The somewhat sad truth is there is no such thing as a perfect listening room. There are horrible listening rooms and really good listening rooms (and of course everything in-between) but there is simply no way to build a perfect listening room. Think of the two acoustical extremes – an anechoic chamber and the gymnasium at your high school. An anechoic chamber has no reflection, no reverberation – no life. You’ll hear detail but in a very dull and unnatural way. A high school gym is pretty much nothing but reflection and reverberation, and while it makes things sound louder and more present, detail and articulation are lost.
Your listening room or home theater is somewhere between the two. The trick is to find the right balance, and I don’t just mean balance between live and dull, reverberant and anechoic, but you have to live in the room too. If you are able to build a dedicated home theater in your home and are willing to spend enough money to do it right, then you can get pretty close to reaching the ideal, but if you’re trying to put together an excellent home theater in your living room and still have it look like a living room, then you’ve got some challenges.
Here are some extremely basic rules to keep in mind as you start to put your home theater system together. These are also a good reference point for those of you who already have home theaters but just can't seem to get it to sound right.
Rooms used primarily for movies have different needs and characteristics than rooms used primarily for listening to music. Don’t expect your 5.1 or 7.1 home theater system to sound the same when listening to music in 2.0 or 2.1 mode. You want your musical room response to be a little lively, allowing some reflections while you want your cinematic listening to be more controlled, and while I won’t say “dead” I think you get the idea. This all stems from how the sounds are mixed and produced, and what we as listeners expect from our experiences with the two. But remember, as with everything else, compromise is the key – with enough experimentation and patience you’ll be able to find the happy medium between a great sounding cinematic experience and a great musical experience.
Sound is just as important as picture. You’ve just spent $1500 on a 60” UHD 4k television. Spending $600 on a subwoofer and soundbar that promises to deliver a spacious soundstage from a speaker array that isn’t as wide as your new television, is, to be frank, kind of dumb. Will it work and will you be able to hear things? Pretty much, but it’s kind of like the drive-in: You have this ginormous screen and this teeny-tiny little speaker. Obviously your grandparents didn’t’ go to the drive-in just for the cinematic experience. Make sure the scale and quality of your sound matches the scale and quality of your video.
The 38% rule is the best place to start when putting your room together. Assuming you have a rectangular room, your listening position should be roughly (or exactly if you’re a fanatic) 38% from either the rear or front wall. You can use your ears as a test instrument to help you understand the 38% Rule and to even tweak your listening position. Play some bass heavy material or a low-frequency test tone and stand at the rear wall (opposite the front speakers) and listen. You’ll hear unnaturally loud, muddy bass against the wall as this is where the bass energy is “piling up.” Walk toward the center of the room and at some point between the wall and the center you will hear natural, flat, crisp bass. Continuing into the center of the room you’ll hear the bass response noticeably diminish and then as you continue walking toward the front of the room you will hear the bass return and sound crisp again. More likely than not, the spot you hear the most articulate and punchy bass will be roughly 38% of the room distance from either wall.
Don’t treat your room until after it's set up. This rule does not apply to a professionally designed dedicated home theater. Designers with calculators, experience and knowledge of such things as pressure maxima and pressure minima et al will be able to attack room problems in the initial stages of the design, but for those of you trying to put a multi-purpose room together on your own don’t treat your room until you know what the problems are. Too many times people start throwing absorptive panels around a room before understanding how the system will interact with the nodes of the room and the result is almost always a dull and unappealing sounding room. A professional installer or designer will be able to figure these things out in advance but if you’re doing this on your own you kind of have to do it backwards. Get your furniture and system set up as necessary for actually living in it, then start attacking the sonic problems.
Contrary to Rule #5 there is something you can do initially that will help you in the long run – make your front wall as absorptive or dry (I don’t like to use the word ‘dead’ when it comes to sound room design) as possible. This will help dampen unwanted reflections and SBIR issues that tend to muddy or diminish the responses of your Left, Center and Right channels. Meanwhile, make your rear wall as reflective as possible (without turning it into an echo chamber (see: gymnasium)) which will help diffuse standing waves in the lower frequencies while helping with the live-ness of the rear channels. Don’t over-do it though, as most real walls in a regular room are already reflective enough.
Reverb is defined as the amount of time it takes a sound to decay 60dB. Reverb for music is good, for movies not so much. In a concert hall, reverb times can approach 1 second or even more (which is actually closer to an echo than a reverberation). For music, reverb helps lift the power and live-ness of a musical passage, while with a film it will only serve to muddy up the dialogue and make sound effects swirl about the room without purpose. If you can get your reverb time down in the 250 – 400ms range you’ll be okay but you’re not going to be able to accurately measure a reverb time for reverbs under a second without a good meter or computer-based app. There is also a formula you could use:
- • RT(60) = reverberation time (sec)
- • V = room volume (ft3 )
- • S = surface area (ft 2 )
- • a = absorption coefficient of material(s) at given frequency
- • S indicates the summation of S times ? for all room surfaces
Since you need to know the a for all the surfaces in your room you're kind of out of luck unless you put a lot of work in or have an in with the acoustic laboratory in your town. But, basically, if you can clap your hands and another person can measure the decay time on a stop-watch, you’ve got way too much reverb in your room and nothing you do to make your room sound good will work until you tackle that problem.
You are simply not going to EQ yourself out of trouble and you are simply not going to DSP yourself out of trouble. Firstly, never, ever, boost the EQ if you sense a lack of a particular frequency in a room. Move your speakers and chairs around a bit, or work on reflections. Boosting EQ adds phase problems to a signal and makes the signal artificial, plus any fix via EQ for one physical area is only going to make matters worse in another. Conversely, cutting a particular frequency via EQ may help for small problems, but again the fix will most likely only be in certain areas of your room. Likewise, using your on-board DSP to fix room problems will likely introduce as many problems as it fixes while only fixing problems in limited listening areas. The best EQ and DSP adjustments are accomplished by simply moving your speakers slightly or adjusting your listening position. You'll be amazed what a slight difference in toe-in or distance from a wall can make to how your room and speakers sound.
These “rules” are meant to be basic general guidelines to get you started on your journey to having the best AV set-up you can afford. While they won’t solve all of your problems, following them from the get-go will ensure that you aren’t unknowingly adding problems as you put your room together. In following posts we’ll cover room design in more detail.