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Distributed Speaker System SPL Calculator

This distributed speaker system SPL calculator takes the guess work out of which speaker tap to use. It will tell you the correct power tap for each speaker in your design. The calculator is really useful when balancing levels of different speakers. Or when the speakers are mounted at varying distances from the listener. It also helps in determining the amplifier power needed to deliver the required Sound Pressure Level (SPL) at the target audience.

For those who have played with this calculator previously, a blank version for use is just below. If you want to play with a working example of the calculator to see it in action, click here. If you need help in using the calculator, below is an explanation of all the fields and some user tips.

For further information on distributed speaker systems (often called 100-volt or 70-volt speaker systems) see my article on Understanding Distributed Speaker Systems.

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Explanation of Distribution Speaker System SPL Calculator

As with any of my calculators, you just need to fill in the white blank fields. A short explanation of each field follows.

Starting with the table at the top of the calculator:

Select the units of measure, either metres or feet. The selected unit of measure will be used for the distance from the speaker to the target audience.

Enter the required SPL at the target audience. This will depend on the ambient noise level, the purpose of the sound system (background or foreground) and how loud the system needs to be among other factors. As a guide:

  • 65dB – 75dB for quite areas like offices
  • 75dB – 85dB for louder areas like shops
  • 85dB – 95dB for loud places like sporting fields

Select amplifier power output. Most SPL calculators tell you the maximum SPL possible with the amplifier running at full power. This distributed speaker system SPL calculator will do that and more, (or should I say, and less). If required, you can select full power if you want the amp to run at full power with no headroom. Simply select “Full”

You can also calculate the SPL and speaker power taps with the amplifier running at less than full power, which is often the case, and mostly preferable. As the amplifier output power is reduced, the effective power of any connected speaker is similarly reduced, as is the amplifier output voltage. For your information, the following table shows the power and voltage levels along with the effective power rating of a 30 Watt 100 volt speaker for various levels.

Amplifier PowerAmplifier VoltagePower of 30 watt
100 Volt speaker
Full100 volts30 Watts
1/2 power (-3dB)70.7 volts15 Watts
1/4 power (-6dB)50 volts7.5 Watts
1/16 power (-12dB)25 volts1.875 Watts
Low power (1/100, -20dB)10 volts0.3 Watts

I suggest you try “1/16 (-12dB)” first. If the speakers can deliver the required SPL at this power setting, then the amp is not going to work hard and you have plenty of room for extra level if required. If this selected level doesn’t deliver sufficient power, try “1/4 (-6dB)”, as this level stills offers headroom.

Select the acoustic space reflectivity/reverb. This is where any pretense of this calculator being accurate goes out the window. The calculation for SPL loss in the air is based on having no reflections to allow the sound the decay naturally. Specially designed rooms or outdoors are common examples of “dead” acoustic spaces. In such cases, select “Dead”, and no allowance will be made for reflections.

In most rooms there will be some reflections which decrease the natural losses. This calculator allows you to select if the reflectivity or reverb time of the room is judged to be Low, Mid or High.

  • Low (+2dB) would be for a space with lots of curtains, carpeted floor and soft furnishing. The small reflections within the room wont allow the sound to decay as well as if in the open air. So the air loss might be around 2dB less than in the open air. Or in other words, the sound level at the target audience might be increased by around 2dB.
  • Mid (+4dB) would be for a space with some reflections which might add around 4dB to the SPL at the target distance.
  • High (+6dB) would be for a space that is highly reflective with substantial reverb. Large rectangular spaces with wood or concrete floors, and acoustically reflective walls will often increase the SPL to around 6dB at the target audience.

The selection you make here is very subjective, and maybe a guess, but at least you have the ability to allow for the different acoustic spaces. If in doubt, choose “dead” for outside open areas, and “Mid(+4dB)” for indoors.

Calculate Speaker Taps

OK, you have now entered the required SPL, amplifier power setting and selected the acoustic space. Now you can now have fun with different speaker arrangements and calculate the tap setting required.

Start with “Speaker 1” and work down the table for each speaker. The calculator allows up to four speaker types to be calculated.

Speaker Description: This field is optional. It can be used to describe the speaker and anything unique to it. For example, a ceiling speaker might be mounted at a height of 3.2 metres. A while a similar speaker might be mounted on ceiling that is 5.2 metres high. There might also be a box type speaker mounted 5.2 metres high. Entering these descriptions for each speaker install is useful to avoid confusion. If entered, these descriptions are also used in the graph at the bottom of the calculator.

Speaker Sensitivity: This figure is normally found in the specifications for the speakers. For example, a ceiling speaker specs might say “Sensitivity: 90dB (1w/1m)”. For an explanation of speaker sensitivity and how we use it to determine the SPL of a speaker, see my article “Understanding Speaker Sensitivity

Distance to Target: This is the distance from the speaker to the listener. For ceiling speakers it would be the height of the ceiling minus the sitting or standing height of the listener.

Once these figures are entered, the following are calculated for each speaker:

SPL loss due to Distance: This is the SPL loss through air. This figure is dependent on the distance between the speaker and the listener, and the acoustics of the room/space (that you entered at the top of the calculator).

Required Speaker gain over 1 watt: We hardly need a spreadsheet to calculate this. It is the SPL of the speaker at 1 watt, minus the air losses, compared to the required SPL. In the example calculator below, speaker 3 will produce 84dB with 1 watt going into it, when you measure the SPL 1 metre away. So 84 minus the air loss (8dB) leaves us 76db. This is 6dB below the required 82dB, so the required gain from the speaker is 6dB. This is how much power over 1 watt we need from the amplifier for the speaker output to reach the required SPL.

Speaker power required: This calculation converts the required dB gain over one watt into watts. It also takes onto account any “de-rating” of the speaker power due to it not being driven to full capacity (see table above).  This figure is the theoretical value the speaker tap should be set at to attain the required SPL at the target distance.

However, most speakers will not have a tap at that precise level. So, on the next line you need to input the value of the closest tap to the theoretical level.

Actual power tap setting: In the example below, the theoretical speaker power required for the ceiling speakers is 1 watt and 4 watts. However let’s say our ceiling speakers have taps at 10, 5, 2.5 and 1.25 watts.  For speaker 1, we could use 1.25 watts instead of the theoretical 1 watt. For speaker 2 we could use 5 watts, instead of the theoretical 4 watts. Speaker 3, the box speaker might have taps at 30, 15, 7.5 and 3.75 watts. Choosing 15 watts instead of the theoretical value of 16 watts, results is only 0.3dB difference in SPL at the target audience distance.

Calculated SPL at target: is the SPL at the target audience distance with the actual speaker power tap. These results are displayed in the graph at the bottom of the calculator.

Difference to required SPL: is the difference between the calculated SPL for the speaker tap, and the required SPL. This number will be positive if the speaker output is above the required SPL. It will be negative if it is below the required SPL. If the difference is only 1-2 dB, it will be barely noticeable. If the difference is over 6dB, then a different speaker or different tap should be used.

The graph reflects any change to any of the values in the tables. It is interesting to change the amplifier power to “Full”, you will see the SPL level raises, as you would expect. However, the dB difference between any of the speakers to each other is the same.

Once you know what taps you will be using for each speaker type, you can multiply the number of similar speakers by the actual tap value to determine what size amplifier you should use. Keep in mind, that good practice suggests to use an amplifier 20-25% greater than the total load.

Further reading

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Please Note: Due to other commitments I'm currently unable to spare the time required to answer further questions in the way I'd like. Hopefully this is a temporary situation. I'm sorry for this inconvenience.