Monitor Mythology

[barefoot]

As some of you might have guessed from my posts, I'm heavily into loudspeakers. I play around with writing and mixing electronic music, but so far I'm mostly into this stuff from the equipment side of things. I don't have the experience or expertise to say what monitors are the easiest to mix with, nor from which ones the majority of people get the best results. I can, however, say a lot about the process of converting an electrical signal into an acoustic signal, and what needs to be done electronically, mechanically, and environmentally in order for that process to be as accurate as possible. From my experience and the many things I have read on this site and other sources there seems to be some basic myths about loudspeakers frequency response with regard to monitoring that I'd like to try and dispel. Here is one.


Myth: Frequency response is the ultimate measure of speaker accuracy.


Here's my own tall tale to help us think about the first:

Once upon a time I got done designing and building a beautiful new monitor and just needed to measure its response. I do my measurements outside in the back patio using the ground plane method. By the time I had everything meticulously set up it started to get dark. But it was spring and I knew the weather would be clear, so I just left everything outside until the next day when I could complete the measurement. In the mean time, however, a swarm of bees had flown into the port of my monitor and built a nest in there.

The next day I didn't even go outside to check the setup. My computer based measurement system is in the garage, everything is in place, so I just start to measure. Unbeknownst to me, when the test signal starts playing through the speakers the bees start to buzz like crazy. Buzzzz! Of course, the system is outside, so I'm blissfully unaware of what's happening. When I look at the response I see an unexpected +6db bump around 250Hz. I try a few different test signal levels, but the relative +6db bump remains. (Little do I know the louder the test signal gets, the more agitated the bees get, hence the louder they buzz.) Having confidence in my test equipment and seeing the response anomaly tracks the input level, I accept the data, and proceed to design a filter to compensate for the bump. Through skillful component selection I design an elegant –6dB notch filter which completely eliminates the bump. Measure the speakers again and I get a beautiful flat response of about +/- 2dB from 30Hz to 20kHz. Nice. Happy with my accomplishment and anxious to listen to my good work, I bring everything back inside and hook up the monitors.

I kick back, pop in my favorite ambient compilation CD on the Pork Records label, but I keep hearing this damned annoying buzz. Turn up the music and the buzz follows. Everything in the system checks out. Maybe the CD is bad? After all, I didbuy it used. So I put in my test CD and play the pink noise track. Still that buzz! Well, I did also buy that one used, and third time is a charm, so I pop in a fresh from the wrapper CD of Rage Against The Machine..........................................


While laying in the hospital bed for the next week I had time to ponder this question:

How was I able to filter out the sound of those bees and make a ruler flat response, yet still able to distinctly hear them later - even with the uniform pink noise signal?

The answer is because frequency response is a time integrated measurement. The temporal information which allows our ears to easily distinguish coherent sound events from the background, like bees buzzing under the noise or a soft conversation at a loud cocktail party, is completely lost. But it wouldn't have mattered if the buzzing had come from bees or a speaker cone resonance. Either way, a well placed filter could make the response look flat and hide it from our eyes, but it couldn't hide it from our ears.

Don't like this tale? Maybe I just still have a bug up my ass? But, the moral of this story isn't really so far from reality. Loudspeaker response peaks and dips are routinely filtered away. Most often they result from time dependent mechanisms which cannot simply be eliminated by a filter. Out of site is not necessarily out of mind.

barefoot

[tubedude]

Odd story...
still not as good as my Ozzy story though

Build me some monitors that match a good pair of Genelecs, and I'll test them for you indefinately, free of charge

Paul

[Gidge]

The use of crack cocaine is painfully prevalent in todays society.....