curtis
09-09-2007, 11:20 AM
I thought it would be a good idea to post/reference a few posts Dave has made the last few days over at Ecoustics and AVS. Some very good reading and explanations:
First, this from Ecoustics in this thread:
http://forum.ecoustics.com/bbs/messages/1/382724.html
Hi Christopher,
You bring up a good point here but there are a few things worth mentioning.
First off, the graph that you refer to is 90dB spl at 2 meters in an anechoic chamber from a single loudspeaker. This is easily equivalent to about 100dB output from a single loudspeaker in-room at 1 meter away, or 103dB output from a pair of speakers. It is also a swept sine wave source, in other words, about as abusive to a loudspeaker as you can get - and not reflective of actual music listening. (far too loud for most people)
Additionally, the graph posted is misleading and requires explanation... Because these measurements are taken in a true anechoic chamber and this is a rear ported loudspeaker, bass response in the port range is not correctly reflected in the frequency response measurement. Much of the bass output of the port is actually absorbed in the wall treatments. THD measurement techniques at the NRC have also changed since the B&W CM1 was tested (about 3 or 4 speakers ago) They did not inform me exactly what changed so I am not sure if comparing newer graphs to older ones is valid.
If you look at any recent measurements of rear ported speakers, you will see that the low end response does not match manufacturers specifications (usually not even close). For example, look at the B&W CM1 (http://www.soundstagenetwork.com/measurements/speakers/bw_cm1/ (http://www.soundstagenetwork.com/measurements/speakers/bw_cm1/)) B&W's spec is -3dB at 55Hz while the graph indicates about -12, -13dB down at 55 Hz. A 10Hz difference. You will also notice that same huge jump in distortion at this frequency, the same with our Sierra-1 and also about the same level of discrepancy in the low frequency response compared to our published spec...
There is a very real explanation why the distortion measurement is so high at this frequency and why it increases so dramatically. As I am sure you know, as you approach the port tune frequency, less output is produced by the woofer and more is produced from the port. In fact, at port tune frequency, the woofer is barely even moving -- with almost 100% of output being produced from the port. As mentioned above, because this is a rear ported speaker and being tested in a true anechoic chamber (I believe tuned to about 50Hz) -- the majority of the output of the rear port is being absorbed in the 3-4 foot thick foam wedges behind the speaker and since there is barely any output from the woofer at all, all that is left to measure is pure noise (the graph is THD + noise). Try placing 5-6 pillows directly behind the port of a rear ported speaker --- where’d the bass go?
I have had a few discussions with some of the people over there regarding this, especially when it comes to including the output of the port in the frequency response measurements (simply a matter of turning the speaker around so the port faces the mic and then averaging this into the measurement) but their response was that since it is the same for all speakers, they are not willing to change. I don't agree with this though, as front ported speakers will provide better looking THD+noise graphs and deeper extension in the frequency response graph.
Here is a link to some deep bass torture tests of the Sierra-1 conducted by Craig Chase in a large room at 3 meters away:
http://img206.imageshack.us/my.php?image=sierra45hzox5.jpg (http://img206.imageshack.us/my.php?image=sierra45hzox5.jpg)
Craig measured 90dB of output at 45 Hz with 2nd order disortion at 7%. Which is much more reflective of how the speaker is truly performing.
While I am all for crossing over at around 60Hz and using a subwoofer, for music listening I think many (even most?) will find the bass of the Sierra-1 cleaner and tighter (more musical) than many subwoofers. I know I do, as do hundreds of Sierra-1 owners. Personally, I haven't used a subwoofer in quite some time. Although I felt a sub was missing when I watched 300 last night. Definitely consider adding a sub for home theater usage...
I am not here to "ruffle any feathers", but there is a reason those distortion measurements are high and they are certainly not reflective of actual performance. Note: The NHT classic 3 you mentioned is a sealed speaker so all bass response is being reproduced by the woofer in the front and the Paradigm Signature you mentioned is front ported (the majority of port output will reach the mic before it is absorbed by the wall treatments). I would say that the only fair way to compare distortion between all three of the speakers is in a frequency range where the port is producing minimal output, from 100Hz on up. Due to the measurement technique employed, distortion and response comparisons at frequencies where the port tube is contributing to output are simply not valid.
