Loudness measurement in sone (DIN 45631  ISO 532B)

Sound can be described with various physical parameters e.g. intensity, pressure or energy. These parameters are very limited to describe the perception of the human ear. A first approach is to take into account that the ear is less sensitive at lower and higher frequencies. The research work around 1920 from Barkhausen result into the well known A,B,C weighting curves. The frequency response of the ear depends on the sound level, that is the reason for different curves, which are valid for a certain level only. A pure sine at lower levels e.g. 40dB(A) has the same subjective loudness over the complete frequency range. However, in practice sine tones are not very common. For complex sounds and noise, this simple model is not valid anymore.

In most sound pressure meters available today you will find these techniques, because they are easily to derive from the physical parameters.

 
Loudness (Sone)

A wideband noise has a different subjective loudness than a pure tone at the same level measured in dB(A). Therefore, dB(A) is very limited in its usage although widely used. Zwicker analyzed various psychoacoustics effects thoroughly. A result was a model for the human hearing. A famous psychoacoustics effect is acoustical masking. A loud tone hides quiet tones. Nobody would expect to hear the sound of a mouse during a symphonic orchestra, although the mouse would audible in quiet phases. A very famous application of this and other effects are audio compression techniques like MP3. Only parts of the signal, which are audible according to a model, are stored.

Based on its countless hearing tests, Zwicker developed a model for loudness measured in sone. For stationary signals you will achieve much better results than the traditional dB(A). In contrast to dB(A) sone is a linear parameter. Two sone has double loudness compared to one sone. The reference level is one sone, which is equivalent to a tone with 1kHz at a level of 40dB.

As mentioned previously, sone is valid for stationary signals only. It is ideally in the judgment of air conditioners or typical PC noise like hard disks of fans.

This measurement describes the subjective loudness. However, it does not allow distinguishing between nice or annoying tones. Most people will agree that the sound of a flute is much nicer than a drill used by dentist although they both might have the same loudness.

The measurement of the loudness is based on the 1/3 octave analyzer. You will require both the loudness plug-in (#3010) and the 1/3-octave module (#3004). In addition, you have to calibrate the system. All psycho acoustic analysis requires absolute sound levels. This can be achieved with a sound calibrator or a sound pressure meter.

A complete measurement system for sone contains the following components:

PC/Laptop running Microsoft Windows

• WinAudioMLS mind. PRO Version
• WinAudioMLS plug-in „1/3 Octave RTA“
• WinAudioMLS plug-in „Sone/Loudness“
• Measurement microphone
• Sound-level calibrator
• Tripod

Measurement with low noise devices

Many sone measurements include analysis of low noise devices (e.g. beamer, notebooks etc.). The normal measurement distance is 1m. You can only perform a reliable measurement if the signal level is above the noise. The noise includes acoustic noise from the environment and noise in the measurement chain from the microphone or preamplifier. In a normal office environment at a quiet noise level of 45dBA it is impossible to analyze a fan with 25dBA. Since the human ear is very sensitive, such measurements require high quality measurement equipment.

For every measurement, we strongly recommend to measure the quiet noise first. Simply switch off the measurement device. The optimum measurement environment is a special anechoic chamber, which reduces reflections and noise from outside. We strongly recommend switching off noise sources if possible (windows, air-condition, computers etc.)

Many measurement microphones have a sensitivity of 30dB, which defines the lower bound for measurements. You can improve the measurement sensitivity by reducing the measurement distance from 1m to e.g. 25cm. This increases the signal level by 12dB. This offset can be corrected with a different calibration factor, which allows WinAudioMLS to show the measurements results with respect to 1m.

We will explain this with an example.

By reducing the distance to the half, the level increases by 6dB. With a distance of 25cm the correction factor is 12dB. If you use a sound level calibrator with 94dB, the reference level has to set to 82dB. A sound level of 82dB will be displayed as 94dB.  This approximation is only valid in the far-field with spherical radiation. In the near-field you will encounter e.g. diffraction, which has significant influence on the signal level. Therefore, we strongly recommend documenting this different measurement distance clearly, because it is difficult to compare different measurement distances.