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First touch


As we plugged in the power, we couldn’t resist and put out our hands to feel the vibration.

The project is getting more and more real by the day!





Mini Prototype Speakers

The speaker drivers have FINALLY arrived and experimenting will very begin very soon!

IMG_0139 IMG_0140 IMG_0141 IMG_0142 IMG_0143 IMG_0144

Now we just have to attach the circuit from Shariq’s old speakers before we begin our experiment!


Our first practical session

On Wednesday the 16th of March, the three of us EPD seniors went down to the IDC workspace to work with my UROP supervisor Dr Ulf Bissbort. Having worked with him on the Stealth speakers, I had gain a little bit of experience and found it really useful for the project. Our goal on that day was to understand the parts that went into the Stealth, and to correct certain flaws in the first iteration of the design.

We had identified a few problems with the current iteration

  1. A background hum on standby mode- likely attributed to noise in the ground loop. Proposed solution- Produce a transmitter and receiver circuit
  2. The second speaker does not turn on with the first from standby mode. Proposed solution- Change the XLR cable to a 5 pin one so that it can accommodate a power relay.
Soldering of amps and adhering them to the heat sink

The completed product
Receiver circuit

Sound propagation, Reflection and Perception

by Daniel Tay

The focus of my research was on the physics behind the propagation of sound in a small room setting, and how the same sound is perceived by the user. This setting best describes the home environment that our group is designing ‘invisible sound’ for. Such a specification of the room setting is vital, as the acoustics of small spaces defer greatly from that of larger venues such as concert halls.


An important aspect of the sound quality is based on perception. Acousticians then conduct experiments to quantify such qualities scientifically. An acoustically good room sounds ‘live’, and the converse, ‘dead’. The most dated method of quantifying this is the Reverberation Time, where a high RT number indicates better acoustic quality. Acousticians such as D’Antonio and Eger (1986); Geddes (2002); Jones (2003); Kuttruff (1998) have questioned this given that smaller rooms typically have smaller RT numbers. They propose that sound reflection takes on even greater importance in the overall listening experience, which brings us to the next section.


Toole (2008) talks about the need to delve deeper beyond simple measurements. He too concurs with the former group of acousticians about the influences of early reflected sounds. He further proposes that the knowledge of ‘directivity and off-axis frequency responses of loudspeakers and the directional reflective, diffusive, and absorptive characteristics of materials at the points of first reflections are essential’.


Toole (2008) writes about a cognitive effect called the ‘precedence effect’ of the brain localizing in the presence of multiple different sound sources. The results gathered from different research is presented as follows- Firstly, stronger reflections shift the position of the source and make it seem larger. Secondly, reflections and delayed reflections change the timbre of the perceived sound, adding coloration and resonance that may affect the listening experience in both ways. Humans perceive such effects most if the reflections are in the median plane.


The timing in between reflections too affect the listening experience. Haas, Gardner, Lochner and Burger all found that the first reflection dominates this experience. However, the timings between subsequent reflections determine if these are heard as ‘spatially separated auditory images’ (long delay) or as a single image (short delay). Also, the volume of the delayed sound determines how ‘spacious’ the room sounds.


In relation to our project, it is clear that for an even perception of sound across the entire living space, it is necessary to consider the acoustics of the individual spaces such as the kitchen, bathroom and hallway that make up the entire living space. Factors such as the volume of the room, the presence of strongly sound-absorbing curtains and furniture in the living room in comparison to hard table tops in the kitchen do affect the listening experience. Sound sources should therefore cater for and smoothen out these localized irregularities to create an excellent, across-the-board listening experience. If possible, the sound system could even take advantage of reflections and utilize them to enhance the listening experience.




Acoustics of rooms

ITU-R Recommendation BS.775–2 (2006). “Multichannel Stereophonic Sound System

With and Without Accompanying Picture.”!!PDF-E.pdf



Toole (1986). “Loudspeaker Measurements and Their Relationship to Listener Preferences,”

  1. Audio Eng. Soc., 34, pt. 1, pp. 227–235; pt. 2, pp. 323–348.


Schroeder, M.R. (1954). “Statistical Parameters of the Frequency Response Curves of

Large Rooms,” Acustica, 4, pp. 594–600. Translated from the German original in

  1. Audio Eng. Soc., 35, pp. 299–306 (1987).


Schroeder, M.R. (1996). “‘Schroeder Frequency’ Revisited,” J. Acoust. Soc. Am., 99, pp. 3240–



Baskind, A., and Polack, J.-D. (2000). “Sound Power Radiated by Sources in Diffuse

Field.” 108th Convention, Audio Eng. Soc. Preprint 5146.


