MATLAB: Confused about dB in MATLAB

Question

Hi;

I am fairly new to dB manipulations and I am getting really confused with it.

So if I produce a tone in MATLAB like this:

Fs = 16000;

duration = 60;

level = -2;

freq1 = 500;

t = 0: 1/Fs : duration*(Fs-1)/Fs;

fa1 = 10^(level/20);

y1 = fa1*sin(2*pi*freq1*t)';

sound(y1,Fs);

The dB level the user needs to input is in dB-full scale because the sound is being produced digitally correct?

So if I measure the sound using a dB sound meter, which measures the level in dB-SPL, I should be getting the max dB-SPL at 0 dB-full scale correct?

However, if I go down to -6dB-full scale, the dB-SPL goes down by 10dBSPL.

How can I modify the code such that -6dB-full scale causes the dB-SPL to be reduced by 6dBSPL.

I hope I did not confuse any one lol.

NOTE: my speakers are at MAX level

Thank you very much. Ali

Answer 1

Depends what you mean by full scale, but yes. As for what could cause a 6 dB reduction in input to give a 10 dB reduction in output there a a couple of things. Running at full scale for the soundcard and amplifier is probably not a good idea and can lead to nonlinear behaviour. Your sample rate of 16 kHz is unlikely to be the native sample rate of your soundcard. You are making a mono signal, but MATLAB converts it to a stero signal. Can you speaker handle this? Are you using your sound level meter correct?

I would connect the soundcard output to a volt meter and confirm with that.