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Subwoofer Technology: Controlling the Bass

The evolution, facts, and theories about subwoofers and the low end of the frequency spectrum. Also check out the gallery for a tour of new subs introduced over the past couple of years.

By Keith Clark May 14, 2018

Over the last few years I’ve been fortunate enough to use some of the best sound systems currently available from many of the leading manufacturers, and one of the things that struck me is how far we’ve come in terms of a subwoofer delivering the bottom end of the mix.

Subwoofer technology and placement techniques have developed to the point where we have a lot more control over how this important band of frequencies is deployed. So I thought it would be good to take a look at how we got to this point and explore how we can deliver a better bottom end.

When talking about bass frequencies we’re typically referring to those below about 250 Hz, but I want to focus on the lowest of the low; in other words, sub bass – frequencies typically between 20 and 100 Hz. The nature of the way in which we hear means we don’t perceive all frequencies equally (this also changes with volume). But generally speaking, humans are much less sensitive to frequencies below 100 Hz, which is why this region requires special attention.

Looking at standard PA loudspeakers or a pair of studio monitors, most of them extend down to a reasonable frequency for reference purposes, but both will always benefit from the addition of a subwoofer to enhance the bottom end. Amplifiers designed to power subs tend to be more powerful than those used for mid-range or high frequencies, and the loudspeakers themselves are always larger (usually 15 or 18 inches in diameter) than their higher-frequency counterparts.

A common misconception is that large loudspeakers are needed to produce low-frequency sounds, but reality says otherwise – a simple proof of this is ear buds and headphones. The issue here is the medium through which the sound travels.

diatone loudspeaker image

A Diatone loudspeaker from Mitsubishi; the first ones were built in the spring of 1945 for the Japanese radio station NHK.

Air is a relatively inefficient conductor of vibrations, and bass vibrations are relatively slow, so much more power is required to produce bass frequencies that are perceived to be the same level as the corresponding higher frequencies. For instance, if we all lived underwater, a relatively low-powered 2-inch woofer would be quite capable of producing pleasing levels down to about 20 Hz.

Early Rumbles

The first acoustic suspension woofer was invented by Edgar Villchur in 1954: the AR1 debuted at the New York Audio Fair and quickly went into production for his newly founded company, Acoustic Research. The design utilized the elastic cushion of air within a sealed enclosure to ensure a linear restoring force for the woofer’s diaphragm, thus producing louder and cleaner bass frequencies.

Acoustic Suspension Woofer

Edgar Villchur, developer of the first acoustic suspension woofer, in the lab circa 1965. (Credit: aes.org)

Loudspeakers capable of producing frequencies below 100 Hz with adequate volume and minimal distortion become more common in the 1960s and 70s but weren’t commonly applied in the way that they are now (i.e., at concerts, in recording studios and for home hi-fi). The main use was in cinema to enhance the movie-going experience.

One such approach was called Sensurround, a process developed by Cerwin-Vega in conjunction with Universal Studios that used multiple subs powered by racks of 500-watt amplifiers triggered via control tones printed on the audio track of the film to produce energy between 17 and 120 Hz.

The most famous application of Sensurround was to add “realism” to the 1974 film Earthquake. The low-frequency entertainment method was credited with making the film a box office success and generated a lot of publicity for subwoofers. (And the film won an Academy Award in the Best Sound category.)

Sensurround pseudorandom generator circuit

Diagram of the Sensurround pseudorandom generator circuit to create low-frequency rumble.

 

One of the main reasons a subwoofer wasn’t commonly used in the home was due to the limitations of the primary playback medium of that time (vinyl records). Loud and deep bass was difficult because it affected the stylus’s ability to track the groove; even a moderate amount of bass (by modern standards) would cause the needle to oscillate excessively until it jumped out of the groove. Truly accurate and deep bass wasn’t really possible until the compact cassette tape became a common medium, followed by the compact disc (CD).


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About Keith

Keith Clark
Keith Clark

Editor In Chief, ProSoundWeb & Live Sound International
     
Keith has covered professional audio and systems contracting for more than 25 years, authoring hundreds of articles in addition to hands-on work in every facet of publishing. He fostered the content of ProSoundWeb (PSW) from its inception, helping build pro audio’s largest portal website, and has also served for several years as editor in chief of Live Sound International (LSI).
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