One very fundamental dimension of “Audiophile Quality” sound experience is the Room acoustics which tremendously affects the listening and psychoacoustics behaviour.
No matter how advanced and elegant system you own, a simple variation in placement and orientation can dramatically influence and alter the overall perception of sound. If we try to answer a basic question, “What is Sound?”, it will uncover several aspects related NOT to sound but to the perception. Leaving aside the scientific explanation of sound, it is nothing but a perception of vibrations travelling through the air and reaching out to your ears coupled with subtle vibrations of your body and close proximities. And the feeling you get by the combined effect of all these phenomenon make you perceive the goodness of it.
Now, coming back to the subject of this page – Room acoustics basically deals with all the factors which can drastically change or influence the overall listening experience. To further emphasize the importance of this, several music system manufacturers (For ex.: BOSE) can be quoted. If you look at the specs of BOSE speakers or music systems, it never tells you the frequency response or the distortion factor. The rationale behind this is, their products as they say guarantees a great listening experience but given that the frequency response hugely depends on the overall room architecture and placement of speaker itself, the frequency response cannot be the judging criteria for a speaker, so they do not provide these details, however, in the interest of the customer, they do provide immense details about placements which actually matters. Needless to say that the frequency response as shown on several speakers are charted out in a dead room surrounded by acoustic foams where no reverb can alter the actual sound and thats is something absolutely irrelevant to a listener’s conditions.
Let’s get into little bit of science. When we are dealing with acoustics, we should keep in mind that ITS ALL ABOUT PHASE. Phase in waves is the initial angle of a sinusoidal function at its origin and is sometimes called phase offset or phase difference. Refer the image below, which depicts a phase difference between two sine waves.
Another important thing to understand is the boundary behaviour of sound waves. This means the behaviour at the end of a travelling medium. As a wave travels through a medium, it will often reach the end of the medium and encounter an obstacle or perhaps another medium through which it could travel. When one observes the reflected wave off the fixed end, there are several notable observations. First the reflected pulse is inverted. That is, if an upward displaced pulse is incident towards a fixed end boundary, it will reflect and return as a downward displaced pulse. Similarly, if a downward displaced pulse is incident towards a fixed end boundary, it will reflect and return as an upward displaced pulse. This can be explained by Newtons third law. This means that if speakers are placed at a certain point in a room then all the reflections off the walls and other solid objects lead to a reflected wave with is inverted that is out-of-phase. This has severe potential implications. Depending upon the distance travelled by the waves, their intensity after reflections, there can be several places in the room where you will observe some frequencies sounding louder and some suppressed or not present at all. This all happens because of constructive/destructive interference of waves.
Before going further, for the sake of simplicity, let us define some symbols and parameters:
- Speed of sound as c
- Wavelength as L
- Frequency as f
There relationship among these there is, f = c/L Let us understand this with diagrams:
A little bit of thinking on what is happening here would immediately bring out the dynamics. We can see from the diagram that at the exaat distance of speaker equal to 1/4 th of a frequency leads to complete cancellnation when the listener is sitting at full wavelength from the speaker. Obviously, sound contains all the frequencies so this will vary from frequency to frequency. Let us bring out our formula, that is, f = c/L, or, here c the speed of sound can be taken as 330m/s. So, the formula can be written as, L= 330/f. The audible sound contains frequencies from 20Hz to 20,000 Hz. For a simple analysis, let us take some anchor frequencies as 1: 20Hz, 2: 50Hz, 3: 500 Hz, 4: 1000 kz and 5: 10,000 Hz. The wavelength corresponding to these frequencies will be, L1 = 16.5 m, L2 = 6.6 m, L3 = 66 cm, L4 = 33 cm and L5 = 3.3 cm. Considering that an average listening room doesn’t go beyond 5 m in length, we can be sure that our 20 hz frequency will not undergo destructive interference in the way shown in the diagram. An obvious attenetion must be given to mid frequencies in the close proximity of 500Hz. Also, changing the distance between the speaker and the wall will significantly change this dynamics and again carrying out a similar exercise we can see how lower frequencies get suppressed and boosted. To illustrate practically (Courtesy of Genelec speakers), following graph witness how the distance of speaker from the wall and listener changes the response.
It is also good to know what are the different frequency ranges in audible sound/music.
|Frequencies in Audible Sound|
|Subsonic frequencies||1 Hz – 20 Hz||Not audible to humans|
|Very low frequencies||20 Hz – 40 Hz||Lowest audible octave to humans|
|Low frequencies||40 Hz – 160 Hz||Music low frequencies, here are the kick drum, bass and low register of grand piano|
|Middle low frequencies||160 Hz – 400 Hz||Middle C of piano|
|Middle frequencies||400 Hz – 2.5 kHz||Low-order harmonics of most instruments|
|Middle high frequencies||2.5 kHz – 5 kHz||Ear most sensitive to this range. Presence, voice frequencies are here|
|High frequencies||5 kHz – 10 kHz||Brightness and harmonics are here|
|Very high frequencies||10 kHz – 20 kHz||Highest harmonics are here|