We have distortion in our equipment. Some of it is wanted and self induced. The unwanted type must be managed and controlled. Our small room acoustical environments also have distortion. It is not electrically induced but it is distortion none the less. All room distortions are unwanted by their very own definitions.
The term distortion is a term that refers to a change from the intended designed for signal and the unwanted sound. Distortion is this difference. It is the change in the original wave form as it moves through the circuitry of a unit or though many units chained together. If the waveform is changed in such a way that it is flattened from its original bell shape, we call this distortion clipping because it “clips” the top of the waveform off.
In our equipment their are two main categories that distortion falls into. Lets call it wanted and unwanted. Wanted distortion is sought after. We twist this knob or that one to try and alter the waveform to produce a desirable, unique, and different sound quality to an instrument or vocal. We can overdrive an amplifier with a guitar to achieve a certain guitar sound.
The unwanted distortion occurs when we have hum or static in our system. This unwanted distortion comes about because of the misuse of electronics in the signal chain. If an electronic circuit in one of our pieces of equipment sees a signal that it can’t handle and then starts to overload, we will get electronic distortion. We have all heard this with ground looping issues. The original waveform that was originally designed to produce by the equipment has been distorted through overloading the circuit.
Sound waves are classified as longitudinal waves. They oscillate or expand and contract like a caterpillar moving across the floor. This raising or lowering of the caterpillars body produces higher and lower pressure regions. Each of these pressure areas minimum and maximum have opposite polarity. The pressure on one side is increasing and the pressure on the other side is decreasing.
Our small room acoustical environments have distortions. Rooms have a certain dimensional ratio that produces room modes. These modes or resonances are the room’s way of reacting to certain frequencies or wavelengths. Certain room sizes like certain frequencies while disliking others. When a room modal resonance is excited, it can blur and smother other frequencies. A microphone placed in a room mode may “hear” some of the intended sound to be recorded, but may miss other frequencies completely.
Room modes come in three main flavors. There are axial modes which are the resonances that occur between two parallel room boundary surfaces. Axial modes are the most powerful of the three forms of modes. Axial modes occur between the ends of our rooms, the sidewalls, and the floor and ceiling. There are also the associated harmonics that go with each fundamental resonance.
Tangential modes are half as strong as axial modes and occur between two sets of parallel surfaces. Tangential modes are formed by four traveling waves that reflect from four walls and move in a manner that is parallel to two walls. Even though they are only half as strong as axial modes the frequencies that they occur at along with their harmonic trails interfere with our middle frequency ranges where our vocals lie.
Oblique modes involve eight traveling waves that are reflecting from all six room boundary surfaces. They are half as powerful as tangential modes and half one fourth the energy of an axial mode. Axial and tangential modes give us the most difficulty when we are trying to manage them. If one treats for axial and tangential modes first, the oblique modes will usually become a non issue.
Speaker Boundary Interference Response
Our speakers and our room walls create another distortion. It is called the speaker-boundary interference response. This is a distortion that is produced as a result of the direct sound from our loudspeakers interlacing with the reflected sound from our room surfaces. This particular type of distortion occurs at lower frequencies more than middle and high frequencies.
The room’s boundaries that are close to our loudspeakers create another speaker if you will. The sound leaves our speakers and strikes the nearest room boundary surface. That energy is then sent back to the speaker through reflections. This process continually repeats itself and we have the speaker producing energy and then all of these virtual reflected energies producing sound. Thus our sound at the listing position will be coming from our speakers and the room boundary interference “speaker”.
Comb filtering is another room caused distortion. Comb filtering is basically a reflection that interferes with our speaker direct sound. The direct sound from our loudspeakers is the sound that travels in a straight line from the speaker to our ears. It is the sound that we need to hear so we get our mixes correct. Reflected energy from our room surfaces contains all the room sound. We do not want room sound in our mixes, so we sit near field to try and take the room sound (reflections) out of our mix.
Comb Filtering Evil
When reflections interlace with the direct sound and we take a picture of this phenomenon, we see a series of peaks and troughs closely grouped together to form a “comb”, just like the device we use on our hair. You know the thing we use to keep next to our pens in the pocket protector. This comb filter does just what the name implies. It imposes its comb like teeth on our audio signal and can filter all of the clarity and definition from our music. A comb filter can also mask whole instruments or vocals, especially harmonies and rhythm sections.
Sound that occurs in our lives that is not absorbed is reflected or diffused. Diffusion is everywhere in our world with sound bouncing and moving off all objects on earth. Our ears are specifically designed to deal with diffusion of energy and our ears localization systems work in a diffuse sound field. When we take a space of air and build four walls with floor and ceiling, we contain that air space and then we minimize the diffusion that occurs naturally in our world.
Diffusion Is Air
A diffuse sound field has “air” in it. Air is that space between each attack note. It is also that space where the note decays in. It is in the reduced and focused reflections from the walls that it works its magic. Diffusion takes a reflection and reduces it in magnitude without altering the signal’s phase or amplitude.
Our electronic equipment produces both unwanted and wanted forms of distortion. Bending waveforms does have its sonic limits. Unwanted distortions that are produced electronically are unwanted because they hurt our ears. Our rooms can also be a source of this acoustical pain. Reflections mix themselves up with the direct sound and then we have this dance of direct/reflected energy. It is a dance between the direct which is the music and the reflections which represent the room sound. If the dance gets to heated, we get comb filtering. Our room is a box and we must let fresh air in our box. One could tear the walls down or simply add acoustic diffusion to create more natural air.