I tend to ride relatively modest gear (Tiagra/Ultegra), but when I'm riding near higher-end bikes, I sometimes notice that the clicking sound that comes from their cassette/free-wheel while coasting seems to be much louder and more distinct than on my bike. Why is this? Intuitively, one would expect that a quieter drive-train would be an indicator of greater efficiency, but that doesn't seem to be the case here. Is it something to do with different construction, or materials? Can you get 'quiet' high-end cassettes, or is being noisy while coasting the price you must pay for being efficient while pedaling?
Most of the noise comes from pawls on the freewheel hitting against the splines on the engagment surfaces which makes up the racheting unit.
Some reasons for the noise between freewheels?
Of course there are exceptions to these. Some hubs don't use your standard racheting mechanism and use a 'roller clutch' instead. The roller clutches tend to be very quiet, but are more prone to failure.Here's a good description of how those work. http://pardo.net/bike/pic/mobi/d.winners-hub/index.html
I noticed this as well. In my experience, on higher end road bikes, the cassette that you put on the freehub body makes the most audible difference, versus the actual inner-workings of the freehub itself in most cases, i.e. normal, ratchet style freehub body.
Example: I went from a Sram PG-1130 cassette to a PG-1170 recently. The lower end cassette (1130) construction is different, specifically the dome and spacers (which are plastic). On the higher end cassette (1170) the dome is lighter and stiffer, and the spacers are steel and attached to the cogs.
The cassette acts like an amplifier for the sounds coming from the freehub. Thus, a cassette like the aforementioned 1170, with a very resonant dome and body, casts way more sound than the 1130, which is expected as it has plastic construction with deadens the sound considerably and a thicker dome, which also hinders resonance.
I noticed immediately after the switch that the sound coming from my bike went from:
click, click, click...
ting!, ting!, ting!...
Personally, I like the 'ping/ting' sound better and associate it with higher quality.
I think loudness is not a good indication of efficiency, since loudness (and pitch as well) can be affected by many more factors at same or similar total energy consumption.
Recently I replaced my 6800 groupset with a Chorus one. I am using a Mavic Ksyrium SLS wheel; that means I had to change my freehub body to a Campagnolo compatible one.
Surprisingly, the new freehub has a different sound when coasting, and the noise is so much more reduced. I supposed the difference in the two freehub bodies is to accommodate different brands of cassettes.
I had never like my Mavic wheels, but now they are so much more lovely.
Not my knowledge to explain why, but it proves that the noise is not a matter of high- or low-end hubs.
Louder means more force has been used to make the noise. A change in tone means that different materials have been used to make the noise. More contact points does not automatically result in more noise - if the sound was generated at the same time it would not increase the volume. If a noise gets louder then it is likely that an amplifier in the system - depending on the design, the space in the axel might act as an amplifier. The gear rings could act as an amplifier - this is not likely as the tension generated by the chain would act as a damper. Going back to the first point: if more force has been used that has resulted in a louder sound I would point to an increase in force being applied at the contact points - one obvious reason for a stronger force would be stronger springs in the freehub.
I had deore xt hubs on my mtn bike, they were silent. I have rebranded Formula on my 2008 Bontrager Race Lites which have a nice clicky sound. Upon servicing I must have put too much grease in and it muted the sound. So perhaps there is a relationship between sound and pawl wear. I have DT Swiss star ratchet on my mtn bike now and they have an excellent sound IMO. Chris king hubs have a legendary sound, describe by customer quote "It rolls good with angry bee sound"
I've got multiple sets of Campy Record freehubs and they are all quiet and no one has ever failed. Chris Kings on my old Klein winter mtb are louder but still somewhat quiet. The DT rear hub internals on my Lightweights are super noisy but the first one failed after only 3K miles (lots of mountain passes). They've been upgraded from the 18T original star ratched to the 36T design so the sound has changed but is still just as loud. So far, they have now been reliable. I don't notice any performance difference between the three systems and I would rate all high as far as quality is concerned.
High end road bikes often use high profile carbon rims which act like a sound box. You'll notice not only the freewheel is louder, other rattling noises the bike makes (like on bad pavement) are louder as well.
Actually, mtb hubs can be just as loud as road hubs. So it really has nothing to do with riding in a group and being audible for the sake of other riders.
As for the reason some are loud. Efficiency has a lot to do with it but It has more to do with the grade of punishment components can withstand and still perform well. I say that because if you have small components with less force being applied to them (i.e. Small pawl size and low spring tension holding them in position), the wheel will coast more silently.
In some higher end components, parts are slightly oversized and/or under more torque or tension. This is to keep the performance of the components tight under stress. Imagine going face first down hill on your favorite double black diamond run trying to use campy components built for road. you will break more than just your campy group. The reason some hubs are loud is because the components are built to perform better than just "okay" and to last.
High end road bikes have a louder free hub so that riders in a peloton can have an audible clue that those ahead of them are coasting or braking.