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AudioAcoustics

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May 19, 2006

Heather alerted me to a Public Radio segment on wayside train horns (used to limit excessive noise at highway/railroad grade crossings).

More info on grade crossing noise and wayside horns available here and here.


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The Villager is running an article on New York City subway noise:

Still, the noise remains piercing. However, there is more the transit authority can do to minimize loud noise, and the impetus for more improvements may start with stricter legislation Ń itŐs the type of legislation transit once knew and by which it has already abided.

Aside from the numerous technical errors in the piece, the article is unfair in reflecting New York City transit’s commitment to noise control. For one thing, they spend a lot of time and money on noise control in the system. However, the biggest problem is that they can only do so much to quiet the system given the size (660 miles of track in passenger service), and the age of the system.

While many at the Spring St. station last week suggested that the transit authority use rubber tires on the trains, the type usually seen on the trains in Europe, Seaton said rubber tires arenŐt possible for New York City trains.

Sure, rubber tires would be nice… if you don’t mind shutting down the system for a few years while you rip apart and rebuild the infrastructure, and acquire new vehicles. Spray-on absorption is another option (and is used by NYCT), but it can only provide so much help. Friction modifiers (i.e. spraying water on the tracks) is only useful on straight track (and may reduce the friction between the wheel/track interface which could lead to safety and stopping problems).

Like anything else, we all wish transit systems could be quieter (and yes, the Federal government takes this issue seriously). However, the reason New York City subways aren’t as quiet as we would like isn’t because they don’t try, it’s because it’s not an easy problem so solve.


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May 11, 2006

In the midst of performing some lab work yesterday, I finally decided to take a few minuts and quanity the noise insulation performance of my Bose QC-1 noise cancelling headsets.

The setup is simple: I used a calibrated ¼-inch microphone (Radio Shack 33-3028, calibrated with a GenRad 1986 OmniCal) connected to a TerraSonde ATB Pro. I placed the microphone outside of the left QC-1 earcup, and played pink noise through my audio system to measure the baseline level. I next placed the ¼-inch mic inside the earcup (inside the pinna, outside the ear canal) with the noise-cancelation circuitry switched off to measure the passive noise reduction caused by the cups. Finally, with the mic still inside the ear cup, I switched on the noise cancellation circuitry.

The measurement results are 10-sec averages (Leq). The overall sound level from the pink noise outside the earcups was ~75 dBA. The pink noise sound level was at least 10 dB higher than the background noise level in the range of 31.5 Hz to 16 kHz.

The results:

The results are fairly consistent with those measured in this report on ANR headset performance measurements. Keep in mind that these measurements are typically performed with microphones inside the ear canal, so there will be some differences between my quick-and-dirty tests, and official measurements. Also, I think the negative insertions losses that show up below 40 Hz may be caused by a poor seal around my ear (my glasses got in the way).


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May 10, 2006

Rick Levine belives that three individuals are responsible for the death of the high-end audio market:

  1. Henry Kloss, who decided not to publish specs of his speakers
  2. Amar Bose, who further eschewed the idea of publishing specs, as well as popularize the concept of Wife Acceptance Factor in home audio.
  3. Bob Carver, who introduced “magic” into home audio (an example Rick pointed out was Carver claiming that one of his amps could generate 3000 watts of power, dispite the fact that U.S. 120VAC power is limited to ~1500 watts of power by law and no amplifier is 100% efficient).

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The Boston Globe published an article about ongoing upgrades of the MBTA’s station public address system:

“Once acoustic engineers install and adjust new speakers over the next year, T officials say, there will be a major improvement in getting information to passengers, who have complained for decades about garbled announcements.”

There are several acoustical engineering firms from Massachusetts, New York, California, Texas, and Maryland working on this upgrade. Given the age of the system it’s been a challenge, but as you can see from the Globe article, progress is being made.

The goal of the new PA systems is to meet National Fire Protection Association speech intelligibility criteria. The Common Intelligibility Scale (CSI, which is directly related to the Speech Transmission Index, STI) is being used to quantify the performance of the current and new systems. These metrics can determine the intelligibility of speech broadcast over the PA system in the presence of background noise (e.g. trains), excessive reverbaration, and system distortion. Acoustical modeling tools like EASE are being used to design the upgraded PA systems and to optimize CSI/STI.

All in all, this should lead to a significantly improved PA system at MBTA systems - given the fact that the new announcements have been received favorably by the public, despite the fact that the optimizations have yet to be completed, bodes well.

[Thanks for the heads-up Lance!]


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