In solving noise issues in buildings, it helps to take an objective, systematic engineering approach. Starting with a good understanding of sound, how it travels and how we hear it, we can then proceed to creatively applying a variety of solutions that adhere with building codes and acceptable standards. Some basics on sound measurement and analysis Sound is wave energy, and it is measured in two basic qualities: Amplitude and Frequency. Amplitude is the intensity of sound (in physical terms, it’s the pressure of air being pushed by sound wave energy), measured in dB or decibels. This measurement helps give an idea of how loud a sound is in relative terms. 1dB is the smallest difference in amplitude that the human ear can discern. Most people have trouble discerning 3dB changes in amplitude. To achieve an easily discernible reduction in noise, a decrease of 20dB to 40dB is required. Points of reference: 0 dB – threshold of hearing 20 dB – whisper 40 dB – quiet neighborhood 60 dB – normal speech 80 dB – busy office 100 dB – city traffic 120 dB – inside a loud dance hall 140 dB – air raid siren 160 dB – jet engine 180 dB – theoretically, a Saturn V rocket Amplitude measurements are often based on a reference standard for a given purpose (for instance, radar, radio, antenna ratings, etc.). For the purpose of taking into account the sensitivities of human hearing over a wide band of frequencies, the “dBA” measurement uses an A weighting filter, which gives more weight to frequencies we are able to hear. We don’t hear low frequencies as well as we can hear higher frequencies, though they might produced at a similar power. A low 60hz tone @ 10dB, for example, will sound to us much softer than a 1,000hz tone @ 10dB. The dBA scale adjusts for those discrepancies. Frequency is the pitch of sound. Physically, it is the rate of vibration in a sound wave. Frequency is measured in Hertz (Hz), or cycles per second. For example: 440 cycles per second is the musical reference note ‘A’ on a piano (A=440hz). Ways to Control Noise We typically use three classes of physical methods to reduce sound:
- Barriers: Dense materials to lessen sound transmission
- Absorbers: Soft materials to absorb specific frequencies
- Damping: Materials vary to deaden a specific materials’ resonant (ringing) properties
- Brick and concrete are great. Lead, a very dense material, is great. CMU is good.
- Gypsum wallboard (GWB) is standard wall building material, and is not great at stopping sound, but doubling the density can decrease sound transmission by about 6dB. In cases where you need to reduce noise, a second layer of 5/8” GWB is worth it.
- Of the GWB assemblies, double stud walls are the most effective, better than staggered stud walls. You might think stuffing the walls with soft fiberglass insulation helps but actually, it does not absorb sound as well as GWB.
- Soft materials on the wall help break up first and second reflections: reverberation. They help make speech more intelligible.
- Acoustic Ceiling Panels (ACP) as a lay-in ceiling help as well. Additionally, the cavity above a lay-in ceiling helps reduce reflected low frequency sound. The same ACP panels glued to walls cannot absorb low frequencies since there is no space behind to mitigate low frequency energy.
- Carpet helps with high frequency sound and deadens footfalls within a space and cuts transmission of sound to spaces below.