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Enclosed floating aquatic 12v car-audio amp system

  1. Jul 4, 2016 #1
    Hey guys so I have had this idea for an electronics project for a long time, and it basically goes like this.

    It's a floating speaker box for floating on the river when we are rafting. Design considerations are that it would have to be unsinkable (as much as possible), also loud and with a longish battery life for its given operational specs. Here's about where I am at with it:

    My current design would be an ice chest enclosure which houses 2 speakers (probably 6", maybe 6x9" if feasisble.) These speakers are mounted from the inside of the ice chest, and only the cones would be exposed from the outside. This means it would be important to keep the whole shebang orie tated peoperly (right side up) to avoid potential problems with leakage or speaker damage from too much water. These will be marine speakers (although some most car audio door-mount speakers are designed to operate in rainy conditions since rain which does not get redirected away from your window's weather stripping would end up dropping water on the door speaker itself. That is a possible area for improvement in the design bht that's my current plan.

    Now, completely inside the ice chest enclosure would be the 12v 2-channel amplifier and a 12v rechargeable battery. Also the music source would be in here as well... I was thinking of using a cheap mp3 player like a Sandisk sansa or something like that which offers good memory storage capacity for cheap. This way, all the essential electronics would be kept fully dry, and I could just throw my music on random and float down the river while BLASTING some feel good tunes as loudly and effixiently as possible.

    Design considerations would be things like making sure that the box is completely watertight sealed comma that the balance of the unit as a whole would keep it orientated correctly in the water as best as possible, and also using some kind of lightweight foam to make sure that the ice chest with float even if it did fill with water God forbid.

    So this is my first issue... does anyone have any suggestions on improvements or tips on things I may have overlooked in this design? All replies are welcome.

    And secondly (and lastly) I was looking at using components similar to these:

    12V rechargeable 7 Ah battery-

    11Vdx Kenwood amp at 75w x 2 channels rms

    So by my calculation, trying to get a rouvh estimate of how the battery would fare, if I am drawing say 150w to power the speakers, lets assume that the amp itself draws 150w rms at high volume. At 12 volts dc, it seems like using watts law, 150w /12 volts is about 12.5ish amps.

    So id the battery I linked to offers 7 Ah, and this system would require 12.5AMPS, then dead battery when only last for a little bit over and half an hour before dyingm. Is thay correct or am Inmissing something?

    Thanks thus
    Last edited by a moderator: May 8, 2017
  2. jcsd
  3. Jul 4, 2016 #2


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    yes, you are missing losses. To generate a power out of 150W, the system is likely to be using 200 W or so, maybe as high as 225W
    nothing is 100% efficient ! :wink: .... somewhere between 60% and 80% efficiency wouldn't be out of the norm

    A lot ( well most) of that additional power will be lost at heat

    [edit] Ohhh and I forgot, the system is going to die long before the battery hits 0V .... somewhere around 10V would be the minimum and still have the system working

  4. Jul 4, 2016 #3


    Staff: Mentor

    I hate loud music on the water, so I'm going to refuse to help.
  5. Jul 4, 2016 #4

    jim hardy

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    Unless it's Handel's Watermusic or Victory at Sea
    i'm with anorlunda.
  6. Jul 4, 2016 #5
    You guys are a bunch of squares haha. I know that a forum of physics enthusiasts will be filled with a lot of introverts and people who are more easily annoyed with stimulation from the external world, but to reply to my post with something like "Oh well I might be able to help but I refuse to because I hate the idea of people having fun" is just rude. You don't have to help, but ir you aren't going to be constructive then don't reply at all.

    And for the first response, yes I understand that the efficiency of a class AB amp would be somewhere near 60 to 80%, but I really am trying to get an idea of the battery life. The 7Ah rating of the battery would be the maximum total charge available until the battery drops below a rated threshhold, such as the 10 volts you mentioned, correct?

    Only reply if you're going to be helpful guys. Thanks
  7. Jul 4, 2016 #6
    And for your information, I would not play Handel's water music but there qould be a fair amount of Beethoven, Mozart, Brahms, Chopin and of course Liszt. Never was much for Handel myself, no offense
  8. Jul 5, 2016 #7

    jim hardy

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    None taken.

    I used to live on the water in the Florida Keys.
    Every weekend morning boats went by blasting reggae in low fidelity/high distortion .
    and i wondered "You get on the water to enjoy the air, the sun, and the gentle motion of a boat underway.... what has happened to these peoples' nervous system that it needs such raucous stimulation? "

    Do the other folks on the river a favor and soup up some headphones instead.

    In Arkansas making unreasonable noise in public is a misdemeanor.

    It would be an opportunity to learn about sealed speaker enclosures though, and Class D amplifiers.
  9. Jul 5, 2016 #8


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    well it seems you didn't understand that fact as you didn't factor it into your original calculations
    and that is important as to how long the battery will last
    Depending on the battery, it's current rating is going to decrease along with the voltage drop, how
    proportional that is will be in the battery specifications
  10. Jul 6, 2016 #9
    Jim Hardy - I mean, it's pretty much a given that this system would be used to play reggae, ska, dance, and basically feel-good summer music. I would always be courteous though, and of course this whole shabang would only be through the highest quality cheap parts I could afford, haha. And I was reading about the different amp classes the other day, class D would be feasible for this since I am not too hung up on the exact quality above a certain threshold of enjoyability.

    And davenn- you are absolutely right. I guess I should rephrase my question...

    Our average river rafting trip lasts about 2 hours, so I would prefer to have a MINIMUM of 3 hours (but would prefer more like 5) of battery life whilst listening at nearly full volume. So if I run an amp which is 75w x 2 channels = 150 w rms, and since P= I V where V is 12 volts, my current draw is 150w/12v = 12.5 amps, correct? This is neglecting efficiency, for the sake of simplicity let's ignore the fact that to make 150w of output will actually require something like 170 to 200w of input.

    If my amp draws 12.5 amps, and the battery provides 7amp-hours at 12 volts, that means that this whole setup would only run for about 40 minutes before the battery dies, correct? This is the actual question, because I feelnlike the batter should last much longer than just 40 minutes. Maybe not though, because yourbl car battery can die if you listen to music for too long without your car running, and a car battery sure has a hell of a lot more storage than 7Ah.

    Any serious replies welcome.
  11. Jul 6, 2016 #10

    jim hardy

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    "enjoyability" and "reggae" in same sentence (or even paragraph) is to me oxymoronic.

    this site will do you some good

  12. Jul 6, 2016 #11


    Staff: Mentor

    OK, I'll bite. What is your plan to be courteous to others who might be out there to listen to the birds singing? If you present a believable plan, I'll agree to help you.
  13. Jul 6, 2016 #12

    jim hardy

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    i'm beginning to comprehend
    this is a ploy to collect "flashes" for youtube.

    why mess around in minor league ? Take it to Columbus Day Regatta in Miami.
  14. Jul 6, 2016 #13


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    What you really need isn't a floating speaker but a floating wifi access point so each person on the raft can listen to their own choice of music or stream a video. Only joking.
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