Room Acoustic Design Exercise

Room Acoustic Design Exercise A) Guide to Acoustics1. Acoustic PrimerIn order to fully understandacoustics, it is important to know what sound actually is on a physical level.

Soundis quite simply a disturbance in a medium, which propagates as a pressure wave (Howard,D.M. & Angus, J.

A.S., 2017, pp.

2-10). To model how that wave actuallypropagates through a medium, see Figure 1. This golf ball and spring modelshows that, when one golf ball is moved (representing the movement of amolecule within a medium), the springs that connect them (representing theintermolecular forces between the molecules within a medium) will pull the nearbygolf balls closer (compression) or will push them farther away (rarefaction).

This method of propagating by compression and rarefaction of molecules within amedium is called a longitudinal wave. This type of wave is how most soundtravels, especially through air; however, sound can travel by other types ofwave including transverse, which is how sound often travels in musicalinstruments such as strings and percussion.  2.

The Importance of Room AcousticsThe acoustical properties of a room can drastically affectthe quality of sounds within the room; therefore, it is important to know howto design rooms that will have favorable acoustic properties and what measurescan be taken to improve the acoustics of a room. For example, bad acoustics ina room in which someone is likely to be speaking in front of an audience canmake it difficult to understand the speaker due to a loss in sound clarity. Badacoustics in this type of environment also have the possibility of making soundproduced where the speaker would be standing within the room not as loud forsome locations in the audience as others. This is also a concern for peopledesigning concert halls, as it is important that the performance sound the sameregardless of where an audience member is seated within the room. Thereverberation within a room is also an important aspect of room acoustics, andwhat is considered the desired reverberation time depends heavily on thepurpose of the room. Lecture halls, for example, need short reverberationtimes, as too much reverberation can make it hard to understand the speaker. Incertain concert venues such as concert halls and churches, however, it isdesirable to have longer reverberation times, as it is pleasing to the ear forthe types of music that are likely to be performed in these venues.

Room modescan also negatively affect the way we perceive sounds within a room, as theycan drastically boost some frequencies and drastically cut others, leaving anunwantedly skewed mix of frequencies.  3. Acoustic TreatmentLuckily, there are things we can do to improve the acousticsof a room without building a completely new room. There are a variety ofmethods and devices that can be used in a room to alter its acoustics qualitiesto better suit the purpose of the room. For example, we may want to treat a lecturehall so that it is less reverberant, making it easier to understand what he/sheis saying. Similarly, a room that is particularly modal can be treated withdevices that absorb a certain range of frequencies, in an attempt to flattenthe frequency response of the room. Additionally, a room can be treated to makeit quieter (as in a restaurant or factory) and can likewise be treated to keepthe noise from escaping the room or to keep outside noise from entering theroom. 4.

Sound Absorbers and Diffusers            One methodof altering the acoustic qualities of a room is by adding absorptive materialsin it, of which there are many different type. One such material is known as aporous absorber, which absorbs sound energy through the friction between thevelocity component of the sound and the material’s surface (Howard, D.M. &Angus, J.

A.S., 2017, pp. 339-341). Because this friction increases withfrequency, the amount of sound absorbed by a porous absorber also increase withfrequency, as shown in Figure 2. One way to get more low-end absorption, or tocontrol what frequencies will be most affected by a porous absorber (if it isbeing used to mitigate the effects of room modes), is to place the surface ofthe material a certain distance away from a hard surface. Since the absorptionof a porous absorber depends on the velocity of the sound wave, the absorberwill work best when the sound wave is at its highest velocity (1/4 of a wavelength).

So, if the surface of the absorber is placed ¼ wavelength distance away from a hard surface, it will do a better jobof absorbing those frequencies which fit within that distance.             Another typeof absorber is known as a resonant absorber and involves encapsulating anabsorptive material between a panel and a hard surface, such as a wall,illustrated in Figure 3. The front panel vibrates causing friction in thepressure component of the incident sound (Howard, D.M. & Angus, J.A.

S., 2017,pp. 341-345).

Due to their resonant nature, these absorbers work best for lowfrequencies, as shown by Figure 2. Because of this, resonant absorbers are agreat accompaniment to porous absorbers, which work best for higherfrequencies. Another type of resonant absorber is known a Helmholtz absorber.This type of absorber is similar in construction to the panel absorber inFigure 3; however, it uses a perforated panel to create resonance of a tube ofair. This alters the resonant frequencies of the absorber, as shown by Figure2.

