SparkFun Forums

[reklipz]

Alright, I've received a request from a buddy that I'm not sure how to fill.

I'm building an airsoft prop for one of my buddies to be used in one of his simulations. Here's the lowdown; He wants me to build a bomb of sorts, that when it goes off, plays the sound of a "huge explosion." We're not talking a simple really loud siren here, he wants and explosion noise that can be heard from pretty far away. They play in a wooded/forested area that is probably 1000acres, and he wants to be able to hear it (if possible, the whole ~2miles), or something like, within 500ft. I know that is very ambitious, so I want to deliver the best I can.

What I've got planned is to use a PIC and possibly some memory media (to store the audio samples), and either drive some amplifier directly (using the PWM module on the PIC, probably limiting me to 8 bit samples (10 bit if i want to get creative with the storage and conversion)), or drive a DAC and then an amplifier of sorts (allowing for whatever the dacs resolution is for... resolution). I plan on using .WAVs (with or without the header info, depending on some factor I've yet to realize, ).

My question is, what makes a speaker loud, and where can I find a reasonably priced one (i'm planning on buying from mouser or digikey, if they don't have it reasonably priced, odds are hes not going to be able to hear across the field). II've read about SPL, the speakers all have variable SPL across the spectrum, but they all have a power rating as well. SPL obeys the inverse square law, so the higher the SPL, the louder the noise (from any distance), and the "further" it will reach. So then, what is the power rating for?

Also of note, the prob will be battery driven, but the batteries are good quality NiMH packs (10C+ drain, >= 7 cell batteries, >= 2000mAh).

Any suggestions or comments are greatly appreciated!

-Nate

P.S., this speaker caught my eye, no idea if it's going to perform how I want, but it was cheap, decent wattage (of the selection), and high SPL.
http://mouser.com/Search/ProductDetail. ... Y41Q%3d%3d

[static]

Unfortunately I don't think you will be very successful in implementing this.

Besides looking at SPL you have to realize one important thing - to achieve the same SPL in lower frequencies, you'll need MORE energy than you would have needed to achieve same SPL in higher frequency. This is one
Two: in order to reproduce the sufficiently low frequencies to sound like real explosion, you need a speaker that can 'comfortably' reproduce at least 20-30Hz freq. The speaker that can do 20-30Hz and be audible from even 500m would have to be gigantic (or ported).

so my advice is look further at physics of sound and explosions before going further into details like what micro you'd be using

[reklipz]

Unfortunately I don't think you will be very successful in implementing this.

Besides looking at SPL you have to realize one important thing - to achieve the same SPL in lower frequencies, you'll need MORE energy than you would have needed to achieve same SPL in higher frequency. This is one
Two: in order to reproduce the sufficiently low frequencies to sound like real explosion, you need a speaker that can 'comfortably' reproduce at least 20-30Hz freq. The speaker that can do 20-30Hz and be audible from even 500m would have to be gigantic (or ported).

so my advice is look further at physics of sound and explosions before going further into details like what micro you'd be using

Roger that. So, it seems the only way of "notifying" someone across the field (via speaker) is going to be to use a pitch that the speaker responds best to (best SPL), which is more than likely going to be > 1KHz.

I pretty much knew right from the get go that hearing the "boom" from across a field is pretty much not gonna happen (as a side note, I have a ported subwoofer laying around from a logitech z-5500 that fried during an electrical storm, sounds like I could have some fun later down the road if i wanted to =).

[jdraughn]

If the frequency is of a certain note, or a "range", you can possibly use a electric motor setup with a push/pull mechanism (like a steam train, or connecting rod on a internal combustion engine) to drive a large diaphram, like say a huge 8' x 8' size. Would have to figure out how to block the rear wave from the front wave and build a flexible surround of sorts too.

[Andrew02E]

You might be strained with a single 2W speaker. To give you an idea of a similar project, one of my company's products is a loudspeaker type device. For omni-directional effect, it uses three 4W speakers (from from Misco Speakers) driven nearly to the max. On a typical day with wind, traffic, bystanders, etc., you can clearly hear it up to about 100' away, and if you've got good ears you can make it out up to a block away (~300'). Keep in mind that you can still hear it at that distance, but hearing it and making out legible speech are different.
Also, generating sound isn't hard, it's amplifying the sound that's rough. The product I mentioned before uses class B amplifiers from NXP. The biggest obstacle there is heatsinking (the amps get hot). If you want a good sound quality too, you'll probably want to use separate speakers for the low/mid/high sound ranges. That complicates things even more.
In any case, good luck.

[theatrus]

Speakers are very size inefficient vs an explosive device

You'll need a BIG driver (or many BIG), possibly horn loaded, and a very hefty amp.

But what about just using a starter pistol?

