Rythmik FV15HP Subwoofer Measurements and Analysis

The first part of testing was to measure the frequency response from a modest output, all the way to the point where the system was clearly compressing and had hit the output wall, so to speak.

Rythmik FV15HP Subwoofer  Frequency Response at Various Output Levels

The Rythmik sub was unique in one way during the compression testing vs frequency.  As you can see from the curve above, at the highest output level (the gray curve, as this sweep, 14 seconds long from 400 Hz to 10 Hz) triggered the sensor or compressor about the time we hit 80 Hz. As the signal continued to sweep lower, in a very short period of time, the compression released, allowing the signal to peak again right at about 60 Hz, and then before it got down to 50 Hz, the compressor becomes active again.  This happens AGAIN at about 32 Hz, and as you can see as the sweep runs lower, the compressor releases and then returns several times in quick succession.  Now it could be argued that music does not have any sweeping tones that range across more than a decade (400 to 40 Hz is one decade) and last 14 seconds in length.  They would be quite correct.  However, I cannot help but relate this to instances where I was listening to music and the output seemed to suddenly jump out as if the volume control was being rotated without any help from my fingers.  I would suggest that the use of a longer hold time or integration time be considered based not so much on the signal sweep test shown above, but on my own sense that the sub seemed to go from too quiet, to just too loud all on its own frequently during its operation.

As we can clearly see from the curves above, frequency response is NOT independent of level, and if the system is pushed hard enough, it will roll off below 30 Hz, yet at lower power outputs, is certainly flat down to 20 Hz. (Red curve) at the bottom.

Editorial Note about the Limiter Feature of the FV15HP Subwoofer

The limiter in FV15HP is defeatable. However, I had chosen to test the subwoofer with limiter ON.   The curves with limiter off should be smoother and without this short hold time issue I noted.  As for this hold time duration issue, it is a design trade-off. Shorter hold time can recover fast and provides more output over the same period of time.  On the other hand, my preference is to have longer hold time duration as I stated above.  I tested the H600PEQ amplifier with my review unit which engaged the limiter with the power switch set to "auto".  Since that time, the amp has been updated to the H600PEQ2 model with a discrete on/off switch for the limiter.

CEA-2010 Test Results

On to the CEA 2010 testing.  To understand the meaning behind the graphics below and testing methods used, please refer to the prior article (Subwoofer Shootout Measurements Overview) on this method. The process is a simple one.  Drive the input to the system to the point where either the amp, its compressor, or its speaker simply refuses to give you more output, or the distortion created at that output level exceeds the CEA “redline”.  In EVERY system tested, the result (if not the peak SPL) was similar in one regard.  At the highest frequencies in the sub's bandwidth, we ran out of amplifier power, or the compressor refused to put out more power than the amp could cleanly deliver.  At the lowest frequencies, the subwoofer runs out of excursion.  This is completely normal and what is to be expected.  It may also explain why the CEA standard does not bother testing higher than the center frequency of 63 Hz.   Below is a spectrum capture of the Rythmik FV15HP system at maximum usable output from 20 to 63 Hz in 1/3^rd octave steps.

Rythmik FV-15HP CEA-2010 Distortion Test Results

Max SPL Output Data of the Rythmik FV15HP Subwoofer (1 meter groundplane)

Update: 3/08/12

In order to make it easier for our readers to compare our CEA data between subwoofers tested prior to our new Subwoofer Measurement Protocol, we scaled our 1 meter peak CEA data to 2 meter RMS by subtracting 9dB for each frequency.  We also corrected our 1 meter data to peak by adding +3db from our first review which more closely matches the test data collected by Josh Ricci on the same review sample.  

Discussion of Measurements

What is noteworthy about this sub, is that the second order distortion components at the three lowest test frequencies, are extremely low.  Usually, 2^nd order distortion components are those which are the most prevalent, and arguably the least objectionable.   Removing them creates the largest measurable reduction in THD (in almost all circumstances) yet second order is the least objectionable of all the harmonics, hence the CEA tolerance of it is in excess of the tolerance for any other components.  (Music is, mostly harmonics after all.) Second order distortion at 20, 25 and 32 Hz were so low in fact that with the normal 1/12^th octave smoothing, the second harmonic is no where to be found in the spectrum curves.  It appears that this is where the servo-feedback excels and Rythmik's claims of low distortion are validated.   

The differences in outputs between 40, 50 and 63 Hz should be considered so small as to be negligible.  Any decrease from one frequency to the other is likely caused by a small amount of VC heating, such that if I had tested from higher to lower frequency, these three numbers would likely have been reversed.  In fact, the repeatability of these numbers should be considered no greater than within 1db.  While I am listing them to three significant figures, we need to remember that precision is not accuracy, and that any other speaker within a db of these numbers is likely to be close enough that we will not be able to reliably or consistently hear a difference.