Vehicles with electric drives are barely audible in city traffic: a great advantage when it comes to traffic noise reduction. Yet this characteristic is also controversial due to traffic safety issues. The automotive industry is working on giving electric cars a pleasant artificial sound.
In addition to zero emissions, it is one of the great advantages of electric cars: electric vehicles produce hardly any noise. Quietly humming, they glide through city traffic. Except for wind and the rolling noise of tires starting at a speed of about 30 km/h, little can be heard of the vehicle. For pedestrians, cyclists, and residents, low noise levels also mean less stress.
However, the sound of combustion engines is also an acoustic warning that has become firmly rooted in our heads over the past decades. “The sound of a combustion engine has established itself as the noise we expect wheeled vehicles to make,” says Tobias Beitz, head of Sound Quality & Sound Design at Daimler AG. “Electric vehicles lack this classic engine noise that signals the approach of a car and which other road users are conditioned to.”
To make low-speed electric cars perceptible for the surrounding environment, engine noises have to be generated artificially. They should be neither too loud nor too quiet; the source should be locatable and the sound pleasant nonetheless. The legal basis for the sound mixture is an EU regulation for electric vehicles. It stipulates the mandatory addition of a so-called acoustic vehicle alerting system (AVAS) by July 2019. By then, all brand-new models introduced to the market will have to be equipped with a system that artificially generates specific perceptible noises. Two years later, the rule will also go into effect for all models that were on the market before the original deadline and are resold after this time.
The sound the electric vehicles must emit is not specified in the regulation. It merely states: “The acoustic signal must clearly indicate vehicle behavior and be similar to the noise of a vehicle equipped with a combustion engine of the same class.” Presently, the artificial noise is mandatory only for speeds up to 20 km/h. European vehicles must also have a button the driver can push to temporarily deactivate the noise.
The European measure is based on regulations from the United Nations. The UNECE World Forum for the Harmonization of Vehicle Regulations has set a minimum volume. An electric vehicle will have to produce 50 decibels of sound at a speed of 10 km/h, and at least 56 decibels will be required at 20 km/h. For comparison, a car with a combustion engine emits an average of between 70 and 90 decibels when driving by, according to the German Acoustical Society. Thus, electric cars are quieter even with artificial sound. Without extra sound built in, slow-moving or idling electric cars emit approximately 35 decibels. “Electric cars run quietly; this a positive characteristic we definitely want to keep, because their lower noise emission reduces noise pollution in urban areas,” sound expert Beitz says.
The manufacturers have accepted the challenge of creating the necessary sounds by means of a sound module. Electric cars should hum and purr. “The sound must be harmonious, be compatible with the car, and not sound like a toy,” says Emar Vegt, sound designer for BMW. For this and other reasons, giving the sound of a combustion engine to an electric vehicle would not make sense to the Netherlands native, who plays piano and the drums. “An i3 or i8 represents the future.” In these BMWs, the artificial sound is produced by a speaker behind the front bumper.
“Yet our focus is also on creating and arranging interior sound because the driver of an electric car also wants to hear what he is doing. The sound of the engine and the transmission is part of the automobile; it is information and emotion. With this in mind, the plan is to have the driver perceive acceleration acoustically, such as by intensification of artificial sound inside the car,” says Tobias Beitz, who is also a hobby musician like his colleague Emar Vegt.
BMW and Mercedes alone have about 300 employees who focus entirely on sound sources and pleasant sound vibrations. The companies have built huge sound studios for this. The fine-tuning of sound done there ranges from the right engine noise to the question of how shutting trunk lids or doors should sound. As with design, the approval of the right sounds is an issue for the top management, which does this personally in the recording studio. “With the help of simulation models, optimization in control rooms, and direct road testing, the acoustic properties of relevant vehicle components are developed into a comprehensive vehicle sound,” says Beitz.
These are questions every automobile manufacturer will have to answer in the next few years. What will the automobile of the future sound like? It will be interesting to see if the different brands actually offer different sounds for their vehicles.