How to measure speaker volume - RMS or SPL

by Andrew Bishop

How Loud Is Loud? Only SPL Can Tell You.

Choosing 2-way, 3-way and subwoofer passive loudspeakers and suitable amplification for your PA system is actually quite a tricky job. You’ll see various power specifications being bandied about for both, from Peak Music Power, Music Power, Peak Power, Program Power, and RMS Power and – for loudspeakers only - SPL.

SPL stands for Sound Pressure Level which (for PA and Hi-Fi purposes) is the pressure caused by a sound wave passing through the air. Pretty obviously, the higher the SPL, the higher the volume or – put the other way round - the higher the volume, the higher the SPL.

SPL is measured in Decibels - a logarithmic unit of measurement named after Alexander Graham Bell that is used to state how much louder or softer one sound is with reference to another – which gives us a way to measure how loud any sound is. Once we can measure the SPL produced by different loudspeakers, we can then use those measurements to compare the performances of those loudspeakers.

To give you an idea of how SPL measurements translate into real life:

0dB is the threshold of human hearing

40dB is a very quiet room in your house

60dB is the level of a normal conversation between two people six feet apart

80dB is the level of noise in a busy street, or someone shouting at you from about 6 feet

90-100dB is a train passing a station platform at speed

120dB Jet Aircraft taking off at 30m

130dB is the threshold of pain – don’t go there!

140dB is what your ears would be subjected to if you stood behind a military jet taking off at full power – and you’d get blown off your feet.

150dB Eardrum rupture

(Please be aware that extended exposure to continuous volume levels in excess of 90dB SPL can cause permanent damage to your hearing.)

How loud is loud?

Watts and SPL
Before we can carry on with SPL, we need to look at amplifier power (Watts) and how that affects SPL.

A change of +3dB in SPL is generally accepted as the smallest volume difference (upwards or downwards) that we humans can detect in amplified speech or music. To make a PA system sound +3dB louder the output power of the amplifier has to be doubled.

To make the same system -3dB quieter the output power of the amplifier has to be halved. This means that to make a 100 Watt (W) PA system sound +3dB louder, you’d need a 200W amplifier. To make it sound -3dB quieter you’d simply turn the 100W amplifier down until the output was 50W.

Remember that this +3dB change is the smallest change in volume that you can hear – so simply doubling the power of your amplifier (if your speakers could take it) isn’t going to make your PA sound louder at a gig.

To make a more noticeable 6dB difference in SPL in this system, the amplifier output power would have to be quadrupled to make it louder or quartered to make it quieter, so you’d need a 400W amplifier (and speakers to match) to go +6dB louder or you’d have to turn the 100W amp down to 25W to go -6dB quieter.

If you wanted a PA system that sounded twice as loud as a 100W PA system – and assuming your speakers could handle it - you’d need a 1000W power amp to deliver the +10dB rise in SPL that you’d need, or to turn your amp down to 10W if you wanted to halve the perceived volume.

What’s Watts?

A Watt is a familiar unit to measure power with – after all, we’ve all probably sat in front of a 1000 Watt electric fire that converts electrical energy into heat – and, because loudspeakers are not exactly efficient, an amplifier connected to a loudspeaker can be thought of as a pretty efficient way to produce heat and a rather inefficient method of converting electrical energy into sound.

Amplifier Watts (Output Power)

In amplifiers, the maximum number of Watts that an amplifier can deliver to a loudspeaker is calculated using the output voltage that the amplifier can deliver into a loudspeaker of known impedance.

Impedance – which is measured in Ohms (Ω) - is the term used to describe the resistance of an electrical circuit to alternating (AC) current. Resistance (also measured in Ω) is the measure of an object's opposition to the flow of electrical current through it.

The formula used to calculate the wattage delivered by an amplifier through a loudspeaker is V2/R (Volts x Volts ÷ Resistance) where V is the amplifier’s output voltage and R is the loudspeaker's impedance in Ohms. The higher the loudspeaker’s impedance, the less power a solid-state amplifier can produce, so you might see amplifier output power specifications that say something along the lines of 1000W into 2Ω, 500W into 4Ω, 250W into 8Ω.

There are several ways of measuring the output voltage produced by an amplifier – RMS, Program Power and Peak Power.

RMS stands for Root Mean Square and is a measure of the power that an amplifier is able to deliver continuously over a period of time, so it is absolutely, definitely, the most honest way of representing amplifier power.

If the amplifier specifications don’t say RMS, you can assume that you’re looking at Program or Peak Power. If the amp specs don’t state either of those, you can assume that the figures are for Peak Power. As a rule of thumb, Program Power is double RMS power and Peak Power is double Program Power.

It’s worth remembering that Peak Power is a measure of the absolute maximum output of which an amplifier that is being pushed to its absolute limit is capable of delivering over a millisecond or so, so please don’t be fooled into thinking that an amplifier with a 4000W Peak Power rating will be as loud as a 4000W RMS amplifier – if it was going to be able to do that it would have to be rated at 16000W Peak Power.

