I am an electroacoustics engineer. We are a rare breed. When I got my M.Sc. my class of six were the only ones in UK. I am now retired from engineering and audio is my hobby. You can find more about me on the about page on my profile.
The behaviour of the sound (acoustics) and how we hear it are very complex to formulate. We are dealing with the physical and the physiological, and at the interaction is our brain. We should not assign objective values to what we experience. They are not solid repeatable values. There are no Precision Instruments, nor Klippel to objectively measure how we hear. Add the complexity of music as the source material and it becomes impossible to formulate the sound coming from a speaker in a domestic environment with any precision. Think of this if you plan to read further. What I am about to explain is not objective but also not subjective. It is based on my knowledge that has been accumulated during 50 years as either an electroacoustic engineer, a recording engineer, a speaker manufacturer, and a Hi-Fi hobbyist.
How loud is loud?
When we hear a sound, depending on where we are, what mood we are in, what age we are, etc., we judge the sound level as low, normal, high, etc. Human
physiology psychology confuses our perception of sound levels. What we complain of to be very loud one day can be perceived as pretty normal the next day. Our hearing system is inherently auto levelling anyway but our brain plays tricks on how we perceive levels. Sitting alone in a room the sound level of a club dancefloor is unbearable but we do not complain about it when we are dancing for hours. This is not dissimilar to how we perceive changes between brightness levels. But unlike our eyes, high sound levels permanently damage our hearing. (Extreme brightness is known to damage eyes but it is almost impossible that we experience such high brightness levels in our daily life.)
There are two aspects of defining loud. One is for short periods and the other is accumulated dose. Short periods, of a few seconds are measured with a Sound Pressure Level (SPL) meter. Accumulation is measured by a Sound Dosimeter. It is the latter that almost all audio hobbyist are unaware of and it is lack of the latter that caused many 70s rock stars to become deaf.
How to measure it?
The sound pressure (P) is the average variation in atmospheric pressure caused by the sound. The unit of pressure measurement is pascal (Pa). Sound pressure level (SPL) is the pressure level of a sound, measured in decibels (dB). It is equal to 20 x the Log10 of the ratio of the Route Mean Square (RMS) of sound pressure to the reference of sound pressure (the reference sound pressure in air is 2 x 10-5 N/m2, or 0,00002 Pa). Or, in other words is the ratio of the absolute sound pressure against a reference level of sound in the air.
Naturally, the purpose of measuring sound is not only for music. If we are measuring noise, it makes sense to apply a filter on the type of frequency range it has as our hearing has different sensitivity at different frequencies. (Remember the Fletcher Munson curve?) Those filters are named A, B & C. dBSPLA is the most used as it is the industry standard to define how noisy any environment is. For measuring the level of music playing we should
not apply any filter. Music has the same range as our hearing system is designed for. Any filter will skew our readings.
SPL meters used to be prohibitively expensive for a hobbyist or so cheap that they were nothing but toys. This has changed with smartphone apps and modern devices, iPhone especially. Huge scale manufacturing and software calibration allowed an iPhone by itself to almost reach Class 2 classification (2dB tolerance suitable for everyday use outside a lab). This level of error is more than satisfactory for hobbyist’s needs. Do not take my word for it, read
the conclusion reached by the US CDC.
My favourite app is called SPLnFFT. It is iPhone only. You can
find it at the Apple App Store. Have I told you that it is free? It is incredible that such a powerful and elegant app is given away.
Will it affect my health?
THX standard says that at reference level (-20dBFS, the broadcast/professional reference level) a Pink Noise should generate 85dBSPL at the audience position. As you can see from the simple chart below this means the theatre's sound system is expected to produce 115dBSPL without audible distortion.
View attachment 173259
Let’s fire SPLnFFT app and start playing some soundtracks at the 95dBSPL average level. (Don’t try Pink Noise as it will be an unbearable experience.) This is what your app screen should look like.
EDIT:
Here are two 12 minutes samples of the Dolby Pro encoded stereo track of the films Team America World Police (2004) on the left and Dune (2021) on the right. You can clearly see that the soundtrack is routinely above the reference level and often stays at 10dB above, which is 95dBSPL.
View attachment 173398
The circles are at 10dB increments and the green to yellow transition is at -20dB
LKFS. Measured with
TC Electronics Loudness Meter.
EDIT ENDS
View attachment 173098
95dBSPL: Very noisy, exposure time should be limited to less than 2.5 hours. And, you are already in amber for the dose of sound your body can handle.
This is at 95dBSPL
average level. Imagine what will your body feel when you increase the level by 20dB to reach the peaks in the soundtrack?
If going to the cinema is bad for your health how come nobody talks about this. Well, a lot of people talk about it in the industry. Many theatres have overridden the calibrated levels of their sound systems because people were complaining. The main reason no action is taken is because we go to a cinema rarely. A frequent moviegoer is defined as someone who goes to the cinema once a month. But is that the same when you have a home theatre? What is the point of having a home theatre if you are going to use it just once a month, after all?
As far I can see, and I hope someone will show me that I am wrong,
watching films at THX levels frequently can be bad for your health.
Let us try music instead. Many posters on ASR talk about listening to music at an average level of 85dBSPL. This what I see while playing IGY by Donald Fagen (Opening track of the
Nightfly Album.)
View attachment 173105
85dBSPL: Too loud, exposure time limited. How long do you listen to music a day? Am I risking my health? I’m afraid all the medical studies available point us to the one answer: yes, you are. 85dB average dBSPL is too loud for enjoying music.
I hope I added something to the great cumulative knowledge of ASR. Feel free to ask questions, but please stay on topic. For instance don’t ask me why I used the Nightfly album. Well if you must; because it was once called "
one of pop music's sneakiest masterpieces," which I totally agree.
A member has posted this video on page 15. I copied it here for your information.
