April 2020

Understanding Digital Room Correction For Audiophiles
Article By Mitch Barnett Of Accurate Sound

  Modern Digital Room Correction (DRC) is an often misunderstood subject area. I have seen many articles written on the subject that don't answer basic questions such as what problem is DRC trying to solve? How does DRC work? And why, as an audiophile, should I care? This article endeavors to answer these questions.

What Problem Is DRC Trying To Solve?
Let's start off with what problem DRC is trying to solve. One can identify the problem fairly quickly by listening to the bass response of your loudspeakers in your room. Try it along with me. Find some music that has a variety of bass notes. The more the variety, the more frequencies we are testing, just by listening to music.

Here I have chosen the song "Spanish Harlem" from Rebecca Pidgeon's The Raven, which has a very nice acoustic bass in the key of G that uses the classic 1, 4, 5 progression. While I have a reason for this particular selection, which will be revealed shortly, pick something you like to listen to and follow along.

What to listen for? Turn up the volume to your preferred listening level. If you have a sound pressure level (SPL) meter or one on your phone, then turn up the volume until the average level is around 77 to 83 dB SPL C weighting at the listening position.

Get comfortable, close your eyes and focus on the bass line and the bass notes being played. For each bass note played, do they all sound the same level in your room? Are some bass notes lower in level? Some higher in level? Is there one bass note that stands out above the others?

It is not an easy listening exercise because we are so used to listening to the uneven bass response we may not have heard equal loudness bass notes before to compare to. So it may take a little while to "tune" into the bass line in the mix and focus on its level variation. This can be further complicated by the rest of the instruments and vocals playing at the same time.

This is why finding a song that has significant bass note variation makes it easier to identify which notes are louder and which ones are softer. In some cases, it helps if the music is sparse, like in "Spanish Harlem." Other times, it helps if the bass notes are loud and sustained like in Madonna's "The Power of Goodbye." Once you tune in, it becomes easier to hear.

Getting back to "Spanish Harlem," the reveal is here are the frequencies of the fundamental notes of the bass melody being played in the song in its 1, 4, 5 progression:

49      62      73                 65      82      98                  73      93      110

This is the beginning of where our subjective listening exercise meets objective measurements that helps us understand what problem DRC is intended to solve. The short answer is, DRC is intended to smooth the bass response in your listening room so that each note sounds even to your ears. And not at just one location. Modern DRC algorithms work across multiple seats to ensure consistent frequency and phase response covering a user-definable target area.

Why do we have an uneven bass response in our listening rooms to begin with? We will get to that answer shortly.

Since we know the bass note fundamental frequencies played in "Spanish Harlem," we can correlate these bass notes to an acoustic measurement of the low-frequency response of the loudspeakers in the room. Here is an example sound system using Purifi's SPK4 demo kit that I have set up in my listening room:

The subs are not engaged. Placing a measurement mic at the listening position and using REW, I measured the frequency response using REW's default window setting of 500ms and no smoothing.

One should understand with these default settings the mic is picking up the direct sound from the loudspeakers, early reflections, and later reflections up to 500ms over a frequency spectrum of 20 Hz to 20 kHz, which are all lumped into one bucket and displayed on a frequency response chart. For this article, we are focused on bass frequencies below 200 Hz:

This is a frequency response chart from 20 Hz to 200 Hz on the horizontal scale and Sound Pressure Level (SPL) on the vertical scale in 5 dB SPL increments. I calibrated my measurement mic with an SPL meter using Pink Noise so the level is calibrated to the SPL chart.

Note the variation in amplitude representing the largest peak to peak variation, which is over 20 dB as shown in the chart. To our ears, we perceive that 20 dB difference as being four times as loud or quiet depending on which end of the variation the bass note lands. In addition, there are level variations between the two channels.

Looking back at "Spanish Harlem's" bass notes, we can see in the chart the bass note frequencies played between 70 Hz and 100 Hz are down in level and then the 110 Hz bass note just resonates in my room as indicated by the 110 Hz peak response on the chart. The level variation that I hear in those bass notes range from 4 times as loud or quiet, depending on which bass note is played. The subjective listening experience correlates to the objective acoustic measurement when one maps the bass notes (i.e. bass fundamental frequencies) to the frequency response chart.

Now that we have subjectively heard and objectively measured bass note level differences, why do we have an uneven bass response in our listening rooms in the first place?

Why Do We Have An Uneven Bass Response Within Our Listening Rooms?
The answer is primarily due to the physical dimensions of a listening room and its room ratio. Room construction and acoustical treatments play a role, but at these long wavelengths, it is more about the room ratio. This article on, "Room dimensions on small listening rooms" gives us insight as to why room ratios affect the quality of the bass sound in one's room.

We can also enlist one of the many online Room Mode Calculators like this one: AMROC Room Mode Calculator to examine our existing listening rooms. Type your room dimensions into the calculator and read the various panels about your room modes.

