THE VOLTI AUDIO VITTORA 3-WAY NETWORK (V1)

North Reading Engineering has developed a crossover network for the Vittora 3-way horn-loaded loudspeaker system manufactured by Greg Roberts at Volti Audio.   The Vittora consists of a folded bass horn, a large format tractrix horn for the midrange frequencies and, for the highest frequencies, a horn tweeter.   The midrange horn is driven by the BMS ND4592 MID neodymium, 2" throat, polyester diaphragm, compression driver.   The high frequency tweeter unit is the Beyma CP-25.  

The network has been developed by extensive iteration between computer model simulations using both LTSPICE and MATLAB simulators and experimentally determined acoustic responses.   The design approach considers the acoustic response limitations of each component and seeks a network configuration that establishes as near a flat, on-axis, near-field, frequency response as is possible within the design practice allowances for useable bandwidth, phase distortion, physical size, part count and cost.   Network simulations are terminated with loads that accurately simulate both impedance magnitude and phase information of the bass, mid and high frequency horns, providing accurate predictions of purpose specific design revisions.   Near-mouth, ground-plane and far-field impulse, frequency and phase data files are generated using CLIO (v. 7.13) acoustic analyzer.

Note regarding acoustic response curves - Real, unsmoothed acoustic response curves are not always pretty and are almost never shown, simply because the data likely fails to substantiate the performance claims made by the manufacturer.   We have characterized, in great detail, the acoustic response of the Vittora system.   We have used computer simulation to develop the crossover network.   The computer models were then calibrated to the actual acoustic response measurements.   This provides us with a powerful tool to refine the networks and establish the desired acoustic response.   The only thing that matters is the acoustic response of the system.   We have refrained from showing the predicted response from simulation software because predictions have no value to the end user.

DESCRIPTION

The V1 network consists of two sections, a bass horn filter and a MID/HF filter.    The bass horn filter section sends low frequency signals to the bass horn whilst the top section filter sends signals to the BMS midrange and CP-25 high frequency drivers.   Capacitors and inductors are Janzen manufactured (Capacitors: polypropylene Cross-Caps, Inductors: #18 and #15 air-core and #15 iron-core).

HIGHLIGHTS

Compatible with any reasonable quality audio amplifier (minimum real component of impedance 4.2Ohm).
Midrange horn output level user modified by switching out optional L-pads.
Midrange filter is 4th order band-pass, establishes well-defined acoustic center at critical mid-band frequencies
High frequency filter is 4th order high-pass.
Bass horn filter is 2nd order low-pass.

Photographs shown below are of the V1 networks.  The MID/HF filter section is to the left and the bass filter section is to the right.   Terminal blocks are CINCH, hook-up wire BELDEN #16GA, spade terminals AMPHENOL.

BASS HORN FILTER RESPONSE

The Vittora bass horn near-mouth, acoustic response, is shown as a function of various filter configurations in the plot below.   The horn near-mouth response is measured by placing the microphone in the geometric center of the upward facing bass horn mouth.   Shown in the plot are a third-order (green), second-order (blue) and no-filter (purple) responses.   The V1 bass filter utilizes the second-order filter.

The Vittora bass horn ground-plane magnitude response is shown in the plot below at 0, 2, and 4m (top to bottom).   The responses were measured outdoors (restricted access parking facility) with the measurement microphone place at ground level and centered on the bass horn mouth.   The measurement conditions used provide an ideal environment to assess the low frequency response of the bass horn section.   Responses are DFFT of MLS stimulus, raw data from analyzer (no averaging).   The responses shown are with the V1 bass filter.

Cumulative Spectral Decay (CSD) response of the bass horn with no-filter (top) and second order, V1 filter (bottom), shown below.  Responses are 2m ground-plane measurements.   Note the attenuation of prominent resonances at approximately 480Hz and above 800Hz with the V1 bass filter.

MID AND HF HORN SECTION

Plot below is the near mouth response of the MID horn (green) and HF unit (purple) connected to the V1 network.   Responses derived using DFFT on MLS impulse using a rectangular window.

MID-RANGE AND TWEETER CROSSOVER QUASI-ANECHOIC RESPONSE

The measurement microphone was placed approximately 1m from the center of the V-Trac horn mouth section and an MLS impulse applied and response recorded.   The mid-range and HF compression drivers were connected to the V1 network.   The quasi-anechoic, frequency response shown is valid for frequencies above approximately 400Hz.   Orange curve is mid horn response, red is high frequency response and purple is the system response.   Frequency domain information derived from DFFT of MLS impulse.   Blue plot is excess phase rotations determined by subtracting numerically derived minimum phase response associated with measured frequency response from the total phase response measured.

CUMULATIVE SPECTRAL DECAY MID-RANGE TO TWEETER

The Cumulative Spectral Decay (CSD) plot of the response shown above is provided below (top plot).   Bottom plot is higher resolution plot of the CSD at the MID to HF crossover frequency located at 3800kHz.    Note smooth transition between the MID and HF unit at the crossover frequency.    Note also the exceptionally uniform decay at crossover frequency.

EXCESS GROUP DELAY

Excess group delay plots provide a quantitative assessment of the time dispersive characteristics of a multi-way loudspeaker system.   For example, if a loudspeaker system is claimed to be time coherent, that implies a flat excess group delay plot across the useful bandwidth of the system.   Large format horn systems exhibit excess group delay values that can be large, especially when compared to well engineered direct radiator systems.   Thus, given the constraints imposed by the physics of the system, the design of the V1 network attempts to establish a well-defined acoustic center in the critical MID range.   This is accomplished using a 4th order MID band-pass and 4th order hi-pass on the tweeter.   As evidenced in the plot below, the magnitude of the excess group delay of the MID lags the tweeter unit by approximately 1.2ms down to about 900Hz.   At lower frequencies, the magnitude then increases with frequency, a typical observation.   The large peak in the plot is associated with the crossover performing the filtering function.   Excess phase derived from DFFT of MLS impulse.

Quasi-anechoic Response

The quasi-anechoic frequency magnitude response of the Vittora with the V1 network is shown below.

IMPEDANCE

The impedance modulus (Z) of the Vittora loudspeaker system is shown below (blue), the phase angle is shown also (purple).   The measurements shown include the resistance associated with the 6ft of #16ga hook-up wire used to connect each of the three drivers to the V1 network,

The Real component (red) and Z (blue) plotted below.   A minimum in Re is observed at approximately 150Hz (Re~4.2Ohm).

ADDITIONAL THUMBNAIL IMAGES BELOW (click on image, 1280x1024px or higher resolution recommended)

PERFORMANCE VALIDATION

Each V1 network shipped with a Vittora loudspeaker system is tested for performance capability and serialized by North Reading Engineering.   They are not sold separately.   Product improvement is ongoing, products shipped may differ from performance data presented and the photographs shown.   For questions about the data shown, please email us at the contact information below.

"'Volti' is Italian for 'turn the page', and turning the page on horn-speaker performance issues is just what the Vittora seems to do.

Marc Mickelson,
The Audiobeat

To contact John Warren of North Reading Engineering: click here