# dB/hz to dB Conversion

I'm have a PSD graph of dB/hz vs. frequency in hz. I want to convert the dB/hz unit to dB, but I can't find any way of doing it. Help would be much appreciated!

Recall from Physics class that density is, mass per unit volume. As an example, the approximate density of water is 1 g/cm3, so 100 cubic centimetres of water weighs 100 grams.

Power Spectral Density (PSD) is simply average noise power per unit of bandwidth. Power is expressed in units of watts. In telephony measurements, power is commonly expressed in logarithmic terms with respect to a milliwatt, i.e., dBm. The unit (volume) of bandwidth is 1 Hz. So PSD may be expressed as dBm/Hz, pronounced, “dBm per Hz.”

PSD is to Total Power as Density is to Weight. All transmission systems have finite power and bandwidth. Let’s consider an ideal system with a flat PSD signature.

At each frequency between f1 and f2 Hz, Let PSD to be a flat -10 dBm/Hz, or 0.1 mW/Hz.

Then

PSD (dB) = PSD(dbm/Hz) * Bandwidth(Hz)

where bandwidth can be defined as f2 –f1.

If you want to calculate the total power there are more info available in the reference.

reference: PSD techniques by Peter Walsh, NCE

• Thank you! That makes sense, however, because of the nature of the data that I have, I can't use that method, as I only have a graph and not the exact data points. I suppose I would then rather have to convert my PSD results (from CFD) from Pa^2/Hz to dB/hz so that I can compare it to the results that I only have a graph for, instead of using sound pressure levels in dB..any idea of how to do that conversion? – clarice Aug 31 '17 at 16:58
• As $n\log{a}=\log{a^n}$, you should first convert from dB to physical units, then multiply by the bandwidth and finally convert back to dB. – user883521 Aug 31 '17 at 19:00