Consciously or subconsciously we are always situationally aware of the acoustic properties of the space we are in, an evolutionary trait fundamental to our survival.
That said, sight is still the most dominant and dense informational sense (in the average human) so we can get a lot of information about a room just by looking around. Making a conscious effort to calculate (roughly) the probable fundamental resonances of rooms you enter is good practice—and fun—and gives you an idea of what to expect in more precise measurements taken later.
Take notice of its shape, size and material properties. See what is hanging on the walls, what furniture or objects are in the room. Is it carpeted, tiled or parquet? Is the ceiling dropped? And if so, what are the tiles made of? Walls? Painted brick? Dry wall with an air gap behind? Is there a window? Where is it? How big is it? Are there curtains? And so on.
After a while you will get a fairly good idea of how any given room might behave sonically, just by looking at it. Of course that is not enough detail, but it is a great starting point.
It is possible to garner a few rough acoustical facts about a room just by approximating its size.
A “back of the napkin” equation to memorize is f1 = c / (2 L).
f1 is target frequency; c is 343 (average speed of sound in meters) and (2 L) is twice the length of the room.
So you divide the speed of sound by twice the length of the space you are sizing up. I am not going to explain why. All you need to know is that this will give you an approximation of the lowest resonant frequency in the room, end to end.
Walk into a room. Judge (or actually measure) its size. Let’s say it’s around 3. 2 meters.
Okay. Plug that into the equation.
This room, (front to back) will probably have a resonant frequency around 53 Hz, and multiples of that; 106 Hz and 212 Hz, and so on.
You can repeat the equation for height and width if you like, but that is overkill, this technique is just a mental practice to develop so that you have a starting point and general idea of how a room will behave.
With this quick calculation we have a reasonable expectation that certain musical notes will be affected or negatively emphasized by the calculated fundamental resonance.
53 Hz is between G♯ 1/A♭1 (12th key on grand piano) and A1, so the notes around G1 - A1, (and their multiples; 106 Hz, 212 Hz, etc.) will get overemphasized in this room - useful info at the back of your mind for mixing; recognizing that some parts of the mix in this range will be acoustically accentuated, especially music in that key.
This is useful in live sound situations too, especially in unfamiliar rooms, because this might be where feedback issues sit and it shortens the time hunting for problematic frequencies.
Waking into a space and giving a single loud clap is a popular, if flawed technique for informally sounding a room.
In a small room, though, a clap is not going to give you much to work with, apart from some ringing flutter echoes, which are the least of a small room’s issues. Small spaces are dependent on localized sound and a clap will give you different information depending on the position of the source and receiver. If you clap loudly once at your mix position and then clap loudly at your speaker position, you are not ending up with any useful auditory or acoustic data. This is because most often one is dealing with room dimensions with early reflections well within the Haas interval—meaning you cannot "listen” to the room's reflections because they cannot be heard as individual signals and are merged with the direct signal.
Typically, with a small room, we are most concerned with flat/even frequency response at the listening/mix position, not the entire room.
Without major investment in acoustic treatment (and even then), a small mix room will always have low frequency issues, it’s physics. However, a good clap, can give you some information to work with. At the very least sitting at your mix position and clapping hard will tell you if the blanket you hung behind you with clothe pegs and wire has made any difference at all to the tone of the room.
So these are rough and dirty, split second, walk-into-a-room-and-gauge-what-we’re-dealing-with techniques, but they are not enough to truly know what is happening acoustically in a space.
Getting more serious with measurements takes us into mathematics. Now, before you rage quit the browser and start crying, some awesome folks around the internet have built, and freely share online, calculators that do all the number crunching for you.
There are two calculators that even a relative newcomer to acoustics can use, and get useful data from; a room modes (eigenmodes) calculator and an RT60 calculator.
Room mode calculator:
A room mode calculator, very simply, will tell you which frequencies might cause problems in your space by calculating probable room modes. The calculator assumes a perfect space with no laundry drying across the window and no messy bed in the corner so the results are idealistic, but helpful.
To use the calculator measure the dimensions of your room—height, depth and width. Plug the numbers in, and the calculator will spit out some useful data.
There are plenty online, this one has a handy graphic of the orientation and number of the room modes.
One aspect of acoustic control is attention to how long sounds remain active in a room after their initial impulse.
A large church can have a long RT60, upwards of 3 seconds. That is to say, if you clap your hands together once, loudly, it will take 3 seconds for the reverberations of the sound to drop 60 dB to silence at that frequency. In complete contrast a small professional recording booth could have an RT60 as low as 0.1s.
RT60 times can be different for various frequencies in an untreated room, meaning a 500 Hz tone may take 1.2 seconds to decay 60 dB, but a 1 kHz tone may take 0.7s to decay 60 dB.
This is what an uneven frequency response means (in relation to RT60).
A good room, therefore, has a calibrated, balanced reverb time (RT) across all frequencies, and that RT is appropriate for the kind of music it plans to host.
The average living room is well treated just by having furniture and curtains in it, and often sits in the 0.8s–0.9s range.
A concert hall favoring classical music will benefit from a balanced RT60 of 1.0s–1.5s or more, but a complex jazz quartet playing in the same room may not fare well in that acoustic space. This is why jazz tends to smaller, intimate venues and choirs tend to larger, open spaces. The lower RT60 inherent in a small club makes for less interference with syncopated rhythms and musical solos, and larger spaces allow the layered chanting and harmonizing of groups to gather and swell.
In a studio control room RT60 would ideally be around 0.5s to 0.7s for each frequency band, because this range keeps a sense of naturalness in the audio without coloring it and lends itself to accurate monitoring of source material.
Use an online RT60 calculator like this one to see the theoretical behavior of your room: http://www.sengpielaudio.com/calculator-RT60.htm
Enter the dimensions and materials of your room for each octave and calculate how your room will behave at each. The numbers will give you a fairly good idea of which frequencies need attention and knowing which frequencies need attention will inform your choice of which treatment to buy.
For the more serious (and adventurous/masochistic) this calculator takes absorption materials into consideration too: http://www.marktaw.com/recording/Acoustics/RoomReverbSabineCalculato.html
Yeah, it’s a lot of work, welcome to the entry-level acoustics. The rabbit hole only goes deeper.
Nothing will beat real world measurement data. All the math calculations in the world will not give you accurate enough data for professional use.
You will need a good omni-mic (Dayton EMM-6 or a Behringer ECM-8000 are highly recommended, even by pros) and very reasonably priced, a sound card with at least two inputs and outputs, and Room EQ Wizard (REW) or Fuzzmeasure.
Used together they will identify the indirect signals that are destructive to intelligibility, localization, and imaging–and what that all means is discussed in depth in their forums.
How to calibrate your hardware (mic + soundcard) with the analysis software, and how to read and analyze the graphs is beyond the scope of this article.
Acoustics is a very daunting and confusing aspect of audio, but with these few tips you can get started on discovering possible issues in your room and make some informed moves to address them. For more in-depth coverage of the topics touched on here, check out Joe Albano”s excellent and accessible series here: https://ask.audio/articles/studio-acoustics-part-1-an-overview-of-room-issues
And to take your productions to the next level check out the in-depth video course on Acoustics and much more in The Ask.Audio Academy.