RMS
Steady power the amp can hold without distorting.
01Key terms
Read the box
Every label on a sound bar box, in one line each. The topic links further down point back here.
Think of a biscuit packet
The front shouts "high protein" in big letters. The small table on the back tells you the grams. PMPO is the front of the packet, and RMS is the table on the back.
The words on the box, in plain English
RMSSteady power the amplifier can hold without distorting. The number to trust.
PeakThe most it can hit for a split second. Usually about double the RMS.
PMPOA marketing number with no agreed standard behind it.
BassLow notes, about 20 to 250 Hz. Played by the subwoofer and needs the most power.
MidsVoices and most instruments, about 250 Hz to 4 kHz.
HighsCymbals and crisp "s" sounds, about 4 to 20 kHz. Played by small tweeters.
dBDecibels, how loudness is measured. +10 dB sounds about twice as loud.
HzHertz, how fast sound vibrates. Low Hz is deep, high Hz is bright.
SubwooferA separate speaker just for bass.
2.1 and 5.1Main speakers, then subwoofers. 5.1 is five speakers and one subwoofer.
02RMS, peak and PMPO
Headroom, not loudness
Watts tell you how much power the amplifier has. More watts means more room for loud moments, not better sound.
Think of a car's engine
A car that can reach 180 km/h rarely goes that fast, but the spare power makes overtaking easy and calm. Watts work the same way: spare power lets a loud explosion in a film come through clean.
Three numbers, one worth trusting
Peak
What it can hit for a split second. About double the RMS.
PMPO
No standard behind it. Often several times the RMS.
A PMPO number cannot be compared
With no agreed test behind it, two boxes that both say 3000 W PMPO can differ several times over in real power. Compare RMS figures only.
How the box number adds up
The box number is a total
- Subwoofer240 W
- Left, centre, right3 × 80 W
- Two rears2 × 60 W
Each small speaker gets 60 to 80 W. The subwoofer takes the most, because bass needs the most power.
Each step is 10× the power for +10 dB
90 dB
100 dB
110 dB
120 dB
1 W10 W100 W1000 W
Every 10× in power adds 10 dB. Double the power adds only 3 dB.
TopicsRMS vs PMPOPeak powerDecibels and loudness5.1 power split
03Bass, mids and highs
Bass eats the power
Sound is air moving back and forth. How fast it moves is the frequency, in hertz. We hear from about 20 Hz to 20,000 Hz.
Think of waves in a bucket
Flicking the water with a finger makes quick little ripples with almost no effort. Pushing the whole bucket's worth back and forth takes your whole arm. Bass is the slow, big push, so it needs the most power.
The hearing range, split into bands
Sub bass
Bass
Mids
Highs
20 Hz60 Hz250 Hz4 kHz20 kHz
Rumble you feel
Kick drum, bass guitar
Voices, guitar, piano
Cymbals, crisp "s" sounds
Human speech, 300 Hz to 3.4 kHz
The scale is logarithmic, the way our ears hear pitch.
Bass takes more than half
Bass and sub
55%
Mids
33%
Highs
12%
Approximate. It shifts with films and music.
Who plays what
One speaker for each range
- BassSubwoofer, 6 to 10 inchMost power
- Mids and voicesMain and centre speakersSome power
- HighsSmall tweetersLittle power
TopicsBass and subwoofersMids and dialogueHighs and tweetersFrequency in Hz
04Room size and use
Room first, then use
The right number depends on your room size and what you watch. The calculator in the next module does the maths for you.
Think of an AC's tonnage
You pick a 1 ton AC for a bedroom and a 2 ton AC for a hall, because there is more air to cool. Sound works the same way: more air to fill needs more watts.
Suggested honest RMS power by room size
Bedroomup to 120 sq ft
50–150 W
Living room120 to 250 sq ft
150–300 W
Large living room250 to 400 sq ft
300–500 W
Hall or open plan400 sq ft and above
500 W+
0 W150300450600 W+
Hard rooms sound louder than soft ones
Hard floors and bare walls bounce sound around, so you need a little less power. Curtains, sofas and rugs soak it up, so you need a little more.
Match the setup to your use
- News, serials, cricketClear voices matter most2.1 or 3.1, 100 to 250 W
- Movies, OTT, gamingSubwoofer and rear speakers3.1 to 5.1, 300 to 600 W
- Music and partiesStrong subwoofer, clear mids2.1, 250 W and up
Topics2.1 vs 3.1 vs 5.1Wireless subwooferWatts for room size
05Room calculator
Your room, in watts
Drag the sliders or type your room size. The TV shows the power to look for, and the speakers show how it splits.
Think of a tailor
A tailor measures you before cutting any cloth. The calculator measures your room the same way, then cuts the watts to fit.
5.1 setup
0–0 WRMS total to look for
5.1
8 inch sub
Box peak ~0 W
0sq ft floor
0cu ft air
Roomroom type
A rule of thumb, not a lab test
It starts from the room table in module 04, adjusts for ceiling height, use and furnishing, and rounds to 10 W. Your ears have the final say.
06Questions and checklist
Asked at the shop
Short answers in plain words, then a checklist to carry to the store.
Questions people ask
Which number on the box should I trust?
RMS. Peak is about double it, and PMPO has no standard behind it.
Is the number on the box per speaker?
No, it is usually a total across all the speakers. Divide it up to see what each one gets.
Is 600 W twice as loud as 300 W?
No. Double the power adds only about 3 dB, a small step. To sound twice as loud you need about ten times the power.
Why does bass need so much power?
Bass is a long, slow wave, so the speaker has to move a lot of air. That is why a slim bar with no subwoofer cannot make deep bass.
Do I need a subwoofer?
For films and music, yes. A 300 W system with a real subwoofer often sounds fuller than a 500 W bar without one.
What do 2.1, 3.1 and 5.1 mean?
The first number is the main speakers and the .1 is the subwoofer. 3.1 adds a centre speaker for clearer voices, 5.1 adds two rear speakers, and 5.1.2 adds two height speakers for Dolby Atmos.
Which specs should I check on the box?
Power stated as RMS, ideally per speaker. Bass down to 40 Hz or lower. A separate subwoofer with its own wattage. Sensitivity of 85 dB or more, if it is listed.
What suits a typical living room?
For 150 to 250 sq ft: a 2.1 or 3.1 bar with a wireless subwoofer, at about 200 to 350 W RMS.
Can I buy too many watts?
Yes. 1000 W in a bedroom is like a truck for your office commute: big on paper, wasted in practice.
Should I listen before I buy?
If you can. Play a dialogue scene and a bass heavy scene. Your ears are the final test.
Before you buy
Clears the ticks above and every module you marked done.