Optimal digital cable length -- science or voodoo?

I'm doing research toward purchase of a new 75-ohm digital cable to feed a DAC, and I am coming across claims that the "optimal" length for a digital cable is 1.5 meters. Some even claim that, given the exact same brand and model of cable, there's a significant audible difference between a 1-meter and 1.5 meter length. Can an extra half-meter make any improvement? Is there science behind this claim, or is it just voodoo?
J, what you are reading about translates to the many low end cable manufacturer's inability to maintain the 75 or 110 ohm impedance across the required frequencies for more than one length. Run don't walk from any brand in this category. Z.
This was one of those observations that was unexpected. As a result, people weren't really looking for it. Sort of like the observation that a copied CD sounds better than the original.

I remember at one time reading an interview with a cable designer. He provided a reasonable scientific explanation for why a slightly longer optical cable was better than a shorter one. Sorry but I don't recall the details. Now whether this was really what was happening or whether it was just a clever argument is not something I can judge. However, there has to be some reason as to why many people report hearing differences.
Here is my white-paper on the subject:

My findings were independently verified in a double-blind test that was published in a past issue of UHF magazine from Canada.

Steve N.
Empirical Audio
It doesn't have to be expensive to test this claim, if you're skeptical.

After reading Steve Nugent's white paper, I used Apogee Wyde Eye and made up two cables, in 1-metre and 1.5-metre lengths, using Canare connectors and silver solder.

(Apogee Wyde Eye is one of the best-regarded inexpensive digital cables and was a Stereophile recommended component. Canare connectors are also inexpensive.)

I listened to the same music, using one of these cables each time. I did this with a pal, first in his system and next in my own. IIRC he was using a Theta transport and a Kora Hermes DAC with Siemens cca tubes. My own system was a TEAC T1 and an Apogee DA-1000E-20 at the time.

In either system, we both felt there was no doubt that the longer cable provided increased resolution. Instrument locations were more defined, soundstage height / width / depth were improved, timbres were more complex and natural.

I reported these results to UHF Magazine. They ran the test that Mr. Nugent refers to above, and as a result no longer recommend any cable length but 1.5 metres.
1.5m might be optimal for particular slew rate of the output driver but it is different from transport to transport. Some drivers keep constant dv/dt (slew rate control) while other don't. Calculation of the reflections effect on the signal shape is pretty difficult since output of the driver is nonlinear. Long time ago people used Bergeron diagrams to evaluate reflections in transmission lines but it isn't simple.

If you hear difference between 1m and 1.5m cables then modding (or at least evaluating) output driver on the transport is probably beneficial.
Kijanki - I used to mod lots of transports, all kinds, to improve the rise-time of the output driver. It usually allowed the customers to use a 1m cable rather than the 1.5m, saving them some money. Of course the mods cost money too. I dont mod ttansports anymore since I am a firm believer that computer audio is not only the future, but can easily outperfrom the best transport on the market at a fraction of the cost.

Steve N.
Empirical Audio
Steve - I feel the same. 500GB external Firewire drive costs about $100 and can store around 1000 CDs. Second external drive can be used as a backup kept in different house. In addition other formats with more bits and higher sampling rate can be supported. All this and convienience.
Pure voodoo. Talk with some of the principles at any of the reputable mfgrs. The answer will be the same. Get the length you need and the best quality you can afford. Cable length just is not an issue at the frequencies being transfered.