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WinISD apparent power question

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Category: Plans
Forum Name: New Projects Forum
Forum Description: Forum for new speakerplans projects, in memory of Tony Wilkes, 1953 - 2014
URL: https://forum.speakerplans.com/forum_posts.asp?TID=108409
Printed Date: 26 March 2026 at 7:10pm
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Topic: WinISD apparent power question

Posted By: donatas7855
Subject: WinISD apparent power question
Date Posted: 14 March 2024 at 6:17pm

Hello,

I modeled a subwoofer in WinISD and looked at apparent power. As imput signal i put 300W (300W RMS driver), but then i put my high pass and low pass filters maximum apparent power is 125 VA. I know VA is not W, but simply speaking my amplifier puts out 125W. Does it mean, if i am not limited by excursion, i can increase my imput signal ?

Or if i look at this graph and see maximum of 125VA, does it mean i can get away by using 150W RMS amplifier, even though my driver is 300W RMS ?

GREEN lines/ driver

https://ibb.co/CBv1nY9

P.S. Driver: Hifonics ZXS8D2, ported volume: 18L, tuned to 28Hz.

Dont mind other drivers;

Replies:

Posted By: fudge22
Date Posted: 15 March 2024 at 5:19am

Loudspeakers are ordinarily driven by amplifiers with very low output impedances which provide essentially constant-voltage operation regardless of loudspeaker impedance. Speakers are also traditionally designed to have roughly flat acoustic frequency response when presented with a constant-voltage electrical input over the desired operating range.

The nominal electrical input power to a loudspeaker driver is defined as the power delivered by the amplifier into a resistor having the same value as the driver’s voice coil resistance This is usually calculated by simply squaring the input voltage and dividing by the driver’s voice coil resistance or rated impedance. This definition of input power yields a nominal efficiency vs. frequency response curve that is similar to the SPL response curve you get when driving the system with a constant voltage source.

However, the impedance of a loudspeaker coil is not constant, so if you plot a graph of the input voltage squared divided by the actual impedance, with relation to frequency, you end up with the curve shown in winisd. This curve is essentially the inverse of the impedance curve, modified by any filters that you have added. It is also an inverse of the real efficiency of the drive unit. At 30Hz (on your graph) it takes 3 times more power than it does a 45Hz - assuming that spl response is flat. That is, your speaker system is 3 times as efficient at 45Hz as it is at 30Hz

If you select the signal tab, where you entered the power input, it also shows the drive voltage that is used to calculate the power, and a series resistance that considers the cable resistance.

Returning to the amplifier power; as the graph shows, with an input voltage that would dissipate 300W into an 8ohm load, your drive unit is only dissipating 125W. You could therefore drive the loudspeaker with a more powerful amplifier and not run into thermal overload. Check the maximum power and cone excursion plots to ensure that you don’t exceed the maximum excursion limits of the cone in your desired frequency range.

Posted By: DMorison
Date Posted: 15 March 2024 at 12:24pm

The only thing I would add to the above is that on the signal tab, WinISD calculates the power based on the minimum impedance of each project, not nominal, so if you want to model the effects of using an actual 300W amp, your most accurate way of getting there is to put 49 in the Driver input voltage field and ignore the System input power field.

To calculate the equivalent Voltage for any nominal amp power multiply the power by the nominal impedance then take the square root of that.

HTH,

David

Edit - clarified terminology.