[Philba]

I was thinking along similar lines. Maybe you could use something pneumatic to produce a popping sound. I have no idea what it would be, maybe a cork coming out a tube under pressure.

[bigglez]

I'm building an airsoft prop for one of my buddies to be used in one of his simulations. Here's the lowdown; He wants me to build a bomb of sorts, that when it goes off, plays the sound of a "huge explosion."

By far the easiest solution is to use an explosive. It
will sound very realistic...

Another thing you could try is to take a sheet of metal
and shake it by hand. Sounds a lot like thunder, and
due to its size is quite loud. Seen this done "back stage"
for sound effects in the theatre.

Even if you could provide a conventional sound
reinforcement amplifier and speakers your explosion
sound will not be very realistic in open space. A
pyrotechnic explosion is a massive release of energy
and a huge pressure wave propogates out in all
directions.

Plus, the lower the pitch (frequency) the harder it is
to identify the direction. Home theatre sun-woofers
can be moved around the sound field and have little
effect on the listeners.

[reklipz]

By far the easiest solution is to use an explosive. It will sound very realistic...

That was my first idea, . Unfortunately, due to how the prop will be used (and the ability to get a new explosive every time he needs one), it's not likely to happen (but, it will have electrodes for a remote detonator if need be).

Lots of good ideas here, but again, there is a size constraint, so it looks like it's simply not possible to get the sound he wants at the range and size he wants. He'll have to settle for some a bit smaller (similar to what Andrew02E mentioned).

Like I said, there's a lot of cool ideas here, would be neat if someone was able to create one or two, I'd be entertained.

Thanks everyone for the suggestions, I'll let you all know how things turn out!

-Nate

[Dibblah]

Commercial farming in the UK at least used to use bird scarers.

http://www.birdscaring.co.uk/acatalog/features.html

These are simply a programmable small propane fuel-air explosion generator.

Cheers,

Allan.

[FartingMonkey92]

Commercial farming in the UK at least used to use bird scarers.

http://www.birdscaring.co.uk/acatalog/features.html

These are simply a programmable small propane fuel-air explosion generator.

Bird scrarers would work really well. (incase your not familiar with them, they work by slowly letting out
propane which, because it is cold, pools in the bottom of the barrel/horn waiting for a spark to ignite it)

http://en.wikipedia.org/wiki/Bird_scarer

[Garak]

The smallest speaker I would consider for something like this would be a 10" 200watt+ in a cabinet tuned for 80hz.

The smallest sub woofer we use outside for small concerts is rated for 700watts continuous 1400watt peak 18" driver. At the smallest concert we use 8 of them. On mid size shows we would use 16 double 18" cabinets loaded with 2000 watt drivers.

On high frequencies 3khz+ we only use 2 100watt drivers to match 4 700watt 18". In other words it takes lots and lots of power to produce low frequencies compared to high frequencies.

The other bit of trouble is coupling that power to the air. You need big air pushing diaphragms, which is basically what an 18" speaker is. Its a big coil attached to a diaphragm.

In a concert situation we want a flat frequency response. Meaning that the speakers will and amplification will take a signal say 1v p-p at 60 hz and produce the same SPL as 1v p-p at 80hz or for any other frequency in hearing range. So the cabinets are designed to help smooth out the frequency response of the speakers.

But in your situation you just want a big boom. So you can tune your cabinet so that it resonates at a particular frequency. My guess would be 80hz or 100hz would be effective. It wouldn't be ground shaking but it would sound like a boom and travel fairly far. It might be as simple as just making the right sized hole in your enclosure.

Lower frequencies don't drop off as much as higher frequencies over long distances outdoors. The amount of moisture in the air plays a huge factor here. At an outdoor festival on a dry day it will sound great at the back of the field, on a moist day it might sound pretty dead and boomy as the high frequencies have dropped off over the distance.

For your application you only need to produce a few cycles of 100hz at say 100watts. You could use a step up regulator to step the voltage up to at least 28 volts to get 100watts across an 8 ohm speaker. A 4ohm speaker would be better because you could use a lower voltage but the current will be twice as high. You would have to slowly charge a large bank of capacitors which would power your power amplifier. I would use a stereo amplifier IC in bridge mode. You might be able to get away with exceeding the specs of the IC a little where your only looking to get a single blast as long as the IC has good thermal protection. Your not going to get high quality audio out of this thing so I wouldn't even bother with using an audio recording chip or anything. You should be able to get away just use PWM and a few filters to convert square wave down to a sine wave. Well if we are going to be that crude you might even be able to get away using a h-bridge to drive the speaker. Just keep the DC pulses short so you don't cook the speaker coils.

I'm kinda picturing a basket ball sized device, maybe a little bigger...