Technical & Editorial
- Written by Dr. David A. Rich
- Published on 20 March 2013
- Subwoofers: A Brief Look at the Effectiveness of Using a Subwoofer in a Music System
- Page 2: The Subwoofer in the Listening Room
- Page 3: How Low Do We Need to Go?
- Page 4: Issues with the Subjective Evaluation of a Subwoofer
- Page 5: The Under-reported Issue When Adding a Subwoofer
- Page 6: Conclusions
- All Pages
The Subwoofer in the Listening Room
In a room, the -3dB point will be extended via model pressure variations of the room and effects of speaker boundary interface where the direct sound of the speaker interacts with sound reflected from the walls. This is usually called room gain, although Tom Nousaine, the veteran writer on the subject, points out in private e-mails there is no "passive gain" and prefers to call this "preservation of sound pressure" (private conversation). Tom's site (www.nousaine.com) has numerous PDF downloads on subwoofers, among other interesting topics. As anyone familiar with in-room frequency response knows, it is anything but flat. A worthy set of graphs to illustrate the effect is available at:
The graphs isolate the room effects from the anechoic response of the speaker for different speaker and room placements. For the large room used in this test, there is an apparent preservation of sound pressure in the 35 – 50Hz range; this will not get the typical floor-stander flat to 27Hz. Smaller rooms offer more extension at lower frequencies for reasons best explained by Tom Nousaine:
The last article shows why the really small space of a car can produce so much SPL at the low end.
Room volume is not a criterion for picking a subwoofer, although some reviews have implied this. Our ears respond to sound pressure (the change in pressure referenced to the atmospheric pressure as a result of the sound wave propagating in space), not sound power. The acoustic intensity is the sound power per unit area. Acoustic intensity is found by squaring the rms pressure and dividing by the specific acoustic impedance. It can be seen that sound pressure in this acoustics equation acts like voltage in an electrical system. Sound power declines with area, but sound pressure declines with distance from the ear. The decline is 6dB with a doubling of distance between the speaker and the ear. In a small room, the speaker may be limited to 2 meters back, while a large room might allow a larger distance of 4 meters, especially for the case of movie room with multiple rows of sets. There is a 6dB reduction of sound power at that rear listening seat relative to the seat placement in the small room in this case. Pressure preservation does vary with room size as we saw in the data in the referenced websites.
The conclusion is the in-room response of a typical passive speaker will not be nearly flat at the desired 27Hz and it will not be possible to achieve 100dB SPL at 10% distortion at 1 meter at 27Hz. A subwoofer can provide added extension and dynamics for music scored with significant activity below 50Hz. The large-scale symphonies of Shostakovich are one example.
The complete set fills 11 CDs. The Haitink performance above is a well-recorded version of the symphonies at a bargain price. Just to cover the basic 20th century repertoire that calls for large orchestras easily requires a collection of more than 100 CDs.
Scores calling for 100 musicians will not have instruments playing in the frequency range a subwoofer is designed to reproduce across a whole movement, as evidenced if you unplug the main channels with a properly level-matched subwoofer active. Many adjust the subwoofer level control far higher than the appropriate level at which it should be set to hear all the money they spent on the subwoofer. If set correctly, one should never hear sound coming from the subwoofer with the crossover held below 80Hz (Toole Sound Reproduction section 13.3.8)