Bethesda Generator Systems – Automatic Backup Generator Systems

Noise Levels

One of the questions we are most frequently asked is, “How noisy is the generator?”

Unfortunately, there isn’t an easy answer to this question as everyone throughout history has interpreted noise differently, just like art and music… That is, until 1928 when workers at Bell Telephone Labs scientifically quantified noise levels as a convenient way to express power losses in telephone lines, and termed it decibel (dB) (literally, one tenth of a bel), in honor of founder Alexander Graham Bell. Bell scientists defined decibel as a unit for expressing the relative difference in power between acoustic or electric signals, equal to ten times the common logarithm of the ratio of the two levels.

For those who understood this definition, please click here on Tech Noise Info and skip the remainder of this section.

For those who, like most of us, do not have a clue about Bell’s definition, we decided to go back to the good old days when everyone interpreted noise as they pleased, i.e., I can’t hear anything (1dB); whisper quiet (15dB); library quiet (40dB); normal conversation (50dB); noisy machinery (80dB); annoying lawnmower or leaf blower (95dB); rock concert enraging (110dB); and back to I can’t hear anything (anymore! 120dB+). Towards that end, we have recorded the noise levels of a variety of generators with a variety of options. As a reference to more familiar noises, we also recorded common equipment like an air conditioner, a lawnmower, as well as the 2004 Cicadas (next update in 2021).

So please click on the tabs below to listen to the sound levels for yourself keeping in mind that, contrary to the suggestion of our sales department, lowering the volume on your speakers will not reduce the noise level of the generators!

An important note which needs clarification is the log scale of dB level ratios, meaning that every 3dB increase is equivalent to doubling the noise level, and that every 10dB increase is equivalent to a tenfold increase in the noise level. For example, 63dB and 70dB noise levels are equal to double and ten times the noise level of a 60dB noise level, respectively.

The most significant factors that determine the noise that we hear are, by order of importance, the type and level of sound-attenuation (enclosures and natural barriers), the distance from the source, and the size, type and specifications of the engine. A generator with a custom sound attenuated enclosure will reduce by about 20 times the noise level of the same generator with a standard enclosure (say from 72dB down to 59dB), and by about 1,000 times the noise level of the same generator without any enclosure (say from 89dB down to 59dB). Doubling the distance from the source will result in halving of the noise level (say from 72dB down to 69dB); factors that also influence the noise we hear are the existing ambient noise, the positioning of the generator, and the type of base and ground.

In summary, we would like to note one last interesting fact while addressing the original question at the top of the page. Every single day of the year, regardless of the weather or of other conditions, at least one of our customers is without utility power, and their generator is actively providing emergency back-up. Over the years, out of our hundreds of residential customers, we have only had one noise complaint from a customer, following a period when our generators had been running continuously for over a week. Accordingly, we can safely state that all our generators emit an acceptable noise level, and some are actually quieter than your AC and meet strict county noise ordinances (typically 65dB during the day and 55 dB at night as measured at the property line).

As a final note , it is important to keep in mind that all standby systems operate (and produce noise) on a temporary basis only when utility power is unavailable.