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1307invert 16th January 2011 06:43 AM

Backpack Sound
Hello audio enthusiasts!

Now, I AM new to this forum, but I'm not new to forum procedure. Just to be clear, I have read all the other forum posts about backpack ideas like mine. However, instead of posting several questions on several different threads, and resurrecting them after 3 or 4 years of inactivity just to answer my own questions, I thought I'd start a new post about my idea and questions!

So, without further ado, here's what I've been looking at:

For a year or two now, I've been thinking about building a backpack that I can bring to events, and blast music out of. I'm just in love with the idea of an inconspicuous backpack having the ability to create flash-dance like music.

Now, for my specifications.

The backpack itself will obviously be custom made to fit whatever I come up with. Here is a tenative drawing i made of what i want it to look like. (enlarge the image to read the descriptions)

In case the drawing is unclear, the backpack would unzip down the front, and the two halves would unfold outward, revealing the main speaker.

I'd preferrably like it to be 30 lbs or less (I'm pretty sure that the battery will make up the majority of this), with a normal, fully charged 12volt gel-cell I'd like it to last for 3+ hours, and I'd like a good balance of sound quality and kicking bass. Also, I'd like to do this as cheaply as possible. That doesn't mean i want to be skimpy and sacrifice quality (too much) but it does mean that if one option is cheaper than another, and the more expensive one only offers something that is relatively extraneous or unnecesary for the extra money, I'll most certainly prefer the cheaper alternative!

And now, for my questions! (and I'm rather new to this, so please excuse any glaringly obvious answers I may have missed.)

1. What kind of amp should I use? I know it will be a T amp, but there are a dizzying array of options. I'm not really a Kit guy, so pre-packaged and ready to go would be nice (although if i had to, I could suffer through a kit if it was the absolute best option). Also, what kind of vibration and shock-proofing, if any, will this amp need? I'd rather not have it fail because I didn't glue down some parts inside of it or give it enough padding

2. What arrangement of speakers do you all think would be optimal? From looking at some of the other posts similar to this, I've seen several different ideas posted about how to go about doing arranging the speakers. My general feeling is obviously described in the picture above, but I make no mistake that I could be very wrong. Is one big driver in the middle the way to go? or would several smaller ones do the trick better? Also, do I need the extra 3-4 inch speakers on either side? What will they add, if anything, to the overall sound? I like the idea of having them there, but if they serve no real useful purpose, they can be dispensed with. Does anyone have another configuration that may work better?

3. Depending on the answer to the above question, what specific speakers should i use? I know that for extended battery life and optimum amp efficiency, I'll need pretty efficient speakers. Probably nothing that is made for car systems, since I've read here that those are very inefficient. What speakers will give me the best efficiency for my money?

4. Finally, what should I do about enclosure? I know that for the main body, it will probably be a plywood box, but should I include those holes for cooling and airflow? And what kind of reinforcement will it possibly need?

Thank you for taking the time to read this!


zerokelvin99 16th January 2011 11:52 AM

going to be brief, but hopefully some help :-)

1. If I were you I would go for a lepai tripath. reason is that its in a strong case, got bass+treble controls and gives you a good amount of power for 12v. either that or the 41Hz Audio:AMP6-BASIC kit (AMP6BASIC ASSEMBLED).

this should be enough power, if not then upgrade the batteries to two in series, and go for either the (google these) sure 2*100, or sure 4*100, AMP9BASIC ASSEMBLED, or any other amp suitable for 24v-30v

im sure other people will be able to recommend good amps!

2. the best way to do it is to go for PA (NOT CAR) speakers. One big one is the way I would do it, with a piezo up above. however, you need to bear in mind the amplifier youre using. with the lepai, one 6" 4ohm PA speaker (bass/mid) with a piezo per channel should do a good job!

again, others will chime in with their preferences :)

3. oops, kinda said that above. good starts are the eminence beta 6/8s, when ported these go down to 90hz (i think) and are v. efficient / loud!

4. plywood, you will want to port it to make the most of the bass response. tripath amps at this power dont need cooling. 12mm will be plenty thick

1307invert 16th January 2011 07:14 PM

Thanks for the response!

yes, I've heard a lot about these Lepai tripaths, and they look like a very good option! but I'd like to wait for any other options to develop as well.

As for the speakers, I looked up the ones you mentioned, and came to this page
Eminence Speakers - Eminence 6" & 8" speakers - Eminence Alpha and Beta 6" & 8" speakers. Eminence 6" & 8" speakers here.

are you referring to the Beta-8A or the Beta-8CX?

Also then, in regard to the speakers, will I need a crossover for this project? (I'm pretty new to this stuff) If I do need a crossover, what do people recommend?

Another question to consider is that I think for running a system that is this small, it doesn't really make sense to run stereo, correct? the speakers will be so close together that it won't really matter which side its coming from (especially if I use only one big speaker and some tweeters), so how would that effect my use of a Lepai tripath amp? (again, sorry if I've missed something very obvious)

Thanks again! 1307invert

gooki 16th January 2011 07:55 PM

1. I would look for a class d amp made to run at 12 volt.

2. I would do two speakers only, one woofer + horn loaded compression driver. Both mounted facing back. Will allow you to maximize enclosure space.