Schultz, T.J.  (1983). “Improved Relationship between Sound Power Level and Sound Pressure

Level in Domestic and Offi ce Spaces,” Report No. 5290, Am. Soc. of Heating, Refrigerating

and Air-Conditioning Engineers (ASHRAE), prepared by Bolt, Beranek, and

Newman, Inc.


Hodgson, M. (1983). “Measurements of the Influence of Fittings and Roof Pitch on the

Sound Field in Panel Roof Factories,” Applied Acoustics, 16, pp. 369–391.

Gover, B.N., Ryan, J.G., and Stinson, M.R. (2004). “Measurements of Directional Properties

of Reverberant Sound Fields in Rooms Using a Spherical Microphone Array,”

  1. Acoust. Soc. Am., 116, pp. 2138–2148.


D’Antonio, P., and Eger, D. (1986). “T60—How Do I Measure Thee, Let Me Count the

Ways,” 81st Convention, Audio Eng. Soc., Preprint 2368.


Geddes, E.R. (2002). Premium Home Theater: Design and Construction. GedLee LLC, Novi,

Michigan, USA.


Jones, D. (2003). “A Review of the Pertinent Measurements and Equations for Small

Room Acoustics,” J. Acoust. Soc. Am., 113, p. 2273 (abstract only). Personal communication:

presentation text from the author.


Kuttruff, H. (1998). “Sound Fields in Small Rooms,” 15th Conference, Audio Eng. Soc., Paper



Benade, A.H. (1984). “Wind Instruments in the Concert Hall.” Text of an oral presentation

at Parc de la Villette, Paris; part of a series of lectures entitled “Acoustique,

Musique, Espaces,” 15 May 1984 (personal communication).

Precedence Effect


Gardner, M. (1968). “Historical Background of the Haas and/or Precedence Effect,”

  1. Acoust. Soc. Am., 43, pp. 1243–1248.



Localization and Listener Perception


Gardner, M.  (1969). “Image Fusion, Broadening, and Displacement in Sound Localization,”

  1. Acoust. Soc. Am., 46, pp. 339–349.




Gardner, M. (1973). “Some Single- and Multiple-Source Localization Effects,” J. Audio Eng.

Soc., 21, pp. 430–437.


Haas, H. (1972). “The Influence of a Single Echo on the Audibility of Speech,” Doctoral

dissertation, University of Göttingen. Reprinted in J. Audio Eng. Soc., 20, pp. 146–

159, 1972. A reprint of a 1949 translation of Haas’s PhD dissertation.


Lochner, J.P.A., and Burger, J.F. (1958). “The Subjective Masking of Short Time-Delayed

Echoes by Their Primary Sounds and Their Contribution to the Intelligibility of

Speech,” Acustica, 8, pp. 1–10.


Benade, A.H.  (1985). “From Instrument to Ear in a Room: Direct or via Recording,” J. Audio

Eng. Soc., 33, pp. 218–233.


Olive, S.E., and Toole, F.E. (1989a). “The Detection of Reflections in Typical Rooms,”

  1. Audio Eng. Soc., 37, pp. 539–553.


Shinn-Cunningham, B.G.  (2003). “Acoustics and Perception of Sound in Everyday Environments,” Proc.

3rd Int. Workshop on Spatial Media, Aisu-Wakamatsu, Japan.



Rakerd, B., Hartmann, W.M., and Hsu, J. (2000). “Echo Suppression in the Horizontal

and Median Sagittal Planes,” J. Acoust. Soc. Am., 107, pp. 1061–1064.


Meyer, E., and Schodder, G.R. (1952). “On the Infl uence of Refl ected Sound on Directional

Localization and Loudness of Speech,” Nachr. Akad. Wiss. Gottingen, Math.

Phys. Klasse IIa, 6, pp. 31–42.


Textbook Source


Toole, F. E. (2008). Sound reproduction: Loudspeakers and rooms. Taylor & Francis.