Sound Limiters Explained

This article is to help you understand how sound limiters actually work so you can ask the right questions to your venue and be able to decipher what the answers mean.

My venue has a sound limiter installed, how will this affect me?

Sound limiters are actually a very confusing and flawed design. When a venue tells you “the limiter is set at 90 decibels”, it doesn’t actually mean anything. There are several factors that can affect a sound limiter installed in a venue:

  1. Over what period of time is the sound being measured?

  2. How far away the measurement microphone is from the sound source (PA speakers & crowd in this instance). 

  3. How the sound is actually being measured (the weighting or bias, does it compensate for human hearing)

How does a limiter work?

Most sound limiters installed in venues will be the ‘cut-off’ type. These limiters have a light-up display on the measuring device (with a meter going from green on the left to red on the right), this is attached to a microphone that will be located somewhere inside the room.

When the sound level goes above the threshold for a given period of time, the electricity will be cut to the stage. Sound limiters can be affected by any ambient noise such as Music from a PA, instruments such as guitars or drums, people cheering/shouting/talking and even things like fireworks outside of the room.

Some venues will have a ‘traffic light’ system to let the DJ/Band know when it’s too loud, others will have a member of staff with a sound meter who will gauge if it’s too loud and ask the performer to turn it down.

 

Sound limiter being triggered by fireworks that are going off a good while away:

Footage of a limiter at its absolute maximum. Notice how you can clearly hear the small audience sing along over the music. Had I continued for another couple of seconds at this volume, everything including lights would have been cut off:


The Science & Jargon

What is a decibel?

A decibel unit used to measure the intensity (or power level) of a sound. There are many many different types of decibels (dBA, dBC, dBZ, dBFS, dBu, dBw, dBm and more)

In our case we are referring to A-Weighted Sound Pressure Level (dBA or dB SPL for short). This is a measurement of the intensity of the sound.

How is SPL measured?

SPL must be measured 3 different ways.

  1. Over a specified period of time (Peak, RMS, Leq)

  2. From a specified distance (How far away is the measurement microphone from the sound source)

  3. Over a specified frequency spectrum (Is the microphone measuring A, C or Z Weighted)

Is 100dB twice as loud as 50dB?

No, Decibels are a logarithmic measurement, not a linear measurement. For simplicity’s sake, a 6dB difference is what makes a sound twice or half as intense (or loud).

Twice as loud as 50dB is 56dB, twice as loud as 56dB is 62dB etc.

Half as loud as 50dB is 44dB, Half as loud as 44dB is 38dB etc.

In Summary

Before you end up boring yourself below. Here’s what you need to know:

  1. Sound limiters will generally measure ‘Peak’ SPL, this means ‘fast’ loud noises such as a drum can trigger them. This is a bad thing.

  2. Each time you double your distance from the sound source, you ‘lose’ 6dB SPL. If a sound limiter’s measurement microphone is placed 1m in front of a speaker, it will cut off a lot earlier than if it is placed at the back of the room 10m away (The music will have to be over 4x as loud for the music to cut off for the 10m away limiter). This will be a good or a bad thing depending on the venue. 2 examples: West Brewery’s limiter is at the back of the room and is never an issue. Melville castle’s limiter is on the stage…

  3. Sound limiters will generally compensate for human hearing & are less biased towards bass. This is a good thing however, some don’t! The Waterside doesn’t compensate for human hearing meaning that the PA system can easily be spoken over when playing full range audio.

 

Learn the difference below…


Decibels over time

Sound pressure levels can be measured either Peak, RMS or LEQ. Peak is a constant instantaneous measurement, RMS is a continuous average and Leq (sometimes written as LAeq) is an average over a long period of time (generally 15 minutes; written as Leq15 or LAeq15).

Unfortunately, Sound Limiters installed in venues will generally measure ‘peak’ SPL.

Rock concerts in Europe are generally limited to anywhere between 95-107dB Leq15 at the sound desk (40m-50m from stage).

What difference does this make in real terms?

Generally speaking, for recorded music playback a peak measurement will appear 6-12dB higher than the RMS level on the same meter. 96dB SPL Peak will be 84-90dB SPL RMS.