It is highly educational if your browser is hooked up to your sound reproduction system so when you hover the mouse over a room mode, you can hear what it sounds like in your room (careful to keep the volume down). It is a real ear-opening experience. Give it a try as there is nothing like hearing the problem with your ears. Try walking around the room while hovering over a mode. There may be locations where it is really low in level and other locations where it sounds like blowing on a Coke bottle, but at a much lower frequency.

The unfortunate reality is that few of us have properly designed listening rooms with appropriate room ratios to evenly distribute the low-frequency room modes. So, we end off with rooms that have the wrong modal density with virtually no modes down low and with others bunched up together. Sometimes this crops up at the most inappropriate frequency, like the usually recommended subwoofer crossover frequency of 80 Hz. Pro tip, cross subs between room modes to your mains.

Further, below a room's transition frequency, also called the Schroeder frequency, room modes, standing waves, room resonances dominate the sound, so much so that the room is in control of the low-frequency response, not our loudspeakers. Some may want to re-read that sentence as it is the raison d'etre for Digital Room Correction.

For example, here is a typical size listening room where a measurement mic has been placed at the listening position and the loudspeaker has been moved to three different locations within a two-foot radius:

As one can see, below the room's transition frequency of about 300 Hz, the bass response varies significantly, not only by location, but also in each location!  Above 300 Hz the loudspeaker is in control of the frequency response that we perceive. Alternatively, the room has substantially less influence on what we hear above 300 Hz. With careful loudspeaker and listener placement, one can get lucky and be in-between the worst of it. But more often than not, it is simply shifting the frequencies and timing of where the room modes are, but they are still there.

The chart above is from Floyd Toole's excellent article on Audio -the science in service of the Art. As Floyd says, "In the investigation of many rooms over the years, I would estimate that something like 80% have serious bass coloration." Further, Floyd's research shows that bass subjectively accounts for 30% of how we judge speakers' sound quality. And "ANY loudspeaker can sound better after room EQ, so long as it competently addresses the bass frequencies - this is not a guarantee, but is not difficult for at least the prime listener." 

Now that we've heard and measured uneven bass, understanding that room ratios play an important role in distributing room modes and that the room is in control of the bass response below the room's transition frequency, let's consider how DRC works in solving this problem.

How Digital Room Correction Works
Here is an example of an ideal loudspeaker's frequency and phase response. Only the band limit towards zero Hz deviates from theoretical perfection. Again, for our purposes, we are interested in frequencies below the rooms' transition frequency, so this chart is from 20 Hz to 200 Hz:

Our modeled ideal loudspeaker has a flat frequency response with a roll offset at 20 Hz in which you can see the -3 dB point at 20 Hz. Note the rising phase response which is following the amplitude (frequency) response as loudspeakers are minimum phase systems. Additionally, both channels are identical in amplitude and phase response.

But, when we place a real loudspeaker within a real room and measure, we "typically" get what I showed earlier from the measured frequency response of the Purifi SPK4 with +20 dB of low-frequency amplitude variation:

While every room is different, more often than not, especially with the number of rooms I have measured over the years, there is almost always a good 15 to +20 dB frequency response variation in the low frequencies in every room I have measured. There have been a few exceptions, but these are "purpose-built" rooms with designed room ratios like studio control rooms.

Here is the phase response at the listening position:

As explained earlier, the room is in control of the low-frequency response, regardless of what loudspeakers are used. The measured, in-room, acoustic response is no longer a minimum phase system because of the low-frequency room reflections, standing waves, resonances, and so we have a mixed-phase response. This is what we are seeing in the phase response above. To further understand the subject of minimum phase systems, I suggest reading John Mulcahy's (author of REW) educational minimum phase article.

Getting back to how DRC works, and in a nutshell, most modern Digital Room Correction software products not only correct the magnitude response, but also the excess phase (i.e. timing) response of low-frequency room reflections.

Once an acoustic measurement is made, the DRC software extracts the minimum phase response. Then by inverting the amplitude response and applying it as a filter to the measured response, the result is a flat frequency response. By EQ'ing the amplitude response, the phase response is also adjusted, as it is a minimum phase system.

Then the DRC software, independently of the minimum phase correction, also corrects the excess phase (i.e. low-frequency room reflections) response towards the minimum phase response so that one ends up with the ideal low frequency and phase response across a listening area.

Important note: The excess phase is the phase difference between the real signal and the minimum phase response.

Here is an example with a loudspeaker system in the same room as the Purifi SPK4 kit measured above. It consists of two 15" woofers in ported cabinets per side plus dual 18" subwoofers that have successfully employed Digital Room Correction:

As one can see, this pretty well matches the ideal loudspeaker minimum phase response shown earlier. Additionally, both channels are virtually identical, even though the stereo system is set up asymmetrically. Unfortunately, REW does not have an overlay display that allows both frequency and phase on one chart, otherwise, I would show it.

Again, this is with the mic placed at the listening position and using REW's default window of 500ms and no smoothing. So, we are letting in the direct sound, early reflections, and later reflections up to 500ms. As one can see, there is little disturbance in the force J. I wish there was an easy way for audiophiles to flip a switch to get this kind of bass response. It is an ear-opening experience.

