Equalisation article v1.1. Copyright © 2002 Adam P. Salisbury.
Equalisation.
Introduction.
Equalisation is the process of making something equal, or uniform. Sonically speaking, equalisation is the process of altering certain frequency characteristics in order to make up for deficiencies with the playback equipment or with the environment. With recording studios and auditoriums, for example, acoustical response is a primary design feature, and thus equalisation is initially a mechanical process that encompasses room shape, size, even the choice of construction methods and materials.
The car is hardly a designed sonic environment, and whilst manufacturers expend much effort towards the reduction of engine and external noise, it is unrealistic to expect the same level of effort with regard to acoustical performance. Mechanical equalisation can be achieved in-car, and properly applied sound deadening is one or the best, and; depending on the material of choice; most cost effective sonic improvements to a vehicle interior. However, even a properly damped car can and will suffer deviations in response due to speaker placement and hard surface reflections.
Electronic equalisation is remedial, if not curative. It can negate/reduce sonic deviations to a point where the response is essentially flat. Flat in that all frequencies are treated equally. Naturally, that rather depends on whether flat is the desired response, and in many respects it is not. Car audio tends to err towards a more dynamic presentation, generally with emphasis on low bass response. Equalisation can therefore be used to tailor the system response to suit that of the target vehicle or it can be used to tailor the system response to the end user.
The two most common types of equalisation used to provide this level of system flexibility in-car are the graphic equaliser and the parametric equaliser.
1 Graphic Equalisation.
So called because of the ‘graphic’ representation of applied equalisation via the unit’s slide controls. Equalisers that use rotary controls are no less effective, if not exactly ‘graphic’.
1.1 Fundamentals of Operation.
The fundamental building blocks of a graphic equaliser are a series of active bandpass filters, usually designed to cover the full audible spectrum (20Hz – 20kHz). Each bandpass filter operates around a centre frequency (fc), and being active, has an associated gain circuit.
The visible elements of a graphic equaliser are the slide or rotary controls, which are slide track or rotary track potentiometers, used to control the gain element of each filter. Gain is usually in the order of ±12dB, meaning a maximum 16-fold increase (boost) or decrease (cut) can be applied to each frequency element of the graphic equaliser.
The number of frequency elements (bands) is determined by the spacing protocol used by the manufacturer. Such spacings are a function of an octave divided frequency range, and can be integer multiples or fractional.
Every frequency has associated octaves, those being a factor of two of the original frequency. 1kHz having octaves at 500Hz (/2), and 2kHz (*2). Naturally, depending on the chosen bandwidth, such octaves (and their relative frequency spacings) can be applied without any reference point. In order to simplify octave selection, and to add a reference to the process, the ISO (International Organisation for Standardisation) issued standardised whole and 1/3 octave spacings. The following table shows the whole octaves (shaded), as well as the additional 1/3 spacings.
20kHz / 16kHz / 12.5kHz / 10kHz / 8kHz / 6.3kHz / 5kHz / 4kHz / 3.15kHz / 2.5Khz2kHz / 1.8kHz / 1.25kHz / 1kHz / 800Hz / 630kHz / 500Hz / 400Hz / 315Hz / 250Hz
200Hz / 180Hz / 125Hz / 100Hz / 80Hz / 63Hz / 50Hz / 40Hz / 31Hz / 25Hz
An octave equaliser therefore has 10 bands per channel, and a 1/3 octave equaliser 30 bands per channel. Equalisers with less than 10 bands are generally rather crude in operation, and 2/3 spacing is felt to be the minimum for worthwhile equalisation.
1.2 Filter Q.
Q, or Quality Factor, is used in most technical and scientific fields. In the field of audio Q has many functions, but is mostly used in reference to the output response from various subwoofer enclosure and filter alignments. The definition of Q relative to the bandpass filters used in a graphic equaliser is the centre frequency divided by the bandwidth. Bandwidth being defined as the area that falls between the high and low frequency 3dB points (see [enter location]):
Where fc is the centre frequency, fh is the 3dB high frequency, fl is the 3dB low frequency.
In practical terms, Q relates to the steepness of the filter slopes either side of the centre frequency. The steeper the slope the smaller the bandwidth. However, in standard graphic equalisers Q varies relative to the amount of gain (see [enter location]). In other words, high gain equals high Q, and hence steep slopes. Low gain equals low Q, and shallow slopes. It is important to bear in mind that this variability is a function of the graphic equaliser design, and is not a user definable parameter.
The importance of filter slope should not be underestimated. Most equalisation takes place with only moderate levels of boost or cut, meaning each filter used will tend to exhibit a low Q characteristic. A shallow slope means that the filter has a far greater bandwidth, and thus affects a wider range of frequencies either side of the centre frequency. This is known as the filter’s ‘skirt’.
When the defined frequency range is double octave or octave spaced (5 bands, 10 bands), the overlap (relative to a low Q filter) into adjacent bands is minimal compared to that of a 2/3 or 1/3 octave design (15 bands, 30 bands). In many respects, the low band-count designs need the wide bandwidth in order that applied boost or cut does not produce a response that simply peaks at each centre frequency.
2/3 and 1/3 octave designs fare less well. The bandwidth attained from moderate boost/cut (low Q) will inevitably overlap adjacent filters. Naturally, finite control is lost, and effective equalisation is less easy. Better quality graphic equalisers therefore utilise a design known as constant Q (see [enter location]).
