The W’s Unpopular Windows

The windows at the W Hotel and Block 21 Residences above have been in the news quite a bit lately.  In the below video, KXAN’s Jarrod Wise gives a good overview of the situation.  It also summarizes a 3rd party study (referred to hereafter as “the consultant”) concerning the Block 21 Residences and Cedar St. Courtyard that was recently released by the City.

In this article I will expand on these 3 points from the port covered by Mr. Wise in the video:

  • Cedar Street Code Compliant
  • COA Measurements Questionable
  • W Residences Not Built to Reasonable Sound Standards

Caveat emptor:  In the report are many measured sound levels referred to as “dB.”  Frustratingly, the consultant never specifies what frequency weightings or time constants were used for the measurements.  It seems reasonable to assume he had his meter set to A-weighted SPL with the “slow” time constant to match the noise ordinance, but I can’t know that.  Reading and recording an instantaneous sound pressure level (SPL) with sound level meter during this type of measurement requires the user to make judgments about what sound level to record, as the actual measured level changes frequently in time, especially with music as the noise source.  Without having observed the measurements myself, I am unable to know what type of methodology the consultant used to interpret the meter’s display, nor can I know how consistent he was measurement to measurement.  My assumption is that he attempted to choose representative values that would be comparable to a time-averaged Leq measurement.  If this assumption is incorrect, some of the conclusions drawn below in the Construction Quality discussion will be less accurate.

Is Cedar Street code compliant?

Mr.  Wise’s news story concludes that Cedar St operated according to code, as measured by the consultant.  This is not strictly true.  The report concludes that “Cedar Street stayed within compliance virtually all the time” and indicates that occasionally they measured levels exceeding “85 dB” (presumably 85 dBA) at the property line.  The noise ordinance makes no allowance for any kind of time averaging or momentary spikes in noise level.  85 dBA (slow) is an instantaneous value, and if the sound level exceeds that value for any period of time, even a few milliseconds, they are not in compliance with the ordinance as it is written.

The City’s Noise Measurements Are Questionable

I won’t spend much time on this topic because any regular reader of this site already knows I agree with this fully.  The report raises the issue that A-weighted broadband level is a poor choice for measuring music, and that is absolutely true.  The nature and frequency signature of live music is simply not compatible with simple A-weighting in all situations.  The report suggests using C-weighting instead of or in addition to A-weighting, which I agree with.  It also suggests using time averaging.  Assuming the author is referring to Leq, I agree with this as well, as it would make measurements far more objective and would eliminate the problem of deciding whether occasional spikes should constitute an ordinance violation (see above).

Note that these objections aren’t really about how the city measures sound, but actually how the noise ordinance specifies that sound should be measured.  On the topic of how measurements are conducted, there are further problems that need repair.  I am unaware of what type of meter the Music Office uses, or how they operate it, but I do know that APD uses B&K 2240 meters, which don’t support the slow time constant.  In other words, with that meter, APD is literally incapable of conducting sound level measurements as specified by the noise ordinance.

W Residences Construction of Dubious Quality

To me, this was the most interesting part of the report, and the part I will spend the most effort on.  The consultant took some basic but meaningful measurements at several residences with windows open and windows closed.  With windows closed (standing next to a window), levels of 55-62 dB (again, presumably dBA) were recorded, and in the same position with the windows open, they recorded levels of 68-73 dB.  Comparing these measurements results in a rough approximation of an A-weighted outdoor to indoor noise reduction of 11-13 dBA.

Simply put, this level of performance is terrible for any type of residence.  As a point of reference, HUD expects that basic residential building shells should provide noise reduction of 20 dBA.  HUD projects are meant to provide low-income housing and are generally not high-end buildings.  It is reasonable to expect exterior partitions of luxury condominiums to significantly exceed 20 dBA noise reduction, particularly when they are placed in a downtown area close to outdoor music venues.  If the reported measurements are accurate, the level of sound insulation provided by these residences is far below what is expected at their price range.

Poor acoustical performance can be attributed to poor design or poor construction, or both.  In the case of Block 21, it appears to be both.

The consultant observed “outside air moving through parts of the windows, doors and mullion assemblies in each of the apartments visited.”  Properly sealed windows and mullions are vital to controlling noise transmission.  Air leaks in windows large enough to allow outside air to be felt permeating the window certainly can explain noise reduction of less than 15 dBA.  In fact, 15 dBA is the expected noise reduction value of a partially open window in the World Health Organization’s Guidelines for Community Noise.  Those observations, along with the falling glass problem, seem to indicate major shortcomings in the quality of glass installation.

