Bad Acoustics In My Living Room (With Sound Samples)
By sharing with you some samples of poor acoustics, you'll be able to recognize and diagnose them in your own environment easier.
I had no idea just how bad the acoustics in my livin room were until I bought an ECM-8000 test mike from Behringer. Listening to things through the mike rather than with my own natural ears, you can hear all the flaws. Conversation and television become too garbled and "smeared" to understand. Boomy at some frequencies, and thin at others, I was surprised at what I heard.
Our ears compensate for a lot of laws, but microphones are cold and unforgiving. It's better to fix the problems in your room than to record them or allow your mixing environment to be influenced by them. These sound samples are designed to open your ears to the problems you may be hearing subtly now, but your ears and brain remove from the equation.
This is the echo that occurs between two parallel walls. High frequency sound bounces back and forth without being absorbed or diffused and rings, very much like the feedback you get when you put an open mic too close to the speaker.
Actually, it's very similar to feedback, only in a purely acoustical environment, so if there's no sound source feeding it (like the amp), it eventually dissipates. Usually in a matter of seconds.
Think of this as like being between two mirrors, and looking at a reflection that goes on into infinity. The better the reflectivity of the two surfaces, the longer the flutter echo lasts.
Bass frequencies also travel back and forth without dissipating. The difference here being that because bass frequencies are so large - some the size of your living room - they create huge "waves" of sound with high and low points in your room. Some people believe that making the walls non-parallel will help eliminate room modes because the wave will be misdirected sleightly each time it bounces and dissipate faster. While there may be some truth to this statement, what this does is create a more unpredictable environment with "hot spots" in different locations.
Fill your sink 1/3 to 1/2 of the way full of water (or a plastic tub of water is even better because you can shake the sides and rock it back and forth). Then use your palm to start moving the water in one spot up and down. Your hand is now influencing the water the same way a low frequency sound source influences the room.
You'll find that different frequencies - different rhythms - influence the water in different ways. Some rhythms will cause the water to start rocking back and forth quickly, while others just sort of splash around. Others work against the wave as it bounces off the opposite wall.
When you have the water going back & forth in a strong motion you'll find some areas are highly affected, while others seem relatively calm. You'll also notice that different frequencies cause different peaks and valleys, and where you start is less important than the dimensions of the tub. Drop tiny pieces of paper in the water if you're having a hard time seeing the movements.
Comb Filtering is reverb that crowds the original signal. The peaks & valleys of it's waveform combine with the original and emphasize certain frequencies while cancelling out others. A lot of people end up with some nasty comb filtering when they apply reverb in a heavy handed manner, or don't add a few milliseconds of delay in before they do it.
The result is a nasty, metallic, thin sounding signal. The extreme version is the "in the toilet" swirly wooshy sound. I discovered a neat trick when recording guitar amps early on. I'd stick an empty coffee can over the mic and you'd get a fun, thin sound. I've been trying to replicate that all day today, but I don't have any empty coffee cans.
That reminds me, this sound is also similar to "putting a seashell up to your ear" or cupping your ears. Very similar to your phase or flange effect isn't it? Well, in a sense, it is a phase effect because it's delaying some of the signal by just enough to cause it to be out of phase with the original signal.
The following samples were made with my Behringer ECM-8000 test mic. It's omnidirectional and reasonably flat. It makes my room sound a lot shittier than my ears normally do, though you can't really tell from these samples what the room sounds like to ears. The noise around 16k is my computer fan. I recorded them all at the same level without changing any settings, and did no post-processing, other than to convert them to mp3's.
Note: The acoustics in your room will affect the way you hear these files. I recommend, especially for the Room Modes file that you listen to them via headphones... and keep your finger on the volume control.
- Flutter Echo
I stuck the mic up by the ceiling, aimed at the corner. Then went to the opposite wall and started clapping.
- Room Modes
Sine Wave Sweep from 200 to 20hz. Around 20 seconds into the sample, a huge sound wave came at me from all around and nearly made my ears pop. The noise you hear is me moving around. Also take into account the limitation of the speakers.
- Comb Filtering
I hung the microphone inside a metallic thermos bottle... no coffee cans being on hand.
Other Articles in this Series
- Bad Acoustics In My Living Room (With Sound Samples)
- Acoustics Crash Course 1 - Modes
- Acoustics Crash Course 2 - Reflectivity
- Room Mode / Standing Wave Calculator
- Room Reverb / Sabine Calculator and Charts
page first created on Saturday, April 19, 2003
© Mark Wieczorek