GoodSoundClub - Romy the Cat's Audio Site

In the Forum: Horn-Loaded Speakers
In the Thread: Impulse response, short notes and midbass horns.

Post Subject: John Hasquin’s commentary on the subject.Posted by Romy the Cat on: 6/20/2011

From: John Hasquin
Sent: Wednesday, June 15, 2011
To: Bessnow, Roman
Subject: Re: Meow

"I think your decay is prolonged because the larger room is providing acoustical pressure feedback into your 45Hz lower bass horn. This feedback is modulating your horn and creating artificial decay that is not supposed to be there. This is almost like echo, but not exactly. Like you said, the large mouth acts as sensitive microphone now. The smaller room you had was not able to provide enough acoustical pressure feedback at a frequency that affected your tractrix midbass horns. Also the tractrix midbass horn’s smaller mouth and smaller woofer would be less sensitive to this effect. The larger room’s feedback frequency is much lower so it affects the lower bass horn with its large mouth instead of the tractrix midbass horns. The larger room volume also boosts the acoustical gain of this feedback just like a larger box boosts the efficiency of high sensitivity woofers. I don’t see room modifications being possible or practical. The room is what it is. The only ways I can think of trying to control this problem is with amplification change or woofer change. It might be interesting to try a transconductance or current amplifier. Something like the Pass Labs F1 amplifier. The reason for this is the transconductance amplifier is a current control amplifier so it sends current to the voice coil in relation to the music signal. In other words it is always controlling voice coil current and if an outside acoustical force tries to modulate the cone the current in the voice coil acts as an electronic brake to prevent the cone from being moved. The higher output impedance of the transconductance amplifier also serves as a load for any energy created in the voice coil by cone modulation. Most amplifiers are voltage controlled and have low output impedance. This low impedance cannot provide any load for the modulated energy to be dissipated so the cone continues to move and exaggerates decay. Some people believe you want a low impedance to serve as an electric brake for the woofer cone, this is not true. In order to provide cone braking, the energy in the voice coil needs a load to dissipate into. This is why unused speakers sitting in a room will act as a sound trap. The below thread shows that the tapped horn acted as a better bass trap when the speaker leads were left open instead of shorted together.

http://www.diyaudio.com/forums/subwoofers/182464-th-bass-trap.html

The other option is to switch to a woofer with a much high BL factor. Generally, a high BL woofer will be more resistant to outside modulation and more closely follow its input signal. This may not be a path you want to take because you would have to find one that works well in your horn. Any woofer change would alter the tone and tuning of your lower bass horn. I don’t know if this would be something you would want to try. I have a feeling that most modern woofers may not result in a satisfactory sonic result even if they solve the extended decay problem."

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