Digital Signal Processing (DSP)

Why DSP?

Because an acoustically designed and constructed listening room is an expensive build, especially in controlling low frequencies. Most of us are constrained by our existing rooms that we listen to music and movies in. DSP affords us a way to mitigate these low frequency issues for a fraction of the cost of a new room build, but gain the benefits.

The two most important aspects of a good sounding bass in a room are smooth low frequency response and reduced low frequency room resonances.

First, let’s look at a before and after frequency response comparison of a 3-way stereo loudspeaker system measured in a moderate size living room, without and with DSP enabled:

Learn about audio Digital Signal Processing (DSP)

Click on image to learn about audio Digital Signal Processing (DSP)

Before & after frequency response with Digital Room Correction for accurate sound sound reproduction

Frequency response calibration

There is a 20 dB sound pressure level (SPL) variation at low frequencies in the before frequency response. This is simply due to the physical dimensions of ones listening room. Virtually all small room acoustics that are our living rooms and home theaters will have this issue of large, uneven bass response variations.

To our ears, a 20 dB SPL variation is perceived to sound 4 times as loud or quiet depending on which frequency range is being compared. The bass will sound boomy on some notes (i.e. frequencies) and all but disappear on other notes.

After DSP, the low frequency response is calibrated to an industry standard operational room response curve within ±3 dB tolerance. The result is even sounding, smooth bass response, at multiple listening positions. By calibrating to an industry standard operational room response, we get the benefit of hearing the same tonal response as the artists intended.

But frequency response is only one half of the equation for good sounding bass in rooms. The other half is dealing with room resonances. Specifically, the excessively long sound decay times of room modes that are audible. While the traditional approach is to use bass traps, bass traps do little to attenuate room resonances below 100 Hz. Using specialized DSP software allows us to effectively attenuate these room resonances.

Let’s look at a before and after DSP spectrogram of the low frequency decay times of a typical size living room with 2-way stereo loudspeakers and dual subwoofers. As with most small room acoustics, the decay times below 100 Hz are especially long at various frequencies (i.e. room resonances). The square red boxes highlights the before and after changes:

Room resonances calibration

This chart shows the before and after Room Shaper DSP that removes low frequency room resonances.

The top spectrogram is before DSP and the bottom after DSP calibration. The red square encompasses frequencies from 10 Hz to 1 kHz and time frame from 250 to 500ms. Note in the top spectrogram that room resonance at around 70 Hz is a horizontal bar, solid greenish blue color (i.e. about -20 dB in level) and does not diminish in level, well past 500ms. In the bottom graph we can see that has been reduced to be in line with adjacent frequencies. Plus all the way down to 10 Hz has been significantly reduced (i.e.  -20 dB of reduction).

In the top room spectrogram, you can also see several solid darker blue lines from over 100 Hz to about 300 Hz stretching from 250 to past 500ms. Again, in the bottom pic using DSP, we can see that area has been reduced in level to be at similar levels to adjacent frequencies that are not long decay times.

The end result is your sound system is finely tuned and fully optimized, maximizing the value of your audio investment. Listening to music the way it was intended by the engineers, artists and producers brings a new level of musical enjoyment to ones listening experience as the loudspeakers and room disappear

DSP Resources

We have written several articles and a book on, "Accurate Sound Reproduction Using DSP." 

The articles and book are intended to be educational showing "step by step" what can be accomplished using state of the art DSP software. 

There are additional topics covered, which are part of our service, such as linearizing and time aligning speakers, designing and generating digital crossovers, and integrating subwoofers:

Acourate Digital Room and Loudspeaker Correction Software Walkthrough

Audiolense Digital Loudspeaker and Room Correction Software Walkthrough

Integrating Subwoofers with Stereo Mains using Audiolense

Advanced Acourate Digital XO Time Alignment Driver Linearization Walkthrough

Home Audio Fidelity – Room Shaper Review

Accurate Sound Reproduction using DSP

This book provides the audio enthusiast with an easy-to-follow step-by-step guide for designing a custom digital filter that corrects the frequency and timing response of your loudspeakers in your listening environment so that the music arriving at your ears matches as closely as possible to the content on the recording.

Industry guidelines, spanning over 40 years of evolution, are referenced throughout the book, providing the recommended target responses for accurate sound reproduction. Correcting the measured response to known target responses is, in effect, matching the acoustic output signal arriving at your ears to the digital audio stored in a media file. The target responses can also be tailored to your own preferences.