Lastly, it might be assumed that sound absorbers can also beused for sound isolation; i.e., keeping sound from escaping a room or keepingoutside sound from entering a room. While absorbers extract energy from a soundwave, they typically do very little in terms of keeping sound from goingthrough them (Howard, D.M. & Angus, J.

A.S., 2017, pp. 351).

            Unlike soundabsorbers, which absorb sound energy, diffusers are used to disperse the sound equallythrough the space (Howard, D.M. & Angus, J.A.S., 2017, pp.

345). Theyensure that the reverberation time and absorption coefficient throughout thespace is constant (or as close to constant as possible). This is done usinguneven surfaces that will reflect the sound in multiple directions, such as thesurface shown in Figure 4.  5.

Room Acoustics Guide            There are some signs that canindicate when certain acoustic treatments are likely to need to be used in aroom. For example, when a room is very rectangular, meaning it is a perfectrectangular prism and all of its sides are parallel to the opposite side, thistypically causes room modes and flutter echoes, both of which can be cured byadding sound absorbers. Flutter echoes are nasty repetitive reverberations thatoccur when a sound reflects back and forth off of two parallel surfaces. Anothertreatment that might be necessary in rectangular rooms is diffusion, whichhelps to spread the sound energy evenly throughout the room and make the roomsound more spacious. Another sign of potentially necessary acoustic treatment isif the room is built of reflective surfaces, such as marble of stone. This willlikely yield a very reverberant room, and sound absorbers might need to be addedto reign in some of the reverberation, depending on the context.

If it is a cathedralor large concert hall, it may be beneficial to have such long reverberation times,as this is often a desirable effect in music; however, if the room is to be usedfor lectures or speeches, it is likely that the reverberation time will need tobe shortened, so that the speaker can be more easily understood.  6. Further Reading·        Kindig, S. (2010) Room acoustics for home audio[Internet]. Available from: [Accessed 24 January 2018].

This source provides further general information about room acousticsgeared more towards home theater/stereo setups. ·        Feinstein, S. (2016) The Importance of Room Acoustics[Internet].

Available from: [Accessed 24 January 2018].This source provides simple, yet practical information about roomacoustics. ·        Perry, T. (2016) Acoustic Treatment Setup 101: How toTreat Your Room for High Fidelity Listening & Mixing [Internet].

Availablefrom: [Accessed 24 January 2018].This source provides practical information about room acoustic treatmentmethods. ·        E-Home Recording Studio (2017) Acoustic Treatment 101:The Ultimate Guide for Home Studios [Internet]. Available from:

com/acoustic-treatment-101/> [Accessed 24January 2018].This source provides in-depth information about room acoustic treatmentand some useful products that are available. B) Technical Report1. Introduction            If a room on campus was to beconverted into a small music venue, there are multiple aspects of that room’sacoustic properties which should be taken into account, in order to get thebest listening experience in the room as possible. One good way to get a handleon the acoustic characteristics of a room is to measure its impulse response.

Fromthis measurement, we can calculate a variety of important properties and evenuse it to convolve various anechoic recordings so that we can hear whatdifferent sounds would sound like if they were played in that specific room.This measurement will also tell us what sorts of acoustic treatments need to beadded to the room, in order to make it a better venue for small concerts. 2. Description of Room            The room used for this report was a simple, rectangular teaching room. Asshown in Figure 5 and 6, the room has carpeted and wood floors, and it seems tohave sheetrock on the ceiling. The walls are also made of sheetrock; however,there is a large chalkboard on the front wall, a whiteboard that takes up theentire wall on the far side, and a window that takes up all of the back wall.

There are long black drapes in front of the back window and in front of the door(on the right in Figure 5) which we had drawn for the taking of the impulseresponse measurement. The room is 8.04m long, 5.9m wide, and 2.65m tall.

We hadthe speaker placed 66cm away from the front wall and the microphone placed 3.4away from the speaker.                         Given its physical characteristics, itis likely that this is a pretty dry room. It has very few reflective materialsin it, the only major one being the whiteboard on the far wall, and the carpetand drapes are going to absorb a lot of the sound energy, leaving very littlereverb in the room.

We can estimate the reverberation time of the room (T60)using the equation,We cancalculate the volume of the room (V) and its surface area (S) using the roomdimensions listed above. As for the absorption coefficient (?), we’ll have to approximate a bit. The sheetrock walls andceilings probably have absorption coefficients of 0.

