Measuring Horn Loading Efficiency at the Throat
Horn-loaded loudspeakers can be spectacular, yet their apparent efficiency depends on the driver, horn, crossover and room. A quick in-room volume comparison may reward a sensitive system while revealing little about the acoustic load at the horn throat.
Evaluating horn loading efficiency with throat pressure measurements shifts attention to that controlled interface. It gives designers a repeatable way to compare compression drivers, horn geometry and termination conditions before a full cabinet is built.
What throat pressure reveals
The throat is the narrow end of a horn, and its acoustic impedance determines how hard the compression driver works. A driver developing high pressure there is delivering strong acoustic drive into the horn, but pressure alone does not reveal how efficiently electrical power becomes acoustic power. The same reading can arise from a well-matched load or a narrow, resonant region.
A useful pressure curve should therefore show a broad, repeatable response with low distortion. Compare systems at identical electrical drive, record the horn and driver used, and examine whether added sensitivity comes from a stable load across the passband rather than one conspicuous peak. Throat gain and horn efficiency are related, but they are not interchangeable measurements.
Preparing a repeatable measurement
Start with a low-mass, calibrated measurement microphone, a throat-compatible coupler, a low-noise preamplifier and an amplifier capable of driving the compression driver’s nominal impedance. A specialist retailer such as Shoku-E audio is one place to investigate measurement hardware, but verify the individual microphone’s calibration certificate rather than trusting a generic specification. The coupler must fill the throat without changing its effective area; an obstructive probe can create a different acoustic load.
Connect the intended compression driver and horn, warm the electronics, then calibrate the microphone at 1 kHz with an acoustic calibrator. Sweep stepped sines through the horn’s intended operating band, keeping level constant and below the onset of nonlinear compression. Record actual voltage and current at each frequency, repeat the sweep and check that the noise floor sits at least 10 dB below the reading. The microphone position must remain fixed for every comparison.
Turning pressure into efficiency data
Convert the measured RMS pressure into sound pressure level with (L_{p,t}=20\log_{10}(p_{\mathrm{rms}}/20,\mu\mathrm{Pa})). Then normalise it to a stated electrical input. A 2.83 V drive equals 1 W for a nominal 4 or 8 ohm driver, but becomes misleading when impedance changes with frequency. Measuring actual input power is more reliable: (P_e=V_{\mathrm{rms}}I_{\mathrm{rms}}\cos\phi). The resulting (S_t=L_{p,t}-10\log_{10}(P_e/1,\mathrm{W})) is throat-pressure sensitivity referenced to 1 W, not the familiar 1 metre loudspeaker sensitivity.
True loading efficiency also needs the acoustic power accepted at the throat. With a reasonably uniform throat field and measured complex acoustic impedance (Z_t), calculate (P_a=p_{\mathrm{rms}}^2\operatorname{Re}(1/Z_t)), then determine (\eta=P_a/P_e). Without that impedance or a standardised horn-loading test, throat pressure supports reliable relative comparisons but not an absolute efficiency claim. Report the microphone correction, averaging method, bandwidth and test level alongside every result.
Comparing results without confusion
Different measurements answer different questions, so keep their labels clear:
| Measurement | Useful for | Does not establish |
|---|---|---|
| Throat pressure at fixed electrical power | Driver-to-horn gain and response shape | Total output in a room |
| Normalised throat SPL | Sensitivity comparisons between matched systems | Absolute efficiency without throat impedance |
| Calculated throat acoustic power | Loading efficiency when impedance is known | Directivity or level at the listening position |
| In-room SPL at several angles | Directivity, room interaction and tonal balance | Intrinsic loading efficiency |
Pressure is most valuable when measured at one watt and compared over the same band. A horn that boosts 2 kHz by several decibels may be lively but inefficient if the increase is narrow or distorted. Conversely, moderate throat pressure can support greater output when the flare, crossover and driver operate within their intended range.
- Normalise every result to the same electrical input.
- Keep the coupler, microphone and horn orientation fixed.
- Compare broad-band averages instead of isolated peaks.
- Repeat measurements to establish repeatability.
Driver and market realities in Australia
Driver selection changes the load in ways that SPL alone can hide. TAD-Pioneer compression drivers with different throat diameters, impedance curves and maximum power ratings require matching horns and terminations. A horn that accepts one driver efficiently may load another near its compression limit. Include the intended crossover, cabinet volume and any damping in the test chain rather than judging the throat in isolation.
Australian buyers also face a practical distance problem. Sydney and Melbourne have established hi-fi dealers, while listeners in Perth, Adelaide or regional NSW may have to choose from written measurements before travelling. Imported Japanese drivers and European horns bring freight, GST, Australian-dollar pricing and warranty support into the decision. Nominal 240 V mains and Queensland heat also belong in the test plan: use stable power and let the driver and electronics reach a consistent temperature before comparing readings.
Applying measurements to real systems
Use the data to set targets for a passive crossover and cable run. Sunship Audio’s speaker wire gauge guide explains how conductor gauge and length affect losses in horn systems; cable does not alter throat loading, but excessive loss can leave a compression driver short of clean voltage. For a custom build, compare the predicted crossover point with the measured pressure response and consider the passive network’s impedance and phase behaviour.
- Keep raw curves, calibration files and test settings.
- Record the driver, horn, crossover and cable length.
- Flag nonlinear compression, resonance peaks and unstable readings.
- Confirm the result with listening-room measurements.
Finally, compare the chosen TAD-Pioneer driver, bi-radial wooden horn and braced birch-plywood cabinet as one system. Sunship Audio’s Berlin demonstration room can reveal directivity and tonal balance that a throat plot cannot, while a local dealer audition confirms the result under Australian mains and room conditions. The strongest assessment comes from agreement between pressure data, distortion checks and listening, rather than one impressive decibel number.