Sound Designer’s Manifesto or Knowledge You Can Drop to Impress Your Friends
What is sound?
Sound is the result of alternating waves of pressure (areas of high and low density) propagating outward from a resonating source. Sound travels as particles within a compressible matter (solid, liquid, or gas). Sound is measured in Frequency, Wavelength, and Amplitude.
Key Definitions and Concepts
Decibel: a unit used to express the intensity of a sound wave, equal to 20 times the common logarithm of the ratio of the pressure produced by the sound wave to a reference pressure. 1928, coined from L. decibus “tenth,” from decem “ten” + bel unit for measuring sound, after Alexander Graham Bell.
Threshold of hearing = 0 dB
Quiet restaurant = 20 dB
Conversation at 1 ft. = 70dB
Vacuum cleaner = 80 dB
Threshold of pain = 120 dB
Jet at take off = 140 dB
SPL: (Sound Pressure Level) is the local pressure deviation from the ambient (average, or equilibrium) pressure caused by a sound wave. We interpret dB to be the loudness of an event. You have possibly heard the term ‘pushing a lot of air’ in reference to speakers at high volumes. The greater number of air particles displaced, the greater the pressure, the louder we perceive the disturbance to be. The lowest sound pressure level (SPL) that we can hear is equal to 0 spl (zero decibels sound pressure level) . Damage to human hearing starts occurring at 90db and above.
90 dB – 8 hrs.
95 dB – 4 hrs.
100 dB – 2 hrs.
105 dB – 1 hr.
110 dB – 30 min.
115 dB – 15 min.
120 dB – RUN AWAY!
Frequency: is determined by the number of oscillations/compressions (“waves”) occurring per second measured in Hertz(Hz).
Wavelength: is the distance between repeating units of a propagating wave determined by frequency in relationship to speed of sound.
Frequency Range: A frequency range or frequency band is a range of wave frequencies. It most often refers to either a range of frequencies in sound or a range of frequencies in electromagnetic radiation, which includes light and radio waves. In music and acoustics, the frequency of the standard pitch A above middle C on a piano is usually defined as 440 Hz, that is, 440 cycles per second and known as concert pitch, to which an orchestra tunes. A baby can hear tones with oscillations up to approximately 20,000 Hz, but these frequencies become more difficult to hear as people age.
Hertz: is a measure of frequency, informally defined as the number of events occurring per second. (Hz)
EQ: Equalization (or equalisation, EQ) is the process of changing the frequency envelope of a sound in audio processing. In passing through any channel, an audio signal will “spread” from its original qualities. The goal of equalization is to correct, or make equal, the frequency response of a signal. Typically one will encounter EQ’s with a 2-31 bandwidth settings. These settings (like those found in cars which are labeled Bass and Treble) control the amplitude
Amplitude: is determined by the intensity of the expansion and contraction of matter by sound waves. Also referred to as “sound pressure level,” or “spl” for short, measured in decibels (db).
Dynamic Range: the ratio between the smallest and largest possible values of a changeable quantity, such as in sound and light. The dynamic range of human hearing is roughly 140 dB. The 16-bit Compact Disc has a theoretical dynamic range of 96 dB. 20-bit digital audio is theoretically capable of 120 dB dynamic range; similarly, 24-bit digital audio calculates to 144 dB dynamic range. Observed 16-bit digital audio dynamic range is about 90 dB.
Noise Floor: average level (amplitude) of ambient room sound. Used to evaluate and quantify sonic environments (important consideration when recording and mixing 40db and below is desirable).
Sine Waves: are the core component for modeling more complex waves. It’s frequency defines audible pitch. Sine wave are used to test speaker configurations and setups for discrepancies and alignment.
Harmonics: A wave whose frequency is a whole-number multiple of that of another. What this means in regards to sound: you have multiples of the “fundamental” or first sound layered on top of each other to produce a harmony. Any deviation from the whole-number multiple results in what is called an inharmonic and it is usually characterized as an unpleasant sound.
Bitrate: refers to the bit per second of audio that in relation to data storage. The higher the bitrate, the greater the audio resolution of the final product.
Samplerates: measure the frequency with which the signal is stored. These are measured in kiloHertz, or thousands of samples per second. The standard samplerate of commercial CD audio is 44.1kHz or 44,100 samples per second. This is the default samplerate used by most encoders and is the samplerate in most MP3 files, whether downloadable or created by others. Audio professionals often work with 48kHz audio and, more recently, 96kHz.
