Thanks guys, it will be a work in progress for at least a little while I get a few things ironed out but I am having a blast with it for sure!
About The Sub Zone: Testing Methodology and Environment
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
Record
Control
Connectivity
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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Wow Erin, that is a great example! Thank you for contributing your knowledge to the conversation!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.
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.
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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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