Terminology can be bad and misleading esp on internet and forums ie Wide fm can be broadcast to some or FM wide\Wide FM as in Land Mobile Radio systems.
Channel Bandwidth and deviation is preferred term rather than 25khz channels as you can have a 25 khz channel steps with narrow deviation ie 145.550mhz (NFM 2.5khz\9khz Bandwidth) or FM/FM-W
I have seen this happen
There are currently 6 bandwidths\deviations used
FM Broadcast Wide Band FM -100 khz bandwidth@75khz DEV
Commercial 2 Way Radio\HAM
FM(sometimes called FM-wide or FM-W or wide FM in USA this can be called FM narrow!)16khz Bandwidth@5khz deviation Max
Mid FM-14khz Bandwidth@4khz deviation (mainly USA commercial land mobile used)Max
Narrow FM 9 khz Bandwidth@2.5khz deviation Max
CB radio ETSI spec
Narrow FM-9 khz Bandwidth@2.0khz Max ESTI specs current (sometime quoted on MPT specs 2.5khz max-note)
to work it out thumb of rule,
cb is limits the max audio freq up to 2.5khz,max dev 2.0 for 10 khz spacing.
so bandwidth=2.5(max audio freq)x2=5 then add 2.0 (dev)x2=4.0
5+4=9 khz bandwidth!
channels steps on radio equipment do not determine channel spacing\dev .
Also beware some radio equipment by well know mfr's only switches the transmit deviation side,not the receive side and that can mislead operators into thinking NFM is sounding quiet to them,they then make assumption the person talking to them has low deviation and in some cases they turn it up in error!,but that's a whole other story to tell.
If a radio\scanner does the full switching on both TX and RX you should not notice the audio level drop at all between wide and narrow fm (note 1.) on both tx and rx assuming both bits of equipment are fully switching
The RX filter correct bandwidth
The RX audio level =either from the fm discriminator or at the demodulator audio side on Wide\Narrow
The tx deviation level
As other users have commented as a general rule in UK using my terminology
25.60-28 Mhz 10 Khz Steps NFM (default ESTI standard) CB band UK\EU-Note I have seen FM-Wide used by ops in UK and USA on some occasions ie old\new ham kit,or operator error not knowing radio will switch to NFM
as a note.
28-29.700Mhz FM-Wide (USA HAMS sometimes referred too as Narrow FM in USA) NFM mainly Europe
25-50 mhz Lowband VHF USA FM-Mid 20 khz steps (mainly Motorola/GE/Vertex/Kenwood kit in 29.7 to 49 mhz band)
66-88Mhz Lowband VHF UK\EU inc 4m band is NFM (with exception to some MOD,GOV uses FM(Wide) in some cases due to history) 12.5khz steps.
2m band can FM(Wide) and NFM mixed in my experiance mainly NFM for repeaters and FM on simplex,but ops can vary depending on kit in use and understanding generally 12.5 or 25khz steps and wide\narrow fm operation of kit.
(THis has caused a lot of problems i can personally vouch for and in
my personal opinion
was badly handled and not thoroughly throughout and implications ie old & new radio kit.
Bad information and info on repeater internet sites not clearly stating as to if actually the repeater is truly running wide or narrow fm and yes have seen this happen even recently with System Confusion Yaesu kit,regardless as to what the RSGB were saying or publishing inc Note 1)
Again these are my own personal opinions I have experienced,hobby wise,job role in radio comms and may this may not be the same for other ops reading this.
Marine VHF 156 Mhz FM-Wide 25Khz steps used.
Land Mobile Radio 138-144 Mhz & 147-174 mhz is usually NFM (MOD,GOV sometimes use FM-Wide)12.5khz steps used.
Trunking Band III 174-220 Mhz NFM 12.5khz steps.
MIL UHF 220-400* Mhz AM-Wide FM-Wide @25khz channel steps *subject to current changes
UHF LO 400-430Mhz generally NFM 6.25khz &12.5khz steps
70cm 430-440Mhz FM-Wide and some NFM used (GOV,MIL use FM-Wide shared spectrum)
PMR 446 Mhz-NFM 6.25khz steps
462/467 Mhz FRS\GMRS NFM USA Only
449 Mhz NFM Business Band 3 channels 12.5khz steps
440-470mhz generally all NFM,some FM-Wide sometimes used for GOV,MIL uses in this band.6.25khz,12.5khz,25khz steps can be used.
list is generalised from experiance but sometimes a odd one comes along,hope this help explains a few things on narrow,mid,wide and FMBC wide is used.
I have seen and commented on quite a few forums as this subject seems a popular question asked a lot.
Just be a little careful when discussing interpretations\meanings of wide fm and narrow FM,esp americans on Ham forums,topics can become quite heated for some reason as well??lol
I blame text books and certain technical books don't explain it very well for the beginner entering the radio world and esp china\english manuals for radio equipment sometimes can leave you very confused.
Hope this provides a little help guide.