Introduction

It's no longer news that "thin-is-in" when it comes to displays and speakers. Once a novelty item, just about every major (and boutique) manufacturer has seen the signs from the marketplace.

Customers want speakers that they can mount flat to a wall, or even better, hide in a wall or ceiling.

For a company like Velodyne, renowned for the aptly named "1812" and Digital Drive series subwoofers, the challenge was to meet the demands of the market and still produce a subwoofer worthy of the Velodyne name. Their solution is the SC-IW, short for the Subcontractor Series-In-Wall subwoofer.

A typical subwoofer relies on the physics of speaker size, driver excursion, internal cavity space and amplifier power, plus the ability to be located in the room where it will most benefit from the acoustic properties of the listening space, to do its job properly. How can those requirements translate into a solution that will fit between 16" studs in a 5" deep drywall space?

With the Velodyne SC-IW, the answer is amazingly well.

The Design

The main challenge to designing an in-wall subwoofer is that if you use a traditional large cone speaker, the sound waves will reflect not only forward into the listening space but also resonate into the space at the rear of the enclosure. In a traditional subwoofer, this problem is dealt with by internal bracing and absorptive materials placed inside the box, plus in the case of a ported subwoofer, "tuning" the enclosure to a particular frequency and giving the sound pressure a place to escape.

However, with an in-wall subwoofer, no matter how rigorous the attempt to create a closed, braced, and damped speaker box, a significant amount of sound pressure will excite the wall at the rear of the enclosure.

This has two detrimental effects: 1) sound waves will reflect off the rear of the enclosure back towards the driver, interfering with accurate sound reproduction; 2) if the speaker is installed in a common wall, sympathetic vibrations from the speaker box will turn the wall on the opposite side of the enclosure into a speaker. Even if there is no common wall, the vibrations still interact within the wall space, creating unknown acoustic impacts.

Velodyne's solution to this dilemma was to design a speaker with a long piston that drives a rectangular diaphragm in the vertical direction (diagram below). This means that the sound energy is directed along the same plane as the structural wall stud members, which are much less prone to resonance and sympathetic vibration than drywall. This design also means less bleed-through to the adjoining space, and the ability to reproduce lower frequencies without shaking the drywall off of the studs!

The driver is housed in a separate enclosure from the speaker box, connected with two sections of large rubber hose.

Due to fire codes, the SC in-walls are passive speakers, with the SC-1250 amplifier located outside the wall.

Despite the ingenious design of the SC-IW series, there are several limitations to in-wall subwoofers in general. For one thing, you lose the ability to locate the sub in the room, where it might address room nodes caused by the dimensions of the listening space. The upside is that you can install the subwoofer inside a wall of a room that otherwise would not have space for a free-standing subwoofer (for example, behind a rear projection TV or main speaker). This, of course, is the main benefit of in-walls, freeing up the space and creating that clean look that spouses approve of in ever-increasing numbers.

Here is a photo of the driver in its enclosure and the second enclosure that is connected to the first with a hose.

The other main design limitation of in-wall subs, and the SC-IW design in particular, is that you can only move so much air within the physical limitations of the driver and the internal space provided by the wall cavity. It's important to understand that in-wall subs are not intended to compete size-for-size and dollar-for-dollar with stand-alone subwoofers. Velodyne is very up-front about this (as they told me while helping move my Hsu Research VTF-3HO w/ Turbo out of the room). If space isn't an issue, you're going to get more thump-for-the-buck by purchasing one of their "Digital Drive" series subwoofers and placing it in a corner. In other words, if you're one those "I can get 104 dB at 18 Hz from my listening position!" guys, in-walls are not for you. However, if for space and/or aesthetic reasons, an in-room sub (or subs) isn't in the picture, then you can expect a pair of SC-IW's to provide similar impact as a pair of Velodyne SC-10 ten-inch freestanding subwoofers powered by the same SC-1250 amp.

Go to Part II.

Terms and Conditions of Use