Zero emission buses highlight a well-known environmental pollutant: NOISE

By Steve D. Johnson

So far in this series we have discussed several areas where the HVAC can have a direct effect on the operation of the zero-emission battery bus (ZEB), including HVAC efficiency, passenger comfort, maintenance, and battery range. While bus cabin noise level has long been a concern for operators, specifications continue to become more stringent. Adding to this is the absence of engine noise making the HVAC noise more noticeable and the main focus of attention. How much noise is too much noise? That can vary a lot depending on the person. How loud a sound is to the human ear is measured in decibels and labeled as dB(A). Figure 1 is taken from the OSHA website and provides comparisons for different sound levels that we can relate to. Note that the inside of a bus is shown at 90 dB and conversational speech is at 60 to 70 dB. The specifications we get today are typically 68 dB to 72 dB for the interior average and as low as 65 dB at the driver’s head, which is important because the driver is on the bus for hours at a time. It seems that the specifications are meant to ensure that riders can speak with each other without shouting, and that makes sense.

Baseline noise testing for the HVAC is done while the bus is stationary with the engine running, and with the engine off, to see what the true contribution of each is. Recent noise testing on a ZEB for a concerned operator revealed a couple of things. First, the HVAC noise (based on more than 20 years of test data) is lower than it’s ever been. Engines are quieter too. Secondly, while it is a major source of interior noise, the HVAC is also not the sole source. One of the tests measured noise levels during a road test with the bus accelerating to 45 miles per hour. When the bus is moving in normal operation, road noise, normal traffic noise and noise from the wheel motors and pumps make a significant contribution. Background noise will always be present and the extent to which it infiltrates the bus cabin is largely related to bus construction. As you know, some cars are quieter than others.

Keep in mind that there will always be HVAC noise as with your car air conditioner or your window air conditioner. Some level of noise is the tradeoff for keeping cool. Just for a moment, consider the bus HVAC as a huge car air conditioner. You get in on a 100-degree day and you immediately hit the maximum A/C button and the recirculation button, or the system does it automatically. It’s noisy. The fans are at the highest speed. You might even have to turn the music up a little. The fan speed will slowly drop as the temperature gets close to the thermostat set point. Sound familiar? The bus HVAC system works exactly the same way. Is there room for improvement? Absolutely!

As with all things bus HVAC related, moving the needle requires a collaborative effort between the HVAC supplier and the bus builder. Of course, the operator has a stake in this as well. The specifications must be reasonable and achievable. Some compromise and tradeoffs must be considered, especially in HVAC system design. For example; at one point in time, operators wanted good aesthetics, high performance and low weight in a HVAC unit. In this case, for example, you could choose any two due to unit design constraints. Today, with all-electric HVAC, operators are asking for low weight, low noise and high performance. Again, you could choose any two based on design constraints. However, two of the three, low noise and high-performance options are perfect opportunities for collaboration and compromise. First, high performance is a given with the constant capacity of the all-electric HVAC. With as much as a 20 percent improvement in capacity at idle, all you would have to do is agree on a pulldown specification that does not require suppliers to overbuild the system. Added power adds noise. Next is the pulldown where the fans run high speed until the cabin is cooled. A properly-sized system will drop in capacity and the noise will drop with it. Surely, we can live with that. In closing, we should focus on more collaboration between HVAC suppliers and bus builders in design and the use of existing noise abatement technologies.


Steve D. Johnson, Sr. serves as product marketing manager, Bus HVAC, at Thermo King, Minneapolis, MN. Thermo King is a world leader in transport temperature control systems for buses. Thermo King also manufactures auxiliary power units, which dramatically reduce engine idling. All Thermo King products are backed by a nationwide dealer network. Visit