frequency response graph

[siluet]

how to convert the frequency response graph from micropascal μPa/V graph to dB graph? I.e. graph showing amplitude variations on Y-axis from +dB, 0, -dB

[jagabo]

dB is the log (base 10) * 10 of the amplitude. 0dB is an arbitrary nominal level, everything else is relative to that. For example if you decide that 1000 uPa/V is 0 dB then:

uPa/V, db
-----------
1000, 0
100, -10
10, -20
1, -30
0.1, -40
0.01, -50

[Verify]

Looked up a few comments:

Decibel (dB) is a comparative reference unit, 1Volt output per µPa at 1m distance (dB re 1V/µPa)

The unit "dB//uPa" represents decibels relative to one micropascal, and the micropascal is a reference standard for intensity in underwater sound.

dB
Decibel. Used to measure logarithmic ratios like signal to noise ratio (SNR), total harmonic distortion (THD), volume relative to a nominal level. Similar to percent (%) in that it has no units. dB SPL is used to measure sound levels relative to 20 micro-pascals (µPa). dBu is used to measure voltage relative to 0.775 V.

So jagaob's comments are right on.

[jagabo]

Algebraically:

dB = log(value/reference) * 10

Where "value" is the measurement in question, "reference" is the chosen value to assign as 0 dB.

[siluet]

how will look this frequency response graph?
http://img94.imageshack.us/img94/5353/frequencyr.jpg

[jagabo]

As it says in the label the reference value is 20 uPa/V. This is the reference level usually used for speaker frequency response and SPL graphs.

http://en.wikipedia.org/wiki/Sound_pressure

[siluet]

what is nominal 0dB level on my graph?

[jagabo]

20 uPa/V

dB = log(value/reference) * 10
dB = log(20/20) * 10
dB = log(1) * 10
dB = 0 * 10
dB = 0

The commonly used reference sound pressure in air is pref = 20 µPa (rms), which is usually considered the threshold of human hearing (roughly the sound of a mosquito flying 3 m away).

Speakers are usually measured in an anechoic chamber with a calibrated microphone placed on-axis, 1 meter away, using a 1 volt input signal. If a single SPL value is reported it is usually for a 1 KHz sine wave. Your graph shows 88 dB SPL at 1 KHz.

[siluet]

this graph isn't too informative.

[jagabo]

It's the standard type of graph used to report frequency response for speakers. What information are you looking for?

[siluet]

what is signal attenuation, dB, at 200Hz, 300Hz, 1000Hz, etc compare with nominal 0dB?

[jagabo]

Everything in the graph is louder than the chosen 0 dB reference. For example, at 1 KHz the output level is 88 dB. If you plug that into the equation:

dB = log(value/reference) * 10

You get:

88 = log (value/reference) * 10
8.8 = log (value/reference)
630,957,344 = value/reference

Meaning the volume produced by the speaker with a 1 volt sine wave at 1 KHz, measured 1 meter from the speaker, is 630 million times louder than the buzz of a mosquito 3 meters away.

Keep in mind that human hearing is roughly logarithmic -- it takes about 10 times more energy to sound twice as loud. So the range of human hearing is about 10 billion (10^10, or 100 dB) times the amplitude between the softest sound you can hear and when it starts to become painful.

Some other things you can tell from that graph:

At the low end the frequency response falls off below ~900 Hz. This means it has essentially no lower midrange and no bass output.

There is a huge rise at the high end, above about 5 KHz. This will make them sound very "bright". Cymbals, for example, will have a lot of "sizzle".

[siluet]

how much dB it makes sense to increase the response at the lower midrange, from 300 to 900Hz?

[jagabo]

Do we have to go over this again? You shape the equalizer curve to the exact opposite (upside down) of the frequency response curve:



One other thing you should keep in mind (assuming this is for the speaker in the cell phone you referred to in another thread): that frequency response graph may be of the speaker in an optimized enclosure, not in the cell phone itself.