I know, I know––It’s been a few weeks. I’ve been in the studio a lot.
Since my music school audition is coming up, and since I’m applying as a composition major, I need to send in recordings of my work. For better or for worse, I don’t separate my work as an engineer from my work as a composer, so the recording quality, as always, is of utmost importance to me.
Both of the songs have piano in them (one of them is only for piano), and we’ve already talked about why piano recording is so challenging. I’ve recorded piano before, but perfection demanded me to do better. Unfortunately, this recording has turned out to be an iceberg; I sailed up to it thinking it was trivial, only to discover that the most significant and troubling parts were underwater where I couldn’t see them.
The more I researched piano recording, the bigger the iceberg became. One day, I stumbled onto a forum thread about the topic, and posters were throwing around all kinds of terms I had never heard in my life, talking about a “reverb radius” and also warning about the dangers of phase problems with near-field mics. Uh-oh!
I looked up “reverb radius” and determined that it was referring to an acoustical principal known as Critical Distance. It’s basically the point in a room where the amplitude of the direct sound source becomes equal to the amplitude of the sound’s reflections. Of what practical use is it? For better intelligibility, it’s best to mic at 50% of the Critical Distance or closer (no more than 30% if you’re using an omni mic), because the direct sound source has to be at least 10-12 dB louder than its reflections for your ear to be able to perceive that the direct sound is louder.
I was just going to crunch the numbers to find out the Critical Distance in my studio, but then I realized that the Critical Distance equation required knowing the reverb time in the room (which varies at different frequencies), and calculating the reverb time required knowing the absorbency coefficients of the room’s materials. I was getting myself into deeper and deeper trouble. What had started out as a “simple” recording project had turned into a physics lesson.
Thankfully, I found a handy reverb time calculator with the absorbency coefficients already programmed in. So I broke out the tape measure and found the area of the walls, drapes, rug, glass doors, and other surfaces in order to fill in the chart and make the calculation. Using the reverb times that I got, I determined that the critical distance in my room was between 0.9 and 1.2 meters depending on the frequency, so I would need to have the mics no more than about 0.45 meters (~1.5 feet) away from the piano strings for clarity. Not much latitude for experimenting with miking!
But my science lesson still wasn’t over. I found out that rooms also have nulls and modes which can greatly affect the sound quality of the room, and thus, the recordings. We could make a whole other post about nulls and modes, so if you’re curious, check out this article about it.
But how did doing all of this math actually help my recording? It showed how and why I needed to deaden the room. I didn’t end up buying any kind of “professional” acoustic treatment. Instead, I just used thick, absorpent materials from around the house. I especially focused on treating the corners of the room where low frequency energy likes to accumulate, and I covered the wood floor to reduce the resonance of the vertical mode. When it was said and done, the room was significantly quieter and less reflective, so I could have the flexibility to mic the piano from a little farther away without loosing clarity.
In the end, I don’t think you have to go quite as crazy with calculations as I did if you want to make a quality recording. Knowledge is power, so knowing something about acoustics is important if you’re an audio engineer, but you always have to trust your ears first. If something sounds good, it probably is good! And while it’s certainly better to have a “treated” room to work in, I might say that the mics you use and your mic techniques make the biggest difference in recording quality. Next time, we’ll talk about the controversial subject of piano miking techniques, and we’ll find out how I ended up miking the piano after all of this…
Until then, let’s hear from you all: What do you do when you have to record in an “untreated” room? What home remedies do you use to fix acoustical problems?
P.S. Oh, and if I’ve made a technical error in explaining these concepts or there’s some other mistake in here, please point it out! I’m still new to acoustics…