June 6, 2014
Illusonic Room Equalization
Room equalization, when needed and done well, brings back fidelity and precision
The weakest link of high quality audio systems is often the room in which they are in. Room equalization, when needed and done well, brings back fidelity and precision.
Full integration of room equalization in an audio system requires that the filtering is done just before the DA converter (DAC), to prevent that signals are converted multiple times between analog and digital. Additionally, the DAC has to have the capability to receive signals directly from all desired sources, motivating addition of a pre-amplifier in the same device. Illusonic Audio Processors are preamplifier, processor (room equalization and more), and DAC in one device. They can receive a wide variety of analog (line, phono) and digital (S/PDIF, HDMI, USB, ethernet) signals.
Manual, Automatic, and Illusonic Way
Many professionals and some ambitious consumers use manual room equalization. Usually a number of fully parametric equalizers per channel are used. The challenge is to optimally set the parameters. This task requires expert knowledge, experience, and is time consuming (requiring many iterations between changing equalizer parameters and re-measuring and listening).
To enable room equalization for the majority of consumers, automatic systems were introduced in recent years. The primary benefit of these systems is, that they are easy enough to use, to enable each owner of a corresponding device to use room equalization.
After several years of implementation, experimentation, and testing of automatic and manual systems, our conclusion was, that the manual approach with an experienced expert is much better. Additionally, the manual approach lets the user transparently see what is being done.
Signal-processing-wise Illusonic room equalization comprises of a high quality implementation of well proven tools: gains, delays, and fully parametric equalizers per channel. For those who use it, also bass management is included.
Except for experts and ambitious hobbyists, measuring a room and setting these parameters is too complex. Therefore, Illusonic developed advanced software, IAP Calibration, and a remote tuning service:
A reseller or customer can conveniently make multi-point room measurements
These may be sent to Illusonic, which expertly sets the parameters (for free)
The customer conveniently loads the parameters into his audio processor
The customer may modify any parameters to his liking
This remote tuning service is made possible by IAP Calibration’s virtual tuning capability. As parameters are set, the resulting frequency and time responses are shown in real-time, without a need for iterating between measurement and parameter setting. Virtual tuning is of course also available to all customers, we use the same IAP Calibration tool as is available to everyone.
Illusonic Room Equalization is a system, enabling top-notch room equalization, otherwise only possible by using an experienced expert for the task.
In the following, we are discussing room equalization strategies for those who are interested. To implement these strategies with conventional manual measurement tools is very complex and time consuming. The goal of our IAP Calibration software is to make it as simple and convenient as possible.
One-Point and Multi-Point Room Equalization
One Point Room Equalization
Sound that travels from a loudspeaker to a point in the room can be precisely modelled with a linear finite-impulse-response (FIR) filter. Inversion of such filtering is under normal circumstances ill-conditioned, i.e. practically not possible.
Except for special simple cases, one point room equalization can only attempt to flatten the frequency response, while attempting to keep damage to the phase response small (compared to the case without room equalization). Usually this means that one does not attempt to correct every frequency response fluctuation, but only frequency-smooth trends. This is also advantageous, because flattening the frequency response in one point does increase fluctuations everywhere else.
If needed, a gain and delay correction between the loudspeakers is a no-brainer and will give clear improvements in the sweet spot.
For consumer audio systems, which usually not only serve listeners precisely in the sweet spot, one point room equalization is of little or no benefit.
Listening Area Room Equalization
Cinema and public address have been domains, where room equalization has played an important role for years. The goal is to provide good sound to as many seats in the audience as possible. An excessive focus on a single center position is counterproductive in this case.
In order to equalize a room for more than one listener position, one has to measure the frequency response at several positions, to see what is going on in the room overall. Given the measurements from each loudspeaker to several positions, the goal is to set the equalizers in a way that the sound on the average position improves.
Well done room equalization can give a benefit, even for the most audiophile systems. It can mitigate negative effects of room resonances, correct undesired asymmetries between loudspeakers, and correct undesired spectral defects arising from walls near loudspeakers and earliest reflections. Below, we’ll discuss strategies to set equalizers for achieving the before mentioned goals.
Illusonic Room Equalization
All our audio processors feature room equalization, based on the same signal processing tools used by sound professionals:
channel gain and delay
fully parametric equalizers for each channel
The channel gain has the purpose to correct possible level errors between amplifier channels and loudspeakers. The delay is used, when different loudspeakers have a different distance from the center position (or preferred listening position).
