boat, however, the "super tweeter," as it's sometimes called, will do little more than annoy the dog, if even he can hear it.
All outdoor and marine speakers are made to withstand the elements. Cones are typically constructed of polypropylene, while the tweeters are made of Mylar or a corrosion-resistant metal, such as titanium. Wire and wire connections are coated for water and corrosion resistance, and enclosures or boxes are stainless, marine grade aluminum, or ultraviolet stabilized plastic. That's if you buy an off-the-shelf marine speaker. What are The Speaker Sizes in My Car
If you buy component drivers on your own, you can build the boxes and even hardwire everything just the way you want it. Be sure you've done your research, though. There's a lot more to it than what's been discussed here. You've still got to consider output from the head unit to the amplifier, impedance matching if you want multiple volume controls at multiple locations, antennas, speaker placement, and much more.
"Entertainment systems are very much an individual thing," says Morris. "There's no perfect system. What's important is finding one that meets your needs and sound good to you."
Bob Bernstein is a head-boat captain and freelance writer living in Maine.