Power

POWER

TLDR is a mistake. You need to know this. Also there is a bit of math.

Power matters. More is better. Why do you want a lot of it? Power is what determines how enveloped in the experience you are – how it feels – how clean, tight, and distortion free the sound is. For movies, having a lot of power makes a huge qualitative difference in your emotional involvement in the film.

Peak power. That is what you care about. Sitting on the couch watching the latest action adventure or sci-fi movie and feeling it, really feeling it is what peak power gives you. For music, high peak power gives the system the headroom for clean, tight, low distortion bass lines. Average and RMS power are of little concern for home listening. Why? Read on…..

Music and movie sound tracks are very peaky. That is, the peak power on the loud parts and long term average power are very different.

The technical measure of how peaky a sound track is is called “crest factor”.

The crest factor is the ratio of peak to average (not RMS) power and is given as a number or a value in dB (decibels). A typical crest factor in a subwoofer track is 10 which is 20 dB in voltage or 10 dB in power. Why that is so is a math lesson.

Perceived Loudness – a 10 dB (10x) increase in power is perceived as being twice as loud. So a 1200 Watt amplifier will sound twice as loud as a 120 Watt amplifier. This is why you need a lot of power. It takes a lot to make it a little bit louder. Also, human hearing loses sensitivity the lower you go in pitch/frequency. So the very low sounds of earthquakes and exploding starships are felt more than heard. It takes a lot of power to shake your booty.

Below is a waveform excerpt from the subwoofer track of a recent action adventure movie. This is typical of movie soundtracks. You can see how peaky it is. The crest factor of this waveform is 22.2 dB or in numeric terms 12.9:1. This means the power difference between the peaks and the average value is 12.9 squared or 166.

That means if you have a subwoofer with a peak power capability of 2000 Watts then it only needs to deliver 12 Watts (2000/166) average. This is why you don’t care about average (or RMS) power. The peak power of 2000 W is what determines the feel and experience of being enveloped in the movie. Your earbuds can practically deliver 12 Watts (not really but you get the idea).

RMS power. This term is mis-used very frequently. Actually it is always misused. What is meant is actually average power. Sorry, we have a little math lesson here. The vertical axis is voltage (volts) or power (watts) and the horizontal axis is time.

There are 5 different things plotted here. The black waveform is the voltage to the speaker. It swings from 2 volts to -2 volts and is referred to as 2 volts peak. For this example we are using a speaker load resistance of 1 ohm to make the math easier.

The blue line is the RMS voltage. Power is volts rms squared divided by the load resistance which is 1 ohm in our example. In the picture volts rms is 1.4 Volts. 1.4 squared is 2, then divided by 1 ohm which is shown as the orange line. That is the average power. This is the amount of energy that is converted into heating the environment by moving air and making sound (as well as making the speaker driver warm).

The brown waveform is the power waveform. It swings from zero to 4 watts. The peak is what determines how momentarily loud it is and what you feel. It is easily seen that it is symmetrical around the orange average line, spending equal time above and below it thus the orange line is the average.

Let’s say it one more time. RMS Voltage or Current is used to calculate AVERAGE POWER.

It is possible to calculate an RMS power. That is the purple line at 2.45 Watts.

It doesn’t have any physical meaning or interpretation like RMS VOLTAGE and CURRENT do.

Any manufacturer who talks about their RMS power does not know what they’re talking about. It has no meaning, it is just a number you can calculate on a rainy day.

If you want more details and math on this, check out this article at Analog Devices, one of the premier analog chip design companies.

Bottom line: Peak Power is what you want. That is what makes the experience. Average power is nice but the requirement is so small it doesn’t really matter. RMS power is a misnomer and shouldn’t ever be talked about.

How Much Power do you Need?

The difference in the experience between having plenty of power and just having OK bass is qualitative – it’s not the difference between normal and loud but envelopment and involvement in the movie. It’s not that it’s loud all of the time. It’s only loud for a moment when you need it and the couch is moving under you.

How much it takes to fill your room depends on size. For 2000-3000 cubic feet we recommend 1200 watts.

For 3000-5000 cubic feet we recommend at least two 2000 watt units. If your room is this big you won’t be sorry with two Bespoke Subwoofers.

Personally, I have a 5000 cubic foot room and have 4, but that’s just me…

-Rocky

CEO/CTO Bespoke Subwoofers