Bass distortion on the C5 and C6 is surprisingly high.
The Swissonic A305 and A306 cost 1/4 as much as the Asci's yet beat them handily in bass (though not at higher frequencies):
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Is this a disadvantage of PRs instead of ports? Or just lower quality woofers?
Several interprets are needed to understand the data.
At first, the high bass distortion below about 50Hz is due to high excursion below the PR(port too) tuning frequency.
A305 and 306 have highpass filter to avoid increasing excursion under tuning frequency, as many of Active speakers do.
For example, this is a excursion sim of a ported box tuned at 35Hz.
Below the tuning frequency, woofer and Port air mass (or PR) have same mechanical polarity. It makes the woofer move much higher excursion, While acoustical polarity has inverted each other so that make the slope 24dB / oct.
So high excursion / low SPL output make much high distortion below tuning frequency.
It is very common for passive speakers that is hard to have highpass filter by passive network at such a low frequency.
Then how about adding 2nd order highpass filter at tuning frequency?
It makes the excursion stable. Though the slope is 36dB/oct, but unnecessary excursion and distortion are eliminated. (A306 and A305 have this filter)
This is the main reason of difference distortion level below tuning frequency.
And second,
Around 50~200Hz, yes, our speaker has higher 2nd HD. But it is rather benign.
2nd HD at low frequency is mostly caused by asymmetric behavior. In this case, It is caused by Kms(x).
Let's see LSI measurements of SB17CAC35-4 that we are using.
These images from Erin's Audio Corner. Thanks to Erin.
SB Acoustics SB17CAC35-4 6 Inch Ceramic Midwoofer Review
www.erinsaudiocorner.com
C6B woofer excursion at 2.83V.
and 85dB SPL distortion of C6B, similar level of voltage.
Interpreting the measurements, Kms(x) symmetric range is much less than Bl(x). and also we can see X C is the main factor that limits the excursion.
(And it can prevent Bl droop causing great IMD over the whole frequency)
Due to less symmetric range and short excursion limit by suspension, Yes, the bass distortion rises faster than Swissonic.
But the reason why I said it is rather benign is Kms(x) causes bass distortion only.
(
https://www.klippel.de/know-how/measurements/nonlinear-distortion/multi-tone-distortion.html)
Distortion from Kms(x) doesn't make IMD around mid to high frequency, only at bass response, While Bl(x) and Le(x) causing lot of AM distortion over the mid to high frequency.
I don't know how the measurements are of Swissonic woofer, but high level of 3rd HD(magnetic hysteresis maybe) over the mid frequency rather mean it doesn't have stable Le(x) curve. And it can occur AM distortion at mid to high frequency though the bass distortion is low. (AM distortion is much easy to be audible)
For summary,
C5B and C6B bass distortion below tuning frequency is common for passive speakers using port or PR.
50~200Hz high bass distortion is rather benign due to not make IMD over the mid to high frequency.
If you use subwoofer with C5B and C6B, their potential is very promising.
References.
purifi-audio.com
purifi-audio.com
purifi-audio.com
Putzeys, Dec. 2019
purifi-audio.com
SB Acoustics SB17CAC35-4 6 Inch Ceramic Midwoofer Review
www.erinsaudiocorner.com
Thomann Swissonic A306 2-Way Bookshelf Speaker Review
www.erinsaudiocorner.com
www.klippel.de
