Acoustics is a complex and difficult area to get right. However, it is science and much is scientifically explained and proven. So it sure is not ‘abracadabra and hocus pocus’.
The acoustic setup of a room or environment depends heavily on the purpose of the room. Live music or live recordings should have a different acoustic arrangement than studios where the mixing and mastering take place. Or the room in which you play your audio system.
Live music needs longer reverb times to make it sound rich and lively. For playback via the audio system, you want your system to act as a time machine. It should bring you to the performance and the sound that the musicians and sound engineer intended it to have. You should feel that you have been transported to the recording location.
No matter how high the performance or price of the audio system, it is the acoustics of the listening room that determines whether you feel that you are right there at the real performance.
The Sabine Funnel
The challenge is to keep the decay and reverb within the required Sabine Funnel specifications for the entire frequency range.
And not only keep them within the funnel, but also ensure that the decay of all notes and harmonics are linear!
In a typical non-treated modern living room, the decay and reverb times are 1.5 to 2.5 seconds (red lines)– way above the target of the Sabine Funnel. (white lines)
The decay time (blue line) within 0,3 seconds, linear with a slight deliberate longer time on the lower frequencies.
Reference Listening room
Click the button below to see the Sabine Funnel from our Reference Listening Room with optimized acoustics.
Ideally the echo time in this large room should be between 0.3 and 0.8 seconds to have the required conditions for musical playback.
Flutter echo is common in modern concrete houses with little to no upholstery like curtains, carpets or fabric covered chairs.
Solving flutter echo
Typical RTFS products for this purpose are Carreau panels and SiRRAH diffusers. The picture shows an example of 3 Carreau panels that solve the problem of flutter.
The graph below shows Flutter Echo visible in the reverb simulation
of an empty hard room. The echo time in mid frequencies rises to 2.4 secs!
Flutter echo can be avoided if the mid notes are absorbed and the high notes are diffused by one or all reflecting walls.
They reflect the sound multiple times. The mid and high frequencies bounce between hard walls and ceilings. You lose focus and the sound takes on echoic, harsh and distorted tones.
Walls act like mirrors
It is important to get the following aspects arranged in the listening room for the whole audible range of frequencies:
Absorption of the low frequencies
Absorption of the low mid frequencies
Diffusion of upper mid and high frequencies
It is very important to avoid reflections of the low frequencies by absorbing this energy derived from the initial sound wave. Else these will cause so called “standing waves”
Absorb and avoid reflections of the bottom end notes of for instance voices, upper notes of bass guitars and mid notes of a tenor saxophone.
By the nature of diffusion the room will remain lively not sounding dull while presenting a holographic stereo image or surround image that has believable focus on every voice or instrument played. This area is typically also of great importance for preserving the correct replay of the harmonics of musical instruments. These harmonics are mostly in the higher frequencies.
The appropriate reverberation times for musical playback and critical listening, are defined by the so called Sabine funnel. The right amount of reverb and decay is defined by the RT60 time, this means how much time is needed to comply with a decay in sound level over the audio band of 60dB.
This is very important because allows discrete sound waves of the speakers to reach your ears perfectly timed. With regard to timing, our ears are very capable and critical. They can easily recognize difference in timing within a few milliseconds.
Reflected(indirect) sound waves reach the ear within the first 15 milliseconds. Our brains think these reflected sounds belong to the original signal, making it very hard for our brains to paint the right picture. The focus will be blurred and the tonal quality will suffer.
This is also the reason why you should never listen within a distance of 1.5 / 2 meters of sound reflecting objects, or near the side and rear walls of your room. Timing and tonal quality will suffer dramatically when listening near walls. It is something you cannot correct by changing the acoustics.
Diffusion of upper mid and high frequencies
Low mid frequencies
Reflected waves will interfere with each other and lift the bass energy substantially in certain frequencies.
Most of the times these standing waves will change the bass sound of systems to out of pace, slow and bulky.
Famous is so called “boom bass”, a standing wave at approximately 60Hz On this frequency the whole room will resonate. Another acoustical error is comb filtering where a reflected sound wave interferes with a direct sound wave and may even cancel the energy of the direct sound wave. Comb filtering will also distort and color the sound dramatically in certain areas of the listening room.
Decay RT60 Time
The larger the room the longer the reverb times may be but in general not longer than 0.5 seconds along the whole audible frequency range. A typical smaller listening room, let’s say 5 x 6 meter, must get a reverb time of approximately 0.3 seconds. Please note that the reverb time of a standard not too much upholstered living room is approximately 1.5 – 2 seconds which is far too long for realistic reproduction of playback music.
3D Acoustical Simulation
Shaping superior sounds with cutting-edge technology.
AcousticMatters uses advanced software for acoustic simulation. The software defines the building features such as the size of the room, the type of ceiling, walls and windows, and the furniture. It then designs the listening room in 3D.
The audio system is placed virtually in the room, the speakers are chosen from a database and the listening position is defined. The simulation engine then calculates the quality of the sound given the acoustics of the room.
This image shows the sound quality of a home measuring 5 m by 9 m and 2.6 m high with hard wooden floors and concrete walls and ceilings. The listening height is 100 cm (3.3 feet).
Press the button to see the colors of our reference listening room.
Dark blue, almost black, is the color of perfect sound. Degradation is shown from light blue to green to yellow to red denoting poor to worst. Just the area between green and red distorts the sound quality by more than 50%! So replacing a CD player or an interconnect to improve the sound in this environment is not much use.
The end result – the tonal quality, imaging quality and the reverb times over the audible band – are shown in colors. The simulation takes all sound reflections into account to the order of 5. In general the simulation gives very accurate and reliable results when compared to real live calibrated measurements.
Our Reference Room
Our reference room has almost perfect sound quality in most of the listening positions. Even black on the sweet spot. The walls show red which proofs the statement that one should never listen near the walls!