Flat Velocity Hourglass Port, The Ideal Speaker Vent

The Regular Port/Vent

The vent (or port) is designed to enhance bass response by leveraging its surface area and length. It increases the low-frequency output by functioning as a Helmholtz resonator.

Traditionally, this is achieved using a tube or a straight square or rectangular shape.

However, the problem with tubes and similar shapes lies in increased distortion, vent noise, and dynamic compression effects, all caused by the way air particles oscillate inside the port geometry.

The Hourglass Vent Demonstration

The Hourglass Vent was first theorized and simulated in 2002 by three AES members: Alex Salvatti, Allan Devantier, and Doug J. Button. This design optimizes speaker vent performance by addressing several key acoustic challenges.

Key Benefits:

This design ultimately improves efficiency and sound quality by minimizing losses and unwanted artifacts in bass reproduction.

However, while the overall shape of the Hourglass Vent is advantageous—even with a 12mm radius at the exit—it still suffers from friction and an uneven velocity profile at the exit:

speaker vent
The published theory serves as a demonstration rather than an absolute solution. Each implementation requires specific simulations to achieve the desired results.

Another important factor to consider is that one extremity of the port is located on a flat panel, while the other is not. In practice, this means the profile cannot be perfectly symmetrical.

The Optimised Flat Velocity Hourglass For Real Application

Our design guarantees an uniform radiation velocity both at the vent’s exit and entrance:

speaker vent

The vent consists of asymmetrical front and rear parts optimized through Finite Element Analysis (FEA). The design accounts for radiation differences — 2π at the front and 4π inside the enclosure — to ensure optimal acoustic performance thanks to flat velocity profiles at both the inlet and outlet.

The vent is screw-mounted from the front or rear of the loudspeaker, providing easy assembly.

For optimal performance, the port should be positioned at approximately 30% of the speaker height from the bottom, preferably on the rear panel.

Measurements

Independent measurement series conducted by Dylan, host of the YouTube channel SPL.

A two-piece, unglued port was used for the Hourglass FV; a glued and smoothed version would have performed even better.

Distortion

This graph shows the distortion measured at the port mouth, compared to a straight tube.

speaker vent

Vent Noise

This chart presents an RTA measurement of airflow noise. The focus is on the noise floor, represented by the level between the peaks, which reflects the broadband turbulence generated by the vent.

speaker vent

Dynamic Compression

These charts show the SPL output difference for the same input signal sent to the woofer.

While the Hourglass FV (Flat Velocity) shows only a modest 2 dB increase, its output signal is much cleaner compared to the straight port, which contains significant noise that artificially raises its SPL.

This explains why the straight port SPL appears to fluctuate and remain relatively close.

speaker vent

To better illustrate the difference, two audio files recorded at the mouth of each port are provided for comparison of the airflow noise, showing the airflow noise for an identical rising 40 Hz SPL:

System & Prices

Our Hourglass operates with tube tuning frequency equivalences to facilitate choice with port simulation software (VituixCAD).

Typical Specifications