Many of us share the experience where we purchase speakers after being impressed by them in an audio shop, only to be disappointed after trying them at home. While there might be many reasons for this, the largest contributor is probably the difference in the listening environment. (It might be placebo as well.)
The effect of the listening environment on sound is multifaceted, but we will limit today’s discussion to 2 things: firstly, the difference in sound due to wall reflections; and secondly, the difference in sound due to resonance.
1. Wall reflections
The sound that comes out of speakers can be delivered directly, but they could also enter the ear after reflecting off a wall.
And when the sound is reflected, the frequency-dependent reflectance changes depending on the shape and the material of the surface.
The diagram above shows the Absorptivity of the “BASWAphon Acoustical Finish System” – the thicker ones show higher absorption coefficient in general, while the relative absorption also changes with frequency. In the above case, the 68mm system absorbs much of the 250Hz noise.
Thus, it is possible to change your environment to make it sound deep and bass heavy or bright with an emphasis on treble.
A general scenario for a listening environment might entail a rectangular room with speakers installed to face the listener from one side of a room. In this case, the sound from these speakers will be reflected by the wall behind the listener, and the distance between the walls determines a specific resonance frequency at which the signal will reverb.
The formula for determining the primary resonance frequency is (in SI units):
resonance frequency = speed of sound / (2 * distance between walls)
which normally comes out to be roughly 170 / distance between walls in meters.
Resonance occurs whenever the distance between walls is identical to integer multiples of the half-wavelength(the distance between nodes), and thus resonance also occurs at integer multiples of the primary frequency obtained above.
Most rooms that have not been treated have varying degrees of resonance, and resonance introduces distortion, notwithstanding the flattest speakers.
In the Listening Condition section of ITU-R BS1116-1 standard, the formula below is presented as a condition to minimize room resonance:
(1.1 w / h) ≤ (l / h) ≤ (4.5 w / h) – 4
where l represents length, w represents width, and h represents height.
Additionally, the conditions (l / h) < 3 and (w / h) < 3 should apply.
That is, the ratio of the width of the room times 1.1 to the height must be less than or equal to the ratio of the length to the height, and this in turn must be less than or equal to the ratio of 4.5 times the width of the room to its height, less 4. As you’ll notice if you plug some numbers in, the ideal shape is neither perfectly cubical nor extremely long on one edge.
There are many other factors that come into play, but these two factors are among the most important.
Something that we could derive here is that a high-end speaker system (say, flat within 1dB) is useless unless it is placed in a corresponding environment as the distortions due to resonance may reach over 5dB. Thus, the room must be treated for true hi-fi playback.
The conclusion here is that your wallet will not be happy 🙂
Listening room examples: