Total loudness tells you how loud a stationary sound is perceived to be. Specific loudness explains where that loudness comes from in the auditory frequency scale. In a Zwicker calculation it is written as N′(z), measured in sone/Bark.
What is the Bark scale?
The Bark scale is a psychoacoustic frequency scale based on auditory critical bands. It runs approximately from 0 to 24 Bark over the main hearing range. Equal numerical steps on this scale are meant to represent comparable positions in the ear’s frequency analysis, not equal steps in Hz.
That matters because hearing does not treat every frequency as an independent narrow channel. Nearby frequencies interact within a limited bandwidth, and strong sound in one region can reduce the audibility of sound above it. Critical-band processing is one reason a simple line graph of dB SPL does not describe perception by itself.
What does sone/Bark mean?
Specific loudness is loudness density along the Bark scale. A value of 0.8 sone/Bark at a particular Bark position means that region contributes that density of perceived loudness. It is not the loudness of one isolated sine tone, and it should not be confused with the dB level shown on the source chart.
How to read a specific-loudness curve
The horizontal axis runs from low to high Bark position. The vertical axis is sone/Bark. Broad areas under the curve contribute more to the total than narrow spikes with the same peak height.
| Curve feature | What it can suggest |
|---|---|
| Broad, low-Bark contribution | Low-frequency energy is making a substantial loudness contribution after hearing corrections. |
| High peak in the mid Bark range | A concentrated mid-frequency region is strongly affecting the perceived loudness. |
| Upper-Bark tail | Higher-frequency content is contributing, subject to threshold and masking effects. |
| Similar total, different shapes | Two sounds may be equally loud overall while having very different spectral character. |
The curve is best used for comparison and diagnosis: for example, to see whether an EQ change shifts loudness toward low, middle, or high auditory bands. It should not be read as a precise map from one Bark value to one single physical frequency.
Specific loudness versus a frequency response
A frequency response charts physical level against frequency. Specific loudness charts a psychoacoustic estimate against Bark. The two curves can look quite different because the Zwicker process accounts for threshold, auditory transmission, critical-band grouping, and upward masking.
This is also why A-weighting is not a replacement for specific loudness. A-weighting applies a fixed frequency weighting to a level measurement. Specific loudness changes with level and with the rest of the spectrum. A spectral component can be affected by energy in nearby bands, especially above it in the auditory excitation pattern.
How the calculator creates the curve
Acoustic Converter uses the declared input spectrum to calculate a 0.1-Bark pattern from 0.1 through 24 Bark. That gives 240 specific-loudness bins. The green chart is the pattern; the sone result above it is the integrated total. Exporting CSV includes both the derived one-third-octave input and every Bark-bin value, so you can inspect or compare results elsewhere.
For the pattern to be meaningful, start with a calibrated spectrum. A relative “0 dB at 1 kHz” curve can show spectral shape, but it needs an absolute SPL reference before it can yield sones or sone/Bark. The Zwicker loudness guide explains the accepted input routes.
Example questions it can answer
- Why did two spectra with similar overall SPL produce different total sones?
- Did an EQ change make the sound’s loudness more low-frequency or mid-frequency dominated?
- Does a visible high-frequency boost meaningfully change perceived loudness at this level?
- Which auditory region changed when a speaker, room, or filter response changed?
For a complementary explanation of frequency-dependent sensitivity, read Fletcher–Munson versus Zwicker loudness.