General comments:
(1)The usual measurement is in dB, but this is a relative measure compared to the standardized threshold of just audible human hearing. The absolute measurement is related to the maximum possible recording value of the instrument. These values are generally minus values. I calculated a correction factor to convert the negative values of the recording instrument into positive values of the usual dB measure.
(2) I made five measurements per shutter speed, measuring the minimum, maximum and average values. In all cases I used the RMS values, because these give a more reliable result.
(3) I noted that there is a substantial variation between samples, not only with the mechanically governed shutters, but also the electronically controlled ones. Therefore I used the average value of all measurements per shutter speed.
(4) The variation between shutter speeds (1/4; 1/60; 1/250/1/1000 sec) is minor, at least at 1 meter distance. Example: the M Monochrom II has max dB values of 47.86; 51.55; 51.58; 51.30. The M3, often accepted as the quietest rangefinder camera has max dB values of 54.88; 54.46; 53.53; 53.53. The higher level noise of the 1/4 sec is caused by the additional sound of the retarding gears. The M7 has dB values of: 52.99; 51.96; 51.88; 51.80.
The loudest camera is the Canon L1. The dB values are: 59.25; 61.73; 60.92; 60.41. The Canon has the loudest shutter, but the variation between samples is negligible. To simplify the results I selected the 1/60 sec as the reference speed.
(5) Some typical sound levels: normal conversation: 60 dB; humming of refrigerator: 45 dB; noise level of typical urban roads daytime is 55 dB and night time is 50dB; a quiet library is 40 dB.
Below are the values of all tested cameras with the max RMS power at 1/60 sec.
Canon VI-L——————————— 61.73
Leica M-A——————————— 54.82
Leica M3———————————- 54.46
Leica M7—————————— 51.96
Leica Mono II———————- 51.55
Leica M8.2 standard———————— 49.09
Leica M8.2 discrete———————— 49.92
Leica M10-D———————— 42.00

It is evident that the M10-D is the quietest. A picture taken with the M10-D in a quiet library is still audible. But every camera, with the exception of the Canon is inaudible in normal conditions, like conversations and urban streets in day lime. The M3 and M-A are for all intents and purposes equal. The M7 has a remarkable low level and compares favorably to the MMono II (and presumably all versions of the M240). The M8.2 in discrete mode is louder than in standard mode. The discrete mode is identical to the standard mode, but with a time lag for the sound of the closing blades.
For a correct interpretation of the figures it should be noted that the equipment has a tolerance of 1 dB and the figures should be rounded to the first decimal. The usual interpretation tells you that a difference of 3dB represents a doubling or halving of the sound level. Note also that the peak noise is the sound of the brake: a rather short sharp click. The cameras with the Copal shutter have a more subdued noise that is more pleasant for the average person.