Soundproofing Principles

To: quiet-list@igc.org
From: Ron Pellegrino <ronpell@microweb.com>
Subject: Soundproofing
Date: 4/6/98

>=== Quiet-List message from shunyata@gol.com (Jim Mack) ===

>Hello,

>I live in Japan and we´re fighting a dump truck invasion of our
>small town. There are many aspects I´d like to say about this,
>but for right now, I need to do something for my bedroom so I
>can get some sleep. Below is what I´m thinking of doing. Any
>comments or suggestions most welcome.

>What I´m thinking of doing is making a room-within-a-room,
>basically around my bed so I can sleep through the noise. I´m
>wondering what would be the best material to use and how to
>use it. I´m thinking of wood panels, about 1/2" thick and
>sandwiching between them some material. What´s best to put
>in between them? Is the thicker the better for the wood? I
>know there might be better material than wood, but I live in
>Japan where everything costs an arm and a leg. Would wood
>do the job? I can get it pretty cheap. I just need to cut the
>decibel level by about 15-20db, don´t need to make it 100%
>soundproof, because foam earplugs take care of most of the
>noise but not the very low engine rumbling noises or the very
>high frequency tire whining noises. I thought of leaving one
>side of the structure open ´cause I need to breath. The open
>side faces away from the noise source. Would this negate the
>whole noise reduction effect?

>Thanks,

>Jim

RP - I´ve been forced to deal with the soundproofing problem a number of times in my life, often short term on the road and occasionally for longer periods with rentals. Like Jim Mack I wasn´t looking for the 100% solution, just a noticeable improvement, so I applied the following fundamental principles of the physics of sound.

1. High frequencies are directional and relatively easy to reflect and absorb. Hard materials reflect and soft materials absorb. Place either type of material in the path of the sound especially in front of and behind windows. Place absorbent materials on all hard surfaces that might reflect stray high frequency waves.

2. Low frequencies diffract or bend around obstacles in their path. Nevertheless, absorbent materials placed in the path of low frequencies will attenuate them somewhat. The greatest reduction in the amplitude of low frequencies occurs when you isolate your bed from the floor. Using thick sound insulating material under just the feet of the bed helps. Even better is a bed of insulation under the bed extending six inches or so beyond the feet of the bed; this also helps to reduce midrange frequencies that are also conducted by most building materials.

3. Earplugs help to reduce high and midrange frequencies.

4. Get rid of everything in the house (your sleeping space especially) that vibrates sympathetically with the noise source. This exercise is great for the education of your ears and will make a huge difference in noise reduction.

5. Discover the exact frequencies of your room resonances and how they relate to the offending sound source; most likely the room resonances are reinforcing the offending sound source or are close enough in frequency to cause beating. I realize that this step will present problems for most folks not in the audio field. One approach is to use a signal generator or an analog synthesizer to generate a sine tone that you send through a sound system in the space. Sweep the frequency of the sine tone from subaudio through supersonic. Using a two channel oscilloscope connect a leg of the source sine tone to the A channel and to the B channel connect a good microphone that´s monitoring the sound of the sweeping sine tone in the space. Compare channels A and B while you´re sweeping the sine tone. On the oscilloscope display try to match up the excursion of the wave at a non-resonant frequency of the room. As you sweep you´ll discover that with certain frequencies the excursion of the B channel (the room) will greatly exceed that of the A channel (the source). When that happens you´ve discovered a room resonance; there are likely to be many room resonances. Conduct this A/B procedure at many locations in the room. Experiment with the room design to minimize its resonant frequencies; in other words, break up the room with furniture, room dividers, etc. Place your bed at the null point of the most offensive frequency (probably the fundamental of the room). Audio engineers do something similar to this with pink noise generators and spectrum analyzers. This step is definitely worth the effort if you have to live in a noisy environment for any length of time. The resonant nature of the soundspace tends to be the most overlooked variable in noise reduction.

In every case I´ve had to balance my fresh air requirements with noise reduction techniques so I´ve used a variation of the room within a room idea. I´ve found that four room dividers can be positioned around a bed to serve as reflectors/absorbers and still allow for air flow. Be creative and experiment with all the principles. It´s possible to make a significant reduction in noise using relatively affordable decorative materials like blankets, mirrors, stained glass, room dividers, etc.; for the slightly less expensive approach you can always make trips to the lumber yard and the harware store.

Ron Pellegrino


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