In a constant Q filter, the bandwidth remains the same regardless of the applied boost/cut. This means that the overlap associated with a variable Q design is negated/reduced, thus restoring finite control to the equalisation process.
1.3 Graphic Equaliser Application.
It has already been said that effective equalisation can only be achieved with 15 bands (2/3 octave), or more. Less than that and there is not the control required to effectively negate/reduce the vagaries in system and vehicle response. However, in order to facilitate effective equalisation, it is important to know the present status of the system/vehicle combination.
Listening to the system is important, but setting up a 2/3 or 1/3 octave equaliser (15 or 30 bands per channel) by ear alone is unlikely to produce satisfactory results. A more professional approach is via the use of an RTA (Real Time Analyser). An RTA analyses the sonic environment using pink noise played through the system. Pink noise has equal amounts of energy per octave, and presents a perfectly flat response over the frequency range. Therefore, any measured vagaries in response can be attributed to the sonic environment.
Those familiar with the use of an RTA will be able to testify to their benefit. Those unfamiliar are advised to seek the advice of a professional installer.
2 Parametric Equalisation.
A graphic equaliser is a device whose individual filters can be user defined over one parameter: amplitude (gain). The centre frequency of each filter is predetermined by the manufacturer according to the spacing protocol used. Bandwidth (Q) can vary on certain types of graphic equaliser, but the variation is not user defined. A parametric equaliser, on the other hand, allows the user to define amplitude, centre frequency, and Q. Unlike graphic equalisers, however, parametric equalisers are rarely used to cover the full audible spectrum (20Hz – 20kHz). In-car, especially, parametric equalisers are function specific, and that function specifically relating to low bass equalisation.
2.1 Fundamentals of Operation.
In a function specific parametric equaliser, as those used in-car invariably are, the fundamental building block is a single active bandpass filter. The visible elements of a parametric equaliser will be a series of rotary controls and/or switches, depending on the functionality of the unit.
2.11 Amplitude.
Amplitude relates to the user definable element of gain. The feature is universal to parametric equalisers as it is to graphic equalisers, and is usually applied in the same way: ±12dB. The nature of an in-car (function specific) parametric equaliser, however, is not to react to vagaries in system response, rather it is used to bolster sub bass output. With the latter in mind, it is not unusual to find parametric equalisers that restrict amplitude variations to boost only, and for that boost to considerably exceed the normal +12dB.
2.12 Centre Frequency.
On a graphic equaliser, the centre frequencies are fixed according to the spacing protocol used, with reference to the ISO standard octave spacings (see section 1.1). On a parametric equaliser, the centre frequency is user definable within a range decided by the manufacturer, according to the unit’s function. In a car audio sense, this invariably means low bass equalisation.
The actual range will be determined by the manufacturer, but expect it to be roughly 20Hz – 150Hz. Bear in mind that the setting relates to the centre frequency, not to an individual frequency. The ultimate bandwidth of the filter is determined by the Q setting.
2.13 Filter Q.
Q (see section 1.2) is defined as the centre frequency divided by the bandwidth. In practical terms Q relates to the steepness of filtering slope, and as such governs the amount of overlap (skirt) into frequencies either side of the centre frequency [figure enter location]. Unlike a graphic equaliser, the application of Q is user definable on a parametric equaliser.
The visible element of user definable Q will either be a rotary control, usually expressing Q as a numerical range (1 – 5, for example), or a simple multi-position switch, expressing Q as High, or Low, for example. The numerical indication is proportional to the worded one in that high Q equates to the high number, and low Q to the low number. The numbers themselves are a factor of the following equation:
Where fc is the centre frequency, fh is the 3dB high frequency, fl is the 3dB low frequency.
Examples:
1. Centre frequency 50Hz, fh 75Hz, fl 25Hz, and 2. centre frequency 50Hz, fh 55Hz, fl 45Hz:
Example 1 produces a Q of 1, which is low Q, but has a wide bandwidth, relatively speaking. Example 2 produces a Q of 5, which is high Q, but has a narrow bandwidth relative to example 1. The practical effect is that boost (or cut) applied to a low Q filter affects a wider bandwidth than that applied to an equivalent high Q filter. The nature of a low Q filter, however, is that boost applied to frequencies forming the filters skirt are gradually attenuated due to the shallow nature of a low Q filtering slopes. Naturally, both high and low Q have preferred applications.
2.2 Parametric Equaliser Application.
The primary application of an in-car parametric equaliser is for low bass equalisation. Parametric equalisers are found as part of the features count either of a power amplifier, or as a standalone unit. The standalone unit will likely be named a bass processor rather than a parametric equaliser, but the fundamental operation is the same.
The basis of parametric equalisation with regard to low bass is to set the centre frequency control relative to the resonant frequency of the enclosure, or in the case of a ported enclosure, to the tuning frequency of the port. Sealed type enclosures tend to work best with a low Q setting, but ported are best used with a high Q setting.
An additional feature of some parametric equalisers, and particularly standalone bass processors, is a subsonic filter. A subsonic filter is in effect a variable high pass filter that operates at very low frequencies, and uses very steep filtering (usually 18 or 24dB/oct). The filter will operate in the region of 15Hz – 35Hz, and when used not only keeps potentially dangerous ultra low frequencies at bay, but in doing so frees up power to be used across the required bandwidth.