Tune A Room For Perfect Audio 27Dec19

Here I'll briefly describe how to "tune a room" for better audio... using only digital signal processing of source output signals with equalization filters. Here I don't consider altering the speakers or their placement, using wall treatments like bass traps, or even the audio source (except I assume it's digital and can be adjusted through equalization). This process is basically equivalent to a pro sound engineer's technique as he/she sets up any PA system in any venue... but one can simply apply it to home audio and theater systems and greatly improve "the sound." The simple tools I'll use are the donation-ware program REW and the $75 acoustic measurement mic miniDSP's Umik-1. The room/speaker setup I will adjust is the Nest study consisting simply of JRiver MC25 playing FLAC files from my MacBook via USB through a NAD digital amp to a pair of NHT SB1 bass-limited bookshelf speakers. This setup has been presented before.

Up front, I know deep bass is missing here and that, from simple listening, the whole low end is bloomy and distorted. It is the latter that I especially want to correct. First I connect the measurement mic to the Mac and REW and place it at my listening position - at the fly tying desk.

After assuring that REW is setup to capture measurement input on the Umik-1 and that my NAD amp is selected for output of test signals through the speakers, I shoot a simple 20-20K Hz freq sweep signal and capture it. I do that a couple times and then smooth the average of the responses. Next I load the desired "target" house curve - an ideal response is mostly flat, and somewhat declining throughout the 20-20K Hz range. I "line up" the house curve with my measured response. Yuck... see the weak sub-bass and see the blooming occurring in a number of spots in the 100-1.5K Hz range.

To adjust for these "problems" with my room and speakers, I use REW's EQ tool which "automatically" creates EQ filters. These will need to be applied to digital source material before it reaches the DAC in the NAD... one can alternatively apply the same filters to frequency bands of an analog signal on its way to amplification - but the digital domain is often easier and most of the audio sources we play these days is digital. Here's REW's EQ tool showing the filters and the resulting new, flatter, non-bloomy anticipated frequency response. The filters are actually a complex set of overlapping adjustments (as shown in the 2nd picture below) which mathematically combine to flatten the source material when played through the SB1s in the Nest study.

I then exported these filters as a text file. I am going to apply these filters through JRiver's parametric equalizer in the DSP section. Unfortunately there is no direct importer in JRiver so I have to type in all the filter adjustments and save the set as associated with Nest Study Listening. Of course I use entirely different DSP settings and adjustments when listening to headphones using JRiver on my Mac and there I employ a fully calibrated adjustment scheme via Sonarworks (this scheme has been presented previously). Unfortunately YouTube or Amazon Music output won't be equalized when played using this JRiver parametric equalizer because it is not applied system wide on my Mac. I could use a Mac sound plug-in like Boom 2 or Hear to apply equalization system-wide, and general solutions probably should do it that way but I really just use JRiver MC for playing music in the Nest Study through the SB1s.

The proof in the pudding is that the sound is so much sweeter and flatter and it's now much easier to listen for longer periods. Sure there are choices and tweaks one might want to make along the way here but in ~30 min I 1) measured, 2) analyzed and 3) adjusted to tune my room for the best audio the system can produce.