Getting into the gory details of how DRC works requires several articles the length of a book, like Accurate Sound Reproduction using DSP. However, one great place to start in understanding why we hear what we hear in small room acoustics is James "JJ" Johnston's "Acoustic and Psychoacoustic Issues in Room Correction." Downloading the PowerPoint and understanding the first 31 slides is a great way to learn more about the theory of Digital Room Correction.

Although the presentation is from 2008 all of the principles are still valid. However, DRC technology has matured significantly over the past decade, so technical issues like pre-ringing and other shortcomings from the presentation have all since been addressed. Now with sophisticated DRC software designers and 64 bit FIR correction filters, the design and implementation of DRC is completely transparent. You are in full control over every design aspect.

One aspect of DRC that may not be so obvious is that we are adjusting the amplitude of the frequency response over time. And usually, just the low-frequency response as some room reflections above the transition frequency is a good thing.

There is an important distinction between regular eq like parametric eq (PEQ) and other forms of eq that do not adjust frequency response over time as compared to "purpose-built" Digital Room Correction software products. Regular PEQ is adjusting the frequency response which includes both the direct sound, early reflections, and later reflections all lumped together as one. The listening results are typically not good and are one of the reasons why audiophiles shun eq as the wrong tool has been used for the job.

Bass frequencies can "build up" in a room over time, so we need to reduce the amplitude of those frequencies over time. Bass frequencies can also produce a maximum phase peak that is louder than the direct sound at a point in time past the direct sound. Again, commercial DRC software products are specifically designed to deal with these low-frequency timing issues. Typical digital room correction filters work in the time domain up to half or 3/4 of a second in a room at low frequencies. By then the sound has decayed to inaudible levels.

Remember we are dealing with standing waves and room resonances where some bass frequencies decay faster over time than neighboring bass frequencies. As mentioned, some bass frequencies may build up over time. And some bass frequencies are so resonant they seem to "resonate" indefinitely like blowing air across a Coke bottle.

While acoustic treatments can assist, the reality is unless one "stuffs the room" with bass traps, there is not much absorption below 100 Hz due to the wavelengths we are dealing with. Aside from the major cost, and potentially aesthetic issues, bass traps are also "passive" devices and cannot discriminate between frequencies that need to be absorbed versus ones that don't need absorption. It is blind to this and therefore the usual consequence is over-absorption in the lower bass and mids, which can take the warmth away from the music. I am not against acoustical treatments, but again, we need the right tool for the right job.

It is further complicated by the fact that not all room correction software products are equal. In my testing of several DRC software products over the last decade, there are only three commercial Digital Room Correction software products on the market that address both the minimum phase and excess phase response in the correct manner. These are Acourate, Audiolense, and Dirac. Note that there is one donationware software DRC product which is Denis Sbragion's DRC: Digital Room Correction which also works correctly.

While DIY approaches are using excellent donationware software, and techniques such as the Moving Mic Measurement and combinations of software like REW and rePhase, the issue is that these software products/techniques are not "purposely designed" for Digital Room Correction. As such, they leave out important room correction functions that reduce the effectiveness of these approaches. To be clear, REW and rePhase are excellent software that I highly recommend for their intended purposes.

The correction filters that are generated by DRC software are typically packaged as a Finite Impulse Response (FIR) filter. This is because the magnitude correction (amplitude and phase) can be independently applied from the included excess phase (timing) correction. The FIR correction filter requires a convolution engine like found in popular music player software like JRiver and Roon.

Conclusion
I hope this article has been educational and provided an understanding as to why 80% of our listening rooms suffer from room resonances that color the sound of the bass response from your loudspeakers. The room is in control of the low-frequency response regardless of which speakers you own. Hopefully through the "critical listening and what to listen for section," your ears have tuned into the bass response variations in your listening room.

Purpose-built DRC software can mitigate these standing waves, room resonances, and modes both in the frequency and time domain by applying both magnitude and excess phase correction independently.

We know what an ideal speaker should measure like, and when you apply DRC to remove the room's frequency response and time-domain distortion, one is rewarded with solid, even bass response that sounds tight and crystal clear. It is a real treat to hear for the first-time accurate bass response reproduction in one's home listening environment. This is a real benefit to audiophiles.

One should think of DRC as removing the room's low-frequency distortion and restoring the loudspeakers' low-frequency response arriving at one's ears undisturbed. The DRC software mentioned in this article ranges from $350 to $430. Coupled with a calibrated USB microphone for $105, this represents tools that can make a significant audible and measurable improvement to one's sound reproduction system.

While this article focused on restoring accurate bass response in one's room, DRC products can also be used for digital crossovers of any type or slope, time aligning drivers, linearizing individual drivers, in addition to being applied to multichannel sound reproduction systems.

Enjoy the music!

Article by Mitch Barnett, Accurate Sound Reproduction Services

Want To Learn More
See part 2 Understanding The State-Of-The-Art of Digital Room Correction.

Manufacturer
Accurate Sound
E-mail: mitch@accuratesound.ca
Website: www.AccurateSound.ca