Film-Tech Forum ARCHIVE: Subwoofers needed relating to cubic feet/metres

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Posted by Amanda Mundin (Member # 3386) on 10-21-2005, 12:13 PM:

JBL Cinema Sound System Manual
E. Subwoofers:
Because of the relative non-directionality of very low frequencies, the number of subwoofers specified for a given cinema must bear some relationship to the total volume in the space. JBL’s experience in specifying subwoofer units has led us to the following empirical recommendations:
1. For each 5,666 cubic meters (25,000 cu ft), specify one 2241 or 2242 transducer in a suitable enclosure.

I make 25,000 cubic feet = 708 cubic metres and 5,666 cubic metres = 200,093 cubic feet, so presuming i'm doing the sums correctly does anyone know which is the correct volume of space JBL recommend one 18" driver for? 25,000 cubic feet or 5,666 cubic metres?

Posted by John Pytlak (Member # 331) on 10-21-2005, 12:27 PM:

Amanda: Are you designing your cinema yourself, or just want to be informed when you are working with your experienced theatre designer?

Posted by Steve Guttag (Member # 268) on 10-21-2005, 12:53 PM:

The reasoning is flawed. The 2241 is a 600-watt driver, the 2242 is an 800-watt driver. How can they be treated interchangeably like that. Furthermore, the enclosure they are used in may be correctly designed (suitable) but not yield the same efficiency to make them equivalent.

For specifying subwoofers (or any other speaker, for that matter), you need to know the distance they will play to (how deep is your theatre), how efficient they are (how loud is it with just 1-watt at 1-meter) and how loud do you need it to play. For subs in a digitally equipped theatre...that is 113-115dB at a point 2/3rds of the way back into the auditorium from the screen.

As you add subwoofers, your efficiency goes up 3dB as you double the quantity.

Posted by Amanda Mundin (Member # 3386) on 10-22-2005, 05:52 AM:

Thanks for that, just want to be informed really John

Posted by Louis Bornwasser (Member # 3063) on 10-22-2005, 09:03 AM:

Subwoofers is an area where "size matters." Really, the more the merrier, if you can afford it, both in terms of number of speakers and amplifier power. Even the most over-the-top system can always be throttled back....tamed. The same cannot be said for an inadequate system. Louis