The generation, propagation and perception of noise is one of those fields where concepts are deceptively simple and yet devilishly complex. Anybody can buy a sound level meter at Radio Shack, but that doesn't make one a noise expert any more than buying a calculator makes one a math whiz. Thankfully, the FRA publishes a well-written guidebook on High-Speed Ground Transportation Noise and Vibration Impact Assessment that describes a standard methodology for determining noise impacts from new high-speed rail projects. While this guidebook was published in 2005, the physical laws of acoustics remain the same as always.
Despite citing the FRA guidebook, there is little evidence in the CHSRA's program-level EIR noise and vibration chapter and its appendices that the source-path-receiver model framework described in the handbook has actually been applied. Impacts are broadly classified as high, medium or low, and crudely bumped down by one level on the peninsula (from "high" to "medium") because horn noise would be eliminated from more than 40 grade crossings.
To make the issue of noise impacts more accessible to the layperson, an implementation of the FRA noise impact assessment model is provided in the form of a train noise calculator spreadsheet (45 kB Excel 2007 .xlsx file or 140 kB Excel 2003 .xls file, no macros, made virus-free on a Mac.) Before playing with this, it's probably best to study up on the basics of noise in the FRA guidebook as well as CARRD's compilation on noise pollution to tell your dBA's from your Ldn's.
By crunching the FRA parameters and equations behind the scenes, the spreadsheet allows a rapid evaluation of noise impacts (None, Low, Moderate, Severe) as shown in the screenshot at right. The blue circle represents the project's impact, and you can see how it moves around the noise impact diagram based on:
- Train speeds (anywhere from 20 mph all the way up to 220 mph)
- Train length
- Train frequency (both peak frequency, and nighttime traffic)
- Train type (diesel, electric, maglev)
- Track configuration (elevated, at-grade, trench...)
- Screening effects from intervening rows of buildings
- Distance from the tracks
- Property use (residential, institutional, outdoor...)
- Existing noise environment (suburban, urban, etc.)
Some interesting findings, which one is free to experiment with in the spreadsheet:
- Elevated viaducts, as currently proposed, are twice as loud (+9 dBA) as elevated berms at the train speeds envisioned on the peninsula, everything else being equal. Viaducts amplify rolling noise because they are hollow structures that act as a resonating chamber. Belmont and San Carlos might be well-advised to leave their berms as they are.
- At-grade tracks can be made half as loud (-10 dBA) with a noise barrier, when train speeds are in the 100 - 125 mph range proposed for the peninsula. Noise barriers become less effective as train speed increases and noise becomes dominated by aerodynamics, but here on the peninsula, they would probably do just fine.
- Trenches are extremely effective at shielding train noise; they are overkill as far as noise mitigation is concerned. With a trench, even very high levels of traffic (several hundred trains a day) would create less noise than Caltrain does today. The trench need not even be covered for this benefit to be realized.
- The proximity to an existing grade crossing with horns has a significant effect on the severity of the impact from the HSR project, as pointed out in the EIR noise chapter. A suburban residence within 500 ft of a grade crossing experiences ambient sound levels twice as loud (+10 dBA) than a suburban residence located away from grade crossings with horns. Since the existing noise level is the basis against which the noise added by the project is evaluated, areas near grade crossings can be expected to rate a lower impact. Nevertheless, not all areas along the peninsula corridor are close to a grade crossing.
- Not surprisingly, Caltrain's fantasy schedule (10 tph peak, 162 trains/day) combined with HSR's fantasy schedule (10 tph peak, 6 tph off-peak, 228 trains/day) results in ~5 dBA worse noise impacts than a more realistic traffic level of 6 tph Caltrain and 4 tph HSR.
- Out in the central valley, urban areas with ~150 trains/day blasting at 220 mph on concrete viaducts would experience severe noise impact within ~500 feet on either side of the tracks. Suburban and rural areas would experience even wider areas of noise blight, especially if the tracks are built on viaducts.