Film-Tech Forum ARCHIVE: Full space, Half space and Very Low Frequencies

This is topic Full space, Half space and Very Low Frequencies in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Antonio Marcheselli (Member # 373) on 01-28-2003, 08:27 AM:

Hi everyone

As someone knows I'm still searching the light for fully understand speaker, particularly to understand Low Frequency ena Very low frequency speaker, their power and their efficiency.

I was looking at some JBL and Electrovoice PDF brochure. I discovered that JBL 4645C sensitivity is rated 97dB but then on the footnotes I've read "half space".

What is "half space"? I believe I can answer by myself: when a Low Frequency speaker is aimed with an infinite wall (like the THX sound wall). Right?

Then I noted the following note: "quarter space (wall/floor junction placement) is 6dB greater".
Does it mean that if I put a 4645C at wall/floor junction the same speaker will have 102dB sensitivity? Is it a "real" parameter?

Then, I read on EV datasheet about TL880D. The full data are not so clear, but on the brochure there is on the "sensitivity" field:

Full Space 99dB
Half Space 105dB

So, according with the JBL datasheet, if I put a TL880D on a quarter space it should increase its sensitivity by 6dB? Does it mean that a TL880D on a quarter space placement will give 111dB with 1W/1m?? Again, is it a "real" situation?

Bye

Antonio

Posted by Rob van Ede (Member # 1457) on 01-28-2003, 11:16 AM:

Hi Antonio,

Under a certain frequency (when the cone diameter is 1/2 or 1/4 wavelength, i'm not sure), a loudspeaker system gets omnidirectional. This means it radiates energy in all directions. When this speaker is situated in free space, the energy that is radiated at the rear of the speaker system is lost. When a speaker system is placed in front of a wall (1/2 space), then the energy from the rear is reflected.
If the speaker system is close enough to the wall, the direct and the reflected sound are approximately in phase and reinforce each other. When the speaker system is in a corner, sound that would normally escapes to the sides is also reflected.

Rob

Posted by Ray Derrick (Member # 1001) on 01-29-2003, 04:47 AM:

Nice explanation, Rob.

Antonio, I think you will find these numbers apply to a perfect world where the reflective surfaces are 100% efficient and do not absorb any energy. At the frequencies involved, this doesn't happen very often in the real world.

Full space is when a speaker is suspended in free space, well away from any floor, walls or ceiling. It is usually measured in an anechoic chamber where there are no reflections. It is rare for a bass speaker to be mounted this way. Half space can be thought of as a speaker mounted in the middle of a very large wall, or sitting in the middle of a large floor area. To get the full 6dB increase, the wall or floor should reflect ALL of the sound energy and the relected sound must add perfectly with the direct sound (as there are time delays involved there will be slight phase differences so it will be close, but not perfect). So in reality you could expect these numbers to be a little less.

Quarter space would theoretically yield a further increase of 6dB over half space, and eighth space (at the junction of two walls and a floor) - another 6dB. But in practice? I don't think so!

[ 01-29-2003, 09:27 AM: Message edited by: Ray Derrick ]

Posted by Evans A Criswell (Member # 381) on 01-29-2003, 09:19 AM:

I guess this explains why my JPL HP520 home speakers have a dial on the bottom with 3 clicks for bass adjustment: 0, -3, and -6 dB.

Posted by Antonio Marcheselli (Member # 373) on 01-29-2003, 10:40 AM:

Ray and Rob,

Ok. I understand that theaters are not mathematical models. But an SPL response improvement can be expected if the same SW is placed on the floor or suspended in free space or placed at floor/wall junction, right? Perhaps not the +6dB but I can expect some dB, right?

Theretically can I expect +6dB for each placement improvement? If 100dB free space, will be 106dB half space and 112dB quarter space?

More: TL880D has really +6dB sensitivity than JBL's 4645C?

Bye
Antonio

Posted by Dave Macaulay (Member # 813) on 01-29-2003, 12:49 PM:

You will get an appreciable increase in sensitivity by placing the SW at the wall-floor junction. Whether it is actually 6dB depends on the particular room, this is the theoretical maximum.
I think you can trust the published sensitivity figures for EV and JBL, I would take consumer manufacturers specs with a few pounds of salt though.
To choose a speaker for any room you have to know how much sound energy you need to get the desired SPL in the space, and find a solution with the desired sensitivity and power handling capacity to do the job. More sensitive speakers can have less power handling capacity and be fine for a smaller room - saving on amp expense - but be overloaded if expected to power a large auditorium.

Posted by Antonio Marcheselli (Member # 373) on 01-29-2003, 01:17 PM:

Thanks Dave

Can I ask someone how much money a 4645C and a TL880D cost?

Since EV is 6dB higher where is the advantages of JBL's? More low freq. end?

Bye
Antonio

Posted by Mark Hathaway (Member # 1044) on 01-31-2003, 01:35 AM:

By placing the sub in the corner, you will also excite all the room modes evenly. While this is not a real problem for auditoriums with dimensions approaching the wavelength of 20-30hz, it can be a real issue in 10 seat screening rooms.

Antonio, I can recommend some good books and links.

"Masters Handbook of Acoustics" by F. Alton Everest, available at www.amazon.com note though it is aimed more at small to medium sized rooms, not at a 400 seat cinema. Nonetheless, well worth reading.

"The Loud Speaker Design Cookbook" by Vance Dickason. also at amazon. No mention of horns here, but still worth reading

and some simple free software to run simulations in
http://www.pvconsultants.com/audio/boxmodel/unibox.htm

Hopefully these will be a good start for you. Good luck, just remember to find a home for the 15th set of speakers BEFORE you build them.

Mark Hathaway
Atlab Image and Sound Technology
Melbourne, Australia