## What DSP Is — and What It Is Not
A digital signal processor in a professional audio signal chain sits between the drive outputs of the mixing console and the amplifiers feeding the loudspeakers. Its job is to divide the audio spectrum between drivers, align their arrival times at the listening position, correct measurable frequency response anomalies, and protect the transducers from damage. Done correctly, DSP makes the loudspeaker system behave like the engineering team intended. Done incorrectly, it creates a system that sounds acceptable at low levels and falls apart under real-world show conditions.
There are four primary processing blocks: crossover filters, delay, parametric EQ, and limiting. Each has a distinct function. Conflating them — using EQ to fix a time alignment problem, or using limiting to compensate for a poorly set gain structure — is the most common category of error in system alignment.
## Crossover Filters
A crossover filter divides the audio spectrum so that each driver receives only the frequencies it is designed to reproduce. A high-pass filter removes low-frequency content from a compression driver that cannot handle it. A low-pass filter removes high-frequency content from a woofer that would beam and distort.
**Filter order** determines the steepness of the cutoff slope and the phase shift introduced. A first-order filter rolls off at 6 dB per octave and introduces 45 degrees of phase shift at the crossover frequency. A fourth-order filter rolls off at 24 dB per octave with correspondingly more phase shift. Steeper filters reduce out-of-band energy more aggressively but require careful attention to phase alignment at the crossover point.
**Linkwitz-Riley** crossover alignments are the professional standard for most live sound applications. A fourth-order Linkwitz-Riley (LR4) crossover — the most common choice — produces flat summed response at the crossover frequency with both filters in polarity, and a combined phase shift that rotates consistently across frequency. Butterworth crossovers produce a 3 dB peak at the crossover frequency when summed, which requires polarity inversion of one section and complicates integration.
**FIR vs IIR filters** represent the most significant DSP architecture choice. Infinite Impulse Response (IIR) filters are computationally efficient and introduce minimal latency, but their phase response is minimum-phase — meaning phase shift is tied to the frequency response curve and cannot be independently manipulated. Finite Impulse Response (FIR) filters can achieve linear phase response, meaning they introduce equal phase shift across all frequencies — effectively a pure time delay with no phase distortion. FIR filters come at a cost: they require significantly more processing power and introduce latency, typically in the range of several milliseconds, which must be accounted for in any system requiring low-latency monitoring.
## Delay: Time Alignment and Fill Speakers
Delay serves two purposes in a professional system: time-aligning physically offset drivers, and delaying fill speakers to maintain the precedence effect.
When a subwoofer is positioned downstage and a line array is flown three metres behind it, the subwoofer's acoustic centre is physically closer to the front of the audience. At the crossover frequency, signals from both systems must arrive at the listener's ear simultaneously for the crossover to sum correctly. Delay applied to the subwoofer — the physically closer system — closes this timing gap.
For fill speakers and delay towers, the calculation is straightforward:
**delay (ms) = distance delta (metres) / 0.343**
If a delay tower is 20 metres behind the main system, sound from the main system has already been travelling for 58 ms by the time it arrives at that position. Add 5 to 10 ms of additional delay to enforce the precedence effect — ensuring listeners hear the delay tower as a continuation of the main system rather than a separate source.
The precedence effect (also called the Haas effect) describes how the human auditory system attributes the perceived direction of a sound to the first arrival, within a window of roughly 35 to 40 ms. Fill speakers set within this window after the main system arrival are perceived as coming from the main system, preserving spatial coherence.
## Parametric EQ: Legitimate vs Illegitimate Uses
Parametric EQ adjusts the frequency response of the system output. Each band has three parameters: centre frequency, bandwidth (Q), and gain.
**Legitimate uses** of EQ in system alignment are correcting narrow-band deviations caused by driver response anomalies, correcting measurable room-coupling issues at specific frequencies, and adjusting the voicing of the system to match a target curve established by measurement at multiple listening positions.
**Illegitimate uses** include boosting broad frequency ranges to compensate for a system that is simply too small for the room, using EQ to chase a subjective preference rather than a measured deviation, and using subwoofer EQ to increase bass weight beyond what the transducers can deliver cleanly.
The critical principle: every boost is a power request. A 6 dB boost at 80 Hz means the amplifier must deliver four times the power at that frequency to produce the same SPL gain. If the system is already operating at or near its thermal limit, that boost converts to heat in the voice coil — and eventually to a service call. EQ headroom is not free.
## Limiting: Three Functions, Three Limiters
A professional DSP limiter is not a single device. It performs three distinct protective functions, and conflating them leads to inappropriate threshold settings.
**Excursion limiting** prevents low-frequency driver over-excursion below the system's resonant frequency. Below Fs, the driver's back-EMF no longer controls cone motion — the cone travels further and further for a given input level, and mechanical damage results. A high-pass filter or a brick-wall limiter at the subsonic limit of the driver's design envelope is the correct tool.
**Thermal limiting** tracks amplifier output power integrated over time — the quantity of heat that will accumulate in a voice coil — and reduces drive level before the coil's temperature limit is exceeded. The attack time on a thermal limiter is measured in seconds, not milliseconds.
**Clip limiting** prevents the amplifier output from clipping, which introduces high-frequency harmonic content into a low-frequency driver and risks tweeter damage if it reaches the crossover point. Clip limiters have fast attack times — typically under 1 ms — and are set to engage just before amplifier clip.
## The Correct Order of Operations
As with subwoofer deployment, DSP alignment has a correct sequence:
1. **Crossover** — divide the spectrum correctly
2. **Delay** — time-align all drivers at the crossover frequency and all fill systems
3. **EQ** — correct measured deviations, starting with measurement
4. **Limiting** — set protections based on transducer specifications, not guesswork
Setting EQ before delay alignment is a common mistake that produces filter settings compensating for phase cancellation at the crossover point — cancellation that delay alignment would have eliminated. Verify each stage with a measurement system before proceeding to the next.
SSOUNDS line array systems and subwoofers are supplied with factory DSP presets that encode the crossover alignment, driver delay offsets, and limiter thresholds derived from the manufacturer's own acoustic measurements. These are starting points, not finished alignments — every venue has its own acoustic signature — but they ensure the system engineer begins with the physical characteristics of the transducers correctly represented in the processor, not approximated.