Hear That? Tonality in Product Design

When designing a product, a lot of thought goes into how it will work. But what about how it will sound? A squealing hairdryer, whistling laptop, or buzzing electric car would quickly be rejected by consumers. To improve the sound quality of their products, engineers analyze a variety of sound metrics, including the measure of frequency components in the overall sound — or tonality.  

What is Tonality?

Tonality, or tonalness, is a psychoacoustic indicator that identifies emerging tones over background noise to determine their contribution in the perception of a sound. The tonality of an acoustic source is one of the main metrics generally looked at to improve the sound quality of a product, together with loudness, sharpness, roughness, and fluctuation strength. 

Quantifying tonality means translating how “tonal” a sound is perceived by the human ear into objective metrics. In industrial applications, such as automotive, aeronautics, railway, or consumer goods, tonality expresses the relative weight of the tonal components over their surrounding frequency contents.

Tonality Example: Two Airplanes

To understand how tonality affects what we hear, listen to the difference in tonal scale between these two airplanes. 

In both files, you can hear the blade passing frequency (BPF) caused by the turbines. This corresponds to the turbine rotational speed multiplied by the number of blades. You’ll notice that airplane two sounds more tonal than airplane one. This is because it contains frequencies emerging from the rest of the sound. When a tonal sound is highly perceivable, simply put, it’s annoying. 

The BPF itself, and the surrounding noise, will have an influence on our perception of the tonal content. The more energy around the emerging frequency, the less perceivable the frequency will be and therefore the less tonal the sound.

Tonality in Psychoacoustics

Several psychoacoustic indicators are commonly used to calculate the tonality of acoustic signals, with international standard calculation methods:

Note: In Ansys Sound software, for each of these psychoacoustic indicators, the computation profile includes an automatic tone detection procedure.

For instance, the tone-to-noise ratio (TNR) is the energy ratio between a tone and the noise critical band (CB) centered on it:

The prominence ratio (PR) is the energy ratio between the critical band around the tone and the two closest ones:

Simulating Tonality

Assessing tonality is critical in a wide variety of applications, such as electric motors, HVAC systems, and construction machinery. But in these complex systems, identifying the single source of a sound can be incredibly difficult. To break noises into different sources and analyze how they each affect the overall sound of the system, engineers can use Ansys Sound to simulate and test data.

Fan Noise Simulation

In this example, engineers used Ansys Fluent and  Sound to improve the sound quality of a fan. The computational fluid dynamic (CFD) simulation performed by Fluent made sure the fan was designed properly, while Sound made sure the noise from the fan was as it should be.

To predict the influence of design on the sound quality of the fan, the second fan was given a modified design with twisted blades. Modifying the design divided the tonality by about 3 tu according to both Aures and ECMA74 models. For theTNR, only the 260 Hz tone emerged in the modified design. Through this simulation, engineers confirmed that changing the shape of the blades would reduce the tonal content of the noise and therefore improve the overall sound quality.

E-motor Noise Simulation

Another example where tonality can negatively affect the user experience is electric motors.

Electric motor noises can be annoying due to the high-pitched tones from the rolling and aerodynamic noise. Those tones, which evolve relatively to the speed of the motor, are called “orders.” 

Thanks to Sound, the tonality can be computed looking at the TNR or the PR of the orders generated by the e-motor. This shows two strong tonal emergences of order 48 and order 60, which would likely be annoying to the driver and the passengers.