The fully parametric equalizers are implemented as high precision (32/64 floating point) minimum phase biquad filters. They allow to spectrally correct each channel precisely, while keeping a corresponding undesired phase modification at minimum.
Each loudspeaker is measured to multiple measurement (microphone) positions. The figure below illustrates different measurement positions:
the center of the sound system is measured
one or more listening positions are measured
a few positions distributed randomly in the room are measured
Remote and Virtual Tuning
The measurement data can be sent to Illusonic. We will manually determine good room equalization parameters, using IAP Controller’s virtual tuning capability.
You can also yourself set (or modify our) parameters, using IAP Calibration and virtual tuning. In the following, we describe room equalization strategies.
Gain and Delay
When doing room equalization without favouring a specific listening position, gain and delay for each loudspeaker are determined relative to the center position. Alternatively, it can be determined relative to the favoured listening position. (Often favoured listening position and center are the same. If you have, for example, a sofa with two seats both not in the center, then they differ).
The figure below illustrates example room resonances. Between each pair of parallel walls, sound is bouncing back and forth, forming a standing wave. The maximum and zero position of such a standing wave has a time invariant position. Thus, the level of the room resonance depends on the specific listening position. The listener in the figure above hardly hears the first (red) room resonance (because he is positioned near its zero), while the second (blue) room resonance is very loud (because he is positioned near its maximum).
Room resonances can compromise the sound quality in two ways:
resonance frequency is too loud or too soft at listening position(s)
resonance is heard between transients (bass booming)
Low frequency absorbers weaken resonances, but are often not a viable option for a living room (cost, size). By varying loudspeaker and listening positions one can optimise the balance between the different resonances. However, often the degree of freedom for choosing loudspeaker and listening positions is not high enough to sufficiently solve the room resonance problem without equalizing.
For each loudspeaker, each room resonance is treated like this:
Identify the listening position where the resonance is most dominant
Attenuate the room resonance as much as needed
Room resonances are best tuned while listening to test signals. IAP Calibration software allows to play back test signals, while modifying parameters:
Play pink noise and attenuate the resonance until level is sufficiently reduced
Play low frequency pulses and further attenuate until bass booming is gone
The above figure shows an example. The blue curve is the listening position’s frequency response with two room resonances. Correction 1 is the one carried out with a pink noise test signal. A low frequency pulse test signal may result in more attenuation, as shown as Correction 2.
Loudspeakers Near Walls
Loudspeakers are often positioned at or near walls. The effect of this is an undesired low frequency boost. Typically, loudspeakers near room corners exhibit a 7 dB boost and loudspeakers near walls a 4 dB boost.
Such a low frequency boost exhibits everywhere in the room. Thus it can be corrected, improving the sound in the whole room.
A correctable low frequency boost (or any other correctable whole room effect) can be identified as follows:
Compare frequency response in the listening position(s) to the average frequency response over all measurement positions
Tune equalizer to correct common undesired trend
The figure above shows an example correction. IAP Controller software allows you to simultaneously view listening position(s) and average frequency responses, similary as illustrated in the figure above.
Dominant Early Reflections
Dominant early reflections, such as floor or ceiling reflections also give often rise to whole room spectral distortions which can be corrected.
A floor or ceiling reflection often reveals as a frequency loss of a few dB’s at 2 to 4 kHz. The correction strategy is the same as for loudspeakers near walls, i.e. correct the common trend in listen position and average frequency responses.
Deviations Between Loudspeakers
Spectral deviations between loudspeakers are corrected with a similar philosophy: correct trends which manifest in the listening position(s) and the average over all measurement positions.
The figure above shows an example of correctable spectral left-right loudspeaker deviation: the deviation of frequency responses from a left and right loudspeaker to the listening position(s) is similar to the corresponding deviation considering the average over all measurement positions. In this case, an equalizer is tuned to correct the undesired peak in the left loudspeaker frequency responses.
So far, we have treated spectral defects in each loudspeaker frequency responses and differences in frequency responses between loudspeakers. Usually, for an audiophile application, this may be all that is desired. That means, the room and loudspeakers shall keep sounding as without room equalization, while undesired frequency response defects are mitigated.
When looking at the frequency responses of the same system measured in different rooms, one will notice differences in the overall shapes thereof. Different rooms result in more or less decay of sound towards the high frequencies, denoted house curve. If a specific room is perceived as being “too brilliant” or “too muffled”, one can additionally apply the same equalizer to all channels to modify the high frequency decay of the frequency responses.