Editor’s Note: This is an excerpt from Audio Production and Critical Listening: Technical Ear Training, available from Focal Press.
In the recording process, engineers regularly encounter technical issues that cause noises to be introduced or audio signals to be degraded inadvertently.
To the careful listener, such events remove the illusion of transparent audio technology, revealing a recorded musical performance and reminding them that they are listening to a recording mediated by once invisible but now clearly apparent technology.
It becomes more difficult for a listener to completely enjoy any artistic statement when technological choices are add-ing unwanted sonic artifacts.
When recording technology contributes negatively to a recording, a listener’s attention becomes focused on artifacts created by the technology and drifts away from the musical performance.
There are many levels and types of sonic artifacts that can detract from a sound recording, and gaining experience in critical listening promotes increased sensitivity to various types of noise and distortion.
Distortion and noise are the two broad categories of sonic artifacts that engineers typically try to avoid or use for creative effect. They can be present in a range of levels or intensities, so it is not always easy to detect lower levels of unwanted distortion or noise.
In this excerpt we focus on extraneous noises that sometimes find their way into a recording as well as some forms of distortion, both intentional and unintentional.
Although some composers and performers intentionally use noise for artistic effect, we will discuss some of the kinds of noise that are unwanted and therefore detract from the quality of a sound recording. Through improper grounding and shielding, loud exterior sounds, radio frequency interference, and heating, ventilation, and air conditioning (HVAC) noise, there are many sources and types of noise that engineers seek to avoid when making recordings in the studio.
Frequently, noise is at a low yet still audible level and therefore will not register significantly on a meter, especially in the presence of musical audio signals. Some of the various sources of noise include the following:
• Clicks: Transient sounds resulting from equipment malfunction or digital synchronization errors
• Pops: Sounds resulting from plosive vocal sounds
• Ground hum and buzz: Sounds originating from improperly grounded systems
• Hiss, which is essentially low-level white noise: Sounds originating from analog electronics, dither, or analog tape
• Extraneous acoustic sounds: Sounds that are not intended to be recorded but that exist in a recording space, such as air-handling systems or sound sources outside of a recording room
Clicks are various types of short-duration, transient sounds that contain significant high-frequency energy. They may originate from analog equipment that is malfunctioning, from the act of connecting or disconnecting analog signals in a patch bay, or from synchronization errors in digital equipment interconnection.
Clicks resulting from analog equipment malfunction can often be random and sporadic, making it difficult to identify their exact source. In this case, meters can be helpful to indicate which audio channel contains a click, especially if clicks are produced in the absence of program material. A visual indication of a meter with peak hold can be invaluable to chasing down a problematic piece of equipment.
With digital connections between equipment, it is important to ensure that sampling rates are identical across all interconnected equipment and that clock sources are consistent. Without properly selected clock sources in digital audio, clicks are almost inevitable and will likely occur at some regular interval, usually spaced by several seconds.
Clicks that originate from improper clock sources are often fairly subtle, and they require vigilance to identify them aurally. Depending on the digital interconnections in a studio, the clock source for each device needs to be either internal, digital input, or word clock.
Pops are low-frequency transient sounds that have a thump-like sound. Usually pops occur as a result of vocal plosives that are produced in front of a microphone.
Plosives are consonant sounds, such as those that result from pronouncing the letters p, b, and d, in which a burst of air is produced in the creation of the sounds. A burst of air resulting from the production of a plosive arriving at a microphone capsule produces a low-frequency, thump-like sound.
Usually engineers try to counter pops during vocal recording by placing a pop filter in front of a vocal micro-phone. Pop filters are usually made of thin fabric stretched across a circular frame.
Pops are not something heard from a singer when listen-ing acoustically in the same space as the singer. The pop artifact is purely a result of a microphone close to a vocalist’s mouth, responding to a burst of air.
Pops can distract listeners from a vocal performance because they are not expecting to hear a low-frequency thump from a singer. Usually engineers can filter out a pop with a high-pass filter inserted only during the brief moment while a pop is sounding.
Hum and Buzz
Improperly grounded analog circuits and signal chains can cause noise in the form of hum or buzz to be introduced into analog audio signals. Both are related to the frequency of electrical alternating current (AC) power sources, which is referred to as mains frequency in some places.
The frequency of a power source will be either 50 Hz or 60 Hz depending on geographic location and the power source being used. Power distribution in North America is 60 Hz, in Europe it is 50 Hz, in Japan it will be either 50 or 60 Hz depending on the specific location within the country, and in most other countries it is 50 Hz.
When a ground problem is present, there is either a hum or a buzz generated with a fundamental frequency equal to the power source alternating current frequency, 50 or 60 Hz, with additional harmonics above the fundamental.
A hum is identified as a sound containing primarily just lower harmonics and buzz as that which contains more prominent higher harmonics. Engineers want to make sure that they identify any hum or buzz before recording when the problem is easier to solve. Trying to remove such noises in post-production is possible but will take extra time.
Because a hum or buzz includes numerous harmonics of 50 or 60 Hz, a number of narrow notch filters are needed, each tuned to a harmonic, to effectively remove all of the offending sound.
Although we are not going to discuss the exact technical and wiring problems that can cause hum and buzz and how such problems might be solved, there are many excel-lent references that cover the topic in great detail such as Giddings’s book titled Audio Systems Design and Installation.
Bringing up monitor levels while musicians are not play-ing often exposes any low-level ground hum that may be occurring.
If dynamic range compression is applied to an audio signal and the gain reduction is compensated with makeup gain, low-level sounds including noise floor will be brought up to a more noticeable level. If an engineer can apprehend any ground hum before getting to that stage, the recording will be cleaner.