Optimizing sound for the web
When encoding for the Web, it’s a good idea to set the amount of normalization to a number below 100 percent, such as to 95 percent or -3dB below 0 to allow for extra headroom. Flash and RealAudio, for example, have a tendency to distort certain sound files that are normalized between 95 percent and 100 percent.
Normalization raises or lowers the overall amplitude or loudness level of a sound file to a selected point, generally up to where the loudest amplitude peak in the file rests just below the 0dB clipping level, as shown in Figure 2. The clipping level is the cut-off point where the audio signal information can be accurately digitized. Audio information beyond the OdB cut-off point becomes severely distorted and results in unpleasant noise. Normalization makes maximum use of the dynamic range within the bit-depth of a digital audio signal, but does not effect the relative dynamic range of the source audio. In other words, the dynamic range between softest and loudest material in the sound file is unaffected after normalizing, but the sound file is louder overall.
Equal Loudness Curve
Nearly 75 years ago, Harvey C. Fletcher and Wilden A. Munson—two Bell Labs engineers studying subjective loudness—changed our understanding of the hearing process. Asking a large number of subjects to compare the relative volume of two tones to a standard 1kHz tone at a set level, Fletcher and Munson defined human hearing awareness at various frequencies.
In the October 1933 edition of the Journal of the Acoustical Society of America, Fletcher and Munson showed that hearing is frequency-selective—more specifically, hearing is most sensitive to pure tones in the 3 to 4kHz range and less so above and below that. To perceive that a 100Hz signal is of equal loudness to a 3kHz tone requires an actual SPL of the 100Hz tone that’s much higher than that of the 3kHz tone, particularly at low volumes. This phenomenon was referred to as “Equal-Loudness Contours,” and although this original research was later updated (most notably by Robinson and Dadson in 1956), Fletcher and Munson’s pioneering work laid the groundwork for industry-standard measurement curves, from the classic A/B/C/D-weighting filters to the current ISO 226:1987 standard.
In general, one should adjust their mix in accordance with the equal loudness contour in order to maintain an acceptable level of acoustic enjoyment.
Phase Cancellation (Beware!)
Phase cancellation occurs when two signals of the same frequency are out of phase with each other resulting in a net reduction in the overall level of the combined signal. If two identical signals are 100% or 180 degrees out of phase they will completely cancel one another if combined. When similar complex signals (such as the left and right channel of a stereo music program) are combined phase cancellation will cause some frequencies to be cut, while others may end up boosted. (http://www.youtube.com/watch?v=sba-sRo3UQI) A simple rule of thumb to follow when using 2 or more mics is called the 3:1 rule, which means to place the mics apart from each other a distance of 3 times the distance of the source.
The same Phasing can also occur with 1 mic as well due to the reflections off objects (walls, people, etc). Basically any obstruction can create the potential for phasing as the reflection of the sound waves running into each other.
Factoids
Sound is vibration transmitted through a solid, liquid, or gas; particularly, sound means those vibrations composed of frequencies capable of being detected by ears. The mechanical vibrations that can be interpreted as sound are able to travel through all forms of matter: gases, liquids, solids, and plasmas. The matter that supports the sound is called the medium. Sound cannot travel through vacuum.
For humans hearing is limited to frequencies between about 20 Hz and 20,000 Hz (20 kHz), with the upper limit generally decreasing with age.
The standard telephone bandwidth is 300 Hz to 3.4 kHz, and can be simulated by using an equalizer to isolate that frequency range.
The dynamic range of human hearing is roughly 140 dB. The 16-bit Compact Disc has a theoretical dynamic range of 96 dB. 20-bit digital audio is theoretically capable of 120 dB dynamic range; similarly, 24-bit digital audio calculates to 144 dB dynamic range. Observed 16-bit digital audio dynamic range is about 90 dB.
Ling Electronics of California makes a noise generator whose gigantic howl, loud enough to tear electronic equipment apart, is used to test the toughness of space-flight hardware.
Scientific tests..reveal that changes in the circulation of the blood and in the action of the heart take place when a person is exposed to a certain intensity of noise. Even snatches of loud conversation are enough to affect the nervous system and thereby provoke constrictions in a large part of the blood circulation system…
I steal my secrets from kittens… 