Hunt Library Stairs: Acoustical Analysis

HUNT LIBRARY STAIRS Acoustical Analysis

Sam Faller, Laura Maund Carnegie Mellon University Fall, 2009

HUNT LIBRARY STAIRS Acoustical Analysis

DN UP DN UP

Stairwell for analysis

EXISTING CONDITIONS Activity

-Stairwell provides access to all 6 floors of the library.

-There is a nearly constant flow of people between basement and third floor, particularly on the first floor (main entry level).

Architecture and Materials

-Cavity through all 6 floors allows sound to travel through the entire space.

-Terrazzo floors, smooth plaster ceilings, and aluminum and glass railings reflect sound around the entire stairwell.

-Walls are slightly textured plaster.

walls: rough plaster on concrete block

Quality of Acoustics

-Space is unnecessarily reverberant, to the point that effective communication is difficult in the space.

-Voices carry and are amplified throughout the entire stairwell so there is no opportunity for private conversations.

-Mechanical systems produce a loud rushing sound, but this is only audible when the stairwell is unoccupied. floors: terrazzo tiles

HUNT LIBRARY STAIRS Acoustical Analysis Conditions for Experiments 5th Floor

Unoccupied

-Mechanical systems produced continuous rushing sound.

-People did not circulate through stairwell.

-Doors remained closed.

Occupied 4th Floor

-People continued to circulate throughout stairwell and talk as normal.

-Doors were opened and closed as people circulated in and out.

Speaker Placement 3rd Floor

2nd Floor

1st Floor

Basement

The first speaker placement is on the basement level in the corner by a payphone. Although the payphone is rarely used, many people stand in this corner to speak on their cell phones because it is out of the way of the doors. However, due to the acoustical performance of the space, conversations held on the basement level can easily be heard thoughout the entire stairwell.

The second speaker placement is on the third floor between the doors, another convenient location to use a cell phone.

Due to the speaker malfunctioning, we were only able to obtain data for B2 and T2 occupied. Because the data we were able to obtain for the second speaker placement is similar to the data obtained for the first speaker placement, we can conclude that the second speaker placement data for B2, L1, L2 and T2 unoccupied would be similar to that of the first speaker placement.

HUNT LIBRARY STAIRS Acoustical Analysis

Speaker placement 1

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1st floor plan

Basement floor plan

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Data Collection B2 occupied:

Microphone placements 1, 2, 5

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B2 unoccupied:

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L1 unoccupied:

Speaker placement 1

Microphone placements 1A, 1B, 1C L2 unoccupied

Speaker placement 1

Microphone placements 2A, 2B, 2C T2 occupied

Speaker placement 1

Microphone placements 2, 3

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3rd floor plan

Speaker placement 2 Microphone placements 6, 7, 8

T2 unoccupied:

Speaker placement 1

Points 2,3,4

HUNT LIBRARY STAIRS Acoustical Analysis Background Noise B2 occupied

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Background Noise (dB)

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Background Noise (dB)

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Background Noise (dB)

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At lower frequencies, the background noise is particularly loud, reaching nearly 70dB when occupied. This background noise is partially due to the movement and speech of people in the stairwell at the time the data was collected, but it is not entirely caused by occupants.

We can tell that speech in the space causes a significant amount of background noise because the frequencies most used in talking (approximately 400Hz and lower) have higher sound levels than other frequencies.

HUNT LIBRARY STAIRS Acoustical Analysis Background Noise B2 unoccupied

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Background Noise (dB)

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Similar to the background noise data for the occupied space, the background noise levels for the unoccupied space are particularly high for lower frequencies, reaching nearly 60dB. This background noise can be attributed to the rushing sound of air circulating through the space as well as the low hum of the mechanical systems.

Because the frequencies of the background noise with the highest sound levels are similar to those used in speaking, even when the space is unoccupied it can be difficult to communicate.

HUNT LIBRARY STAIRS Acoustical Analysis Background Noise

B2 occupied vs. unoccupied

B2 occupied vs. unoccupied Background Noise (dB)

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Comparing the average background noise when the space is occupied versus unoccupied, the two sets of data follow the same general trend. The average background noise when the space is occupied is slightly higher than that when the space is unoccupied, as would be expected. The difference between the two sets of data is roughly 5dB at frequencies lower than 315Hz and roughly 15-20dB for frequencies higher then 315 Hz. This means that the sound contributions of people in the space are relatively low in comparison the total amount of background noise in the space. This is confirmed by the fact that when standing in the stairwell while it is unoccupied the sound of the air movement and mechanical systems is extremely audible.

HUNT LIBRARY STAIRS Acoustical Analysis Noise Criteria

B2 occupied and unoccupied

Average B2 occupied- NC45 Average B2 unoccupied- NC36

HUNT LIBRARY STAIRS Acoustical Analysis Room Criteria

B2 occupied and unoccupied

Average B2 occupied L[MF] = 1/3 (49.5 + 44.6 + 41.6) = 45.2 dB

RC-45 environment average B2 unoccupied L[MF] = 1/3 (33.2 +26.4 +23.8) = 27.8 dB

RC-28 environment

HUNT LIBRARY STAIRS Acoustical Analysis Airborne Transmission Loss

L1 and L2 unoccupied

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Transmission Loss (dB)

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Transmission Loss (dB)

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The data for the different points for measuring L1 and L2 closely follows the same trend. Due to the small size of the space, we were only able to obtain data for L2 at a distance of 3 meters.

Because the data collection points were in such close proximity to each other, there was minimal opportunity for variations in the data.

HUNT LIBRARY STAIRS Acoustical Analysis Airborne Transmission Loss L1 vs. L2 unoccupied

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Transmission Loss (dB)

L1 vs. L2 unoccupied A- Speaker 1

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Transmission Loss (dB)

L1 vs. L2 unoccupied B- Speaker 1

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Transmission Loss (dB)

L1 vs.L2 unoccupied C- Speaker 1

Comparing the L1 and L2 data for each mircophone location, we can see that there is a slight increase in transmission loss as the distance decreased between the microphone and the speaker.

This relationship is explained by the Inverse Square Law, which states that when sound energy is twice as far away from the sound source, it is spread over four times the area, and thus is one fourth the intensity. The differences between our data for L1 and L2 are very minimal however, due to the fact that we could only place the microphone up to 3 meters away from the speaker.

HUNT LIBRARY STAIRS Acoustical Analysis Reverberation Time

T2 occupied and unoccupied

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Reverberation Time (sec)

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When measuring the reverberation times while the space was occupied, we encountered many difficulties with the equipment obviously not measuring correct times.

This is due to the fact that when the space is occupied, there is so much sound being produced and reverberating around the stairwell that the equipment could not discern which sounds to measure. The overall trend however, is that at lower frequencies around 630 Hz, the reverberation time is roughly 3.5 seconds and at higher frequencies around 4000 Hz, the reverberation time is roughly 1.25 seconds.

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Reverberation Time (sec)

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At lower frequencies around 400 Hz, the reverberation time is roughly 3.0 seconds and at higher frequencies around 10000 Hz, the reverberation time is roughly .75 seconds.

These reverberation times are not suitable for speaking on cell phones or even communicating in person, considering the frequencies of speech.

HUNT LIBRARY STAIRS Acoustical Analysis Reverberation Time

T2 occupied vs. unoccupied

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Reverberation Time (sec)

T2 occupied vs. unoccupied

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The reverberation time data for the unoccupied space follows the same general trend as the data for the occupied space (higher reverberation times for lower frequencies and lower reverberation times for higher frequencies). However, the reverberation times are lower in the unoccupied space than in the occupied space for low and high frequencies, and slightly higher for middle frequencies.

This is counterintuitive because generally an occupied space would have a lower reverberation time due to people absorbing sound. However in this particular space, the noise that people generate has more of an effect on the reverberation time than the actual sound absorbing capacity of the occupants.

With reverberation times upwards of 3 or 4 seconds at lower frequencies, communication is very difficult in the stairwell when it is both occupied and unoccupied. In order to make the space suitable for talking, the reverberation time would have to be lowered to under 1 second.

Due to the equipment malfuntioning during our experiments, we were not able to obtain L1, L2 or T2 data for the unoccupied space with speaker placement 2.

However, because the background noise data for the third floor closely matched that of the basement, we can assume that the airborne transmission loss and reverberation time data for the third floor would also closely match that of the basement.

HUNT LIBRARY STAIRS Acoustical Analysis Reverberation Time and SPL

Our results were quite a bit different from what the Bose Modeler predicted; in all frequencies our measured reverberation time was higher. This was somewhat expected considering the amount of reverberance was very high and the background noise was fluctuating with people and wind. The general trends of the two graphs however, seem to coincide, especially in the higher frequencies, where the background noise was less intense.

Green: Occupied Background Noise Red: Predicted SPL The Bose Modeler predicted that the greatest sound levels caused by reverberation were at around 2000 Hz. This is due to the sound-reflective materials and the shape of the space. This distinction can be seen even more clearly in some of the model images in the following pages.

HUNT LIBRARY STAIRS Acoustical Analysis Sound Pressure Level: Direct and Reverberant

HUNT LIBRARY STAIRS acoustical analysis Sound Pressure Level: Direct Only

It’s clear from these images that regardless of what frequency of sound is emitted, the whole stairwell hears it at a single amplitude. Only the area directly surrounding the speaker is any louder, and even then it’s only a difference of 2 to 4 decibels from the reverberant sound. The perceptual feeling of being heard by everyone in the stairwell is very well evidenced by these charts. It’s also evident that frequencies in the 1-3 KHz range reverberate more than the others; the difference in the audible sound might be even seven decibels greater if it is a 2KHz sound compared to a 500 Hz sound. Also, the sound reached all the way into the fifth floor cavity from the basement, so it can be assumed that any sound created from one of the other floors would have a similar propagation throughout the entirety of the stairwell.

It’s clear that this reverberant sound is truly the dominating sound characteristic of the space, since the direct sound barely travels to other floors, but the sound is still heard throughout the space.

HUNT LIBRARY STAIRS Acoustical Analysis Speech Intelligibility: Occupied

HUNT LIBRARY STAIRS Acoustical Analysis Speech Intelligibility: Unoccupied

Speech intelligibility is a measure of the ability for someone to understand speech within a space. This project is concerned with speech intelligibility because the stairwell is intended to be used for cell phone conversations. We observed that it was sometimes difficult to communicate even at a distance of a few feet; talking on a cell phone could be even harder given the sound conditions. What this model shows, is that speech intelligibility is fair to good (0.4 – 0.7) if you and the person you are talking to are on the same floor, but it is generally poor (< 0.4) outside of that small space. This is satisfactory since you wouldn’t normally talk to someone who isn’t walking with you or on the phone, however when other people are in the stairwell, speech intelligibility decreases significantly. This creates difficulty when talking on the phone or speaking with other people in the stairwell.