3. Pro sound drivers. Maybe even consider a pro audio coaxial driver so you don't waste space, and compromise you box with many cutouts. Can you squeeze in a 10 inch driver? This one is only 8.6 lb, same weight as their 8 inc coax:

4. I would make the enclosure out of fiberglass with a thin ply frame to get the correct curves. Have a look around some car audio forums to see how people use fiberglass. It's not that hard.. all you really need is a frame, some fleece, some resin, and a bit of fiberglass.

Additional comments. If you go with the single woofer + horn tweet setup, You'll be wanting to run in mono. I would consider adding the minidsp to your setup to give you an active xover, and stereo to mono down mix, and peq for frequency response shaping, all with minimum current draw.

For more ideas look on this forum for some of the boom box builds.

zerokelvin99 16th January 2011 08:42 PM

no worries

if you have a piezo you will NOT need an xover (just a capacitor with the piezo)

if you use the 8CX (coaxial) it has a compression driver (tweeter) built in. you will need a crossover for this, but it gives you both the main woofer and the comp. driver in one very compact package. you can get away with a cheap eminence crossover - each coax driver has a specific crossover freq (which they will say)

converting a stereo to mono signal is dead simple - in this way you will have a mono (left+right) going to each channel.

in your situation i would try for 2 beyma 8cx, one off each channel, but this is quite big for a backpack

proposed diagram:

stereo signal -> mono signal -> channel A and channel B inputs -> amp -> channel A goto crossover 1, channel B goto crossover 2 -> crossover 1 goto first 8CX, crossover 2 goto second 8CX

hope this gives some food for thought

zerokelvin99 16th January 2011 08:42 PM

edit: piezos are the preferred option though, look at the "boominator" thread .. V helpful!

v-bro 16th January 2011 09:24 PM

You might find this interesting:
41Hz Audio - Project: AMP11 Backpack unit

1307invert 16th January 2011 11:01 PM

First off, @gooki ,

I really like the fiberglass idea! to my mind, that sounds much more rugged and forgiving. However, I have no idea how to make the enclosure optimized for the best sound response. Is there an easy, brief tutorial so to speak on how to do that, or is it more trial-and-error?

as for your recommended speaker, it does look pretty sweet! unfortunately $331 seems a bit much... I'm sure its very good quality for the money, but I think that there are cheaper options that I would be just as happy with.


Additional comments. If you go with the single woofer + horn tweet setup, You'll be wanting to run in mono. I would consider adding the minidsp to your setup to give you an active xover, and stereo to mono down mix, and peq for frequency response shaping, all with minimum current draw.
Now, unfortunately, I have NO idea what most of those abbreviations are or mean. I know that xover is a cross over, and I'm guessing that the minidsp is a type of crossover? But other than that, I don't quite understand what you're getting at here, sorry :o


so, correct me if I'm wrong, if I simply wire a piezo tweeter and one of the 8" speakers together, and pass mono through them, it should work, without needing a crossover?? that'd be great! How would I wire them though? parallel or series?

and those bayma's are pretty expensive too! $229 seems a little high for what I'm looking at here, don't you think?

what about these full-range speakers? Beyma Speakers - Beyma 8AG/N fullrange 8" speaker - Beyma 8AG/N 70 watt 8" fullrange speaker for hifi speakers, organ and loudspeaker applications. Beyma 8AG/N fullrange speakers here. they're a little shy on the watt-rating, at only 35 watts, but with a Lepai pumping out only 20 watts per channel, that would be 40 watts, which is kinda close... what do you think?

regardless of that, I think that this speaker Eminence Beta 8A speaker. The Eminence Beta 8A is a 8" 8 ohm speaker for all bass applications including bass guitar cabinets. Beta-8A is a 225 watt RMS 8" woofer., with a piezo tweeter or two, would be the way to go.

but i could be wrong! Whats your opinion?


that backpack looks pretty awesome! However, i don't know if i need anything quite that high-tech. I'll be happy if it works! But I do like the LED lighting he uses for effects.

anybody have any suggestions for amps not mentioned yet?

zerokelvin99 17th January 2011 02:16 AM

apologies now for spelling mistakes..

piezo and woofer connected in PARALLEL, with the capacitor (cant remember the value) connected across the terminals of the piezo.

those beymas look to have really nice response upto 10k, perfect for a cheap full range solution. if you model them in winISD to have a bump around 80-100hz then they should bump out a bit of bass too!

the lepai will give out more like 10watts, at 8 ohms a channel, dont worry about the power handling..

remember, more cone area = more sensitive = louder!

two 8"s are going to need 1/2 the power to reach the same volume as one 8". Doubling the cone area = 2 to 3db more output, as well as a more solid lowend.

Also, with 4 ohm woofers you can get twice (3db) the power output

1307invert 17th January 2011 03:14 AM

okay, sounds like that could be a pretty nice way of doing things... any idea where i can find what kind of capacitor would be needed?

yeah, they look like a nice option... what is in the 10k-20k range that I will be missing with those speakers though?
would a setup similar to this be more useful then? with a tweeter or two adding to the high range?

winISD is a program for making enclosures, correct?

okay, thats good to know... so those 35 watt speakers would work nicely with that amp...

as to doubling the cone area, that extra sound and low-end kick would be pretty nice! but doubling the cone area also involves doubling the amount of money spent on speakers :) so its kinda a tossup there...

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