[chris (OP)]

When listening to a radio station that broadcasts on medium wave(AM), why does the signal cut out when going through a tunnel, while FM stations remain perfectly audible?

Chris

[syhprum]

Because many tunnels have a relay system to maintain the VHF signal but not the MF signal.

[tommya300]

Can this be possible?

 AM radio is confined to a band from 535 kilohertz to 1,700 kilohertz (kilo meaning "thousands," so 535,000 to 1,700,000 cycles per second).

Amplitude Modulation inherits static interference and at the low frequencies can be absorbed by solid structured matter

All FM radio stations transmit in a band of frequencies between 88 megahertz and 108 megahertz.

Partial advantage Keep in mind planks law
E=hc/(λ) lambda
Higher frequency higher energy

Higher the frequency permits the wave to pass through most solid structured matter, with less or no interference.

Added features...Individual "Radio transmissions" ride on a Center frequency or true frequency, with a given bandwidth.
Higher frequencies have wider bandwidths, this permits FM to run sterio on the upper and lower sidebands, without clipping the audio signal. AM has a lower bandwidth, which is not wide enough to do so. 

[chris (OP)]

Hi syhprum - so why does the relay system work for FM but not AM? I thought it largely consisted of a "leaky" coax cable running inside the tunnel.

And Tommya - thanks for the thorough answer...

[Geezer]

I would think it would rather depend on what you "shove" (that's a technical term) down the leaky coax. Presumably there has to be some sort of wide bandwidth receiver that picks up the RF signals and drives them into the coax. If it only operates in the VHF spectrum, it's not going to retransmit MW spectrum signals.

There are probably many more reasons than that. I'll bet syhprum has a much better answer!

[tommya300]

I would think it would rather depend on what you "shove" (that's a technical term) down the leaky coax. Presumably there has to be some sort of wide bandwidth receiver that picks up the RF signals and drives them into the coax. If it only operates in the VHF spectrum, it's not going to retransmit MW spectrum signals.

There are probably many more reasons than that. I'll bet syhprum has a much better answer!

Maybe there is Geezer, I mentioned Partial advantage, can this be explained that the same happens when going across the open steel Girder bridges.
FM does not depend on amplitude gain AM signals do.
AM carrier wave envelops the audio the component that is susceptible to noise. The path of transmission can add of subtract to the signal's original Amplitude.
 
FM audio component is within the Frequency Phase of modulation, less susceptible to noise. 

[tommya300]

More to add...
Chris, I think this will answer your question better...

AM signal amplitude is there half the time

 [ Invalid Attachment ]


http://en.wikipedia.org/wiki/Wave

AM probagates using a ground plane and the ionisphere, an AM signal enters a tunnel or a sheilded steel frame it just will probagate to surround ground.
FM travel in a straight line, non dependent on ground conductivity, works its best when no obsticals are in its path, but it also reflects off of the ground or objects at some glazing angle.
Just as a light beam does.

So if there is a Leaky Coax type of antenna used, this would explain the AM signal loss.
I guess I never experienced going into a tunnel which was long enough to need a Leaky Coax fit, I never had lost FM reception that way.

[Geezer]

I did a bit of poking around, and it seems you have to inject signals into the cable with a transmitter for each station. Combining AM and FM on the same cable does seem to be a bit tricky, but not impossible.

So, it's possible the main reason for not getting MW stations in a tunnel might simply be because no one was willing to pony up the money for the necessary equipment for those stations. Presumably they concluded nobody actually listens to the MW stations while driving  []

[tommya300]

I guess so. Since they have cable everything these days and wifi. Buck talks a heck of allot.
Like all things have their limits.

FM will probagate into a tunnel by reflection of some glazing angle, and travel for a distance.

However, it depends on how close the towers are, the power of transmission, the altitude of the Transmission antenna and the path of the transmission, line of sight.
AM is ground conductive dependent, it will look like a short circuit to an AM wave form as the signal attempts to enter the tunnel.
Most likely if you are receiving a strong FM signal as you enter the tunnel, the signal will bounce through the tunnel for a good distance.
For this distance end, I think the FM signal will not fade, it may exhibit something like " Falling Off A Cliff"
 

[JP]

Higher the frequency permits the wave to pass through most solid structured matter, with less or no interference.

Isn't it usually the other way around?  Waves tend to go through/around things that are smaller than their wavelength without being disturbed.  Higher frequency means shorter wavelength, which means it would pass through less matter undisturbed.  Lower frequency means longer wavelength, which means it can pass through more matter.

This is why you can hear the bass beat of your noisy neighbor's stereo, but not the treble.  It's low frequency so it goes through the wall easily.

[Geezer]

Higher the frequency permits the wave to pass through most solid structured matter, with less or no interference.

Isn't it usually the other way around?  Waves tend to go through/around things that are smaller than their wavelength without being disturbed.  Higher frequency means shorter wavelength, which means it would pass through less matter undisturbed.  Lower frequency means longer wavelength, which means it can pass through more matter.

This is why you can hear the bass beat of your noisy neighbor's stereo, but not the treble.  It's low frequency so it goes through the wall easily.

I think that's right JP. Lower radio frequencies are absorbed less by matter.

