Recording Studio Acoustics

The desired acoustic properties of a recording studio are in many ways the opposite of those of an auditorium. Instead of enhanced reverberation, it is usually desirable for the recording studio to be acoustically "dead", having a very short reverberation time. Not only does this require the enclosure itself to be very absorbent of sound, but soundproofing becomes very important. In order to prevent the passage of low frequency sounds such as traffic noise, aircraft noise, etc., the recording enclosure is often isolated from the main structure with a double wall. Since low frequency sounds are much more efficiently borne by solid structures than high frequencies, the suspended "room within a room" strategy minimizes the structural linking of the recording room to the foundation of the building. Careful sealing of the enclosure and careful design of the heating and air-conditioning system are necessary. Sometimes additional bass traps are employed to further reduce low-frequency background.

Soundproofing

Soundproofing of a room involves the isolation of that room from audible sound from the outside and may be taken to include the acoustic damping of the room itself. Preventing the entrance of sound from the outside is accomplished by sealing openings, making the walls absorbent of sound, and minimizing the passage of sound energy through the solid structures of the walls. Sound absorbing materials such as foam insulation in the walls help with both the sealing and absorbing of mid-range to high frequency sounds, but bass frequencies tend to be efficiently borne through solid structures. To isolate a room such as a recording studio from low frequencies, the "room within a room" approach is often taken. A double wall structure can greatly reduce bass transmission.

The difficulty of soundproofing is increased by diffraction of sound through small openings, so sealing of the structure is important. Also adding to the practical difficulty is the fact that an offending sound may have to be diminished by a factor of a million or more to make it inaudible because of the nature of human hearing.

The Ear and Soundproofing

Soundproofing is rendered more difficult by the way the ear responds to sound intensity. The ear's response is roughly logarithmic, and a commonly used rule of thumb for sound loudness suggests that an offending sound must be reduced by a factor of ten in intensity to be reduced to half as loud. The reduction of the sound power entering a room by a factor of a thousand would certainly seem to be sufficient, but the rule of thumb suggests that its loudness would be one-eighth of the original and still audible.

A reduction by a factor of over a million may in fact be necessary for an intense external sound. Such reductions are typically expressed in decibels, with a factor of a million corresponding to 60 decibels.

Bass Traps

The damping of unwanted sounds is an important feature of recording studios. The bass frequencies are often troublesome because they diffract through small openings and are more efficiently structure-borne into the recording area. Bass traps which make use of the nature of cavity resonance are used to advantage. If a particular bass frequency range is troublesome, a large cavity can be constructed which will offer resonant absorption in that frequency range. Some recording studios have closet-like carpeted enclosures which have a movable baffle as the opening. Positioning the baffle can tune the absorption, since the area of the opening is one of the variables which determines the resonant frequency of a cavity. A tuned cavity which is reflective can enhance the sound at the resonant frequency, as in a bass-reflex speaker, but if the cavity is covered with absorbing material, it will selectively absorb sound energy at that frequency.