As a followup to the Wisconsin GTG, I just posted this on AVSForum and thought it might be of interest here:
Over the years, we have received a fair number of emails and phone calls from customers having issues with their speakers. After spending a good deal of time trying to troubleshoot the source of the individual problems, they most often turned out to be elsewhere in their systems. The following is a good example of what might be just this type of situation.
Last week we attended a speaker GTG in Wisconsin. Since a reviewer in Milwaukee had a pair of SoundScape 10's we had to pick up anyway, it seemed like a good opportunity to pick them up and take them to the GTG.
Long story short, when we played the SoundScape 10's at the GTG, listeners noted that the left speaker distorted terribly. When I listened, I was convinced there was a damaged midrange driver in that speaker and issued an apology to all those in attendance.
Naturally I forgot to mark which speaker had the damaged driver. So when we got back to the shop, I had to determine which driver needed replacement. Upon visual inspection, I expected to see a cracked cone but could not fine one. So I fired up the system and played them uncomfortably loud so as to expose the flawed driver. Again, I was unable to duplicate the issue.
Before I proceed further, allow me to provide a little background to help illustrate what I think lies at the heart of this situation.
About five years ago, a friend named Peter Smith conducted a very interesting test at Rocky Mountain Audiofest. He set up a system with both an average-reading meter (standard RMS VU meter like you would find on most audio equipment) and a peak-reading meter. He played the system with the VU meter registering an amplifier output of 5 - 8 watts. To everyone's surprise, the peak meter read as high as 250 watts during transient peaks in the music being reproduced. The bottom line is that if you were using a 100-watt amp to play back music at an average of 5 - 8 watts, it would be clipping during these transients. In may not be all that noticeable since many of these transients would be things like drum hits that are essentially noise anyway. None-the-less, clipping would be taking place.
OK, back to the damaged driver mystery. Why was I not able to re-create the issue with the damaged SoundScape 10 midrange? Perhaps the reason is that there was no damage.
Thinking back to the Wisconsin GTG, I looked up what I think were the monoblock amps being used. If I identified them correctly, they were 180 watts per channel.
At the event, each speaker was hooked up and measured so that they would be played at exactly the same SPL level. For the instrumental cuts, this seemed like a reasonable volume level. But with many of the vocal cuts, it seemed like the recorded level and/or level of compression in the tracks being played was much higher. Some of these cuts seemed too loud for my taste and louder than I think most people would choose to listen. So I would guess these cuts were probably played at an average of 10 - 15 watts RMS. Which means transients may have been hitting 350 watts or more. In other words, depending on the sensitivity of the speakers involved and the resulting power required to play them at the SPL levels involved, the amps could be clipping intermittently or even fairly regularly.
Most of the speakers played at the GTG were probably around 88db sensitive, but ranged anywhere from 84db (SoundScape 10's) to 98db (JTR). (The Seaton Catalysts were self-powered so they were not subject to any potential amp-related issues.)
Now realize that each 3db represents a doubling of power. So an 87db sensitive speaker will require twice the power to output the same SPL level as an 84db speaker. What's more, the deeper the speakers play, the more power is required to move more air at lower frequencies. So a speaker that plays down to 70Hz (JTR) will require less power than a speaker that plays down to 22Hz (SoundScape 10's) even if they were the same sensitivity.
Some listeners at the GTG sensed some compression with the vocals on most speakers at the event (which, again, tended to be about 88 - 90 db sensitive). Since a 98db sensitive speaker would require about 1/8th the power to output the same SPL as an 88db sensitive design, it is not unreasonable to assume you would not hear this same level of compression on the high sensitivity speaker if the issue was related to the amps and not to the speakers themselves.
The SoundScape 10's were designed to play very deep without a subwoofer. The trade off in order to achieve this bass extension is sensitivity. At 84db, they would require as much as 16 times the power of the 98db sensitive speaker to output the same SPL level.
Played at the relatively high SPL levels used at this GTG, there is a good possibility that the "damaged midrange" we were hearing was not related to the speaker itself, but to rather severe amplifier clipping.
The only thing wrong with this theory was that it only seemed to effect the left speaker. Certainly one would assume that if we were driving the amps to this level of clipping, we should have heard the problem in both speakers. Initially this had me stumped.
I emailed Nuance to run my theory by him. He emailed back today to say that he talked to TJHUB and was informed that the left channel amp was biased to output less power than the right channel amp. Bingo.
The bottom line is that if my theory is correct, the listening sessions tended to bias the results in favor of higher sensitivity speakers which require less power, to the detriment of lower sensitivity speakers which require more.
In the case of the SoundScapes, the clipping was severe enough to cause me to think the midrange was damaged (which does not appear to be the case).
Had I brought a pair of SoundScape 8's, there probably wouldn't have been an issue (outside of perhaps compressed sounding vocals just like most of the other speakers) since they are 87db sensitive. But in this case, it appears the amps used were simply under-powered for the SoundScape 10's and the results were catastrophic.
The amps I use are typically 250 - 350 watts per channel and I have never heard this level of clipping. So it was not something I expected.
I should point out that this is not an issue for TJHUB. His speakers (HT2-TL's) are 88db sensitive and he is off-loading much of the bass duties to his dual subwoofers. So the demand on his amps are not all that great and they are well-suited to the task at hand.
The bottom line is that if you are having issues with your speakers, keep in mind that the problem may not be related to the speakers themselves. We discover this all the time with our customers and now I have experienced the same thing myself.
All that said, we still had a great time and I want to take this opportunity to thank TJHUB and NUANCE for a great event.
- Jim
