Whether it’s an emergency announcement or page in a school, how do you determine the speaker and amplifier required for a specific location? IP paging systems can provide exactly the right sound level to exactly the right location, but what is the right sound level?
How loud does the emergency paging system need to be? How much sound is needed to hear an announcement in a classroom or in a sports field where there is a lot of background noise?
This article provides the technical background and practical information needed to help you determine the right power and speakers required.
What does “loud” mean?
Just how loud does sound need to be to hear it and how loud is too loud? If you ask the people in the city, too loud is the noise from traffic or the noise from the garbage collection or in the suburbs the sound of the lawn mower too early in the morning. Sound levels can even be loud enough to damages your hearing. On the other hand is the sound loud enough to be heard?
We measure the loudness of sound in decibels (dB). It is sometimes referred to as the Sound Pressure Level (SPL). 0 dB is the minimum sound level a person with good hearing can hear. 130 dB is the point where the sound is painful. Even sound levels above 85 dB can be a problem. Most experts recommend that you use earplugs when continuously exposed to 85 dB and above. But what does 85 dB mean? The following chart shows common sounds and their associated sound levels.
|20 dB||Ticking watch|
|30 dB||Quiet whisper|
|40 dB||Refrigerator hum|
|60 dB||Normal conversation (about 3 ft. away)|
|70 dB||Loud singing or Washing machine|
|80 dB||Alarm clock (two feet away)|
|85 dB||Average traffic|
|100 dB||Blow dryer, subway train|
|105 dB||Power mower, chainsaw|
|110 dB||Screaming child|
|120 dB||Rock concert, thunderclap|
Sound Levels and Distance
The further away from the speaker the lower the sound. Sound level is reduced using the law of squares. As an estimate, the sound level is reduced by 6 dB for every doubling of the distance.
The following chart provides an example of how the sound level is reduced.
|Distance||Sound level||Is about like the sound of|
|1 M (3.28 ft.)||100 dB||Blow dryer|
|2 M (6.56 ft.)||94 dB||Loud traffic or diesel truck|
|4 M (13.1 ft.)||88 dB||City traffic inside the car|
|8 M (26.25 ft.)||82 dB||Telephone dial tone or loud singing|
|16 M (52.49 ft.)||76 dB||Vacuum cleaner, shower|
|32 M (105 ft.)||70 dB||Single Passenger car.|
|64 M (210 ft.)||64 dB||Loud conversation|
|128 M (420 ft.)||58 dB||Normal conversation|
Selecting the Right Amplifier
A speaker is specified by the amount of sound it can provide at a distance of 1 meter and a power level of 1 watt. It is also defined by the nominal speaker angle and the maximum output at a certain power level.
By adding power to the speaker (measured in watts) we can increase the sound level. The sound level increases by 3 dB for every doubling of power (watt). Each type of speaker has a maximum limit to the power that can be applied.
The following chart provides an example of the sound level based on power applied to the speaker.
If a speaker is rated at 100 dB at /1 w/1M, then the following sound levels apply:
|Power in Watts||Sound level at 1 Meter away from speaker||Is about like the sound of|
|1 watt||100 dB||Blow dryer|
|2 watts||103 dB||Power mower at 3 ft. away|
|4 watts||106 dB||Chain saw|
|8 watts||109 dB||Screaming child|
|16 watts||112 dB||Jet engine at 325 ft. away|
|32 watts||115 dB||Sandblasting|
|64 watts||118 dB||Hearing damage possible|
|128 watts||121 dB||Rock concert, thunderclap (close)|
What is the right sound level?
The lowest sound level most of us can hear is about 20dB. Normal conversation is at between 58 dB and 65dB. The threshold for pain and hearing damage is about 130 dB. We want to select sound levels that are between these extremes and hopefully above the background noise. The optimal volume is the point where everyone hears the page.
How to do it
To select the right components for the paging system, we start at the point where people will be listening to the sound, and then work backwards to the speaker and amplifier. Here’s an example that shows how to select the right sound level in a classroom. To make sure everyone hears the page in a noisy room, the sound level needs to be at least 10 db above the ambient noise level.
If we are in a classroom that has a noise level of 60 dB, then we should provide a minimum sound output of at least 70 dB. If the speakers are located in the ceiling which is 3 meters (10 ft.) high, and the children are sitting at their desks, they are about 2 meters (6.5 ft.) away from the speakers.
To calculate the right sound level from the speaker we can use the chart of distance vs. sound level. If we start with 78 dB at 1 meter from the speaker then at 2 meters (6.5 ft.) the sound level is 72 dB.
One other thing to consider is the angle of the speakers. In the classroom, we would like to select speakers with as wide an angle as possible. This allows us to use fewer speakers and still have everyone hear the page. As an example, there are some ceiling speakers that fit into a drop ceiling with 100-degree angles. This means that the maximum sound will be heard in a circular area with a diameter of about 15 ft. In most situations, we can get away with one speaker in a classroom that’s about 25 ft. x 25 ft. in size. The power required is usually only 0.5 watts.
The sound level of the paging system needs to be loud enough so that people can hear the announcement over the background noise, but not loud enough to hurt their ears. We can select speakers for the wall, ceiling, or even mount them on a pole. We just have to know the sound required at a certain distance and we can work backward to select the speaker and amplifier power.
If you need help with your emergency paging or classroom notification system, just contact us. We have a lot of experience with these systems so I’m sure we can be helpful.