Weather you are in a recording studio, home theatre or concert hall, the ideal listening environment is generally thought to incorporate a smooth, dense reverberant field that is free from prominent resonance and primary reflections. This is generally achieved by applying a combination of absorptive panels and reflective diffusers into the room.

Absorbers are generally used to control primary reflections and excessive bass. But using too much absorption can pose problems: Overly absorptive environments can sound listless and unnatural. By retaining a sense of air or spaciousness, a performance can be transformed from artificial to exciting. Diffusers have the important task of dissipating sound energy without deadening the room.

In the 1970s, famed acoustician M. Schroeder proposed methods of designing diffused surfaces based on mathematical 'quadratic residue' sequences and showed that these periodic sequences had the property of balancing out the harmonic amplitude of the audio source. In other words, audio peaks caused by room geometry or primary reflections could be spread over time and be smoothed out.

Since then, quadratic residue diffusers (QRD) have been primarily incorporated into the design of professional recording studios and concert halls. This begs the question: Why, with over a quarter century of scientific proof, has the use of QRDs been limited to high-end installations? Why are they not used in project studios, home theatres, auditoriums or even movie theatres? The answer quite simply has to do with cost.

For a diffuser to work properly, you need to combine various elements in effort to manage all of the variables imposed by sound. For instance, the lower the frequency, the more powerful the sound wave. If bass travels through walls made of heavy sheetrock, you can be absolutely sure that it will travel through light weight plastic. Furthermore, if one applies standard quarter-wavelength calculations to the frequency you want to diffuse, you will quickly come to the conclusion that that the depth of the diffuser well is directly proportional to the diffuser's low frequency performance.

Some simple math: Say you want to diffuse 500Hz: simply take the speed of sound (1130 ft/sec) and divide by 500 to calculate the wavelength (= 2.26ft). Now divide by 4 for the quarter wavelength (= 0.565 feet) and multiply by 12 to convert to inches (= 6.78"). This means that your diffuser must be at least 6.78" deep to diffuse 500Hz.

The following table shows the frequency and required diffuser depth at the low frequency cutoff point:

	Frequency	Well Depth		Frequency	Well Depth		Frequency	Well Depth
	300Hz	11.29" (287mm)		800Hz	4.24" (108mm)		5000Hz	0.68" (17mm)
	400Hz	8.47" (215mm)		1000Hz	3.39" (86mm)		10,000Hz	0.34" (9mm)
	500Hz	6.78" (172mm)		1500Hz	2.26" (57mm)		15,000Hz	0.23" (6mm)
	600Hz	5.65" (144mm)		2500Hz	1.36" (35mm)		20,000Hz	0.17" (4mm)

Since sound is generally considered to 'beam' or become directional above the 300Hz to 400Hz range, the diffuser must sufficiently deep to work and be sufficiently heavy to actually reflect lower frequency energy back into the room. And diffusing energy in the all important voice range - 400Hz ~ 2000Hz - is arguably the most critical. This unfortunately requires the diffuser to be large, heavy and deep which invariably makes them costly to build.

To circumvent this 'reality', various manufacturers have attempted to offer solutions that are either injected molded, milled or 'simulated' by creating quasi-diffusers. Although most of these will cause some affect on the sound, you can see that products that boast minute ½" wells will have no effect at all below the 6000Hz region. This makes them completely useless in the critical and often problematic mid (voice) range. And as stated previously, if they are light weight, lower mid and bass frequencies will simply pass through them without effect.

Over the past several years, the Razorblade quadratic diffuser has gone through multiple revisions in effort to find the most cost-effective method possible to deliver a product that achieves the quality and performance needed to work, while reducing costs. To this end, we have invested heavily in the use of computer controlled routers and saws in effort to reduce expensive labor. And because we mass produce one model, in one color (following Mr. Ford's Model-T approach), we are able to deliver a true quadratic diffuser at roughly 1/3 the price of our nearest rival.

With the level of performance and affordability presented by the Razorblade, diffusers are now beginning to find homes in more modest home project studios, cable TV broadcast facilities and municipal soft-seat theatres. Even home theatres are now beginning to employ these devices in their construction. Who knows… maybe one day, diffusers may even become 'standard' in classrooms…

The Primacoustic Razorblade: true quadratic diffusion made affordable!