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Driver burn in facts
A loudspeaker burns in several ways. They burn in electrically and mechanically. The greatest amount of change in the sound of a speaker can be from the electrical burn in. A majority of this is from capacitors, but some of the electrical burn in has to do with the wire, and voice coils of the drivers. What I will be focusing on here is the mechanical burn in process and changes in actual driver parameters due to this process. Typical observations regarding speaker burn in involves an opening up or relaxing of the presentation. Vocals will appear less strained or congested. The sound smooths out. Bass response appears cleaner, tighter, less distorted, and even deeper. These same observations are reported day in and day out by people all over the world and is very consistent. Still there are those who claim that it is the listener that does the burning in as one gets used to the sound. Funny thing is that if a speaker has a bright, edgy, or fatiguing sound to it, you will be more fatigued the longer that you listen to it not less fatigued the longer you listen. On to some hard data: I pulled a fresh M-130 woofer randomly from our most recent shipment. I took three consecutive measurements (for consistency and accuracy) of the T/S parameters using the latest 7.01 Clio measuring system. I used the added mass system using 25 grams of added mass. The measurements were very consistent. These measurements were made on the woofer cold and with no play time and no stretching of the suspension. The suspension is still stiff in this form. MANUFACTURER GR MODEL M-130
We use this use this woofer in a .39 cubic foot enclosure with a 1.8" diameter port that is 4" long in our A/V-1 loudspeaker kit. Without any time on the woofer the numbers show that it will hit a -3db down point of only 61.9Hz. These figures will change after the first time the woofer is worked any at all as the second test will show. The second test involved slowly stretching the suspension all the way in and all the way out holding it briefly at the extremes. This does stretch of the suspension and causes some initial compliance change that will clearly be seen in the measurements but it in no way breaks in the driver. There has been no electrical change made. I made three sets of these measurements to in order to see consistency. MANUFACTURER GR MODEL M-130
Note that the Fs dropped by over 2.5Hz. The Vas increased, and the Q factors dropped. Typically these figures off set each other and have a marginal effect in optimal box volume. The -3db down point dropped to 61Hz. For the third test we ran the woofer hard for 10 seconds with a 50Hz sine wave. Many claim that this is all that is needed to break a woofer in and that no more change occurs beyond this point. This is far from the truth. With this test though we have changed it in two ways. We loosened up the suspension (very slightly) and we briefly heated up the voice coil. See measured parameters: MANUFACTURER GR MODEL M-130
Most of the changes we see from this test are due to the slight amount of heat we put in the voice coil. Fs dropped a little more. Vas was unchanged. Q factors dropped slightly. -3db down point will now be 59.8Hz. Next I let the woofer cool and took the measurements again. MANUFACTURER GR MODEL M-130
We can see that the 10 seconds of hard play with the 50Hz sine wave had no lasting effect on the compliance. The Fs went back up. The Vas stayed relatively the same, and the Q factors went back up slightly. By the numbers this one will hit a -3db of 62Hz. Next I played the woofer hard for 20 hours with the same 50Hz sine wave. This is about 1/2 to 1/3rd of the needed burn in time, but enough to make considerable differences in the way the woofer will sound. Note the considerable difference in the measured parameters. These were also taken right after the hard play so there was still some heat involved. MANUFACTURER GR MODEL M-130
Note that there has been a considerable drop in Fs. Vas has increased, and the Q factors are considerably down as well. Again one set of numbers will counter the other to a large degree in maintaining a similar low end extension. Still the -3db down point is down to 57.2Hz by the numbers. Lastly I allowed the woofer to cool for several hours and then ran the numbers again. Clearly there is a marginal change in the parameters as it has cooled down but much of the change has been maintained. Parameters did not revert back to figures prior to burn in. MANUFACTURER GR MODEL M-130
These changes are consistent with every other time that I have measured a drivers parameters before and after burn in. This woofer still has a way to go though and a slight amount of change still ahead. 20 hours is not enough to reach a settling point. Since this woofer was first pulled from the box the Fs has dropped by over 6Hz. Vas has increased by over 4 liters, and the Qts has dropped from .3581 to .3141. Now I wonder if the woofer burned in or if it was my test equipment?
Dan Wiggins of Adire Audio responds. Dan is a well known driver designer responsible for designing the XBL^2 motor structure and holder of patent number 7,039,213 Danny, You're spot on with the mechanical changes. The biggest reason there is a break-in period for drivers is the spider. The spider - for those who don't know - is a piece of cloth, permanent pressed, and dipped in epoxy. When you break in a driver, you introduce micro-cracks throughout the epoxy, which will make the spider softer. ALL mechanical systems will wear and all mechanical springs will get softer; in this case, it's by design. You break/crack a lot of the epoxy bonds (phenolics are used as well) that permeate the spider, and thus it becomes softer. We often see a 20% drop in Fs over a lengthy break-in, and we quote numbers for drivers broken in. Out of the box all our drivers measure high; beat on them for 40-50 hours, though, and they will be permanently lowered (Fs, that is). One thing to note is that it would be expected Fs would drop, Qes and Qts would drop, and Vas would increase; all these are exactly what happens when you raise Cms, which is the same thing as making the spider softer. Dan Wiggins 7-14-06
Paul Roth of Credence Speakers Inc. responds. Paul is a design engineer at Credence. http://www.credencespeakers.com/ They have been in business since 1976. Paul pointed out to me that on the measurements made right after running it hard for 20 hours, I either failed to show the slight changes in Re (changes from being hot) or I rounded back to the first decimal and did not change it. This is likely one of the reasons for the slight changes in Mms and BL on that measurement. A smaller but similar effect can be seen when I tested the woofer after running it hard for 10 seconds. Once cooled those numbers shifted back. He said that he usually allows 10 to 12 hours of cool down time before running T/S parameters following a break in period. Here is his response that he said I could quote: At any rate, excepting the tests run right after the break-in, where I question the numbers, esp. Mms, your data looks ok to me & consistent with THOUSANDS of measurements I have done. Anyone who thinks a woofer's parameters don't change due to break-in / burn-in is either ignoring the data, has lousy data, or is not doing much of a break-in. Granted that the effect on the in-box measured curve, due to break-in / burn-in, may or may not be significant, depending on the box. Paul Roth 7-17-06
New data now after 40 hours of burn in time, measured on 7-18-06. First here they are right after burning it in and still hot. MANUFACTURER GR MODEL M-130
We can see that heat still has a big impact on the parameters. Fs is down to below 47Hz. VAS is high and Q factors are low. Next I let it cool for several hours so it could return back to room temperature. MANUFACTURER GR MODEL M-130
We can see that the figures shifted back after allowing to cool, but that the additional burn in time still continued to change the compliance. Fs at 20 hours cold was 50.47 and it is now 49.90. VAS at 20 hours cold was 18.64 liters and is now 19.99 liters. Qts at 20 hours cold was .3141 and is now .2996. As noted also the greatest amount of change will take place in the beginning of the burn in period and gradually gets less and less, finally reaching a point at which it stabilizes and shows little change at all from continued use. Most all manufacturers recommend 40 to 50 hours of burn in time to reach an acceptable level of stabilization.
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Recently there
has been some debate regarding speaker burn in. Some even go so
far as to not only question this reality but to even refute it.