Let’s say that we just gotten started in our aquisition of recording equipment. We haven’t been able to bankroll the next piece of outboard gear we desire, such as the compressor/gate we’ve been eyeballing online. Additionally, either we can’t immediately afford the cool looking professional sound absorption panels that we desire, or our spouse (or mom) won’t have it. Chomping at the bit but don’t want to wait to get everything looking ‘proper’ before we begin recording? We’re not going to allow such a trivial matter to sidetrack us, are we?
However, room ‘noise’ can dramatically reduce the impact of an otherwise professional sounding recording, and something is going to have to be done.
Early room reflection, or echo, can be troublesome when trying to achieve good instrument and/or vocal recordings at home. Most rooms exhibit a boxy sound in the midrange on account of their size and the fact that the walls are smooth and flat, both properties of excellent reflective surfaces. These reflections are almost never ideal and, especially when recording with an omnidirectional microphone (one which picks up signals originating from any direction), one ends up ‘miking the room’. This situation also makes it nearly impossible for one to monitir and mix recordings within the same space, which is something that many of us choose to do rather than go to the added expense of constructing a seperate control room. Early reflection is a result of the sounds hitting the wall and ceiling and then bouncing back to the microphone and/or the ears of the listener. It isn’t anything that one usually notices unless one steps into a cavernous room, or a house devoid of furniture and draperies. But even a reflection of 10 or 15 milliseconds can make a recording sound smeared and cheap. Every instrument that is recorded with even the tight pick-up pattern of a cardioid mic will reproduce the same frequency of reflections which, depending on size, the room naturally resonates at.
Allow me to explain how this can quickly become a problem that must be addressed.
At an air temperature of 64 degrees F, sound travels at 1123 feet per second, or 13.476 inches every thousandth of a second. If our room is 12′ x 12′ (144″ x 144″) with 9′ (108″) ceilings, and (for the sake of simplicity) we plop ourselves down perfectly in the centre of it and play a single, stacatto note, this is what will happen: The sound of the note will take 5.343 thousandths of a second to hit each of the four walls and take roughly as much to bounce back to the point of origin, that’s 10.69 thousandths of a second, total. The same sound will reach the ceiling in 4.005 thousandths of a second and return in as much for a total of 8.01 thousandths. These return rates, being unequal, will be translated as room sound. Wall reflection 10.69 divided by ceiling reflection 8.01 is 1.3345817. All of those decimals mean that the reflections do not return to the microphone at an agreeable return rate.
Stay with me, because I’m really simplifying this for the example. The example assumes that we are sitting dead centre of the room…which will rarely be the case in reality. If we move in any direction, the return results will vary from several milliseconds sooner or later. And the speed of sound increases as air temperature rises, so at 70 degreesF it would be 1180 feet per second.
On top of that, a room of this size will actually resonate at two different fundamental frequencies: The distance from one wall to the next, and the distance from the floor to the ceiling. These two fundamental tones which the room dimensions correspond to will be accentuated, as well as, on a different note, the first and second overtones. (I made a funny.) In other words, most rooms are ‘tuned’ to at least a couple of tones because architects and builders have a tendency to work with 90 angles, meaning that the other walls are either perpendicular or parallel to one another. If the room is rectangular, rather than square, there will be three fundamental tones, each corresponding to the length, breadth, and depth of the room. Oh, dear.
Remaining on the subject of our example room, here’s how we find those fundamentals:
Where ƒ represents the frequency in question, and C represents the speed of sound, merely divide the width of the room into C to arrive at ƒ. That is, 12 (room width in feet) divided into 1123 (s.o.s.) equals 93.58 Hz. That is close to F# below C3. Using the same formula, ceiling to floor measurements correspond to 124.77 Hz which is B below C3.
I could give you the math for arriving at the wavelength frequency, but you really don’t want to know, right? Here is a good website to help you determine yours:
Of course, in physics, things are never that simple. To put it bluntly, we’ll not only be dealing with early reflections, there will also be the late reflections, or reverberation: those sounds which bounce off of one wall corner and then another wall corner before (finally) making their way back to the point of origin. Of course they don’t stop there. They keep on bouncing like a Superball possessed until the room air returns to the inertial state. On top of that, the corner where the ceiling (or floor) and the two walls meet are natural pressure zones, and will accentuate bass frequencies. Don’t believe it? Put on some music, climb a ladder, and jam your head in the corner facing the room.
So, what to do? Well, you can spend $1000. and have your bedroom or den looking like a padded cell, or you can use a little resourceful ingenuity. After all, our goal is to break up the reflections, thereby deadening our room, nothing more. I’ve been in studios that were so dead that I nearly lost my balance, seriously. But most rooms sound okay if they can be kept a little lively, we’re just after a bit of control. Furniture does an excellent job of breaking up these reflections; couches, beds, and bookshelves with books of all sizes in various order are great. Empty walls (and those damned 90 degree angles) as well as windows present the worst problem. Draperies drawn across the windows will do a good job in killing the reflections caused by the glass in the windows. Walls can be carpeted to soften some reflections, or, barring that, we can create obstacles that either stand, hang, or both. Generally referred to as baffles or ‘gobos’, these items, placed in various areas of the room will accomplish our goal.
I’ll begin by sharing different approaches in Part 23.