This is Part 4 of a series on a quest to hear music the way it was intended to be reproduced. In the last 3 posts, we have “voiced” and calibrated our speakers to an equilateral triangle, took some frequency response measurements, analyzed the results, and introduced digital room correction (DRC). Let’s look at the frequency response measurement results and DRC in more detail. Is DRC ready for prime time? I think this post will show conclusively, yes, DRC is ready for audiophiles to take full advantage of their sound system investments. Once you hear correct timbre, you won’t go back ;-)
When I say DRC, I mean the subject area: http://en.wikipedia.org/wiki/Digital_room_correction and not the software with the same name: http://drc-fir.sourceforge.net/ I have not used this DRC software and can only provide you with my experiences using Audiolense.
Here is the frequency response of my sound system measured at the listening position:
Here is the frequency response with DRC enabled, using the B&K house curve as the target, again at the listening position:
How was this DRC accomplished? Within Audiolense, you click on generate correction filters, which produces the inverse of the measured frequency response like this:
You save the filter to a file location and then load it in a Convolver like the one hosted in JRiver Media Center:
And within a few minutes, you are listening to music the way it was intended to be reproduced. More on this later.
Now let’s zoom in on these graphs so you can see more detail. I will use the same relative vertical and horizontal scales so that the two graphs can be compared.
Without DRC at the listening position:
With DRC (and the B&K house curve applied) at the listening position:
Let’s analyze this. Given the graphs presented, I would conclude that DRC works very well. It's quite the controlled difference between the before and after with DRC, especially the tight variation tolerance. After 200Hz, the variance is +- 2.5db. Tighter tolerance than the manufacturers anechoic chamber specs.
Here is what it means. Most speaker manufactures will produce a frequency response curve (plus speaker sensitivity) @ 1 watt @ 1 meter in an anechoic chamber. The tech spec usually includes a variance limit on the frequency response, like +- 3db across the measured frequency range. Note that the anechoic chamber is designed to eliminate the room effects on the measurement. Having been in one, it is an interesting experience, completely void of relected sound.
Let’s take the popular B&W CM8 and look at its specified frequency response. From their online manual, http://www.bowers-wilkins.com/Downloads/Product/InfoSheet/ENG_FP300741_CM8_info_sheet.pdf, the frequency response measures 69Hz to 22Khz +- 3db variance. My speakers, which are a tech modernization of the Klipsch Cornwall, but custom designed and built by Bob Crites, called Cornscala Type C http://www.critesspeakers.com/cornscala-style-c.html They will have a similar frequency response to the Cornwall III’s of 34hz to 20Khz +- 3db.
My point is that while my speakers measure, 34Hz to 20Khz with a +- 3db variation in an anechoic chamber, the moment that I put them in a real listening room, of any sort, all bets are off. Look at the frequency response variations of my listening room again. Note the maximum amplitude deviation.
I get +- 12 db from about 34Hz to almost 20Khz. Having measured several studio control rooms and critical listening environments, this looks typical. In fact, regardless of your audio electronics and speakers, you are likely to get similar measurements in your own listening room – it’s all a function of room modes and there is no escape. I could certainly improve mine by moving the speakers/listening position around a bit more and adding acoustical treatments to the room. Ultimately, I could build a seperate critical listening room with more favorable "golden room ratios", but that isn't in the cards for me at this time. Side note, to be sure, if you ever have the opportunity to build a room, get the golden rule ratios as it does make a fundamental difference in the low end.
If I look at the DRC “calibrated” frequency response above and not taking into account the slope of the B&K house curve, I get +-3 db from about 34Hz to almost 20Khz. Now that is about the same frequency response specification I get from the manufacturer, when they measure in an anechoic chamber.
Effectively, what this means is the DRC is not only doing its job by eliminating or minimizing the room acoustics, it is also applying (i.e. calibrated by) the B&K house curve which renders the right tonal quality (i.e. timbre) at the listening position. This is even without the most basic of acoustic room treatments. I could use a carpet on the hardwood floor in front of the speakers. Note how closely the left and right curves match each other. This means that we get a solid dead center phantom image produced by the speakers as any amplitude imbalance of the electronics/speakers/room interface has been calibrated by the DRC.
I have been using Audiolense (and the DRC filter it produces) in my audiophile system for about 6 months. I have not had any issues with the digital filters or any other critical listening artifacts arising from their use.
I look at DRC as a must have to fully realize your audio system investment. Every attention to detail and calibration throughout the audio chain will pay off in the end. However, given that modern day electronics can have frequency responses from 10Hz to 100Khz with +- .25db variations, the speaker to room interface is the biggest variation, by far, on the quality of sound (read: timbre) than any other component in the audio chain.
You are not locked into the listening position using frequency DRC. You can walk anywhere around your listening room and notice the major improvement in sound quality. As mentioned earlier in my first post, my wife commented on how the recorded piano I was listening to still sounded real (i.e. correct tone quality) when she was out in the attached garage. Correct timbre, once you hear it, you won’t go back.
Or at least that has been my case. I am still blown away listening to my studio mixes from many years ago that I produced in a multi-million dollar recording facility and hearing it sound “identical” in my home. I would suggest that with DRC, and the B&K house curve, you hear music reproduced as close as possible to exactly what the mastering/mixing engineer and producer wanted you to hear, with the proper timbre. With the advent of 96/24 resolution recordings, you are hearing as close to the master tape as possible. It is great to be a computer audiophile!
We can get even more sonic improvements. In the case of Audiolense, we can achieve time domain correction in addition to frequency correction. What does this mean? We will look at this in my next post.