The difference widens for live music (if you have a live band, a snare drum can peak at 120dB but the overall RMS level of the band may be around 100dB). Wedding/Function bands will generally switch to an electric drum kit to allow the dynamic range to be controlled and reduce the difference between the peak & RMS levels.


Decibels over distance

If you can think of a speaker like a lightbulb or a torch. (For simplicity’s sake) Low frequency sounds (bass) are omnidirectional like a bare lightbulb, great for filling a room with light but useless at focusing in on one narrow spot. High frequencies (mid and treble) are more like a focused beam from a torch & will travel further inside the room. However, Bass frequencies will travel through solid surfaces such as walls, windows etc. This is where most noise complaints start.

Why is that relevant?

In the sound world, any single object omitting an omnidirectional sound is known as a ‘point source’.

According to theory, each time you double your distance from a point source, you will lose half the intensity (6dB SPL). 

For example: If a speaker is recorded at 100dB SPL at 1 meter, it will be 94dB at 2 metres, 82dB at 8 metres and so on....

This is because the sound is concentrated in a smaller area the closer you are to the speaker.

If the microphone for the sound meter is closer to where the PA is located, a lot less SPL headroom will be available.

This is known as the ‘inverse square law’

Ignore the numbers on this one, it’s just there to demonstrate how a light will become less intense as you get further away as it is spread over a wider area.

Ignore the numbers on this one, it’s just there to demonstrate how a light will become less intense as you get further away as it is spread over a wider area.


So how loud actually is 90 dB SPL?

decibels.jpg

Sitting in a busy pub/restaurant (150-200 people) with everyone having a conversation will typically sit between 85-96dB. Think about the bar area of The Counting House in Glasgow on a Friday afternoon.

90dB (RMS) is about as loud as a hairdryer measured from 1 meter.

A rock concert measured from 40m is usually around 115dB peak. As mentioned above, Rock concerts in Europe are generally limited to anywhere between 95-107dB Leq15 at the sound desk (40m-50m from stage). This will allow the engineer to ‘build up the set’ really slowly. For a 30 minute set it’ll likely come in loud for the first 2 songs then quieten for the next 2 (say 3.5 min per song plus a minute’s talking in-between), the second half will start off quiet and build up for the last 2 songs so the act can go out with a bang.

A sound limiter set to cut off at 90dBA (Peak) 8m away from the performing area will generally allow the DJ/Band to play at around 80-84dBA (RMS). That’s far quieter than the average you’ll hear at a concert and not much above talking level.


Decibels over a given frequency response

Sound meters can be set to measure using A, C, and Z frequency weightings. A weighted measurements compensate for the human ear’s frequency response (how the ear actually hears things at different levels). C weighted is flat response but takes into consideration the limits of human hearing (20Hz-20Khz), Z is completely flat.

What does that mean?

‘A weighted’ measurements are less biased towards bass frequencies, C and Z are more biased.

The good news is that most Sound Limiters will be set to measure ‘A weighted’ sound which means they don’t register as much in the low frequency region (similar to human hearing). This means the DJ/Band will be able to play adequate levels of bass through their speakers without worrying about the limiter cutting off. This ensures the sound remains balanced and won’t become harsh, tinny or ‘in your face’ sounding.

A weighted Decibels are represented as dBA or dB(A).

If your venue has a ‘C weighted’ limiter and tell you it’s set to cut off at 90dBC, this can equate to as low as 65dBA (as loud as a face-to-face conversation between 2 people). The Waterside’s Limiter is a great example. Set to 95dBC which equates to less than 69dBA when converting at 63Hz (a bass frequency).

This is bad.

See where 69dB lands on the graphic above. Quieter than a city street, bearing in mind your venue (without music) will be as loud as a busy restaurant purely with people having a conversation.

Any C weighted limiter should be set at least around 110-125dB. The West Brewery’s C weighted limiter is set to 120dBC (which equates to around 95dBA and is more than adequate to create an atmostphere)

 

Below is a graph showing what different weightings pick up across the frequency spectrum. Human hearing is limited from 20Hz to 20000Hz

Look at the difference in curves in the sub-bass and bass regions, A weighted shows bass between 10dB and 50dB quieter for the same signal.

Look at the difference in curves in the sub-bass and bass regions, A weighted shows bass between 10dB and 50dB quieter for the same signal.


And a little bit of fun…