Equalization means adjusting an audio system’s output so that the response measured in the listening area aligns with a target curve. It is used to correct for characteristics of the audio system, or for the effects of the room (aka “room correction”). Since it is normally difficult to separate audio system from room, we are usually correcting for both at the same time.
When sound leaves an audio system, it interacts with the room, but this interaction changes with frequency. Below the Schroeder frequency, or about 300 Hz, room resonances have a dominant impact on what is heard in the listening area. At these frequencies, the room is in control. Above the Schroeder frequency, direct sound from the audio system begins to dominate with contributions from room diffusion, absorption and reflection. In this region, the audio system is mostly in control. Thus, when equalizing a typical home or car audio system, we are correcting for the room below the Schroeder frequency and for the audio system above it (for an excellent deep dive, please read Floyd Toole’s paper).
When applying equalization, it is best to start at the low frequencies and work your way up. At low frequencies, room resonances will create large peaks and dips in the magnitude. These will be quite audible, so focus on smoothing them out to the target curve. Moving toward higher frequencies, peaks and dips become much less audible and should not be adjusted. Instead, shift your focus to adjusting wide regions of the magnitude response towards the target curve (an octave or more). At the very upper end (10 KHz), it may not be worth making any adjustments at all (A great video on the topic is here).
HouseCurve supports automatic and manual equalization:
Automatic equalization, HouseCurve generates a set of filters to correct a measurement to a target curve. If your audio system has a parametric equalizer, the filter parameters can be imported from a file, or manually entered. Even without a parametric equalizer, the feature is still useful - you can see what HouseCurve would adjust and use that as a guide.
Manual equalization is the most straight forward approach. It works by iteratively measuring the audio system, adjusting it, then measuring again. It will work with any audio system, even ones with limited adjustments (ex: you can move furniture and see the impact).
For best results, average several measurements from the listening area and then save. Averaging provides HouseCurve with a better picture of how sound changes in the listening area.
The image below shows a saved measurement (grey) being corrected to the target curve (yellow). The filters that perform the correction are shown in magenta. The predicted magnitude response (when filters are applied) is shown in cyan.
HouseCurve allocates filters to regions with the largest deviation from the target curve, starting with lower frequencies and ignoring blanked regions (see Coherence Blanking). The algorithm’s behavior can be controlled by tapping to display the equalize setup screen. Plot settings, such target curve fit and blanking threshold also impact the algorithm. Changes are reflected immediately.
HouseCurve uses a form of IIR filter known as a peaking biquad filter. These are commonly used in graphic and parametric equalizers to boost or cut at various frequencies. You can view the individual filters by tapping to bring up the Filter Detail screen, shown below. This screen can be used to manually enter filter settings into your audio system.
The filters can be exported to a file by tapping . This file can then be imported into a compatible parametric equalizer. HouseCurve has been tested with the following hardware/software:
- miniDSP 2x4HD
- Volumio3 with FusionDSP
- HifiBerry DAC+DSP
- Equalizer APO
- moOde Audio Player (manual entry)
- Roon (manual entry)
It is recommended that the same filters be applied to the left and right channels of your audio system. Separate filters may be generated for your subwoofers.
When the filters have been loaded into your audio system, you should find that measurements are reasonably close to the predicted response. The screenshots below show the predicted response (cyan) and actual measurements (green) after applying the filters.
If your results are radically different from the prediction, check that the filter sample rate is correct - a mismatch will cause the filters to be applied to the wrong frequency. The filter file format may also be incompatible with your equalizer. If you have questions or experience problems, please reach out.
Equalization means changing the volume level at different frequencies. There are many ways an audio system can be changed to make this happen:
- Change speaker position in room
- Adjust the bass or treble controls
- Change relative levels of speakers (ex: subwoofer level)
- Adjust the graphic or parametric equalizer settings
- Change the room (ex: sound dampening, adjust furniture)
When equalizing, the goal is to adjust the audio system such that the average magnitude measurement ends up within the +/- 3 dB band that surrounds the Target Curve.
Adjust the audio system in small steps, collecting the same measurements after each change. Saved measurements are helpful for seeing the effect of an adjustment as can be seen below. You can also take a look at the Automatic Equalization screen to see what HouseCurve would equalize (based on the saved measurement).
When most of the average magnitude measurement is within the target band, the audio system is sufficiently equalized. Further adjustments may not be perceptible.
Avoid adjustments that are beyond the capability of the audio system. Doing so will lead to distortion and possibly audio system damage. Keep in mind that a +10 dB adjustment means the audio system has to output 10 times more signal power. A change of +20 dB is 100 times more signal power!