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In Reply to: Re: Measuring horns posted by John Sheerin on February 5, 2007 at 18:42:41:
No need to have poor Ivan schlepping cabs and all that gear into the parking lot so he can measure at 10 meters anechoic. Unless he needs the exercise anyway.
Follow Ups:
Are you being sarcastic? I'll assume so.It seems you're confused on several different things. Real companies designing large prosound equipment have very different measurement requirements than most people around here trying to design their own hi-fi systems. A prosound company doesn't know where their design will be listened to from, but can usually assume it's further away than 1m. I (and most others here) know exactly where I will listen to my speakers from, so I can measure at that location. That may be 5m. It may be 1m. I know someone who set up an awesome nearfield system where you listened from about 10 inches. So saying "You have to measure from at least 2 meters" is not good advice.
And if you read what I wrote about gating, you'll notice I said you won't be able to use it to measure subwoofers at low frequencies in a small room. But that wasn't what Jeff was asking about - he was asking about midrange horns, which could be measured indoors using a gated measurement depending on the mic / speaker / room combination.
Hi BillWell it depends.
The 1W 1M spec is sometimes used to estimate how loud the speaker will be at another distance and power. For that use, it is desirable to have an accurate measure.The inverse square assumption for a point source assumes the source is small and occupies none of the volume in the space. Digging a big hole to drop the cabinets into is not practical but having enclosures which occupy a significant fraction of that 1 meter radius with an inflexible obstruction or have a large source area, is also not desirable so far as accuracy.
In addition, close to an lf source, one can measure “psudo sound” error which is a microphone signal caused by kinetic reaction of the moving air hitting the element and NOT radiated acoustic pressure.
All of those concerns are essentially eliminated when measuring at a larger distance, say 10 meters where 100Watts drive equals the 1W 1M level.Also, in answer to the original post, yes one normally does see some distance related changes in response. Up close, one has more low end than farther away.
For example, an SH-50 measures a gradual rise in low end response at one meter where as by the time one measures past about 6 meters, the low end is slightly depressed.
In general, the more a sources DI changes with frequency, the greater the response changes with distance.
Best,
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