About Room Acoustics and the LASSIE System

Room Modes

Room Modes are caused by sound reflecting off the walls, floor and ceiling. When a sound wave bounces off a surface, it is inverted, and at certain frequencies related to the dimensions of the room, the original wave and the inverted wave cancel each other out. When a cancellation occurs at the location of one of the sound sensors, the LED on the sound sensor will go dim or completely off. This indicates that sound is being cancelled at that location. Response bumps due to constructive interference show up as brighter lights.

A primary motivation for developing the LASSIE system was that many otherwise splendid Home Theatres and listening rooms have poor low frequency response. Audiophiles may spend $10 K, $100 K or more on equipment and furnishings, only to be disappointed in the results of the audio reproduction. There is one primary reason for their displeasure, with many names: Standing Waves, Room Modes, Eigentones, Room Resonances. Another motivation is that room alignment and subwoofer placement can be hard to do correctly and are very time consuming! While parametric EQ can assist in taming resonance peaks, NO amount of signal processing can fix acoustical cancellation. But knowledge of room acoustics can allow you to select the best seating locations, and avoid missing that exciting low bass! The LASSIE System is the key to seeing and controlling your listening environment!

Using the LASSIE System: FAQs

Q. What is the best spacing for the LASSIE sound sensors?
A. The optimal spacing is determined by the speed of sound in air, and the Nyquist criterion, which requires at least 2 sampling points for each full cycle of the highest frequency to be measured. On a practical basis, it is good to oversample (that is, place more sound sensors than required by Nyquist). Since Room Modes tend to be most troublesome below about 200 Hz (wavelength about 6 feet or 2 meters), we recommend a spacing of 18 in. (45 cm). Alternatively, if you are not using the Image Analysis Software, you may like to place sound sensors at seated ear height in a home theater, or cluster a number of sound sensors together near the primary listening location (as in a recording studio engineer's seat). Often a combination of placements will be most informative.


Q. OK, so I've done some sweeps, and have some nasty response notches at 31 Hz and 47 Hz. What now?
A. There are various techniques for dealing with acoustical problems in a room. Sometimes relocating your subwoofer(s) can do the trick, and the LASSIE System makes it easy to see the improvement. Absorptive panels, bass traps, wall isolation, false ceilings, and other techniques may be used if the situation requires. Sometimes, simply moving furniture is the easiest solution! Depending on where in the room these notches occur, it may be possible to relocate the primary seating area away from them. Good frequency response is important where your ears are, but doesn't matter much at your feet or in the far corner!

Q. Why not just use a graphic or parametric equalizer to even out the response?
A. Room Modes are an acoustic phenomenon, and no amount of electronic signal processing will remove them. In fact, increasing gain at a modal frequency will tend to make the problem worse, not better!

Q. I see a lot of narrow response notches above 200 Hz, well into the kHz range. What about these?
A. Surface treatments such as carpeting, wall coverings, and dispersion panels can help to make these less of a problem. The audibility of narrow notches at the higher frequencies is not well understood.

Q. Can I use the LASSIE System to set up my surround speakers?
A. Yes, since the LASSIE sound sensors are responsive up to about 16 kHz, one can send pink noise to a single channel, and observe the radiation pattern of the loudspeaker. Optimizing coverage is now easy!

Q. What about PA systems for large venues?
A. Seagrave Instruments would be happy to sell you as many sensor systems as you require, even for large auditoriums or stadiums.

Q. What is the time constant used in the sensors?
A. The LASSIE sound sensors do not contain any filtering or circuitry for integration. The LED is on for the positive half cycle of a waveform, and off when the pressure is below ambient. The human eye and the exposure technique in the webcam provide the integration. Because the response is instantaneous, one interesting application would be to record very high speed video images with a balloon pop or other audio impulse excitation, and then replay the video at much lower speed. This would allow observation of the transient wave fronts propagating from the source, and reflecting at boundaries.

For more information, print or view our LASSIE brochure.