Batboard

What is the best way to wire in an additional speaker for the crew cab in a fire engine?

I have a Spectra A7 up front with a telephone handset.

Do I need an amplified speaker or can I hook it up with the existing speaker (Parallel, Series)?

This is what we have done at the here in Charlotte for our trucks. We would take two speakers splice them together in polarity before the connector on the speaker side then connect the connector to the radio. I'm not saying this is the best way but for time involved and what not it has worked for us. Thus far we have not had any problems with the amps being over drawn but the fire fighters don't have to turn the radio all the way up either. So ar so good. But if anyone has a better sugestion please let me know so can get corrections made here if need be.

By thomsoningreen at 2009-08-28

I just didn't want to load up the audio amp and have it fail. I figured I would see how someone else has done it - Thanks

If a person chooses to completely ignore Ohms Law and connects multiple speakers in a way that causes a different load on the radio's audio output then serious problems may follow. Do it right the first time to avoid this possibility. Here's how it's done:

Assume the radio's output is 8 ohms, so you take two 4-0hm speakers and connect them in series, thus matching the 8-ohm load the radio wants to see.

Assume the radio's output is 16 ohms, so you take two 8-ohm speakers and connect them in series, thus matching the 16-ohm load the radio wants to see.

In the example shown in a previous post (the parallel circuit), the total load presented by the speakers will be equal to the impedance of each speaker divided by two, so if each speaker's impedance is 8-ohms then the total load is 4 ohms.

Mismatches may work or they may not, but more important, an improperly engineered system might fail at a critical time, which is not good for anybody.
"Curbstone Engineering" should always beavoided when dealing with life safety applications.

Tom,

There was an option for the Spectra radios that supplied a 4 ohm speaker for more audio output power. The audio chip used in the Spectra can handle 22 watts. Note: earlier Spectras had a different audio PA chip, and do not do well with a 4 ohm load.

So, two 8 ohm spectra speakers wired in parallel would work ok, as they equal 4 ohms.

Will wrote:Tom,

There was an option for the Spectra radios that supplied a 4 ohm speaker for more audio output power. The audio chip used in the Spectra can handle 22 watts. Note: earlier Spectras had a different audio PA chip, and do not do well with 4 ohm load.

So, two 8 ohm spectra speakers would work ok.

Yes, but isn't there two version of the speaker for the Spectra?
HSN4018B - Speaker, 8 Ohm, 15 watts
HSN6001A - Speaker, 3.2 Ohm, 10 Watts

The OP didn't say which types he had, and the person that provided the example didn't say, so I was just curious and don't want the OP jumping to it.

I have the 6001, so even though lower on watts, the ohms are quite low, which make it louder. So this would change everything if he is using two 6001A right?

And I thought there was a Speaker A/B option for the spectra? I think that was for buses though for an internal PA and I think you had to switch between Speaker A and B and couldn't have both on at the same time IIRC, which isn't what you want in this application.

My post was directed at familiarizing people with Ohms law, and was not specifically about the Spectra and its possible audio connections. Just because something works when you first power it up doesn't mean you've done a correct engineering job, because if you haven't, the thing can go dead/bad on you when you least expect it.

Tom in D.C. wrote:My post was directed at familiarizing people with Ohms law, and was not specifically about the Spectra and its possible audio connections. Just because something works when you first power it up doesn't mean you've done a correct engineering job, because if you haven't, the thing can go dead/bad on you when you least expect it.

X1000

Ya, just like you can get away with a lower gauge wire running too many amps through it (like a 110 Watt Spectra), but if you run that amps though it at a long duration, then things can go bad quick.

Btw, what does a Spectra allow as an acceptable ohm rating? Since they sell different speakers, one at 3.2 ohms and one at 8 ohms, is it like what Will said that you can go up to a 4 ohm speaker on the newer Spectra? I was just curious.

I wasn't sure of what the Spectra amp was looking for. I'll try and keep it at around the 8 ohm area. Thanks for all the replies - Mike

Tom in D.C. wrote:If a person chooses to completely ignore Ohms Law and connects multiple speakers in a way that causes a different load on the radio's audio output then serious problems may follow. Do it right the first time to avoid this possibility. Here's how it's done:

Assume the radio's output is 8 ohms, so you take two 4-0hm speakers and connect them in series, thus matching the 8-ohm load the radio wants to see.

Assume the radio's output is 16 ohms, so you take two 8-ohm speakers and connect them in series, thus matching the 16-ohm load the radio wants to see.

In the example shown in a previous post (the parallel circuit), the total load presented by the speakers will be equal to the impedance of each speaker divided by two, so if each speaker's impedance is 8-ohms then the total load is 4 ohms.

Mismatches may work or they may not, but more important, an improperly engineered system might fail at a critical time, which is not good for anybody.
"Curbstone Engineering" should always beavoided when dealing with life safety applications.

Thanks for the lesson will remember that in the future. and make correction for the past. I know about Ohms Law was aware of it but wasn't applying it obviously. Learning to trust my basics, thanks.