Deadening an electric car

When deadening a car we are fighting against three key noise areas: engine / exhaust, rolling noise and wind.

The most obtrusive noise starts with the engine, so if we remove it do we have a whisper quiet car? Not quite. I remember a Top Gear Nissan Leaf review where they mentioned that the headlamps had to be redesigned to channel the wind away from the mirrors because the wind noise was too audible, something that would have not been an issue had an engine been there masking the wind noise.

Electric car manufacturers need to take special care about making a car as aerodynamic and deadened as possible to make it comfortable for daily use. So is an electric car perfect for someone like me who is always in the pursuit of a whisper quiet car? Indeed!

Meet the Ampera

I didn't want to buy another car that wasn't an hybrid or electric. A pure electric (or BEV) was out of question as the range is very limited for my usage. So it had to be a PHEV or EREV.

The Opel Ampera was the best that the used car market had for me. A decent electric range and a small petrol engine that works as a generator and can be used with the same performance as when driving in pure electric mode. It looks good, the interior is also nice (but no cream leather option unfortunately) and they were built in small numbers, so an exclusive car that I won't see everyday.

On the 7th of December I got on a flight to the Netherlands and drove it back to Portugal.

Opel Ampera
Opel Ampera

I was apprensive at first, I know the car is very quiet in town, Chris Harris tested it and said that it is quieter than a Rolls Royce at low speeds. I even found a 2014 article titled In search of the world's quietest car and the Ampera came fifth behind an Audi A3 (I'm impressed), a Bentley Mulsanne, a Rolls-Royce Ghost and a Mercedes S-Class. For comparison, in 9th place came the Zoe, another electric car where there was less attention to sound deadening from Renault.

I also found some documents from Chevrolet talking about some of the measures taken to make the Volt quieter without adding too much weight. The car was built from the ground up with deadening in mind. The Opel Ampera is the European version of the Chevrolet Volt, exactly the same car with different bumpers and lights.

On this 2500+ km journey I got to know the Ampera pretty well, and it felt almost as quiet as the C6 on the highway, but winning hands down in town with absolute silence.

A very quiet car

Working in electric mode it is amazing in town and up to 80km/h. It feels as quiet as the C6 and after measuring, it really is. The C6 after the sound deadening (that was not finished) had 59dB at 80km/h and 62dB at 120km/h. The Ampera has the same result at 80km/h and only 1dB more at 120km/h.

A great result, and shows that GM knew what they were doing. Let's not forget the C6 had thick laminated side windows, lots of deadening material and is two car segments above the Ampera, the Ampera being a C-segment and the C6 a E-segment.

Of course these numbers are only indicative, and there is more rolling noise and a bit more wind noise than on the C6. The engine would mask this, but the engine was somewhat nice at highway speeds, a quiet low frequency rumble.

Making the Ampera even more quiet

The most obtrusive noise is rolling noise. I'm sure the tires play a big part in this, as the car is equipped with Michelin Energy Saver tires rated at 71dB, on the C6 I had the Michelin Primacy 3 rated at 69dB. Also the wheel well covers are made of plastic instead of fabric.

There is also some wind noise at speed, and that's it. The doors have a nice sound when closing, there is no resonating panels because Chevrolet applied liquid deadeners to the body structure, tuned to meet the Volt/Ampera metallurgical resonance needs. This material is lighter than butyl based products and easier to apply by a machine.

Liquid deadeners used in the boot floor
Liquid deadeners used in the boot floor
Liquid deadeners used in the boot floor
Liquid deadeners used in the boot floor

This is great but will make my life harder as it will be very difficult to apply Dynaliner on those areas, primarily in the boot. Ampera is a hatchback and doesn't even have a proper parcel shelf, it's made of fabric! So a lot of noise that originates in the boot gets easily into the cabin, mostly tire roar.

Also the subwoofer is there as there is a ventilation unit for the batteries, so I can't close off that area of the car.

The boot floor metal doesn't resonate at all, sounds very solid. There is also a plastic cover that has some deadening material applied underneath, looks very similar to 3M Thinsulate. The same material is applied on other areas of the car.

Boot floor plastic cover with deadening material
Boot floor plastic cover with deadening material

With the plastic cover in place there is still some metal visible. On top of this is the boot floor mat that isn't as thick as I would like it to be.

Boot floor plastic cover with deadening material
Boot floor plastic cover with deadening material

Another issue that I didn't have in the 406 or the C6 is the noise of things inside the car moving about on corners. The glove compartment and the all the storage areas had fabric all around, so things would stay in place and even if something moved there would be no noise.

The Ampera only has a rubberized mat on these areas, the rest is plastic, so at some corners I can hear things moving about in the armrest console and on the dash.

Planning the sound deadening

Rolling noise, petrol engine when accelerating hard, wind noise, things moving about on the interior. These are the key points I want to improve on the car.

Unlike the 406 and the C6 I will only apply Dynamat Extreme in sections to stop panel resonance, unless there is a clear advantage to do so (blocking heat for example).

Dynaliner will be used to block heat from the outside and keep the car cool in the summer, as the interior is all black. This will most likely be applied in the doors, pillars and maybe the roof.

I will probably start with adding Dynamat to the bonnet / hood as it might be the easiest step, and then see what I can do about the rolling noise.

Opel Ampera
Done in 10 of February, 2018