Best Practice - Dispelling some acoustics misconceptions
Summing up the science of acoustics in a short article is impossible. There are, however, some common misconceptions around acoustics that perpetuate without any engineering or scientific rationale to back them up.
Here’s my top six -
Concert halls are reverberant spaces, so it’s OK for listening rooms and home cinemas to be so as well.
Wrong. Yes, some of the most magnificent concert halls in The World are shoebox shaped made up internally of mostly reflective surfaces and with midrange reverb times often over two seconds. I have heard some in this industry say that ‘if it’s good enough to perform in, then it’s good enough to listen to the recording in’. Er, no.
When a concert is recorded, it is done in a way that captures the unique acoustic of the space as well as the direct sound of the orchestra and individual instruments. This is what the great recording engineers spend a lifetime perfecting, and that balance between direct and reflected sound is baked into the recording for posterity. When reproduced, we absolutely do not then want to add even more reverberation to the sound - The job of the system is to reproduce the recorded acoustic of the performance space without adding any character from the room or system it’s being reproduced in.
To reproduce the acoustics of a recording made in a great reverberant concert hall, you do not want to add additional excess reverberation from the listening space
Room acoustics do not matter as my AV processors EQ will correct for all of it
Wrong. Yes, modern DSP based electronic electro-acoustic optimisation systems are remarkable and do now give both speaker designers and room designers more freedom in the knowledge that there is a lot that can be done with DSP. A lot does not, however, mean everything. Some things that DSP cannot do are -
Turn a poorly designed speaker into a good one.
Compensate for excess room reverberation
Compensate for very early reflections where a speaker is placed too close to an adjacent surface. An example of this is a centre channel speaker that has the top of a cabinet immediately under it, or a speaker fitted into a recess where gaps around the speaker have not been filled thus creating a resonant cavity.
Sitting your customers in deep modal nulls.
If a fantastically designed speaker has been put into a normally furnished room where specular reflections and RdT(Reflection decay Time) are controlled by ‘The Acoustic Treatments of Life’ such as sofas, curtains, bookshelves, plants, rugs and people then its possible that no additional acoustic treatment is required. But to say that room acoustics do not matter and do not need to be considered? Sorry, no way.
Modern DSP based electro-acoustic optimisation is amazing, and works even better when the room is optimised in the physical domain in the first place.
My RT60 time is 0.3s so I have a perfect room
If someone tells you that the RT60 time of a room is 0.3s, the question to ask is ‘at what frequencies’. If they either do not know, or it’s just at the commonly stated 500Hz to 1KHz octave band then that really tells you little about the room.
RdT should be as linear through the audible frequency range as possible but pragmatically have a rise at low bass frequencies. Calculations, and an acoustics strategy should be considered from 63Hz to 8KHz.
It’s possible for a room to have an RT60 time of 0.3s at 500Hz, but 0.15s at 4KHz and above. More on that in the next myth.
Acoustic treatment is easy. You just put 25mm foam behind fabric all over the room.
Sure. if every time you walk into the room you like to feel as if you have a head cold causing your ears to be blocked. High frequencies are really easy to absorb with a thin layer of absorbing material such as curtains, or the foam that’s often used as a backing for fabric. If you just absorb above 2KHz but ignore frequencies below that (Which are not very affected by 10mm of foam) you end up with a room that sounds dull, lifeless and extremely uncomfortable for humans to spend time in. The science of acoustic treatment is NOT just putting foam, rock wool or specialist acoustic panels everywhere and hoping for the best. You can’t go into a room, clap your hands then have a smug look on your face because your customer does not hear any slap echos illustrating you’ve done your job properly. No room should ever have any audible slap echos, but that’s not the only acoustics conversation in town.
The best sounding spaces have their acoustics considered and engineered from below 63Hz to above 8KHz. These spaces not only sound amazing for reproduced sound, they are also very human centric and feel fresh and lively due to high frequencies not being over absorbed.
If you use a stretched fabric system or so called ‘Acoustic Panels’ that have thin foam under the fabric, you will end up with unbalanced acoustics that over absorbs high frequencies, without controlling mid and bass ones.
My seats and subwoofers can go anywhere as bass is omnidirectional
Wrong. Every set of room dimensions will deliver a certain set of room modes caused by when a frequency wavelength (or a simple fraction of it down to a quarter) fits exactly within a room dimension. At these frequencies, the standing waves combine to cause modes (Lots of energy) or nulls (cancellation). These are distributed around a room at different positions for different frequencies. The six things that make a difference to mitigate these are -
Where you place the seating
How many subwoofers you have. 2 are better than one. 4 are better than 2. These should be identical subs and positioned optimally.
Where the subwoofer(s) are placed. With one sub optimally placed and EQ’d and the listener positioned correctly, you can achieve a very flat frequency response, but only for one listener. The real trick, and one of the hardest things in cinema design, is to achieve a uniformly excellent experience for all listeners.
How much bass absorption you have in the room. The more bass absorption you have, the less bass reflection there is and therefore the lower the peaks of the modes and nulls will be. This makes if far easier to then tweak the room using electronic EQ.
Use a Double Bass Array. That’s a subject of a future article!
Skilful calibration
If you want to hear room modes for yourself, get a speaker or subwoofer that can play down to around 50Hz and play a 70Hz sine wave tone through it reasonably loudly. Then walk around the room and you will experience that 70Hz sound really loud in some places, and almost inaudible at others. You’ve just witnessed the unavoidable physics of room modes at work.
Acoustic treatment is soundproofing.
Firstly, no it isn’t. Secondly, ‘soundproofing’ is almost impossible to achieve; Sound Isolation is the more proper term, and then the degree of isolation required must be discussed, then a method for achieving it designed.
Acoustic Treatment is managing the sound within a space
Sound Isolation is managing the sound leaking in and out of that space
Acoustic treatment affects how a room sounds. Though isolation can affect how a room sounds, it is more concerned with making a room quiet by isolating noise from other rooms, mechanical plant or traffic.