Why the Hum gets louder at night

One of the great mysteries regarding the Hum heard around the world is the question of why it seems to be louder or more prevalent at night. The answer is in fact quite simply explained.

Gas Days and Gas Nominations

For anyone who has been affected by the Hum for any length of time you will probably have noticed that not only does the Hum become more prevalent during night time hours, it also conforms to certain patterns. There are some exceptions but for the most part if you take note you will find that the Hum has a daily "routine" or a pattern that it follows. Many reports talk about after midnight and the early hours of the morning being the worst in terms of sleep disturbances and the feeling of being assaulted by low frequency sound waves. There is actually a rather simple explanation why this happens.

Gas days

In order to be able to account for how much gas is moved through a pipeline, gas transmission companies have what is know as a "Gas Day" A gas day is a 24 hour period with a start time that may be different depending on the region that you are in. This gas day allows the Gas Transmission Company to calculate and confirm how much natural gas has been moved through the system. In some areas the Gas Day runs from 8 am to 8 am. This actual start time may vary. A detailed explanation of a Gas Day can be found here What is a Gas Day ? and how it contributes to the Hum getting louder at night.
 

Line Packing

Gas can be temporarily stored in the pipeline system, through a process called line packing. This is done by packing more gas into the pipeline by increasing the pressure. During periods of high demand, greater quantities of gas can be withdrawn from the pipeline in the market area than is injected at the production area. This process is usually performed during off peak times to meet the next day’s peaking demands. This method provides a temporary short-term substitute for traditional underground storage. As " Line Packing" begins the pressure in the pipeline builds and the Compressors are forced to work harder and consequently generate more noise including low frequency noise during off peak hours. When demand rises at the downstream end as the world awakens the pressure drops and the Compressors don't have to work as hard. Consequently the noise diminishes as the pipeline unpacks.
 

A more Technical description of "linepack" 

"Linepack" refers to the volume of gas that can be "stored" in a gas pipeline. Thinking back to basic chemistry, gas can be compressed (unlike liquids). Think of the classic example of compressing air in a bicycle pump. The air can be compressed into a smaller volume, or more air could also be squeezed into a fixed volume - for example a tyre.

The operational implications of line pack mean that the volume of gas injected into a pipeline (at the inlet), can bd greater than the volume of gas withdrawn from the pipeline (at the outlet). This frequently occurs due to the unpredictable nature of end-user operations and hence, their gas demand.

However, when gas is 'stored' in the pipeline by compressing it, the pressure exerted on all parts of the pipeline increases. The quantity of additional gas volume that can be stored in a pipeline depends on the pressure rating of the pipe, flanges, non return valves, compressors etc, as well as the ability for equipment upstream (before the inlet) and downstream (past the outlet) to respond to a sudden surge in pressure if inlet or outlet valves failure. This event is called a high-pressure/low-pressure breakthrough. When the pipeline pressure is high, it becomes increasingly difficult to inject additional gas into it. Compressors are used to increase the injection pressure of the gas.

An in depth description of Line Pack from 

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Putting Line Packing in perspective

Our investigation of the Hum has given us absolutely no doubt that Natural Gas Compressors and Pipelines are at the heart of the matter. Through a careful and open minded process of elimination we have identified this as the source of the majority of Hum Reports around the world. One of the last questions to be answered was that of why the Hum has been reported as being more prevalent at night ? Most people of the people who have been adversely effected seem to agree that the Hum is responsible for sleep disturbances and many reports include accounts of people actually going outside at night to look for the source of the Hum and being unable to find anything. Also common to most Hum descriptions is a low rumbling sound similar to a Truck idling outside.

Natural Gas Compressors and Pipelines are quite capable of emitting low frequency sound waves that travel great distances. The sound waves are very long and often do not cause any disturbance until they reach a considerable distance from the source. Consequently it can be very difficult to pinpoint the source of low frequency noise.

Most people assume that the world becomes quieter at night as daytime activities cease. In the particular case of Natural Gas Transmission via Pipelines. Line Packing commonly occurs during off hours and periods when demand is low. That means at night and in the early hours of morning , Pipeline Compressors are working hard to pressurize natural gas and load pipelines to the maximum allowable pressure so that when demand increases the next day there is a full supply. The result is an increase in low frequency noise pollution and in particular pulsations and a deep rumbling noise that for some people makes it impossible to sleep. This process of "Line Packing" is the reason that the Hum becomes louder and more prevalent at night.