Aerial Acoustics Model 5 loudspeaker
Bass extension works against good sound quality in small rooms or rooms with poor dimensional ratios. Any volume of air in an enclosed space will reinforce certain frequencies when that air is excited (such as by a woofer cone moving back and forth). This reinforcement is called resonance, and the frequencies at which the resonances occur are the room's resonance modes. The frequency of a resonance mode is determined by the distances between the room's walls. The greater the distance, the lower the resonance frequency (footnote 1).
When you blow into a bottle, you are not producing the pitch you hear; the bottle's resonant modes are. You are simply exciting the air in the enclosed space nearly randomly; the bottle reinforces certain frequencies that you hear as a pitch. Similarly, a loudspeaker exciting the air in a listening room causes the room to reinforce certain frequencies. The result is smeared and boomy bass (footnote 2).
The larger the room, the lower the room's fundamental resonance and thus the more closely spaced and dense the resonance modes. Closely spaced resonance modes are a good thing; if the resonances approximate a continuum, individual resonances are less likely to be audible. Conversely, a small room's resonance modes are spread out more widely over the band. With large gaps between resonant frequencies, those resonant frequencies are rendered much more audible. That's one reason why it's hard to get good-sounding bass in a small room.
The solution, then, is to choose a loudspeaker that will minimally excite the small room's resonance modes. The modes exist only when the air in a room is driven by a sound source. The less hard you drive the air—such as with a small loudspeaker—the less you'll hear the room's resonant signature.
Choosing a small speaker for a small room has musical as well as acoustical advantages. It's far easier to overlook a lack of bass extension than it is to try to ignore persistent boom and bloat. If we hear harmonics of the missing fundamentals, the brain tends to fill in what should be present. Conversely, boomy bass is a constant reminder that we're listening to a mechanical facsimile of the music.
When I moved into a small listening room temporarily, I was faced with choosing loudspeakers that would work well in a room measuring only 11' by 15'. These dimensions could spell disaster for many loudspeakers. I therefore acquired review samples of two loudspeakers that I thought held promise for my room's size, and also fit my current mandate only to review budget equipment. Those loudspeakers, the $1800/pair Aerial 5 and $799/pair Linn Tukan, have played tag-team in my system for the past five months. Here's how they performed under critical scrutiny.
The Aerial 5
Aerial Acoustics made a name for itself in 1996 with the superb Model 10T (reviewed by Wes Phillips in Vol.19 No.4 and named Stereophile's joint Loudspeaker of the Year in Vol.19 No.12). I had heard the 10T at shows—once when driven by the Mesa Baron tube amplifier—and was mightily impressed by the system's smoothness and neutrality. Although I knew nothing about the Aerial 5, it seemed like a promising candidate for review and pleasure listening in my small room.
The 5 is the smallest loudspeaker in the Aerial line. The sealed enclosure houses a 7" woofer and a 1" dome tweeter. The woofer is rather large for a cabinet of this width, but the enclosure volume is increased by the substantial depth.
The woofer is a 7" cast-frame unit with a damped polypropylene cone, custom-made for Aerial by Vifa in Denmark. The sealed enclosure trades extension for tighter bass and a cleaner midband. When a reflex-loaded woofer becomes unloaded below resonance, its large excursion can introduce Doppler and intermodulation distortion, according to designer Michael Kelly.
The 5's tweeter is a custom-made titanium dome sourced from MB Quart in Germany. It features dual chambers with a flared connecting vent and a large surround. The tweeter is recessed in the 5's baffle and surrounded by absorbing felt.
These drivers are crossed over at 2.5kHz, but Aerial doesn't describe the crossover slopes. The crossover is mounted on a board nearly the size of the rear panel. Air-core coils and polypropylene-film capacitors are used, along with custom 99.997% high-purity copper Litz wire and silver solder. The crossover circuit board is made from glass epoxy with double-thickness copper and solder plate finish.
Footnote 1: The fundamental resonance is supported between two walls at a frequency whose half wavelength equals the distance between the two walls. Another resonance mode occurs at one wavelength, one and a half wavelengths, and so on at every multiple of half a wavelength.
The formula for calculating the fundamental resonance mode is 1130/2D, where 2D is twice the distance between two walls and the speed of sound in air at "room temperature" is 1130ft/s. A 20' room thus has a fundamental resonance at 28.25Hz (1130/40), with further modes at 56.5Hz, 84.75, 113Hz, and so on.
Each pair of room surfaces (length, width, height) will exhibit its own set of modes. A room with suboptimal dimensional ratios causes the modes from different pairs of walls to occur at the same frequency, reinforcing that mode. A room with good dimensional ratios spreads the modes out evenly.—Robert Harley
Footnote 2: At the recent CES in Las Vegas, every demonstration room at the Alexis Park Hotel was plagued by a nearly identical midbass bloat.—Robert Harley