Boundary's and measuring Speakers: What about this....

[jeffsco]

I was reading an article by Pat Brown (Syn-Aud-Con) on another Forum about the effects of Boundary's on different speaker types. (Look up ProSoundWeb Forum/ Tools of the Trade / "How Boundaries Affect Loudspeakers"...if anyone wants the link....I'll post it.....). He measured these speakers in 4 different ways....Free Space, laid flat on the ground , Up against 1 wall and floor , and placed in a corner. The effects of these boundaries on the speaker response was then shmeasured and shown. Interestingly...the response of a horn loaded full range speaker is not effected by boundaries...except in terms of db output at lower frequencies (Subwoofers etc.)

I am finishing up my first WH10 and will be moving on to other WH's and a set of DR 250's. I want to measure the response of these cabinets but I do not have any open fields to do a free space measurement in. My back yard has too many boundaries within 25 ft. of any given measuring point. I read about reflections off the ground and such (even though I have grass). I am looking to get accurate measurements but it seems as if there are a multitude of factors that can render the readings innacurate. I was thinking....what about placing the speaker on it's back ..elevating the measurement mic up above it 10 or 15 ft...and taking a measurement that way. According to the Pat Brown article..that is what is known as a 1/2 space reading. I realize I would get some reflections from various Omnidirectional Low and Low mid frequencies but I'm hoping that the majority of the frequency response will be fairly uncoloured by the effects of the Comb filtering. I read on the DBX Forum that you could even bury the speaker into the ground , level with the ground surface and that would give the most reliable reading as it is considered the same as an free space measurement. In fact ..for the Pat Brown article, they placed the speaker in a baffle recessed into the wall to obtain this measurement.

I really want to capture the response of these cabinets and then develop various curves for them to be stored in my SAC mixing system. Any pertinant information would be appreciated...

[Bill Fitzmaurice]

I was reading an article by Pat Brown (Syn-Aud-Con) on another Forum about the effects of Boundary's on different speaker types. (Look up ProSoundWeb Forum/ Tools of the Trade / "How Boundaries Affect Loudspeakers"...if anyone wants the link....I'll post it.....). He measured these speakers in 4 different ways....Free Space, laid flat on the ground , Up against 1 wall and floor , and placed in a corner. The effects of these boundaries on the speaker response was then shmeasured and shown. Interestingly...the response of a horn loaded full range speaker is not effected by boundaries...except in terms of db output at lower frequencies (Subwoofers etc.)

The effect of a boundary is frequency dependent, and it's also affected by the size of the speaker radiating plane. For instance, a baffle or horn mouth one wavelength across is radiating into half-space even when suspended in the air, and putting it against a wall won't have any effect above that frequency. Put the same speaker on the ground and above this baffle step frequency the speaker is in quarter-space, while below that frequency it's in half-space. There's nothing in that link that hasn't been well known to the acoustical engineering community for fifty years. Brown could have saved himself a lot of work by just reading a book like this one:
http://www.audioxpress.com/bksprods/products/bkpa1.htm

I read on the DBX Forum that you could even bury the speaker into the ground , level with the ground surface and that would give the most reliable reading as it is considered the same as an free space measurement. ..

That would be a half-space measurement. But digging a hole in the ground is a royal PITA. The way professionals do it is to take a ground plane measurement below the baffle step frequency and a free-space measurement above the baffle step frequency and combine the two results to arrive at the half-space result, unless you happen to have a large enough hole in your back yard.
Where it comes to measuring speakers this work is definitive:
http://www.audioxpress.com/bksprods/products/bkaa45.htm

[jeffsco]

Thanks for commenting on this Bill. Given the variables that affect the frequency response.....what would you recommend for establishing a baseline response...how would you advise me to get the measurements?

[Bill Fitzmaurice]

Thanks for commenting on this Bill. Given the variables that affect the frequency response.....what would you recommend for establishing a baseline response...how would you advise me to get the measurements?

Take it outdoors, put it on the ground, measure it on-axis at ten feet away. Boundaries 25 feet away will mostly affect readings below 100 Hz and once you go indoors response below 100 Hz is mainly room sourced anyway.

[jeffsco]

Bill: The more I learn....It seems as if your horn loaded/ Designed speakers systems really do eliminate a lot of the problems direct radiating speakers have to contend with, in terms of acoustical physics, room coupling, lobes, nulls etc. Our culture today doesn't want to know about it unless it's the" latest and greatest, fastest newest thing". The fact that you are using design principles first expounded in the 30's and 40's....and that these principles can actually surpass the designs of today in terms of functionality and quality of sound....well..I can't wait to get my DR 250's done....Thanks again.

[WB]

I can't wait to get my DR 250's done....Thanks again.

The design of the DRxxx series is pure genius. I'll stop now, I'm gushing.