Yeah, the lyre of building has never been very explicitly defined in any edition. It has been used in a couple of 3.x campaigns I've been in, and we used these guidelines:

- The effect you get from 30 minutes of playing is what you'd get from 300 man-days (2400 man-hours) of unskilled human laborers using common tools appropriate to the job at hand. (In theory, the lyre could accomplish more complex constructions if its player had several ranks in Knowledge: Architecture/Engineering or a construction-related Profession, but we never ran into that situation.)

- The lyre supplies the effect of the tools as well as the muscle power but does not supply raw materials.

The effective range of the lyre wasn't clearly defined as a house rule and depended somewhat on what you were trying to do with it. One rule of thumb was, if you weren't close enough to personally direct the work (as a foreman would) then you weren't close enough to use the lyre.

 

Great guidelines Damon!

To get more perspective on an item, I always look at the creation prereqs (in this case, the 'fabricate' spells, which has a material component of "the raw material necessary for the construction of the item")

In this case, if you are making a bridge with the lyre of building, this means you must have a neat pile of materials on nearby pallets inbound straight from the Home Depot or else you get a bridge of woven grass and twigs! LOL! So, anything above a CRAFT/PROFESSION/KNOWLEDGE DC OF 10 would require a skill check... (look at the guidelines of "unseen servant" to see what an unskilled magical servant can do...) --> see below for interesting tangent...

Great catch on the engineering ranks! the spells fabricate also mentions that the necessary skill checks must be made. For at 13,000gp item... it would be perfectly reasonable for a DM to apply all these mitigating conditions.

I would also add that you need at least one rank of Perform String or Peform String DC 13 for the 1/day function (the items states "IF THE PROPER CHORDS ARE PLAYED") just based on the fact that the 1/week function requires DC 18 (a high CHA bards/sorcerers could do this untrained btw...)

And I would apply the same limitation that most bardic powers have: "must be able to hear the performance". In the case of this lyre, ok, at best what? 500 ft radius? (i.e. if performed during hours of low background sound / i.e. not during rush hours -- sure you can hear a guitar or lyre further than 500 feet, but the acoustical engineer in me would rule that the sound pressure level after 500 feet is too low for the magic to take effect... back in 3.X there was an item, horn of volume or something, that doubles that distance -- slotless item to booth... so with that I'd allow a bard to build in a 1,000 foot radius)

--> interesting tangent: craft checks are required to make *things* like swords, armor, clothes, shoes, etc. (see fabricate spell too); profession checks are required to perform a job, i.e. according to core rules, "a vocation requiring a broader range of less specific knowledge. The most common Profession skills are architect, baker, barrister, brewer, butcher, clerk, cook, courtesan, driver, engineer, farmer, fisherman, gambler, gardener, herbalist, innkeeper, librarian, merchant, midwife, miller, miner, porter, sailor, scribe, shepherd, stable master, soldier, tanner, trapper, and woodcutter." Please note that "architect, engineer, miner, and woodcutter" are specifically mentioned. This means, IMO, that someone with ranks in these professions could make direct use of their Profession skill via lyre of building, so this means, as per Profession skill core rules "You know how to use the tools of your trade, how to perform the profession’s daily tasks, how to supervise helpers, and how to handle common problems. You can also answer questions about your Profession. Basic questions are DC 10, while more complex questions are DC 15 or higher." Thus, with lyre of building, you want a simple bridge? DC 10 Profession Engineer. Complex bridge? (i.e. draw bridge or suspended bridge or something with arches that is more complex than a simple cantilevered slab of stone / wood-trunks that is directly supported at regular intervals by beams that can touch the river bottom) then DC 15. Please note that the Craft rules allow for +10 DC to speed up construction: I would allow the same for Profession checks. So a DC 10 item a check of 20 yields 200sp worth of craft time a week; upping the DC to 20 gives your 400sp... (x2 modifier... x1.66 if the base DC is 15, x1.5 if the base DC is 20, etc. less and less at higher DCs, etc.)

So take this: someone who can pull of DC 30 Profession Engineer can build a Masterwork (DC 20) bridge with the lyre of building 1.5 times faster than someone who can only made a DC 20...

To sum up we have this: say a simple bridge requires 8 hours to do using the lyre (100 men working for 48 days normally). Someone with a +20 Profession Engineer can use the lyre for 5.33 hours only, AND the resulting bridge is Masterwork (i.e. nice feature like drawbridge added, or will withstand the ages due to craftsmanship, or can span double the usual distance, such as reaching an island in the middle of an especially large lake, for instance... OR if I was the DM: allow simple bridge in lieu of MW bridge and the PCs don't pay the raw material cost, so in KM speak, instead of a redux of 2BP, allow redux of 4BP... mind you, if the PC doesn't have a total of +20 to Profession Engineering, i.e. if he must *roll* to get the 30, then I would work out a penalty for failure, like no BP redux at all and you've wasted your time for this Kingdom Building turn... maybe not a full vacancy penalty but something along the lines of half the penalty OR regular BP cost for building...)

I would say that knowledge can substitute Profession checks but you take -5 to the check (i.e. knowledge engineering at minus 5, and use the result as if you had done a Profession check...)