Film-Tech Forum ARCHIVE: Amplifier Power Requirements

This is topic Amplifier Power Requirements in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=004323

Posted by Tom Wienholt (Member # 1510) on 12-17-2002, 01:53 AM:

I am having trouble calculating the the amplifier power requirements to power 3 JBL3677 stage speakers and 6 JBL3310 surround speakers. Is there a basic formula for figuring out amplifier requirements?

Posted by John Gordon (Member # 523) on 12-17-2002, 04:34 PM:

Hello Tom,
Yes, there is a basic formula for figuring power requirements. Actually, you need to use multiple formulas to figure amplifier power size. Use 20LogD + 105 - S to determine power in dBW (power referenced to 1 Watt), where D is 1/2 the length of the auditorium in meters (for feet, divide by 3.3 to convert to meters), and S is speaker sensitivity. We want our final power figure to be in Watts, not dBW. To convert dBW to Watts, use 10(dBW/10) to determine the size of amplifier you will need.

This formula is not an exact science--there are too many variables to consider, such as: room volume, impedance ratings, acoustics, and on and on. What you do not want to do is under power your system. If you have to err, it is always best to err on too much power as opposed to not enough. Being I do not know your room size, let's create an example, rounding numbers off of course.

For the above formula, we ignore room width and volume, and instead concentrate on room length and speaker sensitivity (remember, this is not an exact forumla, but it does yield excellent results). So, say the auditorium length is 55 feet, and speaker sensitivity is 100. Using 20LogD + 105 – S, D = 8.5 (55ft/3.3 = 17, 17/2 = 8.5) and S = 100

20log8.5 + 105 – 100
= 19 + 105 – 100 = 24dBW
10(dBW/10)
= 10(24/10) = 10^2.4
= 251 Watts

For best results, use an amplifier that is rated for 251W or higher, the QSC DCA 1622 fits this perfectly at 300W a channel. One thing of note, when determining speaker sensitivity, always use the lowest sensitivity rated driver (almost always the woofer).

The goal with sound is to be able to easily reach reference levels to achieve a uniform SPL level of 85dB throughout the auditorium from the screen channels. But what is even more important is the ability to simply achieve a headroom of +20dB above reference, or 105dB SPL for screen channels, 102dB for left or right surround, and 115dB for subwoofers.

Posted by Tom Wienholt (Member # 1510) on 12-17-2002, 07:05 PM:

Hi John,
Thanks for all of your help.