How to compare subwoofers.
In this section we will be discussing how to compare subwoofers in terms of all three performance metrics: output, compression, and distortion.
Currently, the only tester that we have found that measures output, distortion and compression is Data-Bass. There may be other third party testing companies that go this far, however we have not found them as of yet.
To accurately compare subwoofer systems on Data-Bass, you need to compare all the factors: not just CEA2010 burst Output. Often key elements of Distortion, and Compression are overlooked simply due to the complexity involved in correctly understanding these results.
To compare distortion levels at an actual output level, you cannot simply compare the same nominal sweep levels (ex. 120dB to 120dB) because that does not factor in response shape or compression. This leaves out critical information and assumes that the drivers have the same response curve, and the same compression levels. These assumptions skew result interpretations greatly and can make systems that are wildly different seem very comparable.
What this method does:
This is a way of normalizing the attribute of sound that you are interested in and seeing how the other attributes of sound affect it. By the end of this example you will be able to compare either on of the three aspects of sound (output, distortion, compression) to the other two, inclusive.
Essentially we are providing a way to qualify what you are hearing.
at A frequency at B dB SPL, we are hearing X distortion and Y compression
at A frequency at X distortion, we are hearing B dB SPL along with Y compression
at A frequency at Y compression, we are hearing X distortion at B dB SPL
The following set of instructions can help you understand and draw accurate comparisons from the massive amounts of data presented on Data-Bass, and in fact, the principals found within can be applied to any third party test that measures all three metrics of audio performance.
To compare two different subwoofers:
Get a pen and paper for making notes and open a spread sheet
Open two web browser windows
Go to Data-Bass in each browser
Select the systems tab and choose the two drivers you want to compare, one for each window
Scroll to the bottom of the page and select "Measurements"
Select "Extended Charts"
Scroll down to "Long Term Output Compression" sweep chart
Choose a frequency - I usually start at 10 Hz and work my way up from there
Pick the appropriate sweep where the output matches (or is close) between each subwoofer
Note that sweep and output
Now click on the "Multi-series Charts" tab
Look at the "Total Harmonic Distortion" sweep chart
Now compare each driver to their corresponding sweep at the frequency you have chosen
This will tell you the actual distortion as it relates to real dB output
Write the THD down as they correspond to each driver
Go back to "Extended Charts"
Now scroll down to "Compression Magnitude"
This will tell you how much the driver is compressing at the actual output, at your chosen frequency specific frequency
Write these compression figures down as well
Work your way through the frequency range to get a good idea of what each driver is actually doing
Now compare your chosen drivers
Reading the charts this way tells you a few things; distortion as a function of output, output as affected by compression, all at a predetermined frequency. dB @ xHz = y compression and z distortion
Lets do an example:
Open two web browsers and go to Data-Bass.com and select the "Systems" tab.
Select the GUJ18v1 sealed for browser 1
Select the Funk 18.0 Passive for browser 2
These two examples are Harbottle product to avoid the appearance of defamatory comparisons.
Select "Extended Charts"
Scroll to "Long Term Output Compression"
Look at the 20 Hz mark.
Look to see where they match output
In this case the 18.0 is putting out 103.4dB on the 120 dB sweep
The GUJ18v1 is putting out 104.5 on the 115 dB sweep
Now go to the "Multi-Series Charts" tab
Look at the "Total Harmonic Distortion" sweep chart.
18.0 will be on the 120 dB sweep and the GUJ will be on the 115 dB sweep, both at 20 Hz.
18.0 = 17.69% distortion @103.4 dB
GUJ18v1 = 16.8% distortion @ 104.5 dB
Now we can safely assume that at the same output level, the GUJ should produce about 2.5% less distortion at 20 Hz.
We can work the opposite way and say that at the same distortion level, the GUJ should produce about 2 dB more than the 18.0.
Go back to "Extended Charts"
Scroll down to "Compression Magnitude" and see how many dB you are losing due to compression.
Both systems are compressing around 0.5-1 dB at 20 Hz.
Repeat with all frequencies that you are interested in studying.
This method can be used to create your own graphs in any spreadsheet software for a visual representation of the data as it relates to including all audio performance metrics.
This method takes as many variables as possible into consideration, but not all things are equal. There is one aspect of sealed subwoofers that one must take into consideration when comparing systems, that is box size, materials, and construction methods, as this component affects distortion at a given output greatly.
One will notice after making comparisons with high performance systems, there is a correlation to how much output you can get with a certain level of distortion with a given enclosure size enclosure, dB @ THD @ CuFt. As the enclosure gets smaller, all else being equal, if you maintain output via adding power, distortion will go up. You can combat this with driver/system design however there is a limit, and eventually the air spring distortion of the box becomes the overwhelming factor in the resultant distortion. No amount of engineering can overcome this physical law.