Believe me, I wish the NRC did things differently as I use there measurements as one point of reference for calibration of our own engineering gear...
Hope you find this useful...
First, this from Ecoustics in this thread:
http://forum.ecoustics.com/bbs/messages/1/382724.html
Hi Christopher,
You bring up a good point here but there are a few things worth mentioning.
First off, the graph that you refer to is 90dB spl at 2 meters in an anechoic chamber from a single loudspeaker. This is easily equivalent to about 100dB output from a single loudspeaker in-room at 1 meter away, or 103dB output from a pair of speakers. It is also a swept sine wave source, in other words, about as abusive to a loudspeaker as you can get - and not reflective of actual music listening. (far too loud for most people)
Additionally, the graph posted is misleading and requires explanation... Because these measurements are taken in a true anechoic chamber and this is a rear ported loudspeaker, bass response in the port range is not correctly reflected in the frequency response measurement. Much of the bass output of the port is actually absorbed in the wall treatments. THD measurement techniques at the NRC have also changed since the B&W CM1 was tested (about 3 or 4 speakers ago) They did not inform me exactly what changed so I am not sure if comparing newer graphs to older ones is valid.
If you look at any recent measurements of rear ported speakers, you will see that the low end response does not match manufacturers specifications (usually not even close). For example, look at the B&W CM1 (http://www.soundstagenetwork.com/measurements/speakers/bw_cm1/ (http://www.soundstagenetwork.com/measurements/speakers/bw_cm1/)) B&W's spec is -3dB at 55Hz while the graph indicates about -12, -13dB down at 55 Hz. A 10Hz difference. You will also notice that same huge jump in distortion at this frequency, the same with our Sierra-1 and also about the same level of discrepancy in the low frequency response compared to our published spec...
There is a very real explanation why the distortion measurement is so high at this frequency and why it increases so dramatically. As I am sure you know, as you approach the port tune frequency, less output is produced by the woofer and more is produced from the port. In fact, at port tune frequency, the woofer is barely even moving -- with almost 100% of output being produced from the port. As mentioned above, because this is a rear ported speaker and being tested in a true anechoic chamber (I believe tuned to about 50Hz) -- the majority of the output of the rear port is being absorbed in the 3-4 foot thick foam wedges behind the speaker and since there is barely any output from the woofer at all, all that is left to measure is pure noise (the graph is THD + noise). Try placing 5-6 pillows directly behind the port of a rear ported speaker --- where’d the bass go?
I have had a few discussions with some of the people over there regarding this, especially when it comes to including the output of the port in the frequency response measurements (simply a matter of turning the speaker around so the port faces the mic and then averaging this into the measurement) but their response was that since it is the same for all speakers, they are not willing to change. I don't agree with this though, as front ported speakers will provide better looking THD+noise graphs and deeper extension in the frequency response graph.
Here is a link to some deep bass torture tests of the Sierra-1 conducted by Craig Chase in a large room at 3 meters away:
http://img206.imageshack.us/my.php?image=sierra45hzox5.jpg (http://img206.imageshack.us/my.php?image=sierra45hzox5.jpg)
Craig measured 90dB of output at 45 Hz with 2nd order disortion at 7%. Which is much more reflective of how the speaker is truly performing.
While I am all for crossing over at around 60Hz and using a subwoofer, for music listening I think many (even most?) will find the bass of the Sierra-1 cleaner and tighter (more musical) than many subwoofers. I know I do, as do hundreds of Sierra-1 owners. Personally, I haven't used a subwoofer in quite some time. Although I felt a sub was missing when I watched 300 last night. Definitely consider adding a sub for home theater usage...
I am not here to "ruffle any feathers", but there is a reason those distortion measurements are high and they are certainly not reflective of actual performance. Note: The NHT classic 3 you mentioned is a sealed speaker so all bass response is being reproduced by the woofer in the front and the Paradigm Signature you mentioned is front ported (the majority of port output will reach the mic before it is absorbed by the wall treatments). I would say that the only fair way to compare distortion between all three of the speakers is in a frequency range where the port is producing minimal output, from 100Hz on up. Due to the measurement technique employed, distortion and response comparisons at frequencies where the port tube is contributing to output are simply not valid.
Believe me, I wish the NRC did things differently as I use there measurements as one point of reference for calibration of our own engineering gear...
Hope you find this useful...