Break-In, Part Deux
Many of you will remember my previous attempt at analyzing break-in from this post. A lot of good comments and suggestions were made after the post, and I'd encourage those not familiar with this previous attempt to check it out.
Deciding that my time away at T.H.E. Show Newport would be the perfect opportunity to run some more break-in tests, I programmed my headphone measurement system to take frequency response, square wave, impulse response, and Total Harmonic Distortion data every hour for 100 hours.
Man! That's a lot of numbers ... I'm glad I have a computer.
I had some unfortunate glitches on my computer that resulted in incomplete tests subsequent to the first experiment, but I did get enough data in these preparatory tests to suggest that, indeed, the headphone pads may be settling onto the head for the first day or two as was suggested by xnor, IIRC. I also came to the conclusion that, however slim, there was a chance that the voice coils were suffering from heat build-up during the previous experiment as pink noise was played continuously right up until the time of each testing sequence. I knew this was going to be a sensitive test, and that I really had to rule out as many things as possible.
In preparing for this round of testing, I placed the headphones on the head 40 hours before beginning the test to let them settle in. I also wrote into the testing program 15 minutes of silence before each test so that the headphones had some time to cool off before the measurement sequence began.
The headphones used were a brand new, fresh out of the box pair of black AKG Quincy Jones Q701 headphones --- a headphone believed by headphone enthusiasts to require lots of break-in.
The test itself was programmed to run for 100 cycles. During each cycle, the system would measure frequency response, 30Hz and 300Hz square wave response, impulse response, and THD+noise spectra. Then the system would play pink noise into the headphones at about 90dB SPL, followed by 15 minutes of silence, and then would run through the cycle again. The complete test took 125 hours ... just over 5 days.
Not wanting to get complaints about "fruit salad" multi-color graphs again, this time I crunched the numbers on my corporate MacBook Pro that has a current copy of Excel so that I could display the results on 3D surface graphs. Oooooo ... so cool! Here we go!
In the graph above, the frequency spectrum goes from 10Hz to 22kHz in 511 steps which you see on the axis to the right labeled 1 through 481. The series of 100 hourly measurements run right to left, and are on the axis closest to you labeled "Series 1" and "Series 51." The magnitude of the vertical scale is in dB SPL of difference and is +/- 2dB maximum.
The first thing to note is that virtually all data (with the exception of some of the highest frequencies) is within 1/2 dB of the last measurement. Really, I couldn't be more pleased with the performance of the system being able to record so accurately and noiselessly the trends observed.
Both charts show remarkable similarity between the left and right channel over the course of the test. The charts show some wavy motion over time, I suspect this is temperature differences between night and day over the course of the test, and I doubt it's actually a function of something actually changing in the headphones themselves.
At the low end of the audio spectrum (closest to you) you can see that the bass rises over time, and then falls fairly rapidly at about hour 70.
The set of ridges at about 150 on the right scale is the primary resonance of the driver. You'll notice for about the first 20 hours there are small ripples, then between about hour 20 to hour 70 the features get larger, and after hour 70 they seem to settle out. Also note the features between roughly 250 and 450 in the upper mid-range and lower treble likewise get stronger during 20 to 70 hour period of time.
In both channels, data features appear in bands: 0 to 20 hours is fairly settled; data between 20 and 70 hours is bumpy; then during the last 20 hours of the test seems to settle down quite a bit.
The differences in impulse response are also remarkably similar between the left and right channel. Notice again the characteristic nature of the data in the bands from hour 0 to 20, hour 20 to 70, and the last 30 hours.
Unfortunately, I had a problem with the right channel data, but it looked fairly similar to the left channel shown above. The only things of note here (to me) are the dip in the distortion in the lows between 20 and 70 hours, and the elevated amounts of distortion between samples 200 and 350 in the last 30 hours. (The line of spikes at about sample 380 is a measurement artifact of the range change relays in the Audio Precision.)
The extra distortion features in the last 30 hours of the test lead me to believe that further burn-in testing might be required to see if these artifacts settle out over time.
Well ... do we see something burning in here? I don't know. What do you think?
It certainly does seem like something is happening between hours 20 and 70. I haven't opened the chamber and taken the headphones off the dummy yet, so I really think I need to run the test for another 100 hours to see what happens. That way we can compare these data with a new set of graphs. It's going to tie up my headphone testing for a while, but I think it's worth it. I'm gonna go push the button.
I'm also going to defeat my automatic thermostat in the house to try to keep the temperature more constant over the test.
Lastly, I want to remind people of two things:
- The magnitude of the changes observed are very small. If the features seen are evidence of break-in, the effect is small. When you buy a pair of headphones and listen to them right out of the box, that's going to be pretty much what they sound like after a year.
- Check out "World Access for the Blind." This is an organization that teaches blind people how to echolocate their way through the world by clicking their tongue and listening for the echo. When they hear the return echo from their environment they can get a "picture" of the physical objects around them, and can navigate using it. These blind folks are riding bikes and playing basketball completely unsighted. I'm pretty sure this almost magical ability is indicative of the extraordinarily exquisite sensitivity and processing power of the human senses and brain. Just because the measured data of headphone break-in may be vanishingly small does not mean it can't be sensed.
Okay, what I'm really interested in is your comments. I got a lot of good ideas last time from them, and I like the feedback. So, if anything strikes you, pipe up, I'm interested --- even if it seems a little off the wall, throw it out there. You never know.
Oh yeah! The contest!
Last break-in article I offered up a pair of Q701s to the comment I found most interesting. This one kept ringing in my ears:
Unlike the headphones, I much enjoyed seeing you run pink noise through them for the sake of at least trying to get to the core of the burn-in argument.
The problem, unfortunately, is this is only one down. The curse this headphone has left is one you and your inbox will feel, as people start questioning whether their own headphones burn-in. "Do the HD800, HE6, LCD-2" they'll cry. The 701's will be long gone, but you will be stuck with requests locking you up in the measuring chamber for eternity.
Oh, and Gatepc forgot the main ingredient for boutique cables: virgin tears gathered on the top of the Himalayas . . . that's how they achieve "quantum tunneling" you know?
Okay buddy, you win. And yes, I know this adventure is going to keep tying up my measurement chamber ... such is life. Someone's got to do it, why not me?
Email on the way! Congrats buddy!