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I'm a volunteer firefighter. We have two kinds of radio traffic. Tones and voice. Tones trigger our pagers, but are also audible when listening to the radio. The trouble is that the tones are much louder than the voice traffic.

What happens is that we turn the radio up loud enough to hear dispatch and other units, then tones are dropped for another call and they are incredibly loud. We scramble to lower the volume (which is not easy on the model of radio we have) and then have to turn it back up quickly as soon as the tones are over.

The radio is connected to a single speaker in the cab.

What can I put in-line between the radio and the speaker that will let me temporarily reduce (ideally not mute) the volume quickly and return it to its original volume?

I'd really like a momentary switch (foot peddle). My initial thought was to use a simple resistor, but now I'm thinking that won't work as expected. I need something simple and robust (firefighters are good at breaking stuff).

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  • \$\begingroup\$ Do this wrong and lives are at risk. Get permission; issue a contract for the design. Implement the modification officially. \$\endgroup\$ – Andy aka Nov 11 '15 at 9:14
  • \$\begingroup\$ Whilst I agree with @Andyaka it is worth discussing some of the options - at least it would help shape the technical content of the contract request. This should be possible automatically, recognise specific frequencies - above 'normal' volume levels and auto mute until the tone goes away. \$\endgroup\$ – Icy Nov 11 '15 at 10:09
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Pass your request up the chain of command to the radio people. Fiddling may be a way to fix it but having a wire come loose later on a DIY modification will likely be frowned upon (or fail some insurance clause) .

An inline resistor of similar power rating to the speaker of same or up to a few times the speaker impedance in resistance value will likely control the output sound level at minimal loss of fidelity. You could then have the foot switch operate to short the resistor with a push to make-push to break type switch. A projected switch failure would leave it in one state but at least you would have some volume.

Modern radios may have clever tricks for remote volume, mute, squelch and road noise inputs for volume control that may be more suitable.

If it uses a very limited number of tones, active notch filters in the guts of the radio may be an option but it is likely to be in a useful part of the audio spectrum and degrade speech quality if attenuated.

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    \$\begingroup\$ Safety does come first. But as a volunteer department we often "adapt and overcome" to get the job done. We always have backup radios in the rig (we take them with us on scene). I will talk with our radio vendor and see if they have a solution for this. \$\endgroup\$ – Joshua Hunter Nov 13 '15 at 5:26
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You do have access to the speaker wires so there is scope to do something .Firstly place some 2 watt idiot resistors across what you do so if your circuit fails the radio will play albeit at reduced but still plainly audiable volume level.I cant tell you the exact resistor value you need.I have only worked with firemen a few times.Now place a small light bulb or PTC across your safety resistor .How this works is that the very loud signals create heat that increases resistance reducing volume .You must use a small light bulb or a small PTC .If the cold resistance is a few ohms you can pretty much get full audio on a weak signal.Really what you have done is a poor mans audio compressor.Probably more reliable than a bells and whistles one due to its simplicity.You can also place a paralell tuned circuit across everything thats in series with the speaker .The LC values of such a filter wont yield unobtainium components .You will have to find out the precise tone frequencies that your fire brigade uses in order to design the notch filter.

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You are looking for something like an audio compressor. The wikipedia article is extensive:

https://en.wikipedia.org/wiki/Dynamic_range_compression

Basically you want an active circuit that samples the incoming volume and adjusts the output volume so that it falls within a certain range. This is not a trivial task and requires some active components.

THAT Semiconductor has quite a few design notes showing how to use their line of OTAs in compressor/limiter applications:

http://www.thatcorp.com/Design_Notes.shtml

If you think there is a way to detect the tones easily (ie there is a carrier tone when they come through) you might be able to design a detector circuit that mutes the audio while the tones are active.

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