1 or less, the drapesprobably have an absorption coefficient of 0.3 or more, and the carpeted floorprobably has an absorption coefficient of 0.3. So, the average absorptioncoefficient of the entire room could be approximated as being about 0.2.Plugging this all back into the equation above, Mode # Frequency (Hz) Mode Type 1 21.33 Axial 2 29.

07 Axial 3 36.05 Tangential 4 42.66 Axial 5 51.62 Tangential 6 58.14 Axial 7 61.93 Tangential 8 63.99 Axial 9 64.

72 Axial 10 68.14 Tangential Additionally, since it is so rectangular, it is likely thatthis room is quite modal. There are three different types of room modes, axial,tangential, and oblique. Axial modes occur across one dimension of the room;tangential modes occur across two dimensions of the room; and oblique modesoccur across all three dimensions. The first ten room modes are given in Figure6 below.

Figure 6 – First 10 Room Modes              Another potential issue with thisroom is sound insulation, mostly through the back window. While we were in theroom in fact, I noticed how easily outside noises entered the space. If thisroom is going to be converted into a small music venue, this would probablyneed to be addressed, especially to keep any loud concerts from spilling overinto neighboring rooms.  3. Acoustic Properties of Room Center of Frequency Band (Hz) C50 (dB) D50 (%) 63 0.

452105 52.6038 125 2.3548 63.2302 250 5.03625 76.

11755 500 7.93015 86.107 1000 10.8335 92.

37455 2000 12.4777 94.6442 4000 15.20115 97.07275 8000 18.63175 98.64825              Overall, this room is quite dry, as can beheard in its impulse response; it has a reverberation time that is excellentfor a teaching room, but perhaps a bit short for a small music venue.

One wayto get a better, more objective understanding of such a subjective acousticquality of a room is to calculate its clarity (C50) and definition (D50), shownin Figure 7. Clarity is a ratio of the early sound energy to the late soundenergy, measured in dB, and is typically used as a measure of how easy it is tounderstand speech within a room. Definition is a ratio of the early soundenergy to the total sound energy, measured as a percentage, and is Figure 7 – Calculated Clarity and Definition of Room typicallyused as a measure of how clear a piece of music played within the room willsound to the listener (PureBits, 2004).

Both the clarity and definition of thisroom are very good; they do decrease as frequency decreases, leaving lessclarity at lower frequencies, but this is to be expected.            Even though this room already hasvery good clarity and definition values, there are still acoustic treatmentsthat could be done. First of all, to combat the room modes that are present inmost rectangular rooms like this one, resonant absorbers, tuned to trap particularlyprominent modes, should be added. This will even out the frequency response ofthe room; however, too many absorbers can make the room too dry. Fewer than tenacoustic panels to be installed along the walls and/or ceiling of the roomwould cost approximately £400. Secondly, since the whiteboard alongthe far wall will most likely want to be removed anyways, as this room is nolonger going to be a teaching room, it could be a good idea to add a diffuserin its place for under £300.

A diffuser will not absorb anysound energy and will not reduce any reverberation in the room, but it willspread the sound out around the room, ensuring that sound in the room soundsthe same no matter where in the room the listener is. Lastly, something shouldbe done to better insulate the large window across the back of the room. Oneoption would be to take out the window entirely and replace it with a solidwall. While this would better insulate the room, it would be more expensive.One alternative is to replace the existing windows with better soundproofwindows, although this is likely to be about as costly. Another option is toadd mass to the windows, which will keep sound from passing through them.

Thiscan be done using a sheet of vinyl insulation for around £50. Additionally, keeping the existing drapes will help withinsulating the windows. 4. Conclusion                        While this room has some beneficial acoustic qualities(such as a low reverberation time, good clarity, and good definition), thereare still some problems with it that need to be addressed before it can beconverted into a small music venue. As with many rectangular rooms like thisone, it is quite modal.

Having prominent room modes in a room makes itdifficult to get an accurate balance over the frequency spectrum; however, wecan treat the room with resonant absorbers to reduce the sound energy at anyunwanted frequencies. Additionally, there are a few different treatments thatcan be done to better insulate the room to keep unwanted sound out and to keepthe sound of the performance in. Lastly, a diffuser will help spread the soundof the performance evenly around the room, ensuring each listener receives thesame acoustic experience.


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