Loudspeaker (Input) Watts

For loudspeakers, the wattages are expressed in the same way – RMS, Program Power and Peak Power. However, this time the figures are telling you how much power the loudspeaker can handle before it distorts due to either heat build-up in the voice-coil (remember that electric fire) or short-term distortion from an amplifier’s peak output. Again, if there’s no indication of the type of figure you’re looking at, assume that it’s Peak Power and not RMS.

What About A Loudspeaker’s SPL?

A loudspeaker’s SPL figure is measured with an on-axis measurement microphone system at a distance of 1m while the loudspeaker is being driven by white noise delivered by 1W of amplifier power.

With passive PA loudspeakers, it is basically impossible to compare like with like as all manufacturers optimize their enclosures differently, sometimes matching an enclosure to the particular driver (chassis loudspeaker) or having a driver modified to match a particular enclosure.

As a result, the SPL produced by passive PA loudspeakers is almost always specified as a “Max SPL”, which is a theoretical calculation for a loudspeaker that tends to be somewhat higher than that loudspeaker’s measured value would be.

Although not representative of a product’s real-world performance, the Max SPL does give you a way to compare the relative efficiency between loudspeakers of identical driver configurations and wattages from different manufacturers.

You’ve already seen that if you double the wattage of the amps in PA system, you get a +3dB rise in its SPL that you can just hear and no more, that you have to quadruple the available wattage to get a noticeable increase in the level of +6dB, and that it takes ten times the original amp wattage to produce the +10dB rise in SPL that is necessary to double the system’s perceived volume.

So, if the difference in Max SPL between two speakers is +3dB, in the real world you‘ll need to double the amplifier power to the quieter one to even them up. If there’s a +6dB difference, you’ll need four of the quieter speakers (and four times the amplifier wattage) to make up the difference. If there’s a +10dB difference (you’ve guessed it) you’ll need ten of the quieter speakers and ten times the amplifier wattage to catch up.

Drivers Are Different

You might think that drivers (chassis loudspeakers) would have SPL figures, and you’d be right. However, a driver depends on its loudspeaker enclosure to “load it” (i.e. to give it a controlled volume of air to push against), and if it was being driven to any level in free air it would simply start to flap about. The solution is to use a standard measurement method in which a driver is driven by 1W of power and the SPL is measured at 1m from the center of its cone. You’ll see the result turning up in driver specifications as (for example) “Sensitivity 97dB@1W@1m” or simply as “Sensitivity 97dB”.

The same rules about “differences in dB” apply – a driver with a sensitivity that is 3dB lower than another, will require 2x the amplifier power to produce the same output SPL, 6dB difference will require 4x the power and a 10dB difference in sensitivity will take 10x the power to overcome.

This tidbit of knowledge becomes very useful when you’re looking to replace a blown driver or upgrade an existing speaker. If you know the sensitivity of the driver that you’re replacing, you’ll be able to maintain the existing performance of your loudspeaker if you pick a replacement with the same sensitivity, or even squeeze a bit more level out of it by fitting a unit with higher sensitivity.

So What Does All This Mean?

When it comes to using Watts to select and match amplifiers and passive loudspeakers, if the specifications that you’re looking at giving you both RMS and Peak power wattages, you can confidently pick the amplifier that matches your chosen loudspeaker, or the loudspeaker that matches your chosen amplifier, just by ensuring that the loudspeaker’s RMS and Peak Power handling figures are the same as, or higher than those of the amplifier.

SPL is an important consideration in the real world because continued exposure to SPL levels above 90dB can cause hearing damage - which is a major reason why every sound engineer should carry an SPL meter (there are several very good mobile phone apps available) and use hearing protection equipment when necessary.

When it comes to selecting passive loudspeakers, the Max SPL figure gives you a comparison of the relative efficiencies between loudspeakers, but it doesn’t really translate into real-world usage as there’s no way that you’d be able to stand in front of a PA system being fed with pink noise at maximum power without significant hearing protection.

It’s worth noting that active loudspeakers are a special case in that their amplifiers and drivers are carefully matched at the design stage in order to produce the best possible performance, so you simply need to decide the power you’re going to need for your gigs and then pick the active loudspeaker setup that will deliver it.

You Don’t Need To Go To 11

Driven at its rated maximum power, every BishopSound PA loudspeaker and sub is capable of approaching or exceeding the threshold of pain SPL of 120dB at 1m.

This means that, if you’re using a well-matched and correctly-configured BishopSound PA system that can handle the size of the venue, you’re never going to have to worry about SPL, power handling or short-term transient peak response. Instead, you can concentrate on creating your mix at the sound quality and volume level that matches your - and your audience’s - expectations.

For down-to-earth, practical advice on choosing and configuring your next PA system, please call Andrew Bishop from BishopSound on 01765 698233.