The True Character of Bass

Reproducing the true character of bass in an ordinary room is particularly challenging both because of the room’s large influence, as discussed in our “Room Acoustics, Psychoacoustics and TCA-M Design Concepts” white paper, but also the difficulty of designing a loudspeaker able to generate deep and loud enough undistorted bass, not least while having a slender visual impression easily blending into a carefully curated home interior.

Instruments with large resonant structures, such as a piano with its soundboard and surrounding case, provides the necessary cubic volume to amplify and project deep sounds. Some of the most emotionally powerful aspects in music and sound construction can be attributed to the prominent and firm presence of deep tones - double bass and pipe organ, as well electronic music come to mind.

Bass quality is fundamental to music perception: research shows that when evaluating loudspeakers in blinded tests, bass quality accounts for 30.5% of listener preference^[1].

[1] Sean E. Olive, “A Multiple Regression Model for Predicting LoudspeakerPreference Using Objective Measurements: Part 2 - Development of theModel”, presented at the 117th AES Convention, preprint 6190 (October 2004).

The unique sound profile of individual acoustic instruments is characterised by the initial transient attack causing a complex mix of frequencies, then a steady tone and its harmonic resonances with the perceived loudness of each of them transmitted and amplified through the instrument’s materials and shapely structures. Taking the piano as an example, alongside the intentional key attack and playing the chords, its soundboard vibrates and hence, creates the spectral and transient sound pressures that identify its distinct character.

Conventional box type sealed or ported loudspeaker designs seek to contain half the sound pressure energy inside for increased efficiency, by avoiding the acoustic cancellation that occurs when in and out of phase sound waves meet.

A side effect of this conventional approach is a completely omnidirectional low frequency dispersion reflecting off all boundaries of the listening room. The result is a build up of room mode standing waves, pressuring the room causing booming and little detail lacking the true character of bass from the same instruments played live in the same room.

With half the low frequency sound energy trapped inside a box, it will resonate with its large surfaces vibrating adding undesirable colouration to the originally recorded sound. Often this means extraordinary effort to stiffen and damping the box, which also needs to be quite large for a low frequency extension.

Both of these fundamental design issues are best addressed by not having a box!

Some argue that the devil is in the detail. Stradivarius violins have long held a reputation of mythical excellence in classical circles. Could the secret to Godly sound be in the varnish – a mysterious key ingredient? Regardless of the truth, it stands to reason that the loudspeaker construction itself must not contribute any sound spoiling the purity of the unique original instrument characteristics.

Patented Air Velocity Transducer Bass System

Treble Clef Audio loudspeakers are designed to accommodate the Room and Psychoacoustic requirements for reproducing the true character of bass.

The TCA-M patented force cancelling air velocity transducer bass system takes advantage of the folded dipole concept, illustrated below, for an exceptionally low and clean bass extension and has no box to vibrate and cause resonances.

The moving parts of the woofers and forces exerted by sound pressure levels within the bass system are both converted to heat avoiding vibration and resonances in the patented TCA-M bass system.

Further to mechanical force cancelling, the acoustic sound pressure forces inside each hemispherical cup are absorbed by a constraint layer dampened Cement based Syntactic Foam (CSF) insert. CFS is highly efficient at absorbing low frequency energy further avoiding excitation of the hemispherical cups made from 20mm thick hardwood.

The TCA-M bass system achieves unparalleled clean and effortless reproduction of the lowest frequencies with no sound coloration and very low distortion preserving original recorded deep sound details with the sensation of physical impact.

Limiting side wall reflections

The Klippel NFS measurement below shows a perfect omnidirectional asymmetric sound dispersion pattern limiting sidewall reflections. The slightly lower sound intensity from the two rear ports is an intentional part of limiting room standing waves as explained next.

Limiting room longitudinal standing waves

The Klippel TCA-M bass system phase measurement below shows in-phase sound waves, pictured in blue, emitted from the front of the bass system facing the listener. On return after being reflected off the room’s end wall nearest the listening position, they will be met by sound waves of opposite phase pictured in red reflected off the wall behind the loudspeaker. Where these waves meet in the room, depending on frequency / wavelength, they will cancel out to reduce the build-up of standing waves.

Acoustic resonance and distortion measurements

The near-field Cumulative Spectral Decay plot below shows how long it takes for a short energy pulse of each frequency to decay in the bass system. Long decay times at certain frequencies represents a resonance often inherent to enclosure design, vibrations excited by the mechanical movement of woofers and the acoustic back pressure asserted inside an enclosure. Throughout the entire operating range from below 16Hz through to the crossover point at 200Hz, the TCA-M Air Velocity Bass Transducer shows exceptionally low-level fast decaying system resonances close to the measurement noise floor and below the bass frequency hearing threshold.

Minimum bass system distortion

At Treble Clef Audio we put immense focus on avoiding distortion inherent to loudspeakers addressed through our principles of sound by design. At 20Hz THD, including up to the 9^th harmonic, is a mere 0.7% and does not exceed 3% across the bass frequency operating range. Exceedingly low distortion for a bass loudspeaker system able to attain a flat equalised steady state room response down to 16Hz (-3dB) at 105dB SPL at 1m.

Sound by design combines boxless patented innovation with our passion for music and design aesthetic using the latest technologies and traditional craftsmanship to deliver an unparalleled subjective auditory, visceral, and visual experience.

To learn more please download our “Room Acoustics, Psychoacoustics and TCA-M Design Concepts” white paper and the Features and Technical Specification.