Sharpness Analysis

Sharpness is a psychoacoustic attribute that describes the proportion of high-frequency energy in a sound. Sounds dominated by high-frequency content are perceived as sharp, piercing, or bright, while those dominated by low-frequency content are perceived as dull or dark. Sharpness is a key contributor to annoyance and sound quality in consumer products, vehicles, and HVAC systems.

Standard

Standard	Method	Year
DIN 45692	Measurement technique for the simulation of the auditory sensation of sharpness	2009

DIN 45692 defines the calculation procedure based on the specific loudness spectrum. It supersedes earlier proposals by Bismarck (1974) and Aures (1985) by specifying a standardized weighting function and normalization.

Theory

Relationship to Loudness

Sharpness is derived from the specific loudness spectrum N'(z) computed according to ISO 532-1 or DIN 45631. The specific loudness values are weighted by a function g(z) that emphasizes contributions above approximately 15.8 Bark (corresponding to roughly 3 kHz).

Weighting Function

The weighting function g(z) is defined as:

g(z) = 1 for z ≤ 15.8 Bark
g(z) = 0.066 · e^{0.171 z} for z > 15.8 Bark

Sharpness Calculation

Total sharpness is obtained by computing the weighted first moment of the specific loudness distribution, normalized by total loudness:

S = k · ∫₀^{24 Bark} N'(z) · g(z) · z · dz / N [acum]

The constant k = 0.11 is chosen so that a narrow-band noise centered at 1 kHz with a level of 60 dB and a bandwidth of one critical band yields exactly 1 acum.

Parameters

Parameter	Options	Description
Loudness Standard	ISO 532-1 / DIN 45631/A1	Underlying loudness model for specific loudness computation
Field Type	Free field / Diffuse field	Ear transfer function applied in the loudness calculation
Weighting Function	DIN 45692 (default) / Aures / Bismarck	High-frequency weighting variant
Signal Type	Stationary / Time-varying	Determines whether running analysis is performed

Output Quantities

Quantity	Unit	Description
S	acum	Overall sharpness value
S(t)	acum	Sharpness vs Time trace for time-varying signals

Sharpness vs Time

In time-varying mode, sharpness is computed at each time step using the instantaneous specific loudness spectrum. The resulting S(t) trace reveals how high-frequency content evolves over time, which is particularly useful for:

Engine run-up analysis where intake or turbo whine becomes prominent at higher RPM
Transient events such as switch clicks or relay activations
HVAC blower noise as fan speed ramps up

Sharpness vs RPM

Sharpness is also available as a vs RPM variant. The result is plotted over the speed of rotation rather than over time, which shows directly at which operating speed the sound becomes bright or piercing — the form needed for target setting, because the speed axis removes the influence of how quickly the machine was ramped.

Applicable to run-up and coast-down measurements; the recording must contain speed information.
Standards: the same basis as the vs Time variant — the underlying loudness model (DIN 45631/A1 or ISO 532-1) and the weighting function (Aures, DIN 45692 or Bismarck) are inherited from the analysis settings.
Speed step: 50 rpm by default; the slope (run-up or coast-down) is detected automatically unless it is set explicitly.
Result: one curve in acum per sound-pressure channel over the speed axis, with the mean over the speed axis reported as the single value.

For the parameters shared with the other speed-resolved psychoacoustic analyses and for advice on reading the curves, see Psychoacoustics vs RPM.

Interpretation Guidelines

Reference value: 1 acum is defined as the sharpness of a narrow-band noise one critical band wide, centered at 1 kHz, at a level of 60 dB.

< 1.0 acum — Low sharpness; warm, dull character (e.g., low-frequency hum).
1.0 – 2.0 acum — Moderate sharpness; typical for broadband noise with balanced spectrum.
2.0 – 3.5 acum — Elevated sharpness; noticeable high-frequency emphasis.
> 3.5 acum — High sharpness; piercing, potentially annoying sound character.

Practical Application Notes

Sharpness is strongly correlated with subjective annoyance. In many sound quality models, sharpness enters with a high weighting coefficient alongside loudness.
Reducing sharpness in a product sound can often be achieved by adding low-frequency damping material or modifying fan blade geometry to shift spectral energy downward.
When using Aures' weighting function instead of DIN 45692, sharpness values include a loudness-level dependency; results are generally higher for low-level sounds.
Always ensure the underlying loudness calculation uses the correct field type, as the specific loudness spectrum directly determines sharpness.

Standard References

DIN 45692:2009 — Measurement technique for the simulation of the auditory sensation of sharpness
G. von Bismarck, "Sharpness as an attribute of the timbre of steady sounds," Acustica, vol. 30, 1974
W. Aures, "Berechnungsverfahren für den sensorischen Wohlklang," Acustica, vol. 59, 1985
E. Zwicker & H. Fastl, Psychoacoustics: Facts and Models, 3rd ed., Springer, 2007
DIN 45631/A1:2010 — Calculation of loudness level and loudness from the sound spectrum — Zwicker method — Amendment 1: Calculation of the loudness of time-variant sound