Suppose the construction quality was up to standards, would the design of the building envelope provide adequate noise isolation?  To attempt to answer this question, I performed a fairly standard outdoor to indoor noise analysis based on the following information.

  • The floor plan of a W Residence unit on the Northeast corner of the building (facing the Cedar Street Courtyard), available on the W’s website.
  • A spectrum for live music sound measured by me on the balcony of a downtown Austin residence.
  • The “windows open” sound levels from the report
  • Supposed STC values for exterior windows discovered by the consultant on an architectural drawing
The floor plan I used for the room dimensions in my analysis is shown below, with some significant acoustical features in text that I added.
The important features to consider are:
  • The area of the windows.  Larger windows are exposed to more exterior sound energy.
  • The size and sound absorption of the room being analyzed.  Hard surfaces, such as windows and wood floors, cause sound to bounce around a room more than soft surfaces.  This leads to higher interior noise levels.
  • The expected use of the room.  People are expected to be sleeping in bedrooms, so more stringent criteria should be applied there than to rooms more likely to be occupied in the day.

Almost half of all the partitions surrounding the living/dining/kitchen area floor to ceiling exterior windows.  This is a challenging situation acoustically.  High performing (read: expensive) windows are required to properly isolate interior spaces from downtown noise.  According to a detail the consultant saw on an architectural drawing, the exterior windows used on this building are STC 35 (he was unable to verify that STC 35 windows were installed).  STC 35 windows are on the expensive side of the cost spectrum, but not quite “high end.”

For my analysis I used typical transmission loss values for large area STC 35 windows taken from a collection of STC test data.

An exterior noise spectrum based on measurements I did earlier this year in downtown Austin.  The location of these measurements was on an exterior balcony with a partial view of several music venues.  The two situations are similar enough for this analysis.  I adjusted the spectrum to 71 dBA, which is the center of the range reported by the consultant for his open window measurements.  In reality, an open window will provide at least a few dB of noise attenuation, so 71 is a pretty conservative number to use here.

Based on the above information, the calculated interior noise level for the Living/Kitchen/Dining room is 48 dBA.  Compared to the WHO suggested criterion for indoor levels of 35 dBA Leq, this is quite loud.  Also consider that low frequency sound typically found in live music is mostly unaccounted for in A-weighting.  Music noise transmitting into this room through the windows is likely to be quite bothersome to occupants, even with no construction defects.  (Remember that I am assuming the SPL measurements taken by the consultant are comparable to Leq).The Master Bedroom is a little more absorptive (carpet instead of wood floor) and has a smaller total window area than the living room.  However, it is smaller in volume and should have more stringent noise criteria, as sleep disturbance becomes the primary concern.

The calculation predicts a level of 45 dBA in the master bedroom.  This is well above the 30 dBA Leq suggested by the WHO for sleeping areas.  It is equivalent in magnitude to the 45 dBA Lmax suggested as a limit for impulsive, maximum levels using the “fast” time constant.  With a varying noise source, “fast” measurements will always be greater than “slow” measurements.  Therefore both the Leq and Lmax aspects of the WHO’s suggested criteria are predicted to be exceeded.  Remember, again, that low frequency sound typical to live music is not well accounted for in A-weighted levels, meaning the intrusive noise is likely to be even more disturbing.

According to these calculations, the exterior partition design and window selection of the Block 21 residences appear to be in appropriate from an acoustics standpoint.

Additionally, the report mentions several instances of wall insets that essentially reduce the exterior partition at those locations to a single layer.  The consultant claimed that noise could clearly be heard coming through these areas.  Another apparent design flaw.

An acoustical analysis should have been performed during the design phase to catch these types of problems.  Units costing half a million to several million dollars and located in a downtown area known to have significant noise from music should provide a high degree of acoustical isolation (for a residential structure).  According to the information in the report, the Block 21 Residences fall quite short of those standards, in terms of both design and construction.

It’s actually very possible that an acoustical analysis was performed.  Recommendations from acoustical consultants are typically some of the first on the “value engineering” chopping block.  The VE phase is when the architect and the developer review the nearly complete building design to determine what features can be removed in the name of cost savings.  Design elements meant to reduce noise exposure are frequently cut out, presumably because their value is more difficult to conceptualize.  Acoustical retrofits are very expensive.  So are lawsuits, which it seems reasonable to predict will occur.  If proper acoustical design was VE’d out of the W project, it is a gamble that does not seem likely to pay off.

In closing, please remember that many of the values discussed in this article are not known absolutely.  I’ve done my best to clearly identify where assumptions needed to be made, and to be as reasonable as I could in making them.

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