Quick intro to SPL testing?

[Rick Auricchio]

I've got an ECM8000 mic on the way, so it's time to run some SPL tests on my omni15TB. And I'll run comparisons against the two Eden 112XLTs while I'm at it. (Both a single 112 and the pair.)

I won't be lifting cabinets off the ground.

A few questions:

1. How far from buildings? In front? Behind? To the side?

2. Grass/dirt surface, or paved?

3. What height for the measurement mic?

Anything else that'll help me get a good measurement. Thanks in advance.

[WB]

I won't be lifting cabinets off the ground.

That'll be ground plane measurements, which will give you accurate measurements up to the cabinet's baffle step.

1. How far from buildings? In front? Behind? To the side?

40' to 50' in all directions.

2. Grass/dirt surface, or paved?

Doesn't really matter, as long as it's flat between the cab and mic.

3. What height for the measurement mic?

1/2"

[Rick Auricchio]

Thanks! Now I just have to find a big open space with AC power.

[bgavin]

We use the parking lot at HP Roseville on a weekend. DaveK is an employee and has access to a sub-building the power. We run a long extension cord and set up in the middle of the parking lot. It really is a great location for this. Nobody is bothered by the noise, and the campus is large enough where we are removed from street noise.

[Rick Auricchio]

Nice to have a few square miles of parking lot!

I think I've found my location. Easy load-in, access to power, lots of pavement, far enough from structures and the street.

It's behind an old two-story grammar school building here in town. It's now used as a theatre facility, and I know the folks in charge. Not near residences either.

I've got to do a few dry runs in my garage even before I get the ECM8000 mic, so I've got all my ducks in a row before I get outdoors.

[DaveK]

> 3. What height for the measurement mic?

1/2"

What is the correct orientation for the mic? Long axis horizontal and pointed at the speaker seems like the obvious answer, but I remember seeing a picture a while back with the long axis vertical so the end of the mic is pointed at the ground.

[WB]

> 3. What height for the measurement mic?

1/2"

What is the correct orientation for the mic?

It doesn't matter since the mic is omnidirectional. This is what I do for ground plane measurements.

[bgavin]

The groundplane method works very well on a hard sursface. Measurement mics are omni-directional, so placement is mostly about keeping the capsule as close to the ground as possible. I try for 1/4" free space off the ground.

Surrounding obstacles should be at least a full wavelength away from the test area. The farther away, the less energy in the reflected wave, and the less invasive to your test results.

Hard surface groundplane measurement is not suitable for tops. This requires a grassy open area, and an elevated DUT. Compare measurements of a top box made under these two conditions, and the results are apparent.

Tom Danley pioneered the 100w at 10 meter test protocol. This produces the same relative chart as 1w/1m. The longer distance filters out Doppler distortion, shows results at real-world power levels, and allows for integration over distance. This technique reveals how anemic commercial cabs really are when the Doppler product is removed. This method also reduces close field baffle step enhancement that would artificially inflate the results. The downside is the large amount of free space required without obstacles.

[Rick Auricchio]

So where do you set voltage?

1. I know it's 2.83v for the 1w/1M measurement.

2. 8.95v for the 10w/10M measurement?

2. At what frequency do you initially set the voltage? Running an automatic sweep doesn't let you measure everywhere along the spectrum.

[bgavin]

For an 8-ohm load, 28.3v at 10 meters.

Watts = (E*E) / R

[Rick Auricchio]

28.3v is 100w.

If we're moving from 1m to 10m, shouldn't it be 10w, which is 8.95v?

At what frequency should I set the initial voltage? 100Hz?

[DaveK]

28.3v is 100w. If we're moving from 1m to 10m, shouldn't it be 10w, which is 8.95v?

100W (inverse square law at work)

At what frequency should I set the initial voltage? 100Hz?

Yes

[Rick Auricchio]

The Inverse Square occurred to me later, but I'm glad to have confirmation. 100w ought to be quite loud for the test!

[bgavin]

I have a pair of Radio Shack SPL meters. They both agree with measured results, and are reputed to be accurate at 100 Hz. I use a 100 Hz sine wave at 2.83v/1m or 28.3v at 10 meters. The SPL meters are at the mic position. The reading is used to calibrate the RTA which is measuring the 100 Hz.

For repeatable results, any changes to the RTA require recalibration.

100 watts does get seriously loud. Having the measurement console 10 meters from the noise source will save your sanity. Hearing protection is mandatory.