Hello Mighty!
The question of LW bandwidth has been discussed before, and I think we figured out that since LW is not officially used in the Homeland there is no set bandwidth.
The next thing I don't remember is what the LW bandwidth is in Europe. Is it 9kHz like their medium wave? Is it 5kHz like I think international shortwave is?
But here on the big island of North America we can apparently use any bandwidth, EXCEPT that Rich has pointed out that the physics of Part 15 will only be able to do about 2kHz, so we seem to have only one choice.
The spacing interval of the carriers of LW and MW AM broadcast stations in some areas of the globe is 9 kHz.
However the FCC does not grant licenses for AM broadcast stations in the LW band. The FCC grants AM broadcast station licenses only in the MW band from 530 to 1700 kHz, spaced at carrier frequency intervals of 10 kHz.
But in any case, the maximum r-f bandwidth that can be transmitted using DSB AM without mutual interference by stations on adjacent channels in their respective areas of the globe is only 1/2 of their channel spacing.
So if the transmit systems of each station supported it, stations with 9 kHz carrier separation would have an upper limit of ~4.5 kHz for their program audio at the output of a compatible AM receiver, without having significant interference from adjacent channels -- if they all restricted their r-f bandwidths to 9 kHz.
For those conditions with 10 kHz spacing and 10 kHz r-f bandwidth, the upper audio limit would be ~5 kHz.
Earlier I posted, "But in any case, the maximum r-f bandwidth that can be transmitted using DSB AM without mutual interference by stations on adjacent channels in their respective areas of the globe is only 1/2 of their channel spacing."
To correct my earlier post, I should have written "... the maximum a-f bandwidth ..."
My apologies.
The maximum allowable audio bandwidth for US AM broadcast stations is 10.2 kHz, which corresponds to an RF bandwidth of 20.4 kHz. Stations that are within ground wave propagation distance are not assigned to adjacent 10 kHz channels by the FCC, but skywave propagation can result in interference between distant stations on adjacent channels. This is acceptable under the rules.
Broadcast stations can optionally elect to restrict their bandwidth to concentrate power in the voice range. This is an option, not an FCC requirement. Studies have shown that the average modern AM radio receiver has a 3 dB audio bandwidth of only 2.5 kHz! So, broadcasting with a 10 kHz audio bandwidth is a waste even for a music station. This situation drives broadcasters to reduce their audio bandwidth significantly below the 10.2 kHz limit.
All this information is documented in the NATIONAL RADIO SYSTEMS COMMITTEE document NRSC-G100-A, Bandwidth Options for Analog AM Broadcasters, September 2012. You can read it here: http://www.nrscstandards.org/SG/NRSC-G100-A.pdf
Be sure to read over the extensive supplemental document in ANNEX 1, after page 18 of the main document. There are detailed results of bandwidth measurements of about 30 commercial AM receivers. It's a real eye opener! Bring back the GE SuperRadio III!