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Old 6th July 2005, 09:29 PM   #1
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Default Unique(?) Portable Sound Requirement -- Advice Requested

Greetings all, and thanks for a wonderful forum! I have been reading here for several days straight, and have learned a ton.

I was not certain of the correct forum for this post, as my questions cross several categories, so I thought I would try it here. My apologies if this is inappropriate -- please let me know if this is so.

A friend has asked me to build him a portable, battery powered device that can play a few 5sec - 10sec sound samples at sufficient volume to be heard clearly throughout a large conference room/convention hall containing ~300 people. My reading here seems to indicate that this is quite a tall order, and I am in one of those "pick any two of..." type situations. It can be portable and battery powered but not that loud, or that loud and battery powered but not very portable, or that loud and portable but not battery powered. None the less, I am trying to achieve the best balance I can while meeting all the requirements.

I am limited by that fact that I simply can not scratch or kit build any of the major components for this project. I am stoked to start building some of the things I have seen here for myself, but I do not have enough experience to feel comfortable doing this for someone else.

Here are the necessary specifications:
  • Battery powered.
  • Less than 50lbs, preferably closer to 30lbs.
  • Able to produce sufficient SPL to be heard clearly in large, crowded space.
  • Absolute minimum frequency response of 100HZ - 8KHz. 80Hz - 12+KHz
    preferable.
  • Runs ~12 hours on a charge.

I have left out requirements for the sound effects board, as I already have that sourced. The only info that is relevant here, I think, is that it only samples at 8Khz (so we are definitely not building hi-fi) and that these sound effects will be played only infrequently. I have been figuring 20 minutes total play-time over the course of the 12 hour operation. This is probably excessive by an order of magnitude, but I would prefer to estimate very generously rather than run out of juice.

With all of these limitations (and the one so far unspoken -- this needs to be done as cheaply as possible) I have come up with only two ideas.

The first: Get a Sonic Impact T-Amp, yank the board and cut off all the external connections, and connect the output of the soundboard to the t-amp through a new pot (to be through-mounted in a panel of whatever sort of box this winds up in, along with a power switch and momentary button switches to play the samples). For speakers, 1 cheap PA horn (6" X 11", 120db sensitivity, 40W max, 4 Ohm, 200Hz - 15KHz) and 1 cheap 4" or 5" 4 Ohm fullrange/midrange driver to fill in what the horn lacks. I would power it with a 7 ah SLA.

The obvious problem with this is that no matter how efficient a little midrange driver I can find, it is not going to be anywhere near as sensitive as the horn. I think that @ 4 Ohms the T-amp will be enough to power the horn to the levels I need, but doubt that will be the case for the midrange. It is possible, I think, that a bandpass box could be made to push the output between the ~80Hz and ~300Hz I care about, and poor sound quality is not an issue as long as the effects are recognizable (animal sounds, foghorn, possibly the odd short music sample, etc.), but designing such an enclosure stretches (let's be honest, exceeds) my capabilities. Further, I need to be concerned with weight, so working with MDF or HDP is pretty much out.

Any help on that? Will it be loud enough, through either speaker, using the t-amp? Is there an easy answer that I am overlooking?


The second: Get a cheap crapola auto booster amp, like the Pyramid PB-110PX for about $30. This thing claims 2x120W mac, 2x50W RMS, which is of course a joke. I figure that I can be reasonably assure of getting a usable 2x15W RMS out of it, maybe a smidge more if I am lucky. "Current consumption" is listed as 5A. I have no idea what the idle draw is, but am figuring that it will be very low. I would power it with a 12 ah SLA.

My hope is that with the very infrequent use of the amp at full power, and the short duration (we're only talking about 5 to 10 seconds, remember) of those uses, I can get by with a lot less juice than one would otherwise need for something like this and still have it last all day. With more power on tap, I can still use the 1 horn 1 midrange scheme, but now I can run the midrange as loud as I need and either attenuate the horn to match, or leave it alone and let it be louder still. (The guy specifically wants a horn of some sort for aesthetic reasons.)

Things that my limited experience does not allow me to answer on this one are... Do I have my head up my butt when figuring the power requirement? Is ~15W/channel STILL not enough to get the required volume? Can a small, inexpensive, speaker even GET loud enough in the 80Hz - 300Hz range to do what I am after?


The only thing I can think to add to all of this is that since I will be mastering all the samples myself, I will be able to control bandwidth and do sort of a poor-mans active XO by filtering out all but the absolutely necessary freqs for any given sample, and control the dynamic range to goose the average level as much as possible through aggressive compression. Once again, my limited experience does not really allow me to predict how much perceived loudness will result from this.


I know from all the reading that I have done here that the clueless are not suffered gladly, and I apologize for adding to those ranks. I can only plead that I am trying very hard, and ask you to be patient with me. I am really stuck at this point. With more budget, I could just start putting things together and see what worked. Since budget is the thing of which I have the least, I could really use a push in the right direction.

lyd
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Old 6th July 2005, 11:10 PM   #2
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Ack. Speaking of being clueless. It only just dawned on me that a sound chip sampling at a rate of only 8kHz probably does not have unlimited dynamic range! Cripes.

