Hardware: how a transducer moves you

A loudspeaker works by running an audio signal, as electric current, through a coil of wire sitting in the gap of a permanent magnet. Current in a magnetic field produces a force on the wire, so the coil moves with the signal, and the paper or plastic cone glued to the coil moves the air (HyperPhysics, loudspeaker). A tactile transducer is the same machine with the cone taken off. In its place is a mass, often the magnet assembly itself, suspended on springs. When the coil pushes the mass one way, the housing is pushed the other way, by Newton's third law, and the housing is bolted to whatever you are lying on (HyperPhysics, Newton's laws). No air is moved on purpose. The bed is the diaphragm.

Shaker versus speaker

That swap changes everything about how the device behaves. A speaker cone is light and moves against a load that barely resists it. A transducer's reaction mass is heavy and moves against a bed, a frame and a body. The industry calls these devices bass shakers, tactile transducers or haptic transducers, and they come in sizes from palm sized units meant for a gaming chair to units the size of a paint can. At the big end, the ButtKicker LFE from the Guitammer Company asks for a minimum of 400 watts and accepts up to 1,500, and is built to shake a whole platform (ButtKicker LFE). Clark Synthesis has made tactile transducers for home theatre seating and boats for decades (Clark Synthesis), and Dayton Audio sells both bass shakers and the smaller exciters that turn a panel into a speaker (Dayton Audio, bass shakers). The AVA bed does not use one large unit. It uses many small ones, and that is a design choice worth explaining.

What 25 watts into 4 ohms asks of an amplifier

Each of the 32 transducers under the wedges is a voice coil unit rated at 25 watts RMS into 4 ohms. Electrical power is voltage squared divided by resistance (HyperPhysics, electric power), so 25 watts into 4 ohms means about 10 volts RMS across the coil and 2.5 amps through it. Those are modest numbers per unit, but the low impedance matters. A 4 ohm load pulls twice the current of an 8 ohm load at the same voltage, and an amplifier that is happy into 8 ohms can overheat or shut down into 4. Douglas Self's Audio Power Amplifier Design, the reference text on the subject, spends chapters on exactly this: load impedance, stability, thermal design and the efficiency of the class D stages that make a compact multi channel amplifier possible (Self, 2013). AVA runs two amplifiers, with the transducers grouped by zone so that every amplifier channel sees a load it is rated to drive.

Heat

The RMS rating on a transducer is a thermal number. It is the continuous power the coil can turn into heat without the glue softening or the wire failing. Manufacturers also quote a peak figure the unit will survive for a moment, which is why product listings carry two numbers (Dayton Audio, exciters and tactile transducers). Heat also changes the sound. Copper's resistance rises with temperature, by roughly 0.4 percent per degree Celsius (HyperPhysics, resistance and temperature), so a hot coil draws less current from the same voltage and pushes less hard. Engineers call this power compression. On a bed it shows up as a session that starts strong and fades, which is one more reason to spread the work across many coils rather than drive one coil hard.

Why 33

The AVA octagon is 16 feet across, built from eight wedges, one per guest. Each wedge carries four transducers, one for each of the upper body zones, root, belly, heart and head, and a single shared unit at the centre of the octagon sits under everyone's feet. Eight times four is 32, plus one is 33. The reason is resolution. A single large shaker bolted to a frame moves the whole frame as one thing, and every part of the body feels the same signal. Four small units along a wedge let the bass line arrive at the root, the drums at the belly, the harmony at the heart and the voice at the head, each at its own level. The zones run from 41 Hz at the feet to 880 Hz at the head. The centre unit is shared because the feet zone carries the lowest band, whose wavelength is longer than the octagon is wide, and one source is enough for eight pairs of feet.

The eight channel interface

The signal leaves the computer through a Behringer UMC1820, a USB audio interface with multiple balanced line outputs (Behringer product catalogue). AVA uses eight of its output channels: two feed the room's loudspeakers with the music as mixed, and six feed the bed, one each for the feet, root, belly and heart zones and a pair for the head. Every channel is clocked by the same converter, which is what makes the alignment described in the sound design piece possible. A kick drum leaves all eight outputs on the same sample.

No air is moved on purpose. The bed is the diaphragm.

The power budget

Thirty two transducers at 25 watts is 800 watts if every coil were driven flat out with a continuous sine wave. Music is not a continuous sine wave, and no zone runs at its limit, so the actual draw is far lower. With the centre unit, the two amplifiers, the interface and the computer, the whole system stays under 900 watts. A standard 15 amp circuit at 120 volts can supply 1,800 watts, and the National Electrical Code, NFPA 70, limits continuous loads on a branch circuit to 80 percent of its rating, or 1,440 watts (NFPA 70). The bed runs on one ordinary circuit with room to spare, which was a design requirement. A listening room should not need an electrician.

Safety limits

Two kinds of exposure limit apply, and the bed sits well inside both. For sound, the federal occupational standard permits 90 decibels A weighted over an eight hour day and sets an action level at 85 (29 CFR 1910.95). The point of felt playback is that it does not need to be loud; the room's loudspeakers play at conversation adjacent levels because the bass is arriving through the wedges rather than the air. For vibration, the European directive on mechanical vibration sets a daily exposure action value for whole body vibration of 0.5 metres per second squared and a limit of 1.15, averaged over eight hours (Directive 2002/44/EC), measured by the method in ISO 2631-1 (ISO 2631-1:1997). Those limits were written for people who drive trucks and operate machinery all day. A one hour session on a bed tuned for comfort is not in the same category. We still ask guests with a pacemaker, a pregnancy or a recent blood clot to speak with their doctor before booking, because a bed that moves the body is still a bed that moves the body.

What this means on AVA

Everything a guest feels at 29 Nassau Ave comes from 33 coils pushing 33 weights against the eight wedges under them, driven by two amplifiers, fed by one interface, on one household circuit. The hardware is ordinary. What is unusual is the arrangement: many small transducers instead of one large one, five zones from 41 Hz to 880 Hz instead of one channel, and the projected visuals drawn from the same eight outputs. None of it makes a claim about health. It makes a claim about resolution, which is that a body can tell a bass line from a drum if the bed lets it. This is wellness content, not medical advice.