Let us move beyond plumbing to radios. The cab radio in the beast works fine parked, or driving through town, but on the highway, the driver can just forgeddaboudit (sp?). The cab's speakers, a pair of mighty 4" wonders, live in the upper front corners of the cab. I replaced them when I bought the vehicle, but even new speakers don't do a lot for the distance between the driver and the speakers multiplied by the noise level at speed.
The radio's rear channel is also connected to the speakers back in the module, and I believe that the radio's primary purpose is to provide music/etc for the patient. If the connecting door between the module and the cab is closed, the ride in back is amazingly quiet (no siren screaming test has been conducted, at least by me).
As part of my overall plan, I intend to install a Sirius-capable radio in the module that will also provide audio for a TV and for a DVD player. Unless I manage to track down a radio with the capabilities that I want at a price that I can justify, I'll have to adapt an off-the-shelf radio. This means that the cab radio won't be connected to the module any more, and that will leave the rear channels on the cab radio available.
I had been considering some sort of headset, except the boys-in-blue may frown on wearing a headset while driving. Then, Pat suggested some sort of a head rest speaker installation, but the Bostrom seats in the cab do not lend themselves to a headrest.
I finally realized that it is very common to see an ambulance with speakers for the 2-way radios installed behind the driver's head so he can hear them. Problem solution? I dug through my collection of 2-way radios, and found a nice inventory of both Motorola and Kenwood 2-way radio speakers. Even though the Kenwoods are rated at 40 watts (at 4 ohms) each, I'm going to dig through Crutchfield (or similar) to see what's available with a decent frequency response (I'm old enough that I have hearing loss, so an audiophile system it totally wasted on me). The 4" Freightliner factory speakers will be my standard of comparison.
The frequency response data for Kenwood and Motorola speakers might be available on-line somewhere, but I'm probably going to base my decision on what they sound like to me.
The plan now is to install 2 speakers behind the driver, and probably 2 more behind the passenger's seat.
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