The board is based on the Winbond ISD2560, which rolls off everything above 3.4 kHz, so please disregard any comments above about frequencies higher than that.

It is the lower info at 80Hz - 200Hz that is more critical to my application, in any case, and low-end rolloff seems to be dependent on the value of one or more external capacitors. I am waiting for an answer from the vendor on the low end of this board, but with any luck I should be able to change a cap or two if it is too high.

Sorry for this oversight in my description of the problem.

lyd
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Old 7th July 2005, 03:26 AM   #3
Stocker is offline Stocker  United States
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You want a big PA speaker, maybe a musical instrument or guitar speaker. 8 to 15 inches, probably 10 or 12. Designed to put out generally those frequencies from a guitar, fits in a smallish box that can hold any amp and battery easily. I have an 18AH SLA battery as a reference for my idea of weight. Build the cabinet out of 1/2" MDF or even particle board. You might even pick up a secondhand unit from a pawn shop or consignment or charity store for a song that will do everything you need already made, less the power supply.

This is a moderately easy goal you have set for yourself. Do not lose heart at all the steps involved. Pick a plan and lay out the steps and do them each one at a time.
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Old 8th July 2005, 04:24 PM   #4
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You want a big PA speaker, maybe a musical instrument or guitar speaker. 8 to 15 inches, probably 10 or 12. Designed to put out generally those frequencies from a guitar, fits in a smallish box that can hold any amp and battery easily. I have an 18AH SLA battery as a reference for my idea of weight. Build the cabinet out of 1/2" MDF or even particle board. You might even pick up a secondhand unit from a pawn shop or consignment or charity store for a song that will do everything you need already made, less the power supply.

Thanks for the reply, Stocker. Do you mean by your alternative suggestion that neither of my ideas above are likely to work very well? Can you offer suggestions for any sources of the sort of driver you are describing? Searching for PA and instrument speakers, the only things I am finding are quite expensive.

You do not mention amplification -- does this mean that something like the goofy car audio amp I described above should work with your suggestions, or would I need more power? If the latter, do you have any suggestions in that regard?

I did initially search for some sort of used battery-driven portable PA, both locally and on the 'net, but did not find anything that was inexpensive enough for my needs.

Thank you again for your help.

lyd
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Old 12th July 2005, 05:53 PM   #5
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Well, the silence in response to my questions has been quite deafening.

If there are no opinions or suggestions regarding the project, how about regarding a better place to pursue this line of inquiry? Would I be likely to receive more input by splitting my questions up and asking them in the amplifiers, loudspeakers, and car audio sections here? Is there a forum out there on the intarwebs more suited to discussing this frankenstein than diyaudio.com?

lyd
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Old 21st July 2005, 03:56 AM   #6
Stocker is offline Stocker  United States
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The silence is likely largely due to your goal being quite an easy one to achieve and a couple of good ideas along with your own already posted should get you very close to done if not happily so.

I think it would not hurt to go to pawn or secondhand shops and look around a bit. A cheap guitar cabinet (cheaper without the head, call it a speaker) a beat-down looking (but still working) Class D, B or low-bias AB amplifier, the battery you have and you are already almost done. Don't let budget hold you back... creativity in finding parts and making-do can overcome *almost* any budget.

I am not saying your ideas wouldn't work, not even that they wouldn't work easily. I was offering mine.

Get a *ahem* "budget" PA or musical instrument driver and slap together a box if you can't find one secondhand for a good price. Even most non-sub woofers will likely get you into the neighborhood of 3kHz. Even a regular speaker if it's light and loud enough.

Knock yourself out, take pictures and don't be afraid of yourself.
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Old 21st July 2005, 05:56 AM   #7
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This or any number of PA, Guitar, or Bass Guitar drivers should fit the bill nicely. Just model them real quick using ISD online using the optimum button to see how big the cabinet will need to be (not counting the displacement of the driver, amp, and battery).
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Old 21st July 2005, 06:52 AM   #8
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If you need a little more "oomph" than a T-amp can provide, I'd recommend looking at one of the UCD modules at http://hypex.nl/. They're around 95 percent efficient- about the same as the T-amp. You can get about 200 watts for 60 euros.

This has actually been a project idea I've had for a while- what's the most extreme "boom-box" we could come up with?

I think that a small quantity of Lithium Ion 18650 type cells could provide the juice, along with a UCD module, to push some neodymium PA drivers (see the Eminence delta-lite series) and horns to earsplitting levels for a long time in an ultra-light package. Beware, though- the lithium ion rechargeables are a major fire hazard if they aren't treated just right.

Realistically- a 12" delta-lite will do from 80-3khz, and weighs 9 pounds with 97db/w efficiency. Cost is $120ish. UCD 180 is less than 5 pounds. If you run six to eight 15 AH 12V lead acid batteries in series, you could get well over 100 watts that would last for several hours continuous use. Did you say you wanted ear-splitting? The peak SPL of this rig would be 115 to 120 db at one meter. It could weigh less than 40 pounds with batteries.
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