Speakers for AP8

[livinlowe]

Jon-
I know this was posted before, but which speakers would you recommend for the playback of sounds?

Thanks

[JonnyMac]

Any enclosed (this is really important), 8-ohm speaker that can handle at least two watts.  I have a RadioShack bookshelf speaker that works well, and I've also picked up speakers at All Electronics (just down the road from me) that work well, too.  A great source is second-hand stores that sell old mini stereo systems.  You can often grab the whole rig for a few bucks and end up with two speakers.

[JonnyMac]

For an up-close-and-personal prop this one might work: http://www.bgmicro.com/index.asp?PageAction=VIEWPROD&ProdID=8959

[davidf]

Radioshack has a nice pair of enclosed mini speakers for $10, but they are 4-ohm, 1W -- are these too small?
Thanks,
David

[JonnyMac]

The amplifier we use (LM386) is designed for an 8-ohm load.

[john@thedevils-den]

Will anything lower than an 8-ohm load fry the amp? I am assuming I can not run 2ea 8 ohm speakers in parallel. Can I power 2ea 4 ohm speakers in series? Just some ideas..........

[JonnyMac]

I'm not an audio components expert, but I know that you want to keep the load at 8 ohms (the amp is designed to drive 8 ohms) -- so don't put two 8-ohm speakers in parallel.  Two 4-ohm speakers in series should be okay.

[menehune]

I have had no problem hooking up large (50-75W) amplifiers with paralleled 8ohm speakers. 

The increased current drawn by the lower load may be a problem with smaller, on-chip amplifiers.  A 4 ohm speaker probably will have a lower volume than an 8 ohm speaker.

[gmacted]

I looked at the datasheet for the LM386 and it can drive a 4 ohm load, but at about 1/2 the power.  It "sweet spot" is an 8 ohm load, however.

[jukingeo]

Hello All,

Ok, now you entered my arena as I am an audio technician and been working with commercial audio equipment for over 20 years.

Here are a few pointers to keep in mind when selecting speakers for general prop use on any small amp.  Since the AP-8 has as a very small amp (1/8th of a watt) you want to do the best you can to get the most out of it.

First we will talk about amplifier loading (8 ohms, 4 ohms, etc).

When you place a speaker across the output terminals, you are 'loading' the amplifier.   Basically the amplifier sees a resistance from it's output to ground.  (In the real world speaker loading is referred to as impedance and not resistance, but that is opening up another can of worms which we don't have to deal with right now.).   Current passes through the output section of the amplifier through the load (the speaker) and completes the circuit back to the amplifiers ground connection.  If you 'raise' this resistance (going to a 16 ohm load), you are decreasing the current through the amplifier's output section.  It will deliver less power this way, but you will also reduce the heat created in the output stage.  If you 'lower' this resistance (hooking up a 4 ohm load) you are increasing the current flow to the amplifier.  This in turn will pass more current through the output stage.  You will get more power out of the amplifier BUT there is a caveat, you will heat up the output stages much more.  In the case of an amplifier that is NOT rated for a low impedance load, you do risk blowing out the amplifier.  In other cases, if you don't blow up the amp, you will drag the power supply down and the voltage will drop across the output, this will generally create less power and distortion.

So with all that in mind, the general rule of thumb for loading is to obey the specifications of the amplifier and go down to the lowest impedance that the amplifier is rated for.  This will ensure that you will get the most power out of the amplifier without burning out the output stage or taxing the power supply.

With all that said, the LM386 is rated for an 8 ohm load.   Now what do you do if you have more than one speaker?  No problem, you can reconfigure the wiring to net you the proper load.   If you have two 16 ohm speakers, you can wire them in parallel (that means + terminal to plus terminal and minus to minus) or 'chain' them.  This will make the amp see an 8 ohm load.   If you have two 4 ohm speakers you can series them  (+ on one speaker, - to the other and then connect the two remaining leads together with a jumper).  This will net an 8 ohm load two.   If you have 4 speakers you can use a combination of the two methods and use what is called a series-parallel circuit.  For this you will have to have all speakers 8 ohms.

Ok, now I am going to talk about speaker efficiency.