Accurate Sound Reproduction Using DSP. Learn how to use DSP for accurate sound  reproduction.
Transient response analysis for modern DSP digital room correction for accurate sound reproduction.

Our ears perceive that music is mostly transient in nature. The DSP software we use analyzes the full frequency range of the transient behavior and this analysis leads to a new frequency response that follows the peaks, but not the dips as can be seen in the frequency response chart above.

If one studies the chart closely, one can see the envelope response is following the peaks but not the dips. Note the marker at just over 100 Hz., one can see a very narrow dip, but the new analysis does not follow that narrow dip. This new frequency response can be considered as an upper envelop of the original magnitudes. The spectral envelope is used as the basis for further calculations.

Given that the envelope of the dips at low frequencies are based on room modes, which are standing waves, we know at different mic positions we will measure somewhere on the "up and down" of each wave (i.e. frequency). By correcting only the envelope of the wave, or "average" over time, is how a smooth low frequency response over a broad listening area is obtained with two measurements at one mic location.

At high frequencies the envelope sits high in the "comb filter," which avoids over correction of the dips. This is key for natural sounding high frequencies, where in combination with Frequency Dependent Windowing (FDW), described below, provides broadband "tonal" correction of the direct sound above the room's transition frequency without over correcting the dips.

The science behind this, and FDW described below, is from James (JJ) Johnston's, "Acoustic and Psychoacoustic Issues in Room Correction." plus PowerPoint presentation. JJ explains why we hear what we hear in small room acoustics. 

Frequency dependent windowing for DSP digital room correction for accurate sound reproduction.

Before the explanation, we do not apply in-room equalization to achieve a flat steady-state frequency response at the listening position. This is not the desired target response, nor is this how we interpret the industry standards operational room response curves.

We use DSP to calibrate sound systems below 500 Hz to smooth out low frequency response and remove room resonances. A well designed loudspeaker will have a steady-state in-room frequency response curve that is tilting downwards. This is because loudspeakers are omnidirectional at low frequencies with narrowing directivity at higher frequencies. The rising bass energy yields a steady-state room curve with a downward tilt.

The state of the art DSP software we use applies Frequency Dependent Windowing (FDW) so that we control the dividing line between in-room steady state response and the direct sound from the loudspeaker. This dividing line is typically around 500 Hz. This means below 500 Hz we are taking the room into account and above 500 Hz just the direct sound from the loudspeaker. This is because below 500 Hz or the rooms transition or Schroeder frequency, the room is in charge. Above the rooms transition frequency, the loudspeaker is in charge.

The chart example above displays an impulse response and three window widths at different frequencies using 15 cycles as the FDW setting. The horizontal scale units are in 100 millisecond increments. We calculate the FDW sizes using these FDW math equations. Using the FDW math equations, a 20 Hz window width is 750 milliseconds (ms). At 100 Hz the window width is 150 ms and at 1 kHz it is 15 ms. 

While this example uses 15 cycles as a single FDW setting, the DSP software we use can adjust both low and high frequency FDW cycle times independently. This allows us to calculate the optimum FDW sizes based on your room's size and statistical analysis of model distribution and early reflections density. This analysis gives us the smoothest integration between "room plus loudspeaker" below your room's transition frequency and "loudspeaker direct sound" above your room's transition frequency.

If the loudspeaker requires a spectral tilt above 500 Hz due to the loudspeaker being a bit too bright or dark sounding in your room, we equalize the direct sound only. We only apply broadband “tone control” like spectral tilts to match industry standards operational room response curves.

Acoustic Room Treatments

It may seem odd to see acoustic treatments under the DSP category. That is because we use DSP software to calibrate low frequency response to ±3 dB tolerance. We also use DSP software to remove room resonances.. This DSP software combo takes care of low frequency room response and decay time to 500 Hz.

Above 500 Hz, we use acoustic absorbers and/or diffusers to control the rooms "broadband" decay time. Only required if the room is out of range of the specification. E.g. a room with all bare walls, ceiling, floor and windows.

If your listening room is furnished, with carpet, drapes, bookcases, couch, chairs or anything to absorb and/or break up (i.e. diffuse) the sound, then more often than not, the room falls within the target operation response time specification for the rooms broadband decay time.

The first set of acoustic measurements will tell us your rooms decay time, above and below the room’s transition frequency.

We are happy to use an acoustics product provider of your choice (or our choice) if acoustic treatment is required. 

Acoustic absorption and diffusion panels to control room acoustics for accurate sound reproduction.