Not sure if the audio analogy is quite right though. I think the reason you can hear the bass is because it has so much more power than the higher audio frequencies that it causes your walls to act as a secondary speaker rather than actually passing through the wall. 

[tommya300]

Higher the frequency permits the wave to pass through most solid structured matter, with less or no interference.

Isn't it usually the other way around?  Waves tend to go through/around things that are smaller than their wavelength without being disturbed.  Higher frequency means shorter wavelength, which means it would pass through less matter undisturbed.  Lower frequency means longer wavelength, which means it can pass through more matter.

This is why you can hear the bass beat of your noisy neighbor's stereo, but not the treble.  It's low frequency so it goes through the wall easily.

Am I confusing this with XRAY frequency? Short wave length hi frequency hi energy wave form, passes through most solid matter?

Ah acoustic percussion beats the walls down hear its echo, shiver me timbers, before passing through...

[JP]

Not sure if the audio analogy is quite right though. I think the reason you can hear the bass is because it has so much more power than the higher audio frequencies that it causes your walls to act as a secondary speaker rather than actually passing through the wall. 

You may be right.  That's the explanation I recall hearing back in my undergraduate days, but I'm not sure it holds up since sound needs a vibrating medium in order to propagate whereas light doesn't.

[tommya300]

Resonance is resonance everything has a frequency...
Frequency spectrum's and antenna responces are from the same formula using different values.
λ lambda
F frequency
V velocity can be the speed of sound
                   or speed of light
                   or a user defined for the best response
                   on a power curve, auto exhaust chamber back pressure related to RPM
                                                                                 (tuned Exhaust)

Speaker cabnet dimensions Volume area inside it, cutoff filter speaker protection
Tuned Headers on an auto mobile top end response low end response
Antenna resonant response
Remember the center frequency upper and lower band pass or the full bell curve 6db per octave 10 db per decade
Just because the media is changed V=λF adapts to all full spectrum

I realize of course sound in a vacuum is obvious.
I should not have said sound passes through...
Sound does not pass through it bangs into the medium and the atomic structure of the medium propagates the pressure like a spring bouncing back and forth and also robs some of the energy during the transition.
That is why it is less loud with the windows and doors are closed.

What makes sound proof sponge sound proof? The reduction or lack of this energy transferring pressure the material absorbs the percussion.
What about anti sound. DSP detects and duplicates the sound and inverts it and transmits it The problem with this is from the time of detection, process time to the output propagation is phase shifted.
A known repetitive value need to be stored to provide anti noised.

Filtering light wave uses different optic medium for a response,  Radio waves tank circuits LRC, Sound use material with shape and dimension and acoustic properties to get specific responses, a Stradivarius for instance

[chris (OP)]

Hi JP

I think that the reason that the bass propagates from next door (or a passing car) is because long wavelengths (like bass sounds) tend to diffract better than short wavelengths, so the sound will find its way into your house more easily. But more importantly, higher frequencies are more rapidly attenuated in air; the higher frequency means that more air is being moved about per wave cycle and hence the losses are greater. This is why foghorns use bass (low pitch) sounds.

Returning to the radio wave question; you say that waves go around things that are smaller than their wavelength - but this isn't true of the grille on the door of a microwave oven is it? The microwaves are trapped within the oven cavity, but the visible light (from the interior lamp) comes out just fine...am I missing something here?

chris

[daveshorts]

In a grill the important scale is the length of the wires, which make it up. These are much longer than the microwave wavelength. In the same way that a long wavelength e/m wave doesn't see a small solid object, it also doesn't see a small hole, so the grill is apparently solid to the microwaves.

To the very short wavelength light, the grill looks like a grill, the diffraction effect is of order a micron, so the grill looks like a grill, light goes through the holes but not the wires.

[syhprum]

When I worked in central London I used to drive home via the Blackwall tunnel which in those days had no 'leaky coax' system but rather smooth reflective walls.
I found this was a useful place to test the efficiency of various car radio antenna systems.
with my Ford I could get about 200m into the tunnel before I lost the signal but when I upgraded to a BMW it was nearer 400m, I think the reason why MF reception is not provided is because of the high level of interference generated at low frequencies by the fluorescent lighting and often car electrical systems.
 

[tommya300]

When I worked in central London I used to drive home via the Blackwall tunnel which in those days had no 'leaky coax' system but rather smooth reflective walls.
I found this was a useful place to test the efficiency of various car radio antenna systems.
with my Ford I could get about 200m into the tunnel before I lost the signal but when I upgraded to a BMW it was nearer 400m, I think the reason why MF reception is not provided is because of the high level of interference generated at low frequencies by the fluorescent lighting and often car electrical systems.
 

Yes and some ignition systems not all also contributed to noise.
For you, did the FM signal just cut out or did you experience a fade?
Ford? Gas prices... did you own a gas station or was it a ford semi with a fuel tanker hitched?
 [] [] [] [?]

[Geezer]

There's a new topic on the LF sound question here

http://www.thenakedscientists.com/forum/index.php?topic=34209.new

[syhprum]

The Ford was just a 2 liter not a gas guzzler, the FM held up for about 200m then flutered out.