Story 1
For MIRAI—For the Future
Chapter 2

Reducing Noise and Vibration—The Esoteric World of Sound and Vibration

Fumihiro Suzuki

Group No. 111
Compressor Division

Pinpointing the Source of Noise

The main challenges faced in the development of air compressors are improving fuel efficiency, responsiveness, and vehicle acceleration, and reducing noise and vibration, with these last two posing the most problems. How can we reduce noise and vibration—also referred to as NV? This issue weighed heavily on the minds of group members right through to the end of the development process. Fumihiro Suzuki was in charge of handling NV.

"Problems with noise and vibration plagued us right throughout development. When a customer was checking our first prototype, they commented that it sounded just like a lawnmower. We had to try a large range of methods to make it quieter."

So, Mr. Suzuki and his team thoroughly investigated the sources of the noise, and determined that there were four areas in which to take measures—reviewing the shape of the housing, reducing vibration in the rotor*5, reducing noise in the intake and exhaust systems, and tuning the sound.

"The first of these was reviewing the shape of the housing. First off, sound is generated by an item vibrating, and this vibration is transmitted into the air, where it is picked up by our eardrums and perceived as sound. Reviewing the shape of the housing and increasing its rigidity are effective in controlling vibration, and consequently in reducing noise."

The first thing they tried was eliminating waste from the housing, and making it more compact. Doing so reduces the area from which sound can be emitted. They also integrated the silencer*6 and intercooler*7, both of which had previously been external. Fitting the intercooler into the cylinder absorbs air intake pulsation*8, which is also effective in reducing housing vibration.

Mr. Suzuki thought back on their efforts. "How can we reduce vibration in a cylinder by connecting one part to another? Simulating fastening locations to optimally reduce vibration was also effective in reducing this."

They tried a variety of ways to reduce vibration, and incorporated those that worked, including positioning a rib*9 in the part of the cylinder that uses the most energy in compressing air, and sandwiching a resin sheet only a few microns thick between vibration-damping steel sheets.

*5: Rotor
Generic term for a rotating part of the compressor. This mainly refers to motors, shafts, and rotors.

*6: Silencer
A muffler. Reduces the loud noise generated when air under pressure is released into the atmosphere.

*7: Intercooler
Air under pressure is high temperature with low density. The intercooler functions in order to cool the air, and increase its density. It not only cools the air, but also functions as a muffler.

*8: Pulsation
A regular, rhythmic, movement.

*9: Rib
A part added to reinforce a plate or other thin section.

Stop Shaking!

What sorts of measures did you use to reduce rotor shaking?

"When a rotor shakes, it contacts the cylinder within a certain range, and this is where sound originates. Reducing shaking is also an effective way to reduce NV."

The rotor shaft is supported by three bearings. Mr. Suzuki thought that shrinking that gap between support points would minimize shaking.

"First, we measured the interference between the rotor and cylinder in a compressor. Then, using CAE*10, we analyzed rotor shaking, and calculated where we could place supports to minimize this. We then incorporated these changes in the machine, and reevaluated. We repeated this cycle over and over."

Were there any more effective ways to reduce shaking?

"Of course, increasing the shaft diameter, or using more rigid materials are easy ways to reduce shaking, but these measures make the housing larger, increasing costs. What we needed was to minimize shaking by changing only the interior, without changing the current size, shape, or materials."

Here we saw some of Mr. Suzuki's pride as an engineer. And so. If the size or shape of the housing is changed, then this will impact how the air compressor fits into the vehicle. Finding solutions given constraints is a problem inherent to development, and at the same time a thrill.

*10: CAE
Abbreviation of Computer Aided Engineering. Initial study of product and process designs, carried out on a computer.

Removing Sound with Barriers

They tried another approach to reducing noise and vibration—dampening sound in the intake and exhaust systems. An air compressor sucks in air from the atmosphere (intake), compresses this, and feeds it to the fuel cell (exhaust). The pulsation of the air vibrates the piping, causing sound, and this is a problem that needs to be addressed.

"First, in the intake system, we tried putting a barrier in the intake flange, to split the air path into two. Air that has been split into two then converges again, and when these flows meet, the pulsation in each is canceled out. This reduced the amount of sound generated of a particularly jarring frequency."

This was a high-pitched keening which after a time, is unpleasant. Even if the volume of this is reduced, it can still cause discomfort. The problem with sound is that improvements to both its volume and its range are required. What about the exhaust system?

"In the exhaust system, we did the same thing—put a barrier called a thermal wall in the exhaust piping in order to reduce pulsation. The aforementioned intercooler and vibration damping steel sheets within the cylinder also help to reduce exhaust noise. We also suggested that Toyota Motor Corporation install a silencer. They have been working to reduce noise and vibration with the help of the Toyota Group."

Create a Futuristic Sound!

Yet another approach is tuning the sound. Efforts up until this point have been in reducing noise and vibration, but the Toyota Motor Corporation is not simply after "a quiet FCV." They want a sound that conveys to the driver that sense of acceleration when putting their foot down, but the sound cannot be too abstract.

"Of course, whether a sound is perceived as good or bad is subjective, but voiced sounds like "ZZZ" lack clarity, and weren't an option. We wanted to create a clear sound, one that increased in pitch with speed. Because Mirai means future, we wanted to provide a futuristic sound."

Together with Toyota Motor Corporation, they set out to create a sound taking into account both the quantitative elements of volume and register, together with the qualitative elements of sensation and emotion.

"Reducing rotor shake was effective in terms of creating sound. Shaking generates a mid-range sound, and measures taken to reduce this have left a fairly linear sound. Other approaches at reducing noise and vibration all involved tuning, and instead of simply reducing NV, we found it very difficult to leave just pleasing sounds."

Complicating reduction of NV, already very difficult—the difficulties in also creating a pleasing sound cannot be overestimated.

Aggressive Development

What did Mr. Suzuki feel throughout all of these initiatives to reduce NV, and what did he learn from them?

"The factors behind us being unable to meet our ultimate objectives in reducing NV. This process was a series of setbacks, but each failure did lead to a new breakthrough. Also frustrating to us was that on our own, we could only carry out bench top testing of the air compressor. Therefore, it was very important that we request that Toyota carry out vehicle evaluations. In reality, we wanted to be able to mount it in the FCV for verification every time we tried something new, but this wasn't something we could do regularly. We did, however, run over 20 listening tests with the compressor in the vehicle, with the FCV driving on the Toyota test track. This involved driving around in the freezing cold, listening to the car (laughs)."

Development is a series of setbacks. Quitting because of failure means that all of your efforts have gone to waste. Of course, if we don't do something right, we will be criticized. But Mr. Suzuki was adamant—"We can use our failures in the next step of development, and as long as we don't give up, we'll get results." There is a strong feeling that this rocky development process is precisely what leads to success.

"This development process has taught me the importance of developing products from a vehicle-wide perspective—even a good product may not give the required results once installed in the car. To avoid complacency, it is important to go directly to the customer and get the necessary information, rather than sitting back and waiting for them to come to us. We've done this much, and it is certainly difficult, but I feel that proposing how we'd like things done, and sorting out materials that will guarantee satisfaction is very important."