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The Fundamentals of Signal Processing - Part 2
Every sound consists of a fundamental frequency and its overtones. The timbre of the sound depends on the volume level of the different partials of the overtone series, also known as harmonics. When you equalize a sound, you are changing the volumes of the partials in relation to the fundamental frequency, which changes the overall effect of the sound. EQ is used to: 1. Make up for defiencies in the sound of an instrument or vocal. 2. Make sounds blend better. 3. Make overdubbed sounds match previously recorded sounds. 4. Increase stereo spread by eliminating unwanted frequencies. 5. To create effects or change the timbre of an instrument or a vocal.
EQ is generally done with a mixing board or with a graphic EQ device, such as a rack mount type or in the case of guitars and bass, with an EQ pedal. Generally speaking, EQ is used to make sounds brighter or darker. The following frequency ranges are within the range of audio tones that can be heard by the average human ear. Low Bass - 16Hz to 60Hz. These sounds are generally felt more than heard. Excessive boosting in this range makes things sound muddy. Hum heard in audio systems is generally at 60Hz and/or some multiple of that frequency. Bass - 60Hz to 250Hz. This range contains the fundamental note of the rhythm section. EQing in this range drastically changes the musical balance. Too much boost in this range makes things sound boomy. Midrange - 250Hz to 2kHz. This range contains the fundamental and first overtones of many instruments. Excessive boost in this range makes things sound like a telephone. Boosing from 500Hz to 1kHz imparts a horn-like quality. Excessive boost in the 1kHz-2kHz range makes things sound tinny. Upper Midrange - 2kHz to 4kHz. This is where the important speech recognition sounds can be found. Boosting in this range makes vocals more intelligible. Presence - 4kHz to 6kHz. Boosting in this range gives things more clarity and seem closer. Cutting in this range makes things sound distant and transparent. Brilliance - 6kHz to 16kHz. Boosting in this range makes things sound brighter, On vocals, it brings out "s" sounds and can cause sibilance.
Different instruments have different EQing needs. Below are some general guidelines for various instruments. Electric Guitar - Fullness at 240Hz, bite at 2.5kHz. Organ Bass at 82Hz-120Hz, presence at 2.5kHz. The highest harmonics an electric guitar can produce are around 3kHz. Acoustic Guitar - Body at 240Hz, clarity at 2.5kHz, 3.75kHz and 5kHz. Bass strings range between 82Hz and 120Hz. Bass Guitar - Attack or pluck at 700-1000Hz, bottom at 80-100Hz and string noise is at 2.5kHz. Piano - Bass at 80-120Hz, presence at 2.5kHz to 5kHz. Honky tonk sound at 2.5kHz. Resonance and echo at 20-50Hz. Vocals - Presence at 5kHz, sibilance at 7.5-10kHz. Boominess at 200-240Hz, fullness at 240Hz. Harmonica - Fat at 240Hz, electric at 2.5kHz, acoustic at 5kHz. Drums --- Bass Drum - Slap at 2.5kHz, bottom at 60-80Hz. Snare - Fat at 240Hz, crisp at 5kHz. Toms - Attack at 5kHz, fullness at 240Hz. Floor Toms - Attack at 5kHz, fullness at 80-100Hz. Conga - Resonant ring at 200-240Hz, presence and slap at 5kHz. Horns - Fullness at 100-240Hz, shrillness at 5-7.5kHz. Strings - Scratchiness at 7.5-10kHz. Fullness at 240Hz. If you want to boost a particular frequency, you generally cut the frequencies around the frequency that you want to boost. It takes some practice to get a good mix, but it can be done once you know some of the basics. So,
now that we have a little background on this stuff, it might be interesting
to examine how a tone control affects the frequencies that are going through
it by looking at the waveforms with a spectrum analyzer. That will hopefully be forthcoming
in a later installment.
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