Interaction with listening environments
A complication is the interaction of the speaker with the listening environment. Most listening rooms present a more or less reflective environment which means that the sound that reaches a listener's ear consists not only of direct sound, but delayed sound that has been reflected off of one or more boundaries. These reflected sound waves when added to the direct sound cause cancellation and addition at certain frequencies, changing the timbre and character of sound. This is part of the reason why a system sounds different at different listening positions or in different rooms. A significant factor in the sound of a loudspeaker system is the amount of absorption and diffusion present in the environment.
If one stands in an empty uncarpeted room and claps one's hands, the zippy flutter echoes one hears are due to a lack of both absorption and diffusion. The additions of hard furniture, wall hangings and shelving will substantially reduce the echoes, due primarily to diffusion. Diffusion is provided by somewhat reflective objects with shapes and textures having sizes on the order of sound wavelengths. This texture breaks up the stark reflections otherwise caused by flat walls and spreads the reflected energy of an incident wave over a larger angle.
Adding carpet, curtains or tapestries, and soft furniture will further lessen the echoes by absorbing the sound and preventing reflections. Absorptive materials absorb sound differently at different frequencies. The thinner a material is, the less likely it will have an effect at bass frequencies. The overabundance of absorption at high frequencies that can be caused by large areas of thin absorbing materials can cause a speaker system to sound deficient in treble, and likewise a lack of absorption can cause an otherwise uncolored loudspeaker to sound too bright or sibilant.
For good sound, a room should have a balance of diffusion and absorption. Most systems will sound best when the speakers are set up more or less symmetrically with respect to the listener and also to room boundaries. Early reflections do the most to color the sound (due to the so-called Haas Effect) so placing speakers too near either the rear or side walls is generally something to be avoided, although judicious use of absorbing or diffusing materials can somewhat moderate an otherwise poor placement location. Mounting a speaker in a wall (or in a bookshelf with books flush with the baffle) removes the reflective boundary concerns, but limits placement flexibility.
Another factor in room acoustics is the phenomena called standing waves. A one dimensional example is sound bouncing between two reflective boundaries. Sound will resonate or increase in intensity if the distance between the boundaries corresponds to an integral number of half wavelengths. Since sound travels at ~345 m/s, a pair of boundaries separated by 5 meters will cause a string of resonances to happen at 34.5 Hz, 69 Hz, 103.5 Hz.... Remember, wavelength is simply the speed of sound divided by the frequency.
In a typical rectangular listening room, this resonant phenomena is happening in three dimensions, and there are even more complex interactions that involve four or even all six boundaries. In addition, the placement of the loudspeakers and the listener with respect to the boundaries affect how strongly the resonances are excited or perceived. I am sure the reader is familiar with certain locations in a room or club which have dramatically more or less bass - most notably near room walls or corners. This is because standing wave patterns are most pronounced in these locations and in the bass frequencies, below the Schroeder frequency - which is typically around 200-300Hz, depending on room size.
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