Outdoor Groundplane Subwoofer Testing
The goal is to remove the effects of the environment on the subwoofer response as much as possible so that each one may be compared relative to each other on as level of a playing field as possible and without the response distorting influence of a room. The basic manner in which the subwoofer and microphone are deployed to obtain the measurements is what is known as an outdoor ground plane test, which is a popular manner in which to achieve approximation of an anechoic test environment.
Briefly summarized this method is one in which the subwoofer is placed outdoors on a flat, firm surface, well away from any large objects or boundaries that will affect the results and the microphone is usually placed at a distance of either 1 or 2 meters from the nearest face of the device under test (DUT). All of our measurements are taken with the microphone at a distance of 2 meters unless otherwise implicitly noted. This allows the total output from subwoofers with multiple radiation points: Drivers, passive radiators, ports, etc, to sum together better than at a 1 meter distance where the difference in distance from the radiation points to the microphone element are a larger percentage. Also measuring from 2 meters gives an easy approximation of 1 meter free field output (4pi space) since the extra doubling of distance lowers the output recorded by 6dB and conveniently cancels out the 6dB of apparent output that would be added in by the boundary the subwoofer is sitting on in a 1 meter measurement, the ground. The further away large objects or boundaries are from the DUT and microphone the better but in general you want to have at least enough space to equal the length of half a wavelength of the lowest frequency to be measured. The lowest frequency that is tested to is 10Hz because very few subwoofers have any meaningful output that low in an outdoor setting or otherwise and it becomes very hard to a get good enough signal to noise ratio above the environmental background noise. No subwoofer that I have yet tested would have any output worth noting below 10Hz in an outdoor setting. Many struggle for meaningful output even a whole octave higher. That being the case it can be calculated that a 10Hz wavelength is roughly 113ft, or 34.5 meters long, so in order to reduce the affect of nearby large surfaces on a 10Hz measurement to a low enough point, we will need half of that distance from the nearest large surface at minimum. At the test site the required 60+ft distance is readily available. There is a telephone pole at about 45ft from the normal subwoofer placement area at the test site but due to its relatively skinny cross section and round shape, its effect on the measurements is for practical purposes negligible. The reason that this type of test is chosen is because it is cheap, simple and relatively practical. The downsides involved include dealing with weather, atmospheric conditions and environmental noise. There is not much that can be done about the weather. The environmental noise can be cut down by averaging multiple measurements. Another way is to increase the strength of the direct signal in relation to the background noise. This can be done by turning up the DUT level which can cause issues of non linearity or compression, or by moving the microphone closer to the DUT. This is one advantage of 1 meter measurements over those taken at 2 meters. There are other measurement arrangements and techniques that can also be used other than ground plane testing, but each has their own not insubstantial downsides as well. Another thing to keep in mind is that the further the microphone is placed from the DUT the further the distance needed from nearby large objects and boundaries to keep their effect on the results minimized. This is one downside of measuring from further away.
The Test Site
The site used for these tests in particular is a very large field estimated conservatively at roughly 300ft wide x 1000ft long adjacent to a parking lot in an industrial area.
Subwoofer Orientation Relative to the Microphone
In some cases there are multiple radiation points on the subwoofer whether from multiple: Drivers, ports, passive radiators, horn mouths, etc. These make obtaining the real unit response and output level more complex especially if the radiation points are on opposite surfaces of the subwoofer. One or more of the other radiators on the subwoofer will be further away than the nominal distance of the microphone so its output will register lower in level. Measuring from a further distance can help out in this regard but a much further distance than 2 meters would be needed to equalize this effect in many cases. In the case of subs with multiple radiation points an effort will be made to give the subwoofer the best placement possible by measuring it from multiple angles until the orientation with the best overall response is identified. Subsequently all measurements recorded from the sub will use that orientation and microphone placement. Obviously this will not please everyone or give a completely fair shake to every sub, but short of doing a full polar response work up for every subwoofer and then averaging the response, which would take entirely too much time and effort, this will never be satisfactorily achieved.
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