This page will detail methods for keeping a Mythbox running quietly and cool.
Modern hardware have power saving states that can be enabled through TLP that sets power saving modes on boot for modern Intel and AMD processors. TLP is typically available in your distribution repositories.
PWM fans are the best option for cooling as the motherboard can control the speed based on the case temperature. These have 4-pin connectors that will appear similar to the header for the CPU fan and may come with rubber mounting dampers which are highly recommended. Ensure the motherboard supports a 4 pin chassis fan before purchasing. The BIOS usually has options to select 'silent' operation where the chassis fan(s) may not spin up until needed.
Another option is to add variable resistors to the fan circuit, altering the voltage supplied to your fans. Normal fans runs at 12V, but by lowering the voltage to 7V you will get lower RPM's and thus quieter fans at the cost of air throughput.
CPU coolers in modern processors (c2014) are typically quiet and no special attention is required.
Quiet and/or adaptive power supplies units (PSU) can also help reduce noise by a considerable degree. There are some premium PSUs that just use a heatsink for cooling, and thus run totally silent. However, they don't do much for the cooling of your case! A better solution are high efficiency PSUs (80 Plus Gold) or a pico-PSU and choose a PSU with a rating closest to your computer usage. It is highly recommended that a power meter is used to check that you will not exceed the supply rating of the PSU.
One of the good things about Myth is that 90% of the common functions don't require a powerful CPU at all, so you can often pick and choose the CPU that runs at the lowest power and heat output.
Modern Haswell era Intel processors are handled by the kernel p-state driver and have multiple sleep states (C1 to C10) enabling low power and low heat CPUs. This is typically handled automatically but the BIOS settings should be checked to ensure the BIOS is able to put the processor into these C-states. Use
powertop to check if these low power states are being reached.
If maximum power usage is a problem, the highest frequency of the processor can be capped in the BIOS resulting in a drop in maximum power draw. The processor will otherwise reduce frequency and voltage automatically. Otherwise, T versions of the Intel range are low power variants but may exhibit the same power draw at idle.
If you have an AMD please add information in here.
Vibration from wobbly components can often cause irritating buzzing noises. Regularly clean your fans by blasting them with compressed air (be careful as you can overspeed smaller fans), and if vibration seems to be a problem you can dampen your fans by placing rubber or silicone washers between the fan and the mounting. Other moving components (such as hard drives and optical drives) should be secured mechanically well. You could consider mounting your drives in a dampening cage or using anti-vibration grommets. Quality cases use the latter to great effect.
Related to vibration are optical drives running at 16x or 52x. To watch DVDs and play CDs, you shouldn't need a drive faster than 1x DVD or 1xCD respectively. The DVD and CD specifications are written such that a DVD should playback video or video perfectly well at the drive's lowest available speed.
As of this edit there are no optical drives on the market that are specifically advertised as being quiet or having quietness as a feature. Some drives have "quiet" firmware/utilities available which will limit the speed to a more reasonable and less deafening level. Since almost all drives require that their firmware be upgraded using Windows or DOS you may run into difficulty attempting this method with a Linux based computer running MythTV. You do have the option of putting a drive into a Windows based computer, applying the firmware alteration, and then putting it back in your Linux based computer (a variation on this theme is to always have a FreeDOS bootable partition).
If you have the choice between an 8x DVD drive and a 16x DVD drive, choose the slower (8x) one. The only place MythTV needs fast disc access is when ripping a CD or DVD. You may also want to experiment with hdparm, setcd or speedcontrol; All claim to be able to reduce the maximum speed of optical drives. Setcd may require a patch to work with newer drives, see debian bug 367008. SPCR has a forum thread on the subject. But there appears to be no definitive study of the comparative quietness of DVD/CD drives.
Keeping all those cables tidy can do wonders for airflow within your case, and do away with the need for more fans. You can tie bundles of power cables together using zip ties, braiding or spiral wrap.
It is possible to soundproof your case with acoustic material. This is essentially sticky-backed foam that does a lot to absorb noise from inside the case, although you'll need a case with enough free space inside to accommodate its thickness. I've never tried this myself, but a lot of people swear by it. Lian Li and Zalman make some very pretty (and pretty expensive) cases fitted with this sort of anti-noise technology as standard, so those of you who want a big, quiet case might want to consider splashing out on those.
Furthermore you can uncouple the case mechanically from whatever is underneath it, or whatever it's underneath. Conducted noise which is then transferred by another device is a major component of the noise which a PC can put out — which explains the rubber mounts you can get for hard drives. They're good for noise, but bad for thermal conduction. (You do have a fan blowing directly on your hard drives, right?) Standing the chassis on a piece of carpet, or even better, underlay will cut down on transmitted noise through the floorboards, plasterboard etc.
For more information on keeping your HTPC (and PCs in general) quiet the website Silent PC Review has some excellent guides, reviews and forums to provide you with all the details you need. Their recommended hardware section contains lots of hard numbers.
SilentPC Review - Contains reviews and guides on quiet PC cases, fans and components.