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About The Sub Zone: Testing Methodology and Environment

13707 Views 36 Replies 13 Participants Last post by  AudiocRaver
Welcome to The Sub Zone​

The Sub Zone is the place to get accurate, data driven test results on how the many subwoofer models compare to the manufacturer's specifications. Additionally a majority, if not all, of the tests will be coupled with a subjective listening review and impressions, which we hope will give the reader a much more complete picture of a particular subwoofer.

The Sub Zone Testing Methodology

This particular thread is to serve as the single source for all questions with regard to The Sub Zone testing methodology. This is the information I refer to before and during my testing. If there are any changes to the environment or methodology, it will be reflected here as well as in the results thread in which a change occurred.

Testing Environment

Tests for The Sub Zone subwoofer testing forum will be conducted outside in an open area with the nearest reflection point being no closer than 35 feet away. I will be measuring frequencies down to 0 Hz and up to 200 Hz. This obviously falls outside of all manufacturer specifications, however, it creates a level playing field for measuring against all manufacturer specifications and will provide more consistent results. This is not a ‘shootout’ or any other competition between subwoofer manufacturers. I will do my best to provide the data that directly compares a subwoofer’s results with the stated manufacturer specs.

This is The Sub Zone​

There are obviously many concerns when performing these tests outside such as ambient and random background noise, trains, planes and yes automobiles. Weather can also play a large factor when testing subwoofers outside. Wind, temperature and barometric pressure are all things that must be taken into account when setting up these tests and pre-test checks are absolutely critical to getting consistently accurate results.

A majority the subs being tested will be accompanied with a subjective listening review in order to paint a complete picture of a given subwoofers potential. The Sub Zone tests will not include any of the information in those reviews however, the reviews will include a section for measured results both outside and in-room.


I will be using the following equipment to conduct all tests. As I update or replace equipment, I will re-visit this thread to update those changes.

Computer: Dell Latitude 6400 Running Windows 7 64-bit Operating System.

Soundcard: Creative X-Fi 5.1 External/USB

X-Fi Specifications
  • USB 2.0: Stereo/Surround: Up to 24-bit/96kHz
  • USB 1.1: Stereo: 24-bit/96kHz Surround: 16-bit/48kHz

  • Up to 24-bit/96kHz

  • Volume Control

  • Mic In/Line In
  • Headphone/Front Left & Right Speaker Output
  • Center/Sub Output
  • Rear Left & Right Speaker Output
  • Optical Digital Output
  • USB

Microphone: IBF-Akustik EMM-8
The IBF-EMM8 electret microphone utilizes a small, accurate omni directional capsule housed in an aluminum body. The EMM8 connects to the MP-r1 using a 50’ shielded digital coaxial cable, which provides bias power to the EMM8 with a minimum current of 0.5 mA.

EMM8 Specifications
  • On Axis Frequency Response - 20 - 20.000 Hz +- 2dB
  • Polar Pattern (Directivity) - omnidirectional
  • Sensitivity - 6mV/Pa/[email protected] 2.5V,2.2k Ohm
  • Power Requirements - 1.5 - 10V / approx. 0.5 mA

  • Output Connector - RF phono jack

  • Noise - S/N ratio >58 dB
  • Dimensions - probe dia : approx. 5/16", 8 mm length : 12 3/8" , 315 mm
max dia : 25/32" , 20 mm
  • Weight - 1 1/2 oz, 42 grams (without clamp)
  • Calibration - individually calibrated. The calibration microphone and preamp for this is the Bruel & Kjaer
 condensor B&K 4133 (1/2" free-field) and B&K 2639 preamp.

Pre-Amp: IBF-Akustik MP-1r
The IBF-MP1r is a portable microphone preamplifier that uses bias supply voltage for electret microphones. The unit was designed for 20Hz to 20kHz audio bandwidth operation and specifically for speaker and room acoustic measurements.

MP-1r Specification
  • Number of preamplifier channels - 1 Mic
  • Connectors - RF phono jacks for both inputs and outputs
  • Input - unbalanced, capacitor coupled, input impedance >2kOhm

  • Output - unbalanced, capacitor coupled, min load 2kOhm, short circuit protected, guaranteed output level without clipping +3dBV (full battery) Note: 0dBV=1Vrms
  • Power output – 3.5mm connector, low impedance drive capability (ideal for driving long cables and head/earphone), reduced output level -5dB

  • Gain Range - up to 52 dB input to output

  • Frequency Response - 20 Hz - 20 kHz, +-0.2 dB (relative to 1 kHz)

  • Output Clipping Level - 4Vpp (2 k• load)

  • Clipping indicator LED - Red indicates clipping short attack time, long release time. Bipolar detection

  • THD < 0,02%
  • THD & Noise < 0.04%
  • SNR > 70 dB Bias Supply - typ. 2.5V coupled via 2.7kOhm resistor
  • Power - 1x 9V battery, plug-in power supply connector
  • Power LED - Green indicates power ON, blinking LED indicates low batt. (Vbatt < 7V)

  • Acoustic REF LED - 94dB acoustic reference LED (off / bright / weak indicates < = > 94__1 dBspl)

  • -10 dBV LED – Consumer Level reference LED (off / bright / weak indicates < = > 10__2 dBV)

  • Polarity - Mic input to line output is non-inverting.
  • Operating Temperature Range - 
0 to 40 degrees Celsius (32 to 104 degrees F)
  • Power up delay – approx. 10 sec

  • Dimensions
 - 34 mm x 68 mm x 120 mm (h x w x d) (1 11/32" x 2 11/16" x 4 23/32")

  • Weight - 120 grams (4 1/8 oz). Battery removed

Calibrator: IBF-Akustik SC-1
The SC-1 is a single frequency dual amplitude, self-contained field calibrator. The unit generates a 1kHz reference tone at 94 and 110 dB SPL.

SC-1 Specifications
  • Output Frequency - 1kHz ± 0.2% (crystal stabilized)
  • Output Amplitude - selectable 94dB / 110dB
  • Accuracy - ± 0.5dB @ 155oF and 760mm Hg
  • Temperature - 90oF to 230oF operating,14oF to 
266oF storage (battery removed)
  • Temperature drift Coefficient of SPL - 0 to -0.012 
  • Temperature Correction - Sound-Pressure-Level of 
the Calibrator decreases about 0.1dB per +8oF temperature change. e.g.: at 203oF SPL is about 109.4dB (93.4dB) at 138oF SPL is about 110.2dB (94.2dB)
  • Humidity - 5 to 95% relative humidity
  • Power supply - 9V standard transistor battery 
(Low Batt control)
  • Construction - Solid state integrated circuitry in 
aluminum/plastic housing
  • Size and Weight - dia 1.57", length 5.24", 5 3/4oz (without battery)

Software: REW


Frequency Response (FR)
Frequency response is measured utilizing a 512K (11.9s) sine sweep from 0 Hz to 200 Hz via REW (Room Equalization Wizard). The microphone is placed on the ground at a 2 meter distance measured from the center of the subwoofer driver. In the event a particular subwoofer requires a deviation from this it will be noted within that particular set of measurements. For example, if a ported/vented sub with multiple drivers requires different mic placement in order to capture accurate results, it will be spelled out within that particular subwoofer test.

A measure of how accurately a system reproduces different frequencies. In the case of audio in a home theater system, it is desirable for the frequency response of a whole system, including speakers and subwoofer, to be from 10 Hertz to 20,000 Hertz ±3dB. This performance requires a very expensive system indeed and, in practice, very few systems will produce bass down to anything like that bottom limit. Manufacturers who claim a frequency response for speakers of, say, 20 to 20,000 Hertz without specifying decibel boundaries are telling you nothing. A tinny two inch transistor radio speaker can reproduce that range, although you won't actually hear it at either extreme because its output will be so low. Even subtle variations of less than half a decibel across the audio band can be quite audible, especially if they're spread over a fairly wide band of frequencies, and can thus change the character of the sound. Indeed, with speakers the single measure most closely related to their sound is the frequency response. (Home Theater Shack Glossary)

Max Output Before Compression
Max output before compression is measured using a 2M (23.8 second) sine sweep from 0 Hz to 200 Hz. The first sweep is taken after level matching 50Hz at 90db. The level is then raised by 5 db for each successive sweep until the output level is clearly compressed. The graph reflects the last sweep 'Before' compression is audible.

Group Delay
Group delay is a good indicator as to whether a subwoofer provides tight bass or a sloppier/looser type of bass. Higher numbers equals 'looser' sound.


The spectrogram takes a lot of complex information and presents it in an easy to read 'pretty picture'. For our purposes, we will be using it to determine how much additional response that a cabinet lends to the measurement. In other words, we are looking to see how much a particular cabinet 'colors' the sound or response of the driver by measuring the intensity of the vibration once the signal has stopped being generated. Keep in mind that this is measured in milliseconds and for our tests we will use 500 milliseconds as a baseline/expected response.

To illustrate what I am talking about, let's look at the measurement below. Given the information presented,you can see that this particular sub suffers from about a 300 millisecond decay at 63Hz when the signal goes from 93db to 53db. As a reference, 300 milliseconds is about as long as it takes to blink your eyes.

The waterfall measurement will be taken from the frequency response measurement and will reflect 70db to 105db. This will eliminate the chances of floor noise which can be had starting at 45db from the wind.

Harmonic Distortion
Harmonic distortion, in very simplistic terms, is the addition of undesirable harmonically related tones to the fundamental frequency. Harmonic distortion by component is measured using different frequency intervals and correspond to the max output level sweeps. The frequency intervals measured are 32 Hz, 40 Hz, 50 Hz, 63 Hz and 80 Hz. Harmonic distortion should be as low as possible across all frequencies.
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Have you considered performing IMD testing? I'm kind of out on HD usefulness in these applications. Notably because it's a lower frequency driven measurement. Until you get to the point of breakup, (which will likely be outside the passband on a woofer (especially the smaller sized woofers)), HD matters little compared to higher frequency content. In some cases, the HD from breakup isn't even relevant because the issues that cause it are the audible distortion issues (ringing, high Q artifacts, etc). IMD will tell a better story for higher frequency distortion content and will show weaknesses of driver design higher in frequency, especially when inductance control isn't implemented. Even when this is outside the passband, it can still have audible negative effects.

Just making a case and asking a question. Not trying to assert a judgmental POV.

As far as these "standards", I say if its logical, who's to stop you from making your own. You (us/we) can't guarantee all manufacturers derive specs the same way. Some follow an IEC spec, some make it up as they go. Personally, there are a handful of companies who's stuff I don't see a need to test because their specs have been proven true numerous times. Of course, there are many others who I feel have downright lied in their spec or were too ignorant to understand their results and therefore mislead consumers. As long as you have logic on your side and are repeatable, do what you need to do.

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Do you mind providing more information on the compression testing? I may have some suggestions here based on what I'm seeing in your results and what you've written. :)
Thats great, we have to show consistency and that is why you should start measuring at 75db. That is what everyone else is holding to at REW forum, unless there is a house curve or for measuring for compression.
Why 75dB? I think it's more important to start somewhere where compression becomes a legitimate concern. If the driver is moderately efficient (mid 80's), compression isn't of concern until you're at least there. So, to me, 75dB testing is moot.

"While 75db is the default for REW it is hardly a standard per se. There has been a lot of emphasis over the years on 75db being a standard of some sort but it is really just a reference guideline for level setting speakers. "

Right, so stick with it! If you started at 75db then you could go up in 10 db increments and have this reapeatable for all of your measurements. Would this not make sense from a sciencetific/reference standard.

Looking at your graphs of the 5 woofers that you measured there are no target lines. If you are not measuring at 75db you then need to state the target level for the measurenent taken. IE: on the graph for the Emotiva where would you place the target level? And in the graph for the SVS is the target level 116db? If so you missed it by 20db!
I have a very fundamental issue with the talk around compression testing here. What's catching me off guard here is people saying to test compression by increasing the output to achieve 10dB steps. This is flawed. Compression shouldn't be solely focused on the FR. Compression is a measurement of input vs output. It is not a measurement of response as you increase the output itself. For example, if you increase the input voltage at a speaker's terminals then you should get that same relational value in dB output by the DUT. Anything less is due to the effect of compression.

Yes, you are looking to see how the response changes but what's the point if a driver is so inefficient at higher output that you're having to feed it twice as much as you should for the same output?

Consider this: What happens if you're increasing SPL by 10dB but your voltage ratio from your previous input voltage of 4v to your new voltage input, which should be 12.6 volts, is actually 14.6v? All you've done is increase the output but you've failed to acknowledge the 2v loss in your test, which equates to about 14watts or 6dB! Yikes! So, yea, the curve at the reference frequency increased by 10dB, but it's not illustrating the fact that you just had to make up more than 6dB by turning the amp gain up higher and higher.

This is what compression testing should tell you. Again, voltage in vs SPL out vs what should be there. Then you get the FR curve, but most importantly, you get to see how efficiently the driver is able to use the power provided to it.

Hope that makes sense. Maybe my assumption on how this kind of testing is being performed is wrong so please correct me if so.

- Erin

Below I've attached a picture of compression testing I did on a Seas w18nx driver. This is the 20-110hz band. As long as the lines are stacked on top of each other, what you're seeing is what is expected; no loss in output vs voltage input. Where the lines deviate, is an indication of how much output (in dB) is lost with the input vs the initial voltage vs frequency. As you go higher in frequency in the chart, you can see a loss of about 0.4dB from 1v to 8v input.

This is the same thing but from 400hz to 6khz. You can see a loss of about 0.8dB at 3800hz. Likely due to inductance issues (yet to verify).

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I really do implore you to consider what I wrote above regarding the fundamental purpose of compression testing. Raising the output by 5dB doesn't really tell us anything about compression other than how the curve changes. That's fine... but that's only one (small) part of compression. We need a way to gauge input voltage for each step vs output SPL. You can do this pretty easily with online calculators. Just figure what each voltage input will be, in steps, and that should tell you what your output should be. Make a table with 6-7 stepped voltages, then the theoretical (what you should get, if compression were not inherent in design), and the real result. Then you'll see how much output in SPL is lost due to inefficiency at your reference frequcney of 50hz and you can still provide your FR curves so we can see how the overall curve is affected. It's not quite as pretty as the examples I gave above, but it'll suffice. Once I get set up, I will be providing the compression testing the same way as I did above. Flying in the face of apparent standard REW convention. lol.

Playing devil's advocate: As far as subjective vs objective, I actually agree. I'm more of a 'let the data talk' kind of guy. I stay away from subjective thoughts regarding sound; I freely talk subjectively on my likes/dislikes with a product interface, usability, etc. I do agree that when a subjective review is given, objective data should be provided to help the both the reviewer and the viewers (possibly) understand why your subjective analysis came out the way it did. Subwoofers are especially prone to placement and as we know placement will dictate response at the seated position. A simple 6 point spatial average of frequency response would suffice in objective/subjective correlation.

Just my $.02....

- Erin
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no worries. shoot me a PM and let me know when is best to call you and when/if I get a chance tonight I'll give you a shout. :)
I take back what I said about doing a spatial average for the subwoofer subjective analysis. If you're only concerned about the low end response, there's no reason to do a spatial average here... not unless the listening room is haunted by ghosts. the response is not going to vary here within a few inch window. If you're measuring multiple seats, it's a different story.

anyway, one simple FR capture with a LONG window (enough to allow fine resolution of resonance/modes) and a good CSD (waterfall) should help with your subjective to objective correlation if you choose to go this route. Personally, I'd say do the subjective, post the data of the driver in the 'ideal' situation as your baseline and then post the listening position response.
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