Tuesday, October 9, 2012

Subwoofer Tuning

So you buy or build a subwoofer, hook it up to your system, and viola! Deep bass and a lot of it - at least that's what your first impression is. Explosions in films sound more real, punches to the face are chest pounding, and emotionally it is a great addition.  Where there once was nothing, not there is something...right? Well, here is where a few simple tools can help you figure out if what your emotions are telling you is what your ears are hearing.
Confirming or denying that what you just added made the change you wanted can take hours of listening during which time you can get more and more satisfied with your system's overall sound.  But there is a short cut for doing this that can reduce the time you spend moving the subwoofer around in the room until you find that perfect spot. There are two things you will need to do this:
  1. A smart phone
  2. An MP3 player (most Blu-ray and DVD players will suffice).
First, go to http://archive.org/details/TenMinutesOfWhiteNoisePinkNoiseAndBrownianNoise and download the Pink Noise file. Put this file on a CD, memory stick, or your streaming music resource.  This file creates what sounds like inter-station noise on an old FM radio and is a random but uniform signal source spattering the entire audio band. I use the FLAC version since my music streamer plays FLAC files.
Next, spend the $5.50 and download the RTA Pro app for your smart phone from the PLAY STORE or from https://play.google.com/store/apps/details?id=radonsoft.net.rtapro&hl=en. The free version works but the paid version offers far better resolution and is worth every penny. Calibrate your phone according to the instructions in a VERY quiet room.
Now, charge your phone and set it on top of a thin vase positioned in your "sweet spot" at your ear level with the microphone end of the phone aimed at your system. Unplug or turn off all electrical appliances (refrigerator, TV, radio, etc.) making your room as quiet as possible. I do this VERY early in the morning to avoid traffic noises and my wife talking to me.
Turn on the phone, start the app, and play the Pink Noise file. Adjust the volume on your system so that the level is somewhere near -10dBFS.  Press STOP on the app and then lightly press START being careful not to jar the phone. You will see now what your playback system is producing in the way of acoustic energy as measured from that particular location in your room.  Notice the deep bass level under 200Hz.
I can almost guarantee that the sound pressure will not be uniform. Using this tool, scoot the subwoofer around the room until an optimum location is found. Use movements of inches first, then fractions of inches, and once the right place is found, the sound will be about as uniform as you can get it.
You can try raising and lowering the subwoofer itself if possible to get that last bit of tuning out of it as you can but more than likely you will not be able to raise or lower it - but if you can, give it a shot too.
Lastly, if there are large peaks somewhere, try moving furniture and adding sound absorbing panels on the midlines of the walls of the room or in the corners. This is where sound soak works best and tweak away to your heart's content. 
I have used this little technique to do a lot of tweaking in my personal system and the results for a total investment of $6 and a bit of time have yielded extremely satisfying results, at least in the bass and deep bass regions.
Although you could use this setup to position your speakers, I have found that this is not the right way to do that. Using your ears and focusing on the height, width, and depth of the sound stage is more reliable and produces far more satisfying results.
Yours for higher fidelity,
Philip Rastocny

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