How Sound Changes in Different Rooms

Figure 1.1: Four spectrograms for G major chord played in different rooms within a house. From left to right, each spectrogram represents the chord played in the: bedroom, bathroom, hallway, and living room.

Introduction

The aim of this project is to examine the effects that different environments will have on the characteristics of a sound. The same sound (G Major chord on the acoustic guitar) will be played at approximately the same volume within each room. The four environments will be (1) my bedroom, which is 8.75 m², has lots of furniture, and hardwood floors; (2) my living room, which is 19.6 m², has lots of furniture, and has tile floors; (3) the hallway, which is 4.61 m², has no furniture, has hardwood floors, and is fully enclosed when all doors to other rooms are closed; and (4) the bathroom, which is 4.03 m², has many furnishings, and has tile flooring. The walls in all rooms are built of drywall, and have a few windows in each. In each recording of the chord in each room, the phone playing the prerecorded chord will be one foot from the laptop recording the sound.

The chord will be recorded and played over in each room, rather than played directly from the guitar for each recording, to avoid any differences in the sound across recordings. This is important to do in order to ensure that differences in sound will be caused by the changing environments, rather than a difference in playing technique. Since I am unable to alter the size, walls/floors, and amount of furniture within each room, recordings were taken from different rooms with different characteristics rather than altering the characteristics of any given room. Implementing all the aforementioned controls, the spectrograms for each room are represented in Figure 1.1

Spectrum Analyses

Figure 1.2: Spectrum analysis of G major chord played in bathroom

The G major chord is composed of multiple frequencies played on the strings of the guitar. The notes played to create this chord are the root (G), a major third (B), and a perfect fifth (D). In the chord played, the notes are G2 & G3, B2 & B3, and D3. The frequencies of these notes, respectively, are 98 Hz, 196 Hz, 123.47 Hz, 246.94 Hz, and 146.83 Hz.

Amount and Make-up of Furnishings

Bathroom vs. Hallway

The bathroom and hallway are very similar in size and material make-up; they differ by 0.58m², and are both made of drywall and have hard flooring. However, the bathroom has tile flooring, and the hallway has hardwood flooring.

Figure 1.3: Spectrum analysis of G major chord played in hallway

Figure 1.4 - Spectrum analysis of G major chord played in bedroom

In the bathroom, the F1 is 142 Hz at -51.3 dB, F2 is 294 Hz (-33.6 dB), the F3 is 428 Hz (-48.0 dB), and the F4 is 586 Hz (-38.9 dB). In the hallway, the F1 is 113Hz (-48.5 dB), F2 is 289 Hz (-42.1 dB), the F3 is 415 Hz (-47.7 dB), and F4 is 586 Hz (-35.4 dB). While the frequencies of each peak were very similar between each room, the distribution/loudness of each frequency differs quite a bit. In the bathroom, the loudest frequency heard was F2 (289 Hz), at -33.6 dB; whereas in the hallway the loudest frequency was F4, heard at -35.4 dB.

The bathroom has notably more glass than the hallway, mainly due to the large glass shower door which spans the width of the bathroom. This is notable because different materials absorb different amounts of different sound frequencies. Glass absorbs higher amounts of low frequency sounds, and lower amounts of higher frequencies . In the bathroom, the loudest frequency was a the second lowest harmonic, at 294 Hz. The third and fourth harmonics, both at higher frequencies than the second harmonic, occur at much lower volumes than the second harmonic.

One other significant difference in the materials within each room is the type of flooring. The hallway has hardwood flooring and the bathroom has tile flooring. Tile absorbs less sound than wood does, but the absorption levels across frequencies is more even across different frequencies than that of glass. This would explain for the variation of frequency distributions across the rooms. In the bathroom, the first four harmonics all occur within a range of 17.7 dB from one another, with the average volume being -42.95 dB. In the hallway, this range goes down to 13.1 dB, with a mean volume of -43.43 dB. Between the rooms, while the volumes are almost equal, the range of volumes of specific frequencies is much smaller. Since the tile is absorbing less sound than the hardwood flooring, the volume range of the four harmonics would be larger. As wood absorbs more sound, especially lower frequencies, it makes sense that lower frequencies of the sound would be quieter in the hallway with wooden flooring than in the bathroom with tile flooring. For example, the F2 occurs at -33.6 dB in the bathroom with tile flooring, but at -42.1 dB in the hallway with wooden flooring.

Living Room & Bedroom

The bedroom both have approximately the same amount of furniture, and most furnishings are made of either wood or fabric (e.g. for duvets, couch, etc.). Likewise, their material makeup is similar, both having drywall and hard floorings. However, the living room has tile flooring, and the bedroom has hardwood flooring. This difference was covered in more detail in the final paragraph of section 2.1.2 .

Figure 1.5: Spectrum analysis of G major chord played in living room

In the bedroom, the first four frequencies of the chord are as follows: F1 is 287 Hz (-41.5 dB) F2 is at 586 Hz (-44.8 dB), F3 is at 739 Hz (46.5 dB), and F4 is at 1182 Hz (-48.5 dB). In the living room, the peaks occur at F1 as 290 Hz (-38.7 dB), at 592Hz in F2 (-49.4 dB), F3 is at 741 Hz (-41.5 dB), and F4 is at 883 Hz (-43.4 dB). The fundamental frequencies occur at a much higher frequency in the larger rooms than they do in the smaller rooms. Each recording was cut down as precisely as possible to the same part of the chord being recorded, for the same duration of time. Additionally, all spectrums were plotted on Audacity at size 1024. Because of these controls implemented, I assume that the difference in the fundamental frequencies being detected were due to changes in the environment, rather than differences in the audio recordings. Additionally, these differences are consistent between the larger rooms, rather than only being seen in one larger room.

There are two potential explanations for this phenomenon. The first explanation being that the amount of furniture in the room would dampen the lower frequencies. The living room and bedroom are both rooms which have softer furnishings than the bathroom. All furnishings in the bathroom were composed of glass, ceramic, or tile. On the other hand, the living room and bedroom had furnishings of softer materials, such as upholstery, blankets, leather, and wood. Across most frequencies, chairs and fabrics will absorb much more sound than tiles and ceramics . These materials absorb proportionally more lower frequencies than higher frequencies when compared to the absorption of other, harder materials. Given that these softer materials absorb such a high proportion of lower frequencies, this absorption could be the reason why the F1 for rooms with softer furnishings occurs where the F2 is for rooms with harder/less furnishings.

Size of Room - Living Room & Bedroom

The second potential explanation for the aforementioned phenomenon is the sizes of the rooms in which the sound was measured. One factor that variations in room size brings is the varying amounts of reverberation—reflected sounds. Frequencies with higher reverberation will be more prominent on the spectrum analyses, as these sounds would last longer within the room they're being played in. Reverberation is caused by many factors, two of which being size of and materials within a room. Since the effect of materials within the room has already been examined, I will now explore the effects of size on reverberation; and the effect of reverberation on frequencies heard. Reverberation time is "the time after the source of the sound has ceased that it takes the sound to fade away": smaller rooms will have shorter reverberation times, and larger rooms will have longer ones. This means that larger rooms will have a sound reverberate for longer after its initial expression than it will in smaller rooms. Additionally, sounds of higher frequencies are composed of waves with shorter wavelengths, which lead to stronger reverberations in a room. Generally, reverberation times at low frequencies (62-125 Hz) are around 1.25 times greater than those at frequencies of around 500-100 Hz . Combining these two factors affecting reverberation times, these times will be lowest in frequencies around 500-1000 Hz played in smaller rooms. However, heavily upholstered furniture will decrease the reverberation times of lower frequencies. In this example, the material of the furniture in the rooms decreased the lower frequencies so much that the effect of room size was not as significant as the effect of furniture within the room.

One notable difference between the recordings in the bedroom and the living room is the frequency of the F4. In the bedroom, the F4 occurs at 1182 Hz, whereas F4 occurs at only 883 Hz in the living room. The furnishings within the rooms were very similar, but the bedroom is much smaller (8.75 m²) than the living room (19.6 m²). Because of this, this effect is more likely due to the size of the room, rather than the furnishings. As previously mentioned, reverberation times will be lowest in frequencies between 500-1000 Hz in smaller rooms, and higher in frequencies in the same range in a larger room. In the context of my measurements, this means that frequencies between 500-1000 Hz will last longer in a larger room than in a smaller room. In the living room, the F4 falls within this range: at 883 Hz. On the other hand, the F4 in the bedroom is at 1182 Hz. Because of the larger size of the living room, the frequency of 883 Hz is more prominent. In fact, F2, F3, and F4 all fall within the range of 500-1000 Hz in the living room. In the bedroom, however, only the F2 and F3 fall within that range. The presence of frequencies ranging between 500 and 1000 Hz was more prominent in the living room than the bedroom due to the higher reverberation times caused by size of each individual room.

Material Make-Up of Room & Furniture

One difference unaccounted for is the material of the furniture and flooring when comparing rooms. In the bathroom, all furnishings were tile, ceramic, or glass; being the toilet, sink, shower door, and mirror. In the living room and bedroom, however, there was a larger variety in the material make-up of the furnishings. In these rooms, there was one large soft piece of furniture (bed in the bedroom, couch in the living room), and every other piece of furniture was made of metal or wood (bookshelves, cabinets, fridge, etc.). I cannot make confident conclusions about the effects of furnitures outside of the effects I've observed in my own house because of these disparities.

Another interesting factor to have measured would be the effects of carpeted flooring. I do not have any carpeted floors within my house, so I could not measure this effect. I assume this effect would be quite significant, as the sound absorption coefficient (at 500 Hz, for example) is 0.01 for tile flooring, 0.01 for wood flooring, and 0.14 for carpet over concrete; where a higher sound absorption coefficient implies a higher amount of sound absorbed at a given frequency. For rooms with carpeted flooring, I would assume that the F1 would be much higher than it is in the bathroom and hallway, because the lower frequencies will be absorbed more readily by the carpet.

Conclusion

From my study, it seems that the material makeup of the room and furnishings in a room have a much greater impact on the expression of sound than the size of the room it is played in. I am still hesitant to extrapolate these findings to environments outside of my own home, as my experiment had many limitations. I was unable to change the amounts of furniture with a room, nor was I able to test the effects of acoustic foams on the walls. Implementing these changes would likely yield new results, perhaps ones that would contradict the findings those I gathered in my home. Given the controls implemented, I would say that my results hold high internal validity. However, to strengthen the external validity of these findings, further experimentation with better resources would be required.