Boundary loading

Got speaker theory questions? This is where you'll find the answers.
Post Reply
Message
Author
Rich4349
Posts: 602
Joined: Wed Jul 30, 2008 10:18 am
Location: Kankakee, IL

Boundary loading

#1 Post by Rich4349 » Wed Jan 31, 2018 8:01 am

I just read the sticky on it (I can paraphrase it decently by now), but I've also run across this:

https://www.prosoundtraining.com/2011/0 ... dspeakers/

Am I misreading or misunderstanding this guy when he talks about corner loading giving 9 dB of gain? The source *appears* well researched.

There are a couple different sections; in the sub section he also mentions 18 dB corner gain.
2 DR250s, 2 27" Lab15 T-60s, 2 30" Neo Titan 39s, 1 Autotuba...and looking for more!

User avatar
Bill Fitzmaurice
Site Admin
Posts: 25843
Joined: Tue May 02, 2006 5:59 pm

Re: Boundary loading

#2 Post by Bill Fitzmaurice » Wed Jan 31, 2018 8:49 am

He's wrong. With a speaker that has 360 degree radiation half space loading gets 6dB, quarter space 12dB and eighth space 18dB compared to whole space. But it's not quite that simple. For instance, to have whole space loading there can't be any boundaries, which includes the ground, within a few wavelengths of the speaker, which is not only impractical from a measurement standpoint but also because speakers are almost never used with a true whole space placement. For those reasons the standard base sensitivity is measured half-space. Another complication is that full range speakers don't have 360 degree radiation patterns throughout their band width. Most will be 360 degree at the lower end of their pass band, transitioning to 180 degrees above the baffle step frequency, ie., where the baffle is a wavelength in dimension. Placed less than 1/4 wavelength in front of a wall they'll get 6dB where their radiation is 360 degrees, but that will transition to 0dB above the baffle step, because in those higher frequencies there's no additional reduction in the space loading. Placed on the ground anechoic/outdoors they'll be radiating into half space in the lows, but above the baffle step they'll be radiating into quarter space. You have to take that into account to establish a base response measurement.
The reason why he got the results he did was because he measured indoors, which brought other factors into play, not the least of which was the ceiling. He couldn't establish a base whole space measurement because he didn't have a whole space condition to take it in.
Unless otherwise noted my measurements are true half space anechoic, taken outdoors, using a two step process that accounts for the baffle step.

Rich4349
Posts: 602
Joined: Wed Jul 30, 2008 10:18 am
Location: Kankakee, IL

Re: Boundary loading

#3 Post by Rich4349 » Wed Jan 31, 2018 9:20 am

So a sub in a corner compared to out in the middle of the floor of a room gets an 18 dB boost and not 12?
2 DR250s, 2 27" Lab15 T-60s, 2 30" Neo Titan 39s, 1 Autotuba...and looking for more!

User avatar
Bill Fitzmaurice
Site Admin
Posts: 25843
Joined: Tue May 02, 2006 5:59 pm

Re: Boundary loading

#4 Post by Bill Fitzmaurice » Wed Jan 31, 2018 10:08 am

Only if the shortest room dimension is a few wavelengths, say 200 feet. :shock:
That's why benchmark measurements are done outdoors, away from boundaries. Indoors even in the middle of the room a sub is in eighth space at all those frequencies where it's less than a quarter wavelength from the corner. At 80Hz that's 3.5 feet, at 20Hz that's 14 feet. And don't forget, the ceiling is a boundary. You can be technically in eighth space in the middle of the room if the room is small enough, and in that case you not only get eighth space loading, you also get cabin gain. That's how auto dB competitions get such silly results.

Post Reply