Sound Pressure Level (SPL)
Sound Pressure Level (SPL) is a measurement of sound intensity level.
The lowest sound pressure which can be heard by humans is called the hearing threshold at 0dB.
The highest which can be endured before its painful is 130dB.
Humans perceive sound to be twice as loud when there is a 10dB difference. Every 10dB increase doubles how loud we perceive it. Every 10dB decrease means we perceive it to be half as loud.
In a home cinema it's important to keep ambient noise levels down as much as possible. By designing and engineering a lower noise floor we ensure that speech is more intelligible whilst we are watching movies. It also allows us to use lower power settings to the speakers, this gives them much better dynamics so they sound great. It’s also much better for our hearing to listen at lower sound levels.
When we measure SPL it is either ambient (as in sitting in a room) or for a ‘thing’, say a chainsaw for example. When we measure for a thing it must include the AT distance to make any worthwhile sense. For instance when we measure the trickle fan in our home cinema room it measures at a SPL of 39.9dB at 10cm away but 34.7dB at 1m away, so this equates to being about 50% louder (to our ears) close up to the fan than 1 metre away.
SPL from our trickle fan at 10cm distance
SPL from our trickle fan at 1m distance
Typical SPL Measurements
It’s useful to have some SPL measurements to gauge how loud things actually are. I’ve listed some measurements below. Each item below is twice as loud as the one above. Each item above is half as loud as the item below.
Hearing Threshold 0
Pin Drop 10
Broadcasting Studio 20
Whisper 30
Quiet Library 40
Quiet Office 50
Conversation 60
Noisy Restaurant 70
Vacuum Cleaner 80
Heavy Traffic 90
Jackhammer 100
Chainsaw 110
Discomfort 120
Pain 130
From the above we can work out that a noisy restaurant is typically 4 times as loud as a quiet office, and 8 times as loud as a Quiet Library. A vacuum cleaner is 128 times louder than the sound of a pin dropping.
Our cinema demo room without any A/V equipment switched on has an ambient level of 31.8dB during the day.
Ambient SPL in our Home Cinema Demo Room
In the video we show how the work we did with our acoustic seals on our entrance doors has a dramatic effect on the amount of sound that escapes/intrudes.
We can see that with music playing inside our home cinema room we measure an SPL of around 35-45 dB with the acoustic doors closed.
When we we open them this rises to an SPL of 55-65dB. Every 10dB increase is a doubling of volume levels (to human ears).
This means that the acoustic doors, seals and Batwings help make it sound only a quarter as loud outside the room with the door closed, or its 4 times as loud with the door open.