Ever wonder how a simple 50watt guitar amplifier can blow away a 2000watt home stereo?  Well that is because a guitar speaker is designed to deliver the greatest efficiency to use every ounce of those 50 watts.   Sure this does come at a cost of fidelity, but then again, an electric guitar doesn't have to reproduce the lowest lows or the highest highs.  A single way speaker (a full range) will do the trick.  A home stereo is designed for hi fidelity and thus efficiency is not a major design factor especially when you have multi-hundred watt amps at your disposal.  Ok so now what does this all mean to a prop building?   Well, now we have to get to the ugly math stuff.

When selecting a speaker, two things people normally shoot for is frequency response and power handling.  However there is a third figure that is probably more important than the first two combined (especially for our application), and that is the speaker sensitivity or otherwise known as efficiency.   This specification determines how efficient the speaker is at converting your output power to audible output.   The spec is usually set up like this:   Sensitivity = 90db at 1w/1m .  Translated this means that for one watt of output with a microphone 1 meter (3 feet) away from the speaker, it produces a 90decibel output.   So what does this have to do with power output?  Quite a bit actually.

Lets take this scenario:

We have a 1 watt amplifier and two 8 ohm speakers that we want to use for our prop.  One speaker has an SPL of 90db and another has an SPL of 96 db.  (We will hook them up individually).

Here is the rule of decibels:  For every 3 db decrease in speaker sensitivity you have to DOUBLE your output power to get the SAME output volume.  YUP! 

So with our example above you would only need .5 watt to drive the 96 db speaker to the same volume as going the full watt into the 90db efficient speaker.  Going another step to a 99 db efficient speaker our little LM386 amp will now match the output of the 90db speaker with it being only driven to 1/8th the power output!!!

Ok, now that you know what to look for in terms of specs, where do you look in terms of finding a high efficiency speaker?

That is easy.  Look for a musical instrument speaker.  I gave you one lead right off the bat, a guitar speaker.  Now you may be thinking about what I said earlier about full range, BUT a guitar speaker's range also overlaps that of the human voice.  Get the picture now?

When looking for a musical instrument speaker, the rule here is bigger is better.  Look at the SPL figure for an 8" speaker, vs a 10", vs a 12", you will quickly see that a 12" is the most efficient (and will also have the greatest output range). 

Let's say now that you find a 12" speaker that is 100db efficient...but you still want more juice?  Guess what, you have another tool in your arsenal.  If you buy TWO 12" speakers and put them close together in one cabinet, now you have the added benefits of acoustical coupling and that will net you an additional +3 db of output.  So now we are up to 103db

You want more?  YOU GOT IT!  Putting 4 - 100 db speakers in one cabinet will net you 106db  Keep in mind now, that with one watt of power and standing 3 feet in front of this behemoth you are now entering the realm of some serious volume. 

Now lets clear up this decibel thing in to actual human hearing terms:

Weakest sound heard    0dB
Whisper Quiet Library    30dB
Normal conversation (3-5')    60-70dB
Telephone dial tone    80dB
City Traffic (inside car)    85dB
Train whistle at 500', Truck Traffic    90dB
Subway train at 200'    95dB
Level at which sustained exposure may result in hearing loss    90 - 95dB
Power mower at 3'    107dB
Snowmobile, Motorcycle    100dB
Power saw at 3'    110dB
Sandblasting, Loud Rock Concert    115dB
Pain begins    125dB
Pneumatic riveter at 4'    125dB


Roughly translated with our above example that would mean our 106db 4 12" cabinet would yield the volume output of a power lawn mower with just 1 watt of power.   So now you can see that if you hook up a 100 watt amp to this speaker, you can easily attain ear damaging levels of power.  With 100watts into this 4 12" cabinet, you may as well stand next to a jet airliner engine :).  So I hope that puts things in better perspective for you.

Oh!  As Jon pointed out in another post, you DO have to put the speaker in some kind of enclosure.  The good thing is that you don't have to be critical with most musical instrument speakers.   The main point of the enclosure in this case is to prevent sound cancellation which is what would occur if you tried to push sound through a bare speaker. 

Most musical instrument speakers are designed to what is called an 'infinite' baffle or open back speaker.  This is a plus because an open back speaker placed in a corner will accentuate the bass output more.  A totally closed box would have to be pretty large in order to get more usable bass out of it.  A good overall choice would be to look for a musical instrument extension speaker...or a speaker cabinet for an old organ in which you have the speaker and cabinet already set up for you (it has to be an old organ because newer ones have the low efficiency type speakers).  Old organ speakers are generally VERY large and have many speakers in it right from the get go. They are very efficient and will have a fuller sound than a guitar speaker.  But impedances for these old speakers are weird so you have to watch that.  Also you have to watch it with old speakers as you may get one that is deteriorated and may need to be re-coned.

While we are on the topic of bass, lets continue along those lines.  When you get into wanting more and more fidelity into the low frequency range then you may have to make sacrifices in the SPL output.  Surprisingly, most 15" and 18"  speakers are not much more efficient than their 12" counterparts, but the usable low frequency range is shifted downward.  So a very efficient single or double 15" cabinet that is in the 70 to 3k frequency response range will do very nicely for your monster growls and other animalistic roars and what have you. If the upper range isn't 'satisfying' enough you can add a smaller speaker (or tweeter) and hook it up to the larger one using a 4 to 8uf non polarized capacitor.  The capacitor will prevent an overlap in frequencies and avoid dropping your overall resistance (impedance).

For sub bass...such as a pulsating droning sound...you will unfortunately need a powered subwoofer to handle this.  This is WAY out of the power range for something the size of the LM386 amplifier.  But the good thing is that many powered subwoofers also have a 'speaker level' input and as such you can pipe the AP-8 directly into it.  Better yet, you can STILL hook up a high efficiency speaker to the subwoofer to 'pass through' the higher frequency part of the sound.  With this in mind, you could also compensate for a slightly inefficient (or the use of a smaller) speaker because a subwoofer will remove most of the bass from the speaker driven by the AP-8 and route that bass to the subwoofer's amp and speaker.

If you purchase a subwoofer you will see that it has two sets of terminals (for stereo of course).  There will be a set of amp terminals and a set of speaker terminals.  You only need to hook up one set to the AP-8 (since the subwoofer is mono anyway).  Take the speaker output of the AP-8 and connect it to the input of the subwoofer, (it doesn't matter if you select the left or right channel).

Next hook up your 'full range' speaker to the output of the subwoofer.  Now turn it all on and adjust the subwoofer so it 'blends in' or matches the full range speaker's output.


Ok, that about wraps this sound lesson up.

Enjoy!

[gadget-evilusions]

Wow. Thanks for all the information. Now I know more than I ever did about speakers.

[jukingeo]

Wow. Thanks for all the information. Now I know more than I ever did about speakers.

LOL!  Yeah, I kinda did go over the top.  BUT it is very useful information when dealing with high volume sound, but you only have low powered amplifiers and you still want to do a surprising amount of damage.   The truth of the matter is that if you want to scare the crap out of people with sound, you need volumes of air to be moved.  If you don't have a high powered amp, you need an efficient speaker. 

So for those of you thinking about using computer speakers on the AP-8....Fahgettaboutit, they just will not cut it.  You have an old Klipschorn laying about?  Now we are talkin'.

[TomS]

    If I am going to buy a speaker specifically to play a "thunder" sound effect from an AP-8, I should get an   8 Ohm, 12", high SLP speaker, in a large, enclosed cabinet, with a frequency responce of 70 - 3K, correct?  Anything else to call for in the specs?    TomS

[TomS]

Here's another question.   If an amp uses 4 ohm speakers, will there be compatability issues with the AP-8. Once the signal is run through an amp does that cancel the AP-8's need for 8 ohms?   TomS 

[menehune]

If an amp uses 4 ohm speakers, will there be compatability issues with the AP-8.

No, the "new" amplifier buffers (or isolates) the 4 ohm speakers connected to itself from the AP8. 

You can run the line level sound from the AP8 into the line in of the amplifier connected to the 4 ohm speakers.  The "downstream" amplifier only cares about the impedance of the speakers directly connected to it.

You may need to adjust the levels on the AP8 and/or the amplifier input control to get minimum distortion.