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Discussion Starter #1 (Edited)
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.

Equipment

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
Playback
  • USB 2.0: Stereo/Surround: Up to 24-bit/96kHz
  • USB 1.1: Stereo: 24-bit/96kHz Surround: 16-bit/48kHz
Record
  • Up to 24-bit/96kHz
Control
  • Volume Control
Connectivity
  • 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
>120dBSPL

  • 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 
dB/oF
  • 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


Methodology


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.

Spectrogram

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.



Waterfall
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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Truly amazing piece of work Dale. :TT

Although the neighbors probably think you're :coocoo:
 

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Curious how you will test down to 10 Hz if the microphone's accuracy is given as 20 - 20.000 Hz +- 2dB. How will you test down to 10 Hz (@-3dB)? Maybe I just don't fully understand the accuracy and Hz thing but if it is out of the realm of the test gear then it seems a stretch.
 

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Discussion Starter #7
Hey Scott, thanks for the question. One of the reason's I am comparing and publishing the manufacturers specs and graphs is to provide a direct correlation to the data that I am collecting. But you bring up a very good point and I need to spell it out better in the methodology, I really appreciate it and please let me know if you see anything else that needs better definition or explanation. Much appreciated!
 

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What he is showing is the manufacturer's stated response, however IBF includes a calibration file so that it is flat to 10Hz. He can upload his Microphone Frequency and Phase Response Measurement Report that he got with his mic. Here is what mine looks like for my IBF mic:

 

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Hi Dale, I had responded to your SVS sub post, thinking that the responses do not look correct to me. Sorry if my "tone of voice" seemed harsh in those posts. It appears to me, as I reread them, they appear curt. (insert "sorry" smilie icon)

Anywho... myself and others appreciate your efforts, I am sure. :T:hsd:

I do have some thoughts and questions though, to help in "our" pursuit of measuring accuracy and its implications.

Since we are using REW and REW has its standards, those standards must be adheard to. The reason for my insistance on your graphs are so myself and others can directly compare and collaborate what we are measuring with what we are hearing.

You should attach the mdat with all measurements.

You should start out measuring at 75db, the REW standard, and then increase your measurements 5 or 10 db till compression sets in. I myself never measure past 75db, so I would need to compare my graps directly to yours at the same level.

Also you should post the SPL level that you are measuring at for each graph. For instance looking at the Emotiva SPL graph what is its measured SPL level? Where would I put my line looking at that graph? IE 100 db or 95db, it makes a difference.

Which SPL meter are you using?

Since you do review the subs giving descriptions of the woofers sound characteristics should you also not give the measured freq response in the listening room along with the measured response of the entire system as you listen too it?:hsd:


Thanks,
Bob
PHP143
 

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Discussion Starter #15
Hi Dale, I had responded to your SVS sub post, thinking that the responses do not look correct to me. Sorry if my "tone of voice" seemed harsh in those posts. It appears to me, as I reread them, they appear curt. (insert "sorry" smilie icon)

Anywho... myself and others appreciate your efforts, I am sure. :T:hsd:

I do have some thoughts and questions though, to help in "our" pursuit of measuring accuracy and its implications.

Since we are using REW and REW has its standards, those standards must be adheard to. The reason for my insistance on your graphs are so myself and others can directly compare and collaborate what we are measuring with what we are hearing.

You should attach the mdat with all measurements.

You should start out measuring at 75db, the REW standard, and then increase your measurements 5 or 10 db till compression sets in. I myself never measure past 75db, so I would need to compare my graps directly to yours at the same level.

Also you should post the SPL level that you are measuring at for each graph. For instance looking at the Emotiva SPL graph what is its measured SPL level? Where would I put my line looking at that graph? IE 100 db or 95db, it makes a difference.

Which SPL meter are you using?

Since you do review the subs giving descriptions of the woofers sound characteristics should you also not give the measured freq response in the listening room along with the measured response of the entire system as you listen too it?:hsd:


Thanks,
Bob
PHP143
"If the first 100db suck, why continue?
Hey Bob, first off I want to say thanks for your input on the SVS thread the other because it helped identify a consistency issue between all of the graphs and I have since gone back and reposted the measurements setting the same limitations on each so again, thank you a bunch it really helped us identify that issue.

Since we are using REW and REW has its standards, those standards must be adheard to. The reason for my insistance on your graphs are so myself and others can directly compare and collaborate what we are measuring with what we are hearing.

You should attach the mdat with all measurements.
While I am using REW as a tool for measuring, the emphasis should be on the methodology. 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.

You should start out measuring at 75db, the REW standard, and then increase your measurements 5 or 10 db till compression sets in. I myself never measure past 75db, so I would need to compare my graps directly to yours at the same level.
The purpose of the Sub Zone is to compare to manufacturer specifications and published measurements not general guidelines such as the 75db reference level.

Since you do review the subs giving descriptions of the woofers sound characteristics should you also not give the measured freq response in the listening room along with the measured response of the entire system as you listen too it?
I actually thought about this and you have convinced me that it would be helpful in the reviews to include this measurement so I will start doing that moving forward.

Thanks again for all of your input Bob, it has been very beneficial to the forum! :T
 

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I want to comment on a few of these as well...
You should attach the mdat with all measurements.
These are extremely large files that we really do not care to host. If Dale wants to offer them on a one on one basis, he is happy to do so.

You should start out measuring at 75db, the REW standard, and then increase your measurements 5 or 10 db till compression sets in. I myself never measure past 75db, so I would need to compare my graps directly to yours at the same level.
Are you measuring at 2 meters, outside in the same environment? This is the only way you can compare graphs.

75db is merely a recommendation... not necessarily a standard. I rarely use 75db when I measure because it is not where I listen. I measure at the level I listen to most.

Since you do review the subs giving descriptions of the woofers sound characteristics should you also not give the measured freq response in the listening room along with the measured response of the entire system as you listen too it?
Dale can include this, but I am not sure how helpful it will be due to the varying listening rooms, yet it might be interesting to see.

"If the first 100db suck, why continue?
This word is not allowed in the way you are using it... please refresh on our forum rules, thanks. :T
 

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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.

Erin
 

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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. :)
 

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Discussion Starter #19
Hi Erin, I can and need to flesh that out a bit more so I should have that up later today. Thanks for mentioning it as I had not noticed that I did not have that out there. Thanks!
 

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Hi Dale, you said "because it helped identify a consistency issue between all of the graphs and I have since gone back and reposted the measurements setting the same limitations on each so again, thank you a bunch it really helped us identify that issue".

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.

"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!

You did not answer my question of which SPL meter you are using, so if you would be so kind....


"I actually thought about this and you have convinced me that it would be helpful in the reviews to include this measurement so I will start doing that moving forward."

Of course it would, you are describing what you hear in your room, we need to see the actual in room freq resp to correlate what is measured with what is heard.

Thanks,